Preface

Extraction is an important separation process that is extensively used in variousapplications in food engineering. Extraction is used either to recover importantfood components, being a main processing stage for the production of certainfood products (sugar, oils, proteins), or to isolate desired components (antioxi-dants, flavors). It also removes contaminants and other undesirable components(alkaloids, cholesterol) from food sources.

Extraction, as a major operation of the manufacturing process in manyfood industries, requires a food engineer to have knowledge of the thermody-namics, theory, methods, and systems of extraction, while the feasible designand efficient operation of an extraction process requires knowledge of the tech-nological parameters of the process and of optimization methods respectively.

Solvent or water extraction, liquid–liquid extraction, and leaching havealready been incorporated in basic food processing, while supercritical fluidextraction (SFE) is a promising alternative method for new or improved applica-tions in food engineering; SFE is recommended for the production of high-valuefood components as natural food products.

Despite the great variety of extraction applications already in use, it isdifficult to interpret and transfer theory to application at the industrial scale.Even in recent applications of extraction, optimal extraction or separation condi-tions are obtained empirically. One primary goal of food process engineering isfinding out how an extraction process works best—achieving the optimum inrespect of yield, quality, or cost—and consequently finding the adjustable pro-cessing factor values that produce the optimum.

iii

iv Preface

Optimization of extraction processes is needed either for extraction pro-cesses that are already in use or for future process designs for certain applica-tions in food engineering. Consequently, it is important for the food industryto optimize food extraction processes; that is, to attain the optimum operatingconditions (yield, quality, operation time, and cost) using appropriate methodsand techniques.

In this book, we review the extraction methods used in food processingfrom fundamental theory to optimum practical application using the relevantequipment and the appropriate methods of optimization. All of the contributorsare experts in the field of extraction, in research as well as in applications inindustrial food processes. This book will be useful for food process engineerswho employ extraction processes.

In order to introduce and apply an extraction process in the food industry,the knowledge of thermodynamic principles and the existing extractive systemsis necessary. Irrespective of the extraction method used, the feasibility of theprocess or the system is affected by the operation parameters of the process orsystem. Consequently, the best result in respect of yield or quality characteristicsof the final product should be achieved. That is the objective of the optimizationof a process or a system.

An elementary review of thermodynamics is presented in Chapter 1, con-taining the basic laws, glossary, concepts, and relations of thermodynamics,particularly of chemical thermodynamics covering mixtures, solutions, phaseequalibria, chemical reactions, etc. Chapter 1 presents fundamental knowledgeuseful for the understanding of extraction theory, operation, and applications infood processing.

Solid–liquid extraction or the leaching proces is described in Chapter 2.The characteristics and steps, as well as the basic variables affecting the extrac-tion operation in relation to the solvent and the microstructure of the solid beingextracted, are described. The relationship between the solid microstructure andthe extraction rate based on the mass transfer mechanism due to liquid diffusioninside the solid is also explained.

Supercritical fluid extraction (SFE) using supercritical fluids (SCFs), anovel method providing the possibility for “green” processing of foods, is re-viewed in Chapter 3. The basic (physical and chemical) properties of SCFs,particularly of the commonly used SC-CO2, and the key factors that play signifi-cant roles in the success of the SFE operation (as solubility of solutes in SCFs,phase equilibria, mass transfer) are discussed; the effective use of SCFs in ex-traction and fractionation of food-related materials is also discussed.

The methods and equipment used in conventional solvent extraction andthe relative extraction systems that have been applied in food industrial pro-cesses are presented in Chapter 4 with the equipment required for solid pretreat-

Preface v

ment and solvent recovery. Similarly, for the alternative SFE method the selec-tion and design of the required equipment are discussed.

Optimization theory and appropriate methods involved in design and pro-cess operation having economical and technological interest for food engineersare discussed in Chapter 5. An extraction process should operate efficiently bydetermining the operation time in order to achieve the best yield. The applica-tions of optimization in food engineering and especially in food extraction (con-ventional or SFE) processing are reviewed.

In the following chapters, the most important applications of extractionprocesses in food production industry are presented under the aspect of optimi-zation of the already operating extraction systems; novel and alternative opera-tions and case studies are also presented.

Vegetable oil extraction of oil-bearing materials is covered in Chapter 6.The most efficient method of solvent extraction and the factors affecting theoperation such as the solvent characteristics, extraction temperature, solid pre-treatment, modes of operation, and equipment are discussed. Commercial unitsused for oil extraction and extraction technology for oilseeds with the pre- andpost-treatment are presented; extraction of essential oils is also discussed.

Extraction processes for protein isolation from various edible proteinsources (oilseeds, cereals, etc.) and production of protein products with manyapplications in food systems are covered in Chapter 7. Commercial processingand advanced methods of vegetable protein isolates are discussed from the view-point of yield and quality (nutritional, organoleptic, functional properties) ofprotein products.

Sugar extraction from sugar beet and sugar cane is presented in Chapter8, as extraction is one of the major stages in the sugar manufacturing industry.Characteristics of sugar beet and cane, parameters affecting the extraction pro-cesses, and the optimization of the industrial process of beet sugar are discussed.Techniques of extraction, extractors, and other equipment required for industrialsugar production are presented. Extraction of starch and carbohydrates fromcorn is also discussed.

The extraction processes for isolation of desired components such as fla-vor and aroma substances or compounds with antioxidant activity (natural anti-oxidants) from plant sources are reported in Chapters 9 and 10, respectively.The main sources of natural antioxidants and synergists, the relative extractionprocesses, and the effect of processing parameters, as well as the conventionalor SFE extraction procedures and purification of extracts, are discussed. Solventextraction is one of the most important methods for producing natural extractsof aroma and flavor compounds; extraction solvents used, SFE processes, andapplications of solvent extraction of flavor and aroma compounds are discussed.

The extraction processes for removal of undesirable or toxic constituents,

vi Preface

such as alkaloids from natural products, or compounds with adverse effect onhuman health (e.g., cholesterol) from various food products with emphasis onSFE processes are covered in Chapters 11 and 12, respectively. The physiologi-cal effects and applications of alkaloids, the commercial-scale operations forextraction and isolation of the most valuable alkaloids, especially the decaffeina-tion of coffee and black tea using conventional extraction or SFE, are described.Similarly, the physiological effects of cholesterol on health and the extractionand fractionation processes of cholesterol from milk, fat, eggs, and meats aredescribed. Both for alkaloids and cholesterol removal, future prospects usingSFE are offered.

The safety, health, and environmental issues of solvent extraction, con-cerning mainly the edible oil and fat extraction industry, are described in Chap-ter 13, along with regulatory concerns and toxicity of solvents (hexane or alter-native solvents) in commercial processes.

Constantina TziaGeorge Liadakis

Contents

Preface iiiContributors ix

1. Introduction: Fundamental Notes on Chemical Thermodynamics 1Petros Tzias

2. Solid–Liquid Extraction 35Jose Miguel Aguilera

3. Supercritical Fluid Extraction in Food Engineering 57Nurhan Turgut Dunford, Jerry W. King, and Gary R. List

4. Extraction Systems 95Rudolf Eggers and Philip T. Jaeger

5. Optimization 137Constantina Tzia

6. Fats and Oils from Plant Materials 181Lei Xu and Levente L. Diosady

7. Proteins from Plant Materials 217Lei Xu and Levente L. Diosady

vii

viii Contents

8. Sugars and Carbohydrates 235Pascal Christodoulou

9. Flavor and Aroma Substances 305Youn-Woo Lee and Youn Yong Lee

10. Extraction of Natural Antioxidants 329Vassiliki Oreopoulou

11. Extraction of Alkaloids from Natural Plants UsingSupercritical Fluids 347Marleny D. A. Saldana and Rahoma S. Mohamed

12. Removal of Cholesterol from Food Products UsingSupercritical Fluids 369Rahoma S. Mohamed, Marleny D. A. Saldana,Alvaro B. A. de Azevedo, and Uiram Kopcak

13. Solvent Extraction: Safety, Health, and Environmental Issues 391Phillip J. Wakelyn and Peter J. Wan

Index 429

Contributors

Jose Miguel Aguilera Universidad Catolica de Chile, Santiago, Chile

Pascal Christodoulou Hellenic Sugar Industry S.A., Thessaloniki, Greece

Alvaro B. A. de Azevedo The State University of Campinas–Unicamp, Cam-pinas, Sao Paulo, Brazil

Levente L. Diosady Department of Chemical Engineering and AppliedChemistry, University of Toronto, Toronto, Ontario, Canada

Nurhan Turgut Dunford Food and Agriculture Products Research and Tech-nology Center, and Department of Plant and Soil Sciences, Oklahoma StateUniversity, Stillwater, Oklahoma, U.S.A.

Rudolf Eggers Technische Universitat Hamburg-Harburg, Hamburg,Germany

Philip T. Jaeger Technische Universitat Hamburg-Harburg, Hamburg, Ger-many

Jerry W. King National Center for Agricultural Utilization Research, Ag-ricultural Research Service, U.S. Department of Agriculture, Peoria, Illinois,U.S.A.

ix

x Contributors

Uiram Kopcak The State University of Campinas–Unicamp, Campinas, SaoPaulo, Brazil

Youn-Woo Lee Supercritical Fluid Research Lab, Korea Institute of Scienceand Technology, Seoul, Korea

Youn Yong Lee Korea Institute of Science and Technology, Seoul, Korea

Gary R. List U.S. Department of Agriculture, Peoria, Illinois, U.S.A.

Rahoma S. Mohamed School of Chemical Engineering, The State Universityof Campinas–Unicamp, Campinas, Sao Paulo, Brazil

Vassiliki Oreopoulou National Technical University of Athens, Athens,Greece

Marleny D. A. Saldana The State University of Campinas–Unicamp, Cam-pinas, Sao Paulo, Brazil

Constantina Tzia National Technical University of Athens, Athens, Greece

Petros Tzias Athena S.A., Athens, Greece

Phillip J. Wakelyn National Cotton Council, Washington, D.C., U.S.A.

Peter J. Wan U.S. Department of Agriculture, New Orleans, Louisiana,U.S.A.

Lei Xu University of Toronto, Toronto, Ontario, Canada

1IntroductionFundamental Notes onChemical Thermodynamics

Petros TziasAthena S.A.Athens, Greece

I. INTRODUCTION

Thermodynamics is a very useful tool for many scientists and engineers, includ-ing geologists, geophysicists, and mechanical engineers, and is considered the sci-entific cornerstone for chemical engineers.

When we study a handbook containing papers from a certain field ofchemical engineering, it is very useful to have some notes on thermodynamicson hand to help us understand what we’re reading. This is the purpose of thischapter; to give a brief, elementary review of thermodynamics, reminding to thereader the basic laws, glossary, concepts, and relations of this important branch ofscience so as to make the study of the present volume much easier and pleasant.

This chapter is intended for chemical engineers and chemists, and for thisreason we devote most of this chapter to dealing with chemical thermodynamics,covering mixtures, solutions, phase equilibria, chemical reactions, etc.

We are going to cite simple definitions of the different thermodynamicfunctions and quantities, and we will not enter to any analytical description orphilosophical discussion about them; similarly we will not prove the mentionedthermodynamic relations, as these are beyond the scope of this chapter.

We will deal mainly with reversible processes, so far as an interest is inchemical thermodynamics, and we will not refer to statistical or nuclear andrelativistic phenomena.

1

2 Tzias

II. SCOPE

Thermodynamics is a branch of physical sciences concerned with the study ofthe transformation of heat, work, and other kinds of energy (electrical, lightenergy, etc.) from one form to another, in the different physicochemical phe-nomena and determines the laws and relations governing and describing theseenergy transformations.

Historically (1), many scientists studied the interconversion of heat andwork, from N.L.S. Carnot with his famous ideal gas cycle, to Clausius, wholaid the foundations of the classical thermodynamics with the expression of thefirst and second laws of thermodynamics.

Later, J. Willard Gibbs extended the application of the thermodynamicpostulates and relations to chemical reaction and phase equilibria putting thefoundations for the development of the field, which we call today chemicalthermodynamics.

Thermodynamics consists of a collection of equalities and inequalitieswhich interrelate physical and chemical properties of substances as well as somephysical or chemical phenomena. These relations are deduced in a mathematicalway from some laws, the thermodynamic laws, which are derived directly fromexperience. The physical quantities used are taken either from physics, or theyare introduced in thermodynamics. Using these relations, we can predict thepossible direction of chemical reactions or the final result of a physical process,as well as the quantities of the different kinds of energy involved in these trans-formations.

Thermodynamics is an experimental science (2). All the physical or chem-ical quantities used in its relations are independently measurable, but some ofthem are easier to be measured than others. Another feature of thermodynamicsthat makes it very important and useful is the capability of calculation, throughits relations, quantities measured with difficulty or low accuracy by others mea-sured easier and more accurately.

Another advantage is that very often from existed data of some physicalquantities we can calculate through thermodynamic relations the values of thephysical quantities we are interested in, in this way avoiding long and difficultexperiments and also saving time and money.

Therefore, it becomes obvious that thermodynamics can be a very usefultool to a chemist or a chemical engineer.

III. GLOSSARY OF BASIC THERMODYNAMIC TERMS (2–6)

There are a variety of thermodynamic terms, the most common of which aredefined below, together with their SI units (3).

Introduction 3

The amount of substance (2) nb of an entity b is a physical quantity propor-tional to the number nb of entities b in the system. The SI unit of the amount ofsubstance is the mol.

A. Pressure, Volume, Temperature

Pressure and volume are concepts taken from physics.Pressure is defined as the ratio of a perpendicular force to a surface by

the area of this surface. The SI unit of pressure is the Pa.Volume (m3) is the three-dimensional space occupied by a substance. Spe-

cific volume (m3/kg) is the volume per unit mass. Molar volume is the volumedivided by the amount of substance. Density is the reciprocal of the specificvolume.

Temperature is a fundamental concept used in thermodynamics, and itsdefinition is a very difficult matter. However we could simply describe it as thedegree of hotness of a substance or as the property of matter which has equalmagnitude in systems, connected by diathermic walls and where thermal flowdoes not exist (3).

B. Temperature Scales (4)

The Celcius scale (°C) is established by assigning the value 0.01°C to the triplepoint of water and the value 100°C to the boiling point of water at an atmo-spheric pressure of 1 standard atm (760 torr).

The absolute temperature scale (T) is a scale which is related to the Cel-cius scale by the relation:

T = t (°C) + 273.15

T is given in degrees Kelvin. In this scale it is enough to define just one pointand this is the triple of water equal to 273.16 K. The freezing point of water at1 atm is 273.15 K.

The ideal gas temperature scale defined as:

T = 273.16K limP273.16→0

� PT

P273.16�

where PT and P273,16 are the pressures of a gas trapped in a gas thermometer atthe temperatures T and 273.16 K. The ideal gas temperature scale is the samewith the absolute scale. Constant volume gas thermometers are used to deter-mine the thermodynamic absolute temperatures.

The International Practical Temperature Scale (IPTS-68). This scale givesthe temperatures at some reproducible fixed points together with some interpo-

4 Tzias

lating instruments and functions, by which we find the temperatures betweenthese points. The fixed points as well as the functions have been determined byconstant volume gas thermometers. The IPTS-68 gives the possibility by theabove mentioned instruments and functions to measure in an easy and accurateway the absolute temperature. In the literature (4), we can find temperatures atdifferent fixed points as well as the instruments used to measure the absolutetemperature between these points. In the SI system for the absolute temperaturewe use the degree K.

C. System

In thermodynamics, we call any part of the real world we are choosing to studya system. All the rest of the parts of the world are the surroundings of thesystem. Practically by surroundings we consider the part of the world aroundthe system which can interact with it.

A system is closed if no matter enters or leaves it during any process westudy. Otherwise it is open. A system is called isolated if neither matter norenergy enters or leaves it.

The state of a system is defined by the values of its properties. Propertiesare physical quantities like temperature, volume, pressure, etc., which are relatedwith a system and they have fixed values at any given state of the system. Inorder to define the state of a system, it is not necessary to know the values ofall its properties, but only a certain number of them, which are called indepen-dent and all the rest (dependent) can be calculated by the values of the indepen-dent ones. Since a property is fixed by the state of a system it is also called astate function. A mathematical relationship between thermodynamic state func-tions is called an “equation of state.”

D. Extensive and Intensive Properties

Considering that we divide a system in different parts, if for one property itsvalue for the whole system is equal to the sum of its values for the differentparts, then this property is called extensive. Extensive properties are the volume,the mass, the internal energy, etc.

If the value of a property for the whole system is equal to the values forits different parts, then the property is called intensive. Intensive properties arethe temperature, the pressure, the molar volume, the density, etc.

E. Phase

If for a system, throughout all its parts, all its intensive properties have the samevalue, then the system is a homogeneous one and is called a phase. One system

Introduction 5

can consist of more than one phases. In that case it is heterogeneous and someof its intensive properties have not the same value at all its parts.

One phase can be open when it exchanges matter either with its surround-ings or with another phase within the system. In the opposite case it is calledclosed.

F. Process

Process is the pathway through which one system passes from one state toanother.

If during a process the temperature of the system remains constant, theprocess is called isothermic, if the volume of the system remains constant, theprocess is called isometric or isochoric, and if the pressure remains constant,the process is called isobaric. If no heat enters or leaves a system undergoingone process the system and the process are called adiabatic.

A process is called reversible if it takes place slowly and in such a waythat at any stage of the process the properties of the system differ from equilib-rium by infinitesimal amounts. Otherwise the process is called irreversible. Allnatural processes are irreversible.

G. Molar Quantities

From any extensive quantity X of a phase, it is defined an intensive quantityXm by the relation:

Xm = X

∑i

ni

(1)

where ∑i

ni is the sum of the amounts of the different substances contained in

this phase.

H. Mole Fraction

The mole fraction Xa of a substance a in a phase is defined by the ratio:

Xa =na

∑i

ni

(2)

where na the amount of substance of a and ∑i

ni the sum of the amounts of all

substances in this phase.It follows immediately from the above definition that

6 Tzias

∑i

Xi = 1 (3)

I. Molality

Molality m is the number of moles of a substance in 1 kg of solvent.

J. Molarity

Molarity c is the number of moles of a substance in 1 L of solvent.

K. Partial Molar Quantities

From any extensive quantity X of a phase we define an intensive quantity calledthe partial molar quantity Xa of the substance a in the phase by the relation:

Xa = �∂X∂na�

T,P,ni≠na

(4)

where ni ≠ na means all n’s except na in this phase.

IV. THE CONCEPTS OF W, PE, KE, U, Q AND S (6–8)

W (work), PE (potential energy), and KE (kinetic energy) are concepts bor-rowed from physics.

Work (W) is produced by a force (F) acting on a system and replacing itby a distance (ds) in the direction of the force and equals to:

dW = F � ds

In the case of a uniform pressure P acting on a system’s wall of surfaceS and replacing it by a distance x

dW = −P � s � dx or dW = −P � dV (5)

By energy we mean the ability of a system to produce work.Potential energy (PE) is the energy possessed by a system because of its

position.

PE = m � g � h (6)

where m is the mass of the system, g the gravity acceleration and h the heightof the system from zero level.

Kinetic energy (KE) is the energy possessed by a system of mass m, be-cause of its velocity (ν) and is equal to:

Introduction 7

KE = mE2

2(7)

Internal energy (U) is the total energy, except potential and kinetic, con-tained within a system at a certain state. It is a magnitude determined by thestate of a system. We can’t measure the absolute value of U, but only differences∆U = U2 − U1 between two states (1) and (2). ∆U depends only on the states (1)and (2) and not on the path followed to pass from one to the other.

Heat (Q) is the amount of energy, transferred from one system to anotherbecause of the difference in the temperature of the two systems. The amount ofheat depends on the followed path and not on the original and final states of aprocedure.

For a system receiving heat Q,

Q = C � (T2 − T1) (8)

where C is the heat capacity of the system and T2, T1 its final and originaltemperatures.

The entropy (S) (6) is an extensive property, depending on the state of asystem. It can be defined as:

dS = dQT

(9)

where dS is the heat received by it in a reversible way at a temperature T.In natural processes S always increases and

dQ ≤ T � dS (10)

The SI unit for W, PE, KE, and Q is the Joule.

The SI unit for S isJoule

K.

V. THERMODYNAMIC LAWS (2, 5, 6, 7)

Thermodynamic laws are laws formed from experience, and there is no excep-tion to them. There are several expressions for them all equivalent with eachother. Below are given the most common ones.

A. First Law of Thermodynamics

In any process the total energy is conserved. In other words, there is no devicewhich can create or eliminate energy. Considering the transformation of heat towork or vice-versa in a system, the first law can be expressed as:

8 Tzias

dU = dW + dQ (11)

where dU is the variation of the internal energy of the system, dW is the workproduced, and dQ is the heat transferred.

dU is an exact differential depending only upon the original and final stateof the system, but dQ and dW are not exact differentials and depend upon thepathway of the process. By substituting Eqs. (5) and (9) to (11), the first lawcan be expressed as:

dU = −P � dV + T � dS (12)

B. Second Law of Thermodynamics

It is not possible to transfer heat from a lower temperature to a higher tempera-ture without the expenditure of work. In other words, in any process the totalentropy of an isolated system increases.

C. Third Law of Thermodynamics

The expression of the third law is not possible without reference to statisticalmechanics (2). As expression for the third law we could give Plank’s postulate(7): “At 0K the entropy of a pure crystalline is zero.”

D. Law of Thermal Equilibrium

If two systems are in thermal equilibrium with a third one, then they are also inthermal equilibrium between them.

This law is not an independent one but it is derived from the first andsecond thermodynamic laws (2).

VI. THERMODYNAMIC FUNCTIONS AND SOMERELATIONS FOR ONE-PHASE CLOSED SYSTEM (6, 7)

We have already met the properties P, V, T and the thermodynamic functionsU, Q, W, and S and for the last two functions:

W = −P � dV and dS = dQT

Furthermore, there are the following very important thermodynamic func-tions:

the enthalpy H = U + P � V (13)the Helmholtz free energy A = U − T � S (14)

Introduction 9

and the Gibbs energy or Gibbs function

G = H − T � S = A + P � V = U − T � S + P � V (15)

All the three above functions are state functions and they have the dimen-sions of energy.

Since they are state functions their differentials are exact differentials andfor reversible processes we have:

dH = T � dS + V � dP (16)dA = −S � dT − P � dV (17)dG = −S � dT + V � dP (18)

By applying the properties of exact differentials we can obtain the follow-ing very useful relations:

�∂U∂V�

S

= −P, �∂U∂S�

V

= T (19)

�∂A∂T�

V

= −S, �∂A∂V�

T

= −P (20)

�∂G∂P�

T

= V, �∂G∂T�

P

= −S (21)

�∂H∂S�

P

= T, �∂H∂P�

S

= V (22)

and also the following:

�∂T∂V�

S

= −�∂P∂S�

V

(23)

�∂T∂P�

S

= �∂V∂S�

P

(24)

�∂S∂V�

T

= �∂P∂T�

V

(25)

�∂S∂P�

T

= −�∂V∂T�

P

(26)

�∂P∂T�

V

�∂T∂V�

P

�∂V∂P�

T

= −1 (27)

10 Tzias

a = 1V �∂V

∂T�P

is the coefficient of thermal expansion (28)

K = − 1V �∂V

∂P�T

is the isothermal compressibility (29)

KS = − 1V �∂V

∂P�S

is the adiabatic compressibility (30)

CV = �∂U∂T�

V

is the heat capacity at constant volume (31)

CP = �∂H∂T�

P

is the heat capacity at constant pressure (32)

VII. FUNDAMENTAL INEQUALITIES

We have already seen (Sec. IV.) that although for a reversible process dQ = T �dS and for natural processes dQ ≤ T � dS. Similarly for natural processes fromEqs. (12), (16), (17), and (18) we derive:

dU ≤ T � dS − P � dV (33)dH ≤ T � dS + V � dP (34)dA ≥ −S � dT − P � dV (35)dG ≥ −S � dT + V � dP (36)

This means that for a natural process at equilibrium the above functions get thelowest value, except the entropy which gets the maximum.

In other words for any closed isolated system (2):

�∂S∂t �

U,V,ni

> 0 (37)

which means, that if anything happens in that system then S is increasing, andif

�∂S∂t �

U,V,ni

= 0 (38)

then the system is in equilibrium.Similarly, from:

Introduction 11

�∂U∂t �

S,V,ni

< 0 (39)

that if anything happens in a system at constant V, S and content then U isdecreasing, from:

�∂H∂t �

S,P,ni

< 0 (40)

that if anything happens in a system at constant S, P and content then H isdecreasing, from:

�∂A∂t �

T,V,ni

< 0 (41)

that is anything happens in a system at constant T, V, and content then A isdecreasing, and from:

�∂G∂t �

T,P,ni

< 0 (42)

that if anything happens in a system at constant T, P, and content then G isdecreasing.

The above (42) inequality is very important for chemists since most chem-ical reactions take place at constant T and P.

If in the place of the above inequalities we consider the respective equali-ties, this will mean that the system to which they are referred is in equilibrium.

VIII. RELATIONS OF THERMODYNAMIC FUNCTIONS INONE-PHASE OPEN SYSTEM (2, 6, 7)

We have already seen the Eq. (12) dU = −P � dV + T � dS, which is the expres-sion of the first law of thermodynamics for a change involving only the transfor-mation of energy. If we suppose that in the system under consideration there isalso addition or removal of matter, then the above equation should be writtenunder the following form:

dU = T � dS − P � dV + ∑i

µidni (43)

where dni is the amount of substance of species i transferred and µi its molarenergy.

12 Tzias

In a similar way, from Eqs. (16), (17), and (18) we derive:

dH = T � dS + V � dP + ∑i

µidni (44)

dA = −S � dT − P � dV + ∑i

µidni (45)

dG = −S � dT + V � dP + ∑i

µidni (46)

Eqs. (43) through (46) are called Gibbs equations.Considering U, H, A and G as the functions U(S, V, ni), H(S, P, ni), A(T,

V, ni) and G(T, P, ni) we can prove in a very easy way that the µi’s in theprevious four relations are equal and that:

µi = �∂U∂ni�

S,V,nk≠i

= �∂H∂ni�

S,P,nk≠i

= �∂A∂ni�

T,V,nk≠i

= �∂G∂ni�

T,P,nk≠i

(47)

where n k≠i indicates all the other species nk except ni.The above defined quantity µi is called the chemical potential of the sub-

stance i; it is an intensive thermodynamic function, it has the dimensions ofenergy per amount of substance and its unit in the SI system is the Joule permole.

By definition:

µi = R T lnλi (48)

where λi is called the absolute activity of the species i in the multicomponentsystem.

Integration of the Eq. (43) by keeping P, T, and ni constant (2) leads to:

U = S � T − P � V + ∑i

niµi (49)

or

G = ∑i

niµi (50)

Differentiation of (50) gives:

dG = ∑i

nidµi + ∑i

µidni (51)

Equating the expressions for dG in Eqs. (46) and (51) yields:

S � dT − V � dP + ∑i

nidµi = 0 (52)

which is known as the Gibbs-Duhem equation.Replacing in (52) µi by its form in function of activity (48) the Gibbs-

Duhem equation takes the form:

Introduction 13

S � dT − V � dP + ∑i

niRTdlnλi = 0 (53)

From Eqs. (46) by simple mathematical manipulations we can derive thefollowing useful relations:

�∂µi

∂T�P,ni

= −�∂S∂ni�

T,P,nk≠i

(54)

�∂µi

∂P�T,ni

= �∂V∂ni�

T,P,nk≠i

(55)

�∂µi

∂nk�

T,P,nk≠i

= �∂µk

∂ni�

T,P,nk≠i

(56)

�∂µi

∂T�V,ni

= −�∂S∂ni�

T,V,nk≠i

(57)

�∂µi

∂V�T,ni

= −�∂P∂ni�

T,V,nk≠i

(58)

�∂µi

∂nk�

T,V,nj≠i

= −�∂µk

∂ni�

T,V,nj≠i

(59)

IX. MIXTURES (2, 6, 7)

A system consisting of more than one substance is called mixture. A mixturemay exist in gaseous, liquid, or solid phase. We shall confine ourselves to binarymixtures, from which the extension to multicomponent ones is straightforward.

By definition a mixture is said to be ideal if for any component i (6):

µi = µ0i + R T lnXi (60)

where µi, µ0i are the chemical potentials of component i with a mole fraction Xi

in the mixture and of pure i respectively, both at the same P and T.In other words if for any component i in the mixture (2):

λi = Xi λ0i (61)

where λi, λ0i are the absolute activities of the component i with a mole fraction

Xi in the mixture and of pure i respectively, again at the same P and T.A mixture for which (60) or (61) are not valid is called a real mixture.

14 Tzias

In an ideal mixture the interactions between like and unlike species are thesame and the components in the mixture they behave as in the pure components.

A. Mixing Functions

For a binary mixture of components A and B with mole fractions (1-x) and xrespectively and for any extensive thermodynamic quantity X, such as G, A, H,S, or V the mixing function is defined as:

∆mix Xm = Xm (T, P, x) − (1-x) Xm (T, P, 0) − x Xm (T, P, 1) (62)

or

∆mix Xm = (1-x) [XA (T, P, 1-x) − Xm (T, P, 0)] + (63)x [XB (T, P, x) − Xm (T, P, 1)]

where Xm (T, P, x) is the molar function of the mixture at (T, P, x), XA (T, P,1-x), XB (T, P, x) the molar functions of A and B in this mixture at molefractions 1-x and x and Xm (T, P, 0), Xm (T, P, 1) the molar functions of thepure components A and B respectively.

From (62) and (63) using Eqs. (50) and (61) and for Xm = Gm we obtainfor an ideal mixture:

∆mix Gidm = R T[(1-x) ln (1 − x) + x lnx] > 0 (64)

From (21) and (64) at constant P, n we obtain:

∆mix Sidm = −R [(1-x) ln (1 − x) + x lnx] < 0 (65)

From (64), (65), and (15):

∆mix Hidm = 0 (66)

and from (21) and (64):

∆mix Vidm = 0 (67)

B. Excess Functions

By definition Excess function XEm is the difference between the real ∆mixXm and

the ideal one. That is:

XEm = ∆mixXm − ∆mixX

idm (68)

C. Thermodynamic Functions of Dilution

If in one binary or multicomponent one-phase homogeneous mixture of nonre-acting species, one component is in excess related to the others, and more of thisis added to the mixture, this process is called dilution.

Introduction 15

The thermodynamic functions of the dilution process are given as thedifference of thermodynamic functions of mixing between the final diluted stateand the original one, that is

∆Xm,dil = (∆Xm,mix)2 − (∆Xm,mix)1 (69)

D. Standard and Reference States of ThermodynamicFunctions (8, 9)

We define as standard thermodynamic function of a component i in a system atany temperature and at a fixed pressure the thermodynamic function of i at agiven composition. The state thus defined is called standard state.

Historically the standard states are defined at the fixed pressure of 1 atm(= 101.325 Pa) and in that case the standard Thermodynamic functions dependonly on the temperature.

Usually, for gases as standard state is accepted the pure ideal gas at 1 atmand for liquids and solids the pure liquids or solids at certain P, T where P canbe defined as 1 bar.

As reference state is called one state that is used as reference for thecalculation of the different thermodynamic functions.

Any thermodynamic function can be expressed in function of its standardor reference state.

X. GASES AND GASEOUS MIXTURES (5, 6, 8)

The PVT behavior of a pure fluid, e.g., a gas, can be expressed by the equation:

P � Vm = R � T (1 + B � P + C � P2 + . . . ) (70)

or

P � Vm = R � T (1 + bVm

+ c

V2m

+ . . . ) (71)

where Vm is the molar volume of the gas, Vm = Vn

, V the volume and n the mol

of the gas, and R is the gas constant, which in SI units is equal to 8.3144 J �mol−1 � K−1.

Eqs. (70) and (71) are called virial equations of state of a gas and thecoefficients b, c, . . . of (71) virial coefficients and they depend on the tempera-ture and on the type of chemical species of the gas. The coefficients B, C, . . .can be calculated from b, c, . . . .

when P → 0 then P � Vm = R � T (72)

16 Tzias

This is the equation of state of an ideal gas. The virial coefficients show thedeviation of a real gas from ideality.

The PVT behavior of the gases is also expressed by the aid of the com-pressibility factor:

z = PVm

RT(73)

For an ideal gas z = 1 and P � Vm = R � TOne other attempt to express the PVT behavior of the real gases is the

van der Waals equation:

�P + a

V2m� (Vm − b) = R � T (74)

which tries to take in account the volume (coefficient b) of the molecules andthe interactions between them (coefficient a).

For an ideal gas we can derive the following relations:

�∂H∂P�

T

= V − T�∂�nRT

P �∂T �

P

= V − nRTP

= 0 (75)

CP = CV + n R (76)

The Gibbs-Duhem equation at constant T and for a pure gas becomes:

n dµ = V � dP (77)

If the gas is ideal, substituting in (77) the volume from (72) we obtain:

n dµ = nRTdPP

(78)

or

dµ = R T d lnP (79)

and for a change at constant T from P1 to P2

µ2 − µ1 = R T ln�P2

P1� (80)

A. Fugacity (7, 10, 11)

To express the properties of a real gas in the same way with an ideal gas, Lewisand Randall (10) originated the term fugacity (f), which has the dimensions ofpressure.

Introduction 17

So, for a real gas or vapor instead of (79) or (80) we write (10):

dµ = R T dlnf (81)

or

µ2 − µ1 = R T lnf2

f1

(82)

where f is the fugacity of the gas or vapor.

The ratio Φ = fP

(83)

is called fugacity coefficient. Since ideal gas behavior is approached as P → 0then

limP→0

Φ = limP→0

fP= 1 (84)

As we will see in the phase equilibrium the fugacity of a liquid or a solidwhich is in equilibrium with its vapor is equal to the fugacity of its vapor.

The fugacity of a real pure gas at a given P, T can be evaluated (7) throughthe relation:

�ln fP� = �∫P

0�Vm

RT− 1

P�dP�T

(85)

if we know the equation of state of that gas.For multicomponent real gases, the partial fugacity fi of a component i is

defined in terms of the chemical potential µi as follows:

dµi = R T d(lnfi) (86)

In this case:

limP→0

fi

xiP= 1 (87)

where xi is the mole fraction of component i and P is the total pressure (not thepartial pressure Pi).

For multicomponent gases, the fugacity fi can be also evaluated (7)through the following relation, if we know the equation of state of that gasmixture:

18 Tzias

lnfi = ln(xi P) + �∫P0�(Vm)i

RT− 1

P��dP�T

(88)

where (Vm)i is the partial molar volume of species i.

B. The Standard State of a Gas Component (2)

The standard state of a gas component in a mixture of gases is given by:

µB (g, T, P, xc) = µ0B (g, T) + R T ln�xB

PP0� + ∫

P

0�VB(g, T, P, xC) − RT

P �dP (89)

where T, P, xC are the temperature, pressure and composition of the gas mixture,xB the mole fraction of B in the mixture and µ0

B (g, T) is the chemical potentialof the pure ideal gas, at temperature T and pressure P0. Historically P0 = 1 atm.

XI. LIQUID MIXTURES (2, 8)

All that was mentioned in Sec. IX and what it will follow are applicable toliquid and to solid mixtures as well.

For a liquid mixture using Eq. (48) we obtain:

µi = µ0i + R T ln

λi

λ0i

(90)

or

µi = µ0i + R T lnai (91)

where µi, µ0i are the chemical potentials of component i with mole fraction xi

and of the pure liquid i at certain P, T, λi, λ0i the absolute activities of µi and

µ0i respectively and

ai =λi

λ0i

(92)

is what we call relative activity of component i in the mixture. Some authorscall ai simply activity.

For the absolute activity, the activity coefficient fi is defined by the rela-tion:

λi = fi xi (93)

Introduction 19

and for the relative activity the corresponding activity coefficient γi is definedby the relation:

ai = γi xi (94)

When ai = 1 from (91) we obtain µi = µ0i , that is the term R T lnai gives

the difference between µi and its reference state µ0i at certain P, T and if P = 1

bar then the difference of µi from its standard state.

From (91) and (94) we get µi = µ0i + R T lnxi + R T lnγi (95)

Since for the case of an ideal mixture µi = µ0i + R T lnxi (Eq. [60]), the

term R T ln γi expresses the deviation of µi from ideality.After defining the standard state for µi the standard states for the other

thermodynamic functions can be derived in an easy way. In particular

S0i = −dµ0

i

dT(96)

H0i = µ0

i − Tdµ0

i

dT(97)

G0i = µ0

i (98)

where S0i , H0

i , and G0i are the standard molar functions of S, H, and G respectively.

XII. EQUILIBRIUM OF PHASES (2, 6, 8, 11, 12)

In many industrial processes there is coexistence of two or more phases. Whenthere is mass transfer from one phase to the other the phases are not in equilib-rium. The study of the mass transfer in these processes requires exact knowledgeof the phases at equilibrium. In this section, we will treat liquid-vapor equilib-rium states. Similar results can be derived from the treatment of liquid–solidand solid–vapor equilibrium.

We say that two or more phases are in equilibrium regarding several inten-sive properties if these properties have the same value in both phases. For exam-ple we have:

Thermal equilibrium, when the temperatures T of the different phases areequal

Hydrostatic equilibrium, when the pressures P of the different phases areequal

Chemical equilibrium, when there is no reaction between the constituentsof all the phases

Osmotic equilibrium, when P, T, and several µi are the sameDiffusive equilibrium, when P, T, and all µi are the same

20 Tzias

We will deal here with equilibrium states between phases where P, T, µi areequal in all phases.

A. The Phase Rule (4, 6, 8, 11)

For a closed, isolated system consisting of several phases a, b, c, . . . in thermal,hydrostatic and chemical equilibrium (fixed composition) and where there is notransfer of mass, at macroscopic observation, from one phase to the other, writ-ing the Gibbs-Duhem equations for each phase, since dP, dT and dni are zero itis derived that, for every component i, µa

i = µbi = µc

i = . . .The minimum number of intensive properties (pressure, temperature, mole

fractions, etc.) needed so that the state of a closed, isolated nonreacting systemis completely defined is called the degrees of freedom F.

Gibbs derived a very important rule involving phase equilibria which con-nects the degrees of freedom F with the number of phases P and the number ofdifferent substances C in the system. This rule is given by the following relation:

F = C + 2 − P (99)

For example in the P, T diagram of Fig. 1 for a pure substance we observethat for the regions of only one phase (P = 1), the relation (99) yields F = 2,which means that both P, T are needed for the definition of the state. For pointson the curves we have two phases P = 2 and F = 1 which means that only oneof P, T is needed for the definition of the state and finally at the triple point T,

Figure 1 P-T diagram for a pure substance.

Introduction 21

P = 3 and F = 0, which means that there is no degree of freedom and only onepair of P, T corresponds to the state of coexistence of three phases.

If a reaction is taking place in the system or we want to take in account apeculiarity of the equilibrium phases, for instance an azeotrope, then the phaserule must be modified.

For a single-component two phase system, following the vaporizationcurve (Fig. 1) up to the end point C, we observe that by increasing graduallythe temperature of the system we pass to vapor and liquid phases which theybecome more and more similar in density and molar volume and the meniscusseparating the two phases becomes more indistinct. Finally at the point C thetwo phases become identical and the meniscus between the two phases disap-pears. Beyond C there is no liquid or vapor phase, but only one single-fluidphase.

At the critical point

�∂P∂V�

TC

= 0, �∂2P

∂V2�TC

= 0 (100)

Point C is called the critical point and the corresponding P, T the critical Pres-sure PC and the critical Temperature TC of the studying substance.

B. The Chemical Potential in Phase Equilibria (7, 11)

In the previous section we saw that between two phases a, b in equilibrium forevery substance i

µai = µb

i (101)

or

µ0,ai + R T ln aa

i = µ0,bi + R T ln ab

i (102)

Since the µ0,ai and µ0,b

i are the chemical potentials of the pure i at the same P, Tthen

µ0,ai = µ0,b

i (103)

and consequently from (103)

aai = ab

i (104)

but this does not mean that necessarily the activity coefficients γai , γb

i will beequal since usually xa

i ≠ xbi

For the case of liquid–vapor equilibrium we have seen for the vapor phasethat

22 Tzias

µvi = µ0,v

i + R T lnfv

i

f 0,vi

(105)

while for the liquid phase

µRi = µ0,Ri + R T ln

λRiλ0,R

i

= µ0,Ri + R T ln aRi (106)

From the previous relations it is obtained the following relation, connect-ing the absolute activity, the relative activity and the fugacity of a component iin a system at equilibrium

fi

f 0i

= λi

λ0i

= ai (107)

The equations relating the thermodynamic functions of phases at equilib-rium are very important since from data of one phase we can calculate theproperties of the other phase.

C. Binary Vapor-Liquid Systems (7, 9, 11)

There are important differences between the behavior of a single componentvapor–liquid system and a multicomponent one. For instance, in a single com-ponent the vapor and liquid phases have the same composition, but not in amulticomponent system.

During the evaporation at constant pressure for a single-component systemthe temperature remains constant, but in a multicomponent system at constantP the temperature changes during the evaporation. It is obvious that the behaviorof a multicomponent system is more complicated than that of a single compo-nent. As an example of multicomponent system we study here a two-phasebinary system.

Fig. 2 shows a vapor-pressure, composition (X = mole fraction) diagramfor an ideal mixture of two liquids. At any mole fraction X between 0 and 1 thevapor pressure Pmix of the mixture is

Pmix = PA (1 − X) + PBX (108)

where PA, PB the vapor pressures of the pure components A, B.Fig. 3 shows the P, X diagram of a real mixture of two component liquids

A and B, completely miscible through the whole range of X. At any pressure acomposition of the liquid phase Xliq corresponds to a different composition Xvap

of the vapor phase, which is in equilibrium with the liquid. The compositionsof the liquid phase form a curve called bubble point line and the correspondingcompositions of the vapor phase form another curve called dew point line. Asimilar to Fig. 3 diagram can be drawn relating T with X.

Introduction 23

Figure 2 Vapor-pressure composition diagram for an ideal mixture of two liquids, Aand B.

Figure 3 Vapor-pressure composition diagram for a real mixture of two liquids.

24 Tzias

Each liquid component, when its mole fraction tends to 1, behaves like inan ideal solution.

Some binary liquid mixtures, at a fixed composition, have identical com-position in both the liquid and vapor phase. This composition is called azeo-tropic and the mixture azeotrope. In these cases, although the compositions ofthe two phases are equal, it does not mean that the mixture is an ideal one.

There are positive azeotropes with azeotropic pressure Paz higher than thevapor pressure of the two pure components and negative azeopropes with Paz

lower than the vapor pressure of the two pure components of the mixture.Fig. 4 shows the P, T diagram of a binary liquid mixture at a constant

composition. Since in this case there is one more degree of freedom from thesingle component system, in order to define the state of the system for the vapor+ liquid region we need both P, T (for the single component system it is neededeither only P or only T).

In Fig. 4, the region included inside the ABCDE curve is in the place ofthe vaporization line of Fig. 1. To pass from the pure liquid to the vapor underconstant pressure the temperature is changing. This passage becomes shorter aswe approach the critical point C. The end temperatures, at the start and the endof this process, are called bubble point and dew point temperatures respectivelyat the pressure of vaporization. At each composition there is one curve ABCDEand one critical point.

Similar behavior is observed for the processes of fusion and sublimationand it is not necessary to study them separately.

Figure 4 P-T diagram at constant composition for a mixture of two liquids.

Introduction 25

D. Principle of Corresponding States (7, 12)

For different real gases at the same pressure and temperature the molar volumes

Vm are different. The compressibility factor z = PVm

R Tdefined in Sect. X expresses

the deviations of the real gases from ideality and it is different for the differentgases at the same P and T.

However, it has been observed, by defining the reduced pressure Pr, thereduced temperature Tr and the reduced molar volume Vm,r by the relations:

Pr =PPC

, Tr =TTC

, Vm,r =Vm

Vm,C

(109)

where PC , TC, Vm,C are the critical P, T and Vm that for equal Pr, Tr the Vm,r ofall gases are approximately equal.

This is known as the van der Waals principle of corresponding states.The critical compressibility factor

zC =PC Vm,C

R TC

(110)

is also found experimentally to be in the narrow range 0.2–0.3 and it can beconsidered as a universal constant.

So we finally have

z = F (Pr, Tr) (111)

where F is the same function for all the gases.

E. Enthalpy and Entropy Change in a Two-PhaseTransition (2, 7, 12)

When there is transition from one phase to another we define a property calledchange of transition of state by the relation

∆ Mabi = Mb

i − Mai (112)

Thus for the case of vaporization we have:

∆ VRvi = Vv

i − VRi (113)

∆ HRvi = Hv

i − HRi (114)

∆ SRvi = Svi − SRi (115)

where by R and v we mean liquid and vapor respectively.Knowing that at equilibrium

µRi = µvi or GR

i = Gvi (116)

for a two-phase one-component system at each phase:

26 Tzias

Gi = Hi − T Si (117)

and dGi = Si dT − Vi dP (118)

From (117) and (118), it is finally derived

dPsati

dT= ∆SRvi∆VRv

i

(119)

dPsati

dT= ∆HRv

i

T∆VRvi

(120)

where Psati is the vapor pressure at the equilibrium of the two phases.

From Eqs. (119) and (120) we can calculate the entropy and enthalpychange of vaporization.

XIII. SOLUTIONS (2, 5, 11, 13)

For several mixtures it is convenient to distinguish some components from theothers, for instance when one solid, liquid, or gas has a limited solubility in aliquid and its mole fraction in the mixture does not cover the whole range from0 to 1.

In this case, by convention we call the liquid which has the higher molefraction the solvent, the component with the limited solubility solute, and themixture the solution. When the solvent is in excess and the solute in low concen-tration, the solution is called a dilute solution.

For a binary solution the chemical potential of the solvent is given as inmixtures of liquids by the relation:

µi = µ0i + R T lnai = µ0

i + R T lnγi xi (121)

where µ0i is the reference chemical potential of pure solvent at certain P, T. ai,γi

are, respectively, its relative activity and activity coefficient.When xi → 1 then also γi → 1, and the solvent behaves as in an ideal

mixture. At these compositions near the pure solvent the partial pressure andthe fugacity of the solvent are proportional to its mole fraction.

Pi = P0i xi or fi = f0

i xi (122)

where Pi, fi, xi are the partial pressure, fugacity, and mole fraction of the solventand P0

i , f 0i are the vapor pressure and the fugacity of the pure solvent respectively

at the temperature of the solution.In this region, the solvent behaves in an ideal way and the relations, Eq.

(122), express what we call as Raoult’s law (Fig. 5).

Introduction 27

Figure 5 Fugacity of a solvent and a solute as function of their mole fraction.

The standard chemical potential of the solvent is that of the pure liquid at1 atm and the temperature of the solution. For the solute, however, it is notpossible to define a similar standard state since there cannot be solutions aftera certain composition, with higher solute concentrations. In this case, it isadopted as standard state for the solute the hypothetical, ideal unit concentrationof solute solution, at certain pressure and temperature (reference state) or at thefixed pressure of 1 atm (standard state).

This solute standard state is derived by the extrapolation of the fugacityof the solute at conditions of infinite dilution (the mole fraction of all the solutesin the solution tend to zero), where the fugacity of the solute fi is proportionalto its mole fraction xi,

fi = K xi (123)

to the hypothetical state of solution with solute mole fraction 1.The relation (123) is called Henry’s law and K is a constant called Henry’s

constant.In the region of very dilute solutions where the relation (123) is valid the

solute behaves in an, by convention, ideal way which is different from the idealconditions near the pure solvent.

From the above mentioned the chemical potential of the solute in a solu-tion is given by:

µi = µ0i + R T lnγixi = µ0

i + R T lnfi

f 0i

(124)

28 Tzias

where γi, fi are the activity coefficient and the fugacity of i at a mole fraction xi

and f0i , µ0

i are the fugacity and the chemical potential of pure i at the referencestate of infinite dilution conditions and at the temperature and pressure of thesolution.

The standard chemical potential of a solvent is given by the relation (2):

µ02(T, P0) = µ*2 (T, P) + ∫

P0

P

V*2 (T, P)dP (125)

where P0, P are the standard pressure and the pressure of the solution respec-tively, * means is pure solvent, and V*2 the volume of pure solvent.

The standard chemical potential of a solute is given by the relation (2):

µ0i (T) = �µ1(T,P,m1) − RTln

m1

m01�∞ + ∫

P0

P

V∞1 (T,P)dP (126)

where ∞ means conditions at infinite dilution, µ1(T,P,m1) the chemical potentialat T, P, m1, m0

1 the standard molality, P0 the standard pressure and V∞1 the volume

of solute at conditions of infinite dilution.All the other thermodynamic functions and relations for the solution can

be derived in a straight mathematical way from the above relations.

XIV. ELECTROLYTE SOLUTION (2, 8, 14)

Electrolytes are a special class of solute substances, which involve several com-plications in their thermodynamic study, not found in solutions of nonelectro-lytes.

The difficulties arise from the fact that the electrolytes in solution arefound under complete or partial dissociation in the ions of which they are con-sisted. Because of the restriction of electrical neutrality in an electrolyte solutionit is not possible to define thermodynamic functions of one ion, for example itschemical potential, since this would imply the change of the amount of sub-stance of this ion by keeping constant the amount of substance of all the restions, which has no physical meaning.

To overcome this difficulty, we consider all the thermodynamic functionswith both the anions and the cations of one electrolyte.

For example, for the case of a strong, completely dissociated electrolyteof the type Mν+Aν−, where ν+ and ν− are the number of positive and negativeions, respectively, in the molecule of the electrolyte, we can write:

µMA = ν+ µ+ + ν− µ− (127)

Introduction 29

and using molalities

µ+ (T, P, m) = R T lnm+ + R T lnγ+ (T, P, m) + µ0+ (T, P) (128)

µ− (T, P, m) = R T lnm− + R T lnγ− (T, P, m) + µ0− (T, P) (129)

Substituting (128) and (129) to (127) we obtain:

µMA = R T lnmν++ mν−− + R T lnγν++ γν−− (T, P, m) +ν+µ0

+ (T, P) + ν−µ0− (T, P) (130)

The mean activity coefficient is defined as:

γν± = γν++ γν−− (131)

and the mean molality:

mν± = mν++ mν−− (132)

where ν = ν+ + ν−

From (131), (132) and (130) and by defining:

µ0MA (T, P) = ν+ µ0

+ (T, P) + ν− µ0− (T, P) (133)

µMA (P, T) = ν R T lnm± + ν R T lnγ± (T, P, m) + µ0MA (T, P) (134)

The m±, γ± and µ0MA (T, P) have a similar meaning as for the nonelectrolyte

solutions when m± → 0 then γ± → 1 and µMA (T, P) is equal to µ0MA (T, P) at the

reference state. To obtain the standard state for an electrolyte solute it is fol-lowed a similar procedure to that of a nonelectrolyte solute providing that theappropriate quantities are plotted (13, 14).

For the case of weak electrolytes the method to obtain expressions for thechemical potentials are the same as for strong electrolytes.

A. The Debye–Huckel Limiting Law (2, 6)

In 1923 Debye and Huckel developed a theory about the behavior of strongelectrolytes in dilute solutions. This theory was a mathematical treatment ofsome ideas previously assumed by Arrhenius regarding the dissociation of elec-trolytes in solution.

By this theory after a series of mathematical calculations we arrive to aformula giving the mean ionic activity coefficient, which for very dilute solu-tions by approximation takes the form:

ln γ± = C I1/2 z+ z− (135)

where C = (2 π N0ρs)1/2 (e2 / 4 π ε K T)3/2

30 Tzias

N0 = Avogadro’s numberρs = density of the pure solvente = the charge on a protonε = ε0 εr, where εr is the dielectric constant of the solvent and ε0 is thedielectric constant of a vacuum

T = temperature of the solution

K = RN0

, R = gas constant

I is called ionic strength and is given for a 1-1 type electrolyte by:

I = 12

(m+ z2+ + m− z2

−) (136)

m+, m−, the molalities of the ionsz2+, z2

−, the electrical charges of the ions.

The Debye-Huckel law is very accurate for very dilute solutions of strongelectrolytes.

For mixed electrolytes the theory is still valid with

I = 12 ∑i

miz2i (137)

XV. THERMOCHEMISTRY—CHEMICAL REACTIONEQUILIBRIUM (11, 15)

The application of thermodynamics to chemically reacting systems is very im-portant. Together with mass balance the first thermodynamic law under certainconditions gives exactly the energy absorbed (endothermic) or released (exother-mic) by a chemical reaction. For instance we can calculate the energy neededto produce from some substances other useful substances or the energy releasedfrom the combustion of a fuel.

The second thermodynamic law can predict if one chemical reaction willproceed to one direction or to the opposite and at which extend it will stop(equilibrium state).

However, in several cases, although from the second thermodynamic lawit results that one reaction should proceed to one direction, this reaction doesnot start and to overcome this hindrance, catalysts or other means are used.

But still, in these cases thermodynamics is useful, since through the pre-diction by the second thermodynamic law of the possibility of realization of areaction, it can allow us or release us from the trouble, to seek finding theappropriate catalyst for this reaction.

Introduction 31

A. Enthalpy of Formation and Enthalpy of Reaction (4, 5)

The heat of formation of any compound is the heat required to form that com-pound from its elements at a certain temperature and pressure. When this forma-tion is considered under constant pressure then the heat of formation is equal tothe enthalpy of formation.

The standard enthalpy of formation, ∆HF, of a compound is defined as theheat required to form the compound in its standard state of 1 atm pressure and25°C from its elements at the same standard conditions.

∆HF = hcompound − ∑i

(νihi)elements (138)

νi is the stoichiometric coefficient hcompound, hi the standard molar enthalpies offormation of the compound and of its elements respectively.

By convention the standard enthalpies of formation hi of all elements attheir more stable state are considered as zero, therefore

∆HF = hcompound (139)

Tables with standard enthalpies of formation of many compounds aregiven in books on thermodynamics.

The heat of reaction is the heat absorbed or rejected by the reaction. If thereaction takes place at constant P, the heat of reaction is equal to the enthalpy ofreaction.

The standard enthalpy of reaction is defined as the change in enthalpyfrom a reaction taking place at a constant pressure of 1 atm and constant temper-ature of 25°C.

The heat of combustion of any compound is defined as the heat of reactionresulting from the oxidation of this compound with oxygen.

Quantities of heat transferred at constant T, like heat of vaporization, orfusion for a single compound at constant pressure are called heat effects.

These heat effects together with the heat of mixing, the heat of solution(heat of mixing for the case of a solution), as well as the heat of reaction arestudied by the branch of thermodynamics, called thermochemistry, and the in-struments used for the experimental determination of these functions are calledcalorimeters.

B. Determination of the Enthalpy of Reaction (4, 5, 7)

For a reaction under constant P, where variations in potential and kinetic energyare negligible and no work is produced the change in enthalpy is given by

HP − Hr = (HP − HP0) + ∆HR − (Hr − Hr0) (140)

32 Tzias

where HP, Hr enthalpies of products and reactants at a pressure P respectivelyand HP0, Hr0 enthalpies of products and reactants at a pressure of 1 atm andtemperature of 25°C and

∆HR = HP0− Hr0

= ∑products

(νh) − ∑reactants

(νh) (141)

where the ν’s are the stoichiometric coefficients and the h’s are the standardmolar enthalpies of formation of the compounds in the reaction. ∆HR is thestandard enthalpy of reaction and can be calculated from the standard enthalpiesof formation of the products and reactants using existed relative tables.

(HP − HP0) and (Hr − Hr0

) can be calculated either from known data, experi-

mentally or by simplification considering, for instance, that the reactants andproducts behave as ideal gases. In several cases we can determine the enthalpyof a certain reaction by simply adding or subtracting other reactions of whichwe know the enthalpies of reaction.

C. Equilibrium Constant-Affinity of a Reaction (6, 7, 11)

Let us consider the reaction

ν1c1 + ν2c2W ν3c3 + ν4c4 (142)

at constant T, P, where ci are the constituents and νi the stoichiometric coeffi-cients.

Eq. (46) at P, T constant gives

dGT,P = µ1dn1 + µ2dn2 − µ3dn3 − µ4dn4 (143)

and for a reaction

−dn1

ν1

= −dn2

ν2

= dn3

ν3

= dn4

ν4

= dξ (144)

where ξ is the extent of the reaction (16).Based on (143), (144), and (42) it follows

dGT,P = (µ1dn1 + µ2dn2 − µ3dn3 − µ4dn4)dξ ≤ 0 (145)

the sign of the parenthesis determines the sign of the ξ and consequently thedirection of the reaction.

At equilibrium where dGT,P = 0 (146)

µ1dn1 + µ2dn2 = µ3dn3 + µ4dn4 (147)

The quantity µ1dn1 + µ2dn2 − µ3dn3 − µ4dn4 was introduced by De Donder (16)and called by him the affinity Af.

Introduction 33

From (147) using µi = µ0i + RTlnai, [Eq. (90)] it is obtained:

Af = A0f − RTln�aν3

3 � aν44

aν11 � aν2

2� (148)

where A0f is the affinity for the µ0

i ’s.

The quantity �aν33 � aν4

4

aν11 � aν2

2� = Qa (149)

is called reaction quotient.At equilibrium where Af = 0, the Qa depends only on the temperature and

on A0f and not on the activities and is called the equilibrium constant Ka.Thus we have

A0f = R T lnKa (150)

Af = RTlnKa

Qa

(151)

Since one reaction proceeds only when Af > 0 this means that:

when Ka > Qa the reaction proceeds to the rightwhen Ka < Qa the reaction proceeds to the left

and

when Ka = Qa there is equilibrium.

REFERENCES

1. SW Angrist, LG Hepler. Order and Chaos. New York: Basic Books Inc., 1967.2. ML Mac Glashan. Chemical Thermodynamics. London: Academic Press Inc.,

1979.3. ML MacGlashan. Physicochemical Quantities and Units, 2nd ed. London: The

Royal Institute of Chemistry, 1971.4. TE Daubert. Chemical Engineering Thermodynamics. Singapore: McGraw Hill,

1985.5. P Rock. Chemical Thermodynamics. California: University Science Books, 1983.6. CE Reid. Chemical Thermodynamics. Singapore: McGraw Hill, 1990.7. JS Hsieh. Principles of Thermodynamics. New York: McGraw Hill, 1975.8. SE Wood, R Battino. Thermodynamics of Chemical System. Cambridge, UK:

Cambridge University Press, 1990.9. JM Smith, HC Van Ness. Introduction to Chemical Engineering Thermodynamics,

4th ed. Singapore: McGraw Hill, 1987.

34 Tzias

10. GN Lewis, M Randall. Thermodynamics. Revised by KS Pitzer, L Brewer. 2nd ed.New York: McGraw Hill, 1961, p. 191.

11. DP Tassios. Applied Chemical Engineering Thermodynamics. Berlin, Germany:Springer-Verlag, 1993.

12. HC Van Ness, MM Abbott. Classical Thermodynamics of Non Electrolyte Solu-tions, with Applications to Phase Equilibria. New York: McGraw Hill, 1982.

13. JM Prausnitz, RN Lichtenthaler, EG de Azevedo. Molecular Thermodynamics ofFluid-Phase Equilibria, 3rd ed. New Jersey: Prentice Hall, Inc., 1999.

14. JW Tester, M Modell. Thermodynamics and its Applications, 3rd ed. New Jersey:Prentice Hall, Inc., 1997.

15. MK Karapetyants. Chemical Thermodynamics. Trans. by G Leib. Moscow, Russia:MIR Publishers, 1978.

16. De Donder, P Van Rysselberghe. The Thermodynamic Theory of Affinity. Stan-ford, California: Stanford University Press, 1936.

2Solid–Liquid Extraction

Jose Miguel AguileraUniversidad Catolica de ChileSantiago, Chile

I. INTRODUCTION

Solid–liquid extraction or leaching is a separation process affected by a fluidinvolving the transfer of solutes from a solid matrix to a solvent. It is an exten-sively used unit operation to recover many important food components: sucrosein cane or beets, lipids from oilseeds, proteins in oilseed meals, phytochemi-cals from plants, and functional hydrocolloids from algae, among others. Solid–liquid extraction (or simply extraction) may also be used to remove undesirablecontaminants and toxins present in foods and feeds.

“Solid–liquid” extraction may be a misnomer for it gives the impressionthat mass transfer occurs at a sharp interface between a “dry” solid and theliquid phase. In most food extractions, either the “solid” naturally contains aliquid phase or it becomes impregnated by the extraction liquid, so that liquidphase diffusion inside the solid is a major mass transfer mechanism duringleaching.

A. Characteristics of Food Extraction

From an engineering viewpoint, solid–liquid extraction of foods is a multicom-ponent, multiphase, un-steady state mass transfer operation. It involves transferof more than one chemical species—the solute—from a solid to a solvent. Thesolute is sometimes referred to as the extract, when the chemical species beingrecovered are ill defined, as occurs in the extraction of phytochemicals fromplants. Commonly used solvents in extraction of food components are water,

35

36 Aguilera

ethanol (or ethanol-water mixtures), hexane, and carbon dioxide, but the trendis toward the use of natural chemicals.

During extraction, the concentration of solute inside the solid varies lead-ing to the nonstationary or unsteady condition. A series of phenomenologicalsteps have to occur during the period of interaction between the solute-contain-ing particle and the solvent effecting the separation (1) as represented schemati-cally in Fig. 1. These include:

1. Entrance of the solvent into the solid matrix2. Solubilization and/or breakdown of components3. Transport of the solute to the exterior of the solid matrix4. Migration of the extracted solute from the external surface of the solid

into the bulk solution5. Movement of the extract with respect to the solid (i.e., extract dis-

placement), and6. Separation and discharge of the extract and solid

Figure 1 Scheme of the main steps in solvent extraction of solid food particles. Masstransfer of solute in liquid occupying pores is an important mechanism. External resis-tance is caused by the static liquid layer surrounding the particle.

Solid–Liquid Extraction 37

As a result of these phenomena, extraction takes place at a rate expressedin terms of (mass of solute leached)/unit time or, more commonly, as change insolute concentration in the solid/unit time (dc/dt or dx/dt). In multicomponentextraction the relative rate at which different chemical species migrate throughthe solid may also be of concern. Since the aforementioned elementary stepsoccur at their own rate and in some cases sequentially, the overall rate of theextraction process is determined by the step having the slowest rate or the rate-controlling step. As discussed later, transport through the solid matrix is usuallythe rate-controlling step in food extractions.

Foods are unique in that the microstructure of the solid plays a majorrole in the rate and quality of the extraction process. To attain a fundamentalunderstanding of the relationship between rate and food microstructure, it isnecessary first to introduce some theoretical notions of mass transfer. A rigoroustreatment on the subject may be found in the classical texts for chemical engi-neering. Gekkas (2) presents concepts of transport phenomena with applicationsto foods and biological materials. A recommended book on mass transfer forbeginners and experts as well is that by Cussler (3).

B. Solvent Selection

Solvent selection is based on several properties:

1. Solubility of the specific compound (or compounds) in the solvent.2. Recovery, since the solvent will be reused in subsequent extractions.

If distillation or evaporation is used, the solvent should not form azeo-tropes and the latent heat of vaporization should be small. Removalof the solvent from the miscella (and from the spent solids) can poseserious problems if the residual level of the solvent must be mini-mized.

3. Interfacial tension and viscosity. The solvent should be capable ofwetting the solid matrix and its viscosity should be sufficiently lowso it can flow easily. Wettability is also important if the solvent mustpenetrate through pores and capillaries in the matrix.

4. Ideally, the solvent should be nontoxic, stable, nonreactive, nonflam-mable, harmless to the environment, and cheap.

Laws relating to extraction solvents for use in foods are concerned primar-ily with human health requirements. Accepted solvents for use in compliancewith good manufacturing practice (GMP) provided the presence of residues areunavoidable are as follows: propane, butane, propyl acetate, ethyl acetate, etha-nol, carbon dioxide, acetone and nitrous oxide. Substances that have been foundacceptable by the Enzyme Commission (EC) when used under specific condi-tions are hexane, methyl acetate, ethylmethylketone, and dichloromethane.

38 Aguilera

II. EXAMPLE OF STRUCTURAL FEATURESOF A SOLID MATRIX: PLANT TISSUE

Food materials of plant origin have an intricate microstructure formed by cells,intercellular spaces, capillaries, and pores. There are four major types of matureplant tissue: (a) storage or parenchyma; (b) conducting or vascular, composedof phloem (transport of organic materials) and xylem (transport of water); (c)supporting: and (d) protecting tissue. Only two of these contribute to the micro-structure of edible parts of plant foods: parenchyma cells and the conductingtissue forming an intricate network throughout. An excellent introduction toplant cells and microscopy techniques can be found at the web site http://www.rrz.uni-hamburg.de/biologie.

The desired solute in the tissue may be present inter- or intracellularly.In the first case, intact cell walls and adhering membranes constitute a majorresistance to diffusion. They affect the permeability of solutes so that smallmolecules pass at a faster rate than larger ones, resulting in a selective transfer.The permeability of cell walls and membranes is increased and selectivityreduced by heat-induced denaturation. Moreover, cell walls forming most ofthe supporting tissues and certain types of conducting cells (tracheids, vesselelements) are lignified to a different extent, further reducing the passage ofmolecules.

During preparation for extraction the solid is reduced to small particlesbecause, theoretically, extraction time varies inversely with the square of thecharacteristic dimension of the solid. As particle size decreases the rupturedouter cells constitute a larger proportion of the particle volume and extractioncharacteristics per unit mass of material change. Size reduction is limited be-cause of pressure drop in the extractor, slow drainage rates, presence of fines inthe extract, flow instability, and entrainment. It has to be kept in mind thatextraction is just one of the unit operations in a separation process and down-stream purification depends strongly on how “cleanly” the extract was obtained.

III. FUNDAMENTAL ASPECTS OF EXTRACTION

A. Extraction as a Diffusion Process

Molecular diffusion is the process by which molecules are transported from onepart of the system to another by random movement as a result of a concentrationgradient. In leaching of foods the interior of the solid cannot be agitated andturbulence is unlikely to occur in small capillaries and pores, leaving moleculardiffusion as the main transport mechanism within the solid phase. Solvent ex-traction may be considered as a diffusion process in the liquid (fluid) state since

Solid–Liquid Extraction 39

solute transfer, even inside a solid, exists as a dilute solution. In some cases,solvent influx may occur due to pressure gradients due to capillary forces or bymechanical relaxation of the cellular matrix.

Fick’s laws provide the semiempirical bases for analysis of molecular dif-fusion. Fick’s first law is useful for defining a diffusion coefficient or diffusivity(D). It simply establishes that under steady-state conditions (concentration doesnot change with time) the unidirectional flux of solute 1 (J1, mol/s) in the rdirection is directly proportional to the diffusivity of the solute, to the areatraversed by the flux and to the gradient of solute concentration between twopoints, expressed in terms of absolute concentration (dc/dr) or molar fraction(dx/dr). Fick’s first law describes diffusion referred to a fixed coordinate systemand for the unidirectional case it takes the form:

j1 =J1

A−cD

dx1

dr= −

Ddc1

dr(1)

where j1 is the flux in moles per unit time and unit area, r is the direction offlow, A is the area across which diffusion occurs, and c is the total molar con-centration (moles/volume). The minus sign gives a positive flux term since thegradient is negative (flow occurs down a concentration gradient, from high tolow concentration). This equation can be regarded as a limit for long times,when transient conditions have disappeared and a linear gradient is establishedin the system. It can be applied without problems whenever the solute is in highdilution in the solvent; diffusion in concentrated systems involves convectionand a more complex mathematical treatment (3).

In practical situations and for short times, unsteady or nonstationary con-ditions exist and the concentration of solute varies with time (t) and position (r)inside the solid. In such cases, Fick’s second law (or the diffusion equation)applies and takes the general form:

∂c1

∂t= 1

rν−1

∂�rν−1D∂c1

∂r �∂r

(2)

where the index ν equals 1 for an infinite slab, 2 for an infinite cylinder, and3 for a sphere. It is easier to understand Fick’s second law when Eq. (2) iswritten as:

∂c1

∂t= D

∂2c1

∂r2= D

∂∂r�∂c1

∂r � (3)

for it says that the flow of solute is directly proportional to the change of theconcentration gradient with position. So, when the gradient is constant (i.e.,

40 Aguilera

linear concentration profile), then ∂c1/∂t = 0, meaning that steady-state condi-tions exist and we are back to Eq. (1).

Analytical solutions for Eq. (3) under several simple initial and boundaryconditions are found in many textbooks (4, 5); solutions to more complex situa-tions and for different geometries are analyzed in the classical book by Crank(6). Schwartzberg and Chao (7) present a detailed analysis of the assumptionsand conditions under which solutions to Eq. (3) can be applied to extraction infoods. In general, solutions relate the dimensionless extent of extraction of sol-ute 1 as X = (c − cf)/(c0 − cf) with time in a series expression represented by Eq.(4), where c is the average solute concentration inside the solid at any time andc0 and cf are the initial and final equilibrium concentrations, respectively.

X = Σ Bn exp�−q2nDt

L2 � (4)

where X depends on α = mE/R (where m is the equilibrium distribution ratiobetween the solute concentration in the bulk solution and inside the solid, andE/R is the extract-to-solid volume ratio), and on Fick’s dimensionless numberDt/L2 (where L is a characteristic length, e.g., the particle size). Parameters Bn

and qn are functions of α. When only the first term of the series is considered(e.g., for Dt/L2 > 0.06), plots of log X vs. t are straight lines with a slope equal to−Dq1

2/2.303L2, from which D can be obtained as an overall diffusion coefficient(effective or apparent diffusion coefficient).

The dimensionless number Dt/L2 can be used as a criterion of closenessto the steady state. If it is much larger than unity (e.g., Dt >> L2) an equilibriumor steady-state condition may be assumed. Also, two parameters may be estab-lished for un-steady-state conditions: a “velocity of diffusion,” √D/πt and a“penetration distance,” √4Dt (3). Note that both parameters involve the squareroot of time.

Solid–liquid extraction of various food materials is controlled by internaldiffusion except in the case of very small particles, poor agitation, or presenceof a skin (8–11). The extent of control between external and internal diffusionis indicated by the Sherwood number NSh = kcL/D, where kc is the liquid-phasemass transfer coefficient, L a characteristic dimension of the solid (e.g., particlesize), and D the internal diffusion coefficient. If NSh > 200, internal control canbe safely assumed (7).

B. Determination of Diffusion Coefficients

Data for diffusion coefficients are necessary to make calculations using the Fick-ean approach. Diffusivities may be determined experimentally or predicted. Ex-perimental methods used for liquids are, among others, the diaphragm cell, the

Solid–Liquid Extraction 41

rotating disk (used for drug dissolution), nuclear magnetic resonance (NMR)spin-echo techniques, and interferometer methods (Gouy interferometer). Fordetails of these methods, see Cussler (3).

Orders of magnitude of these coefficients are important to remember (allunits in cm2/s): for gases, 10−1; for liquids, around 10−5; and for solids, between10−8 and 10−30 (if m2/s is used as unit, values must be divided by a factor of 104).Consequently, perfume in air diffuses 10,000 times faster than the tea extract ina cup of hot water! Diffusion coefficients of polymers in solvents under diluteconditions may be as low as 10−6 to 10−8 (cm2/s) and of gases through syntheticmembranes vary widely between 10−8 and 10−11 (cm2/s).

Numerous attempts have been made to predict diffusivities in liquids. Ac-cording to the Stokes-Einstein equation, the diffusion coefficient of large spheri-cal solute species in a pure solvent of viscosity η is given by:

D =kT

6πηrs

(5)

where k is the Boltzmann constant [gas constant divided by Avogadro’s number(R/N0)], T the absolute temperature, and rs the effective radius of the diffusingmolecule. This expression is misleading in the sense that in practice viscosityand solute radius effects are more important than those of temperature. Also,adaptations of Eq. (5) for dilute solutions of macromolecules must encompassconsideration of their size and shape. Nevertheless, the Stokes-Einstein equationhas provided the foundation for several useful semiempirical equations.

Most data for diffusion coefficients of liquids or gases (vapors) in solidfoods come from solutions to the diffusion equation for standard geometriescombined with experimental results, so they are “effective diffusion coeffi-cients.” The main problem is that all steps involved in the diffusion of a solutethrough a solid matrix are ignored and the process is characterized by a singlecoefficient that is highly dependent on experimental conditions, solid geometry,and microstructural arrangement. It is not surprising, then, that “diffusion coeffi-cients” for solute extraction vary by several orders of magnitude, even for thesame solute.

As explained before, effective or apparent diffusivities appear as a calcu-lated value of D obtained from a transient diffusion experiment. Defining M asthe total amount of solute that has diffused in (impregnation) or out (extraction)of a solid of regular geometry (slab, cylinder, or sphere) at any time and M∞ asthe amount transferred after equilibrium is reached, the ratio M/M∞ may becalculated by integrating Fick’s second law under appropriate boundary condi-tions. For a slab of thickness L when 1 − M/M∞ is plotted against t, the slope is−π2Dapp/4L2 (the expression changes for other geometries) from which a valueof Dapp can be determined. The term “apparent” confirms that we do not knowthe exact mechanism of transport, which in most cases may be complex.

42 Aguilera

C. Diffusion Through the Solid Matrix

It should be evident from the previous analysis that microstructure influencesmolecular diffusion through its effect on the diffusion coefficient. Solute diffu-sivities can be defined in the liquid phase (DL) or in the wet solid (DS), andexpressions for Fick’s laws can be written accordingly. Differences between DL

and DS can be attributed to factors such as membrane resistance, complexity of thediffusion path, sorption of the solute by the inert solid, etc. If all of these factorswere accounted for, DS could be related to DL by an expression of the form:

DS = Fm × DL (6)

where Fm is a correction factor including all phenomena related to microstruc-ture. Schwartzberg and Chao (7) have reviewed extensively the subject of solutediffusivities in foods. DL values for various food solutes at infinite dilution inwater at 25°C vary from 5.4 × 10−6 for sucrose to 1–7 × 10−7 cm2/s for someproteins. Selected DS values listed in the literature where microstructural effectsappear important are presented in Table 1. Extraction experiments from whichthese data were obtained do not lead to easy determination of a single, constantdiffusion coefficient. Typical curves for hexane extraction of oil from soybeangrits and flakes of different particle size and a theoretical curve are shown inFig. 2 (12). The slope of the curves becomes less steep as extraction proceeds,meaning a decrease in the value of DS so that usually an initial as well as a finaldiffusion coefficient has to be determined. The initial DS may be influenced bythe previously mentioned release of surface solute (washing); thus, it is not truly

Table 1 Diffusion Coefficients in Foods and Reference Values

Temp. DAB × 106

Food material Solute Solvent (°C) cm2/s

Dilute solution Sucrose Water 25 5.4Sugar cane (across grain) Sucrose Water 75 5.1Sugar cane (with grain) Sucrose Water 75 3.0Sugar beets Sucrose Water 24 1.6–2.5Gelatin gel Sucrose Water 5 0.1–0.2Dilute solution NaCl Water 25 16.1Pickled cucumbers NaCl Water 25 5.3–11.0Dilute solution Lactose Water 25 4.9Small curds Lactose Water 25 3.0Peanut slices Oil Hexane 25 0.007Tungseed slices Oil Hexane 30 0.006Dry solid matrix Glyceride — 50 5 × 10−4

Source: Ref. 1.

Solid–Liquid Extraction 43

Figure 2 Kinetics of oil extraction from soybean flakes (------) and ground particles( ) and theoretical behavior. Numbers on curves are the thickness of flakes orparticle size (in microns).

representative of diffusion of solute inside the solid. Consequently, data on soliddiffusivities listed in Table 1 should be analyzed with caution, always referringto the original work if further conclusions are to be inferred.

DL values from the literature and diffusivities calculated for extraction ofthe same component may be used to determine the order of magnitude of thecorrection factor Fm in Eq. (6). The DL of caffeine in water is 6.9 × 10−6 cm2/swhereas the diffusivity of coffee solubles during extraction from grinds is of theorder of 1.1 × 10−6 cm2/s (13). In sucrose extraction from beets the calculateddiffusivities are one-half to one-third the DL at the same temperature. Diffusivi-ties of pure linoleic and oleic acid in hexane are 3.6 × 10−7 and 2.6 × 10−7 cm2/s,respectively, while inside the seed they are reduced to 2.6–6.2 × 10−8 cm2/s (14).Hence, the correction factor in Eq. (6) appears to be of the order of 0.1–0.9 forsmall solutes, with higher values occurring when membranes have been dena-tured, as in sugar beet processing.

For macromolecules Fm is obviously larger. In fact, Schwartzberg andChao (7) report that solutes exceeding a molecular weight of 2000 cannot dif-fuse into intact cells of coffee grounds. The reduction of the diffusion coefficient

44 Aguilera

for zein (corn protein) in dilute solution and inside the endosperm is about 1000-fold (15).

Sorption of solvent and/or solutes by the solid also retards diffusion. Mi-crostructural entrapment seems to play a role at least as important as physico-chemical sorption. Lignocellulosic materials are known to sorb water that is nolonger available as solvent, particularly if ground into a fine powder. Sorptionis critical when organic solvents must be removed from spent solids after extrac-tion, in an operation known as desolventizing. Residual hexane left in rapeseedmeals concentrates preferentially in the hulls, dissolved in the residual oil, en-trapped inside thick-walled cells (16), or simply adsorbed. In this latter case,internal diffusion coefficients for hexane during adsorption into the solid matrixare very small (about 10−10 cm2/s) and increase with hexane content, probablydue to swelling of the structure.

IV. THE MICROSTRUCTURAL APPROACH

A. Simple Correction Factors

On examination of Eq. (3) and data previously presented it can be concludedthat the influence of food microstructure on extraction rate is predominantly byits effect on the diffusion coefficient. Chemical engineers use the effective (orapparent) diffusion coefficient Deff when dealing with impermeable porous sol-ids with fluid-filled pores:

Deff = Dετ

(7)

where D is the diffusion coefficient of the solute in the fluid filling the pores, εis the void fraction or porosity of the solid, and τ is the tortuosity of the pores,which attempts to account for the longer distance traversed by the solute alonga sinuous path. Porosity may be very low in potato tissue (�2%) or high as inapples (�20%). For solid materials used in chemical engineering (adsorbents,porous catalysts) tortuosity varies between 2–6, and porosity between 0.3–0.8,thus Deff may be 6 to 15 times lower than D.

When the size of the pore and the solute are of comparable magnitude(e.g., in some membranes), Eq. (7) is corrected by a factor λ that depends onthe ratio of solute radius to pore radius. The restricted diffusion of sphericalmolecules within cylindrical pores (Dp) is usually modeled by the so-called Ren-kin equation (17):

Dp = D(1 − λ)2f(λ) (8)

The squared term of the equation is a partition coefficient and accounts forsteric hindrance at the pore entrance. The factor f(λ) is a polynomial function

Solid–Liquid Extraction 45

of λ and corrects for the friction between the diffusing molecule and the wallsof the pore.

The previous results may be expressed in a more general form. For a two-phase composite where spherical particles of a material 1 are dispersed in acontinuous phase 2, an effective diffusion coefficient may be obtained from theexpression:

Deff − D1

Deff + 2D1

= φ� D2 − D1

D2 + 2D1� (9)

where D1 is the diffusion coefficient through the interstitial pores and D2 is thediffusion coefficient through the particles.

So far we have assumed that pores are quite large. When the size of poresis of the order of magnitude of the distance between molecular collisions, Fick’slaws no longer applies and the so-called Knudsen diffusion takes place in whichmolecules collide with the pore walls. We leave this section with the impressionthat a much better work in using Fick’s laws could be made to predict extractionrates if we had an idea of the microstructural architecture of a solid food particle.Microscopy can assist in developing such structural models and in finding pa-rameters such as tortuosity, porosity, and pore size.

B. Introducing Architectural Effects

Other correction factors where the diffusivity is corrected taking into accountthe architecture of the structure have been discussed by Cussler (3). A flakewith uniformly distributed platelets impermeable to the solute, parallel or per-pendicular to the diffusing stream where the volume fraction of the platelets isφF and their aspect ratio is α, results in an effective diffusivity given by theexpression:

Deff

D= � 1

1 + α2φF2/(1 − φF)

� (10)

A plot of Deff/D vs. φF having α (long dimension divided by short dimension ofplatelets) as parameter is shown in Fig. 3.

When platelets are arranged parallel to the diffusion (segmented line),tortuosity is 1 and the effective diffusivity varies linearly with φF, underscoringthe unidirectional nature of the diffusion. When platelets are arranged perpendic-ular to the diffusion Deff/D depends strongly on α and a nonlinear dependenceon φF is evident. For a constant value of α, Deff/D becomes smaller as φF in-creases. It is also interesting to note that when α is very large, say α = 30, evenat low values of φF the ratio Deff/D can become very small. Thus, this modelshows an important effect of microstructure and architecture on the rate of trans-

46 Aguilera

Figure 3 Curves showing the correlation between Deff/D and volume fraction φF ofimpermeable platelets in a slab (φF) for different aspect ratios α of platelets. The effectivediffusion coefficient varies sharply with φF when platelets are placed perpendicular tothe diffusional flow.

fer of solute. However, it does not consider the possibility of nonuniform spatialdistribution of the impermeable elements.

Another model that considers the situation of impermeable spheres occu-pying a volume fraction φS and uniformly distributed within the matrix givingrise to the expression of Deff/D given by Cussler (3) is:

Deff

D+ �2(1 − φs)

2 + φs� (11)

Evidently the size and distribution of the spheres is not considered in the model.In summary, although some expressions like (10) and (11) are available for

Solid–Liquid Extraction 47

biphasic systems with uniformly distributed elements, they do not account fornonuniform architectural arrangements.

V. MATHEMATICAL MODEL FOR EXTRACTIONFROM A TWO-DIMENSIONAL MATRIX

Fick’s first law for the steady state and second law for the transient or unsteadystate in two dimensions of a homogeneous and isotropic media are described bythe following equations:

J = −D�∂C∂X +∂C∂Y� (12)

∂C∂t

=∂∂X�D∂C

∂X� + ∂∂Y�D∂C∂Y� (13)

Applying the precedent equations to a structure with two different phases A andB, continuity of mass flow must be observed at every boundary between thephases. Crank (6) proposed a discrete form (finite difference) for the unidirec-tional diffusional case of a two-phase compound:

1

2{∆XA + ∆XB}

C(i,n+1) − C(i,n)

∆t=DB

∆XB

(C(i+1,n) − C(i,n)) −DA

∆XA

(C(i,n) − C(i−1,n)) (14)

where i is the position of the element or node and n the iteration index. Basedon this equation and applying explicit finite difference discretization to theFick’s second law in two dimensions using ∆X = ∆Y = ∆L while taking intoaccount the continuity between phases, equal distance increment in the phases(i.e., ∆XA = ∆XB = ∆X) and constant diffusion coefficients at each point through-out the extraction process, the following expression is obtained:

C(i,j,n+1) = C(i,j,n) =∆t

(∆L)2× [D(i+1,j,n)(C(i+1,j,n) − C(i,j,n))

− D(i−1,j,n)(C(i,j,n) − C(i−1,j,n))+ D(i,j+1,n)(C(i,j+1,n) − C(i,j,n))− Di,j−1,n)(C(i,j,n) − C(i,j−1,n))] (15)

which is based on the application of the Schmidt method of the finite differenceapproach to heterogeneous compounds (e.g., different diffusion coefficients ateach specific spatial locations) as presented in Crank (6). The subindex (i,j,n)refers to the position (i,j) of an element in the matrix at the iteration step n,equivalent to a time t = n × ∆t.

48 Aguilera

Modeling extraction with Eq. (15) starts by defining a matrix of M × Nelements representing the object being extracted bidirectionally and introducingthe specific architectural elements in the form of pores, walls, or platelets. Geo-metrical arrangements of the structure are modeled assuming different diffusioncoefficients for each phase or element.

The boundary condition for the model assumed that the object was sur-rounded by pure solvent at all times (bulk concentration of solute in solventequal to zero or infinite volume of solvent). This is equivalent to saying that theinternal resistance is controlling or the Biot number for mass transfer is infinite.The concentration at each point of the matrix and the corresponding diffusioncoefficients at time 0 were the initial conditions for this model. Extraction wassimulated for different architectures of the matrix. Extraction curves are semilogplots depicting variations in time of the ratio of the instant mass of solute re-maining in the solid to the total initial mass (q/q0).

A. Modeling Extraction from Cellular Material

An example of the previous approach has been applied to extraction from acellular structure modified by various pretreatments (18). Figure 4 shows thearchitectural arrangements associated with four pretreatments. The basic struc-ture considers that all cells are intact and completely surrounded by a thick cellwall (A). Enzymatic treatment with a cocktail of cell wall–degrading enzymesmay pierce the wall of outer cells at different places, leaving openings thatconnect the cytoplasm directly to the solvent (B). A third situation may be thatinduced by blanching, resulting in degradation of the cell wall membrane (andpossibly the cell wall itself), diminishing the overall internal resistance for trans-port of solute as most cells become interconnected (C). Another conceivablearchitecture is that of a totally connected cytoplasm and broken outer cells ex-posed to the solvent due to extensive physical destruction of the tissue (D). Inall cases, the proportion of cell wall material to cytoplasm is the same.

The simulation used data obtained from Schwartzberg and Chao (7) forinfusion of salt (NaCl) in pickled cucumbers and water. Data for the cell wallcomplex were Dcw = 53 µm2/s and Ccw = 1 g/L, and for the cytoplasm D = 219µm2/s and C = 3 g/L. ∆L was 20 µm and ∆t was 0.2 s. The overall size of thematrix was 380 × 380 µm in a grid of 19 × 19 elements.

The resulting extraction curves for the simulations are presented in Fig.5. The fastest extraction rate corresponded to pieces having complete breakageof the cell wall and a layer of cytoplasm exposed directly to the solvent. Wash-ing of solute from the outermost layers removes 20% of the solute almost in-stantly. Since it was assumed that the cell wall complex also contained soluteand had a finite diffusivity, even the arrangement containing intact external cellsshowed some extraction. However, if the cell wall complex had been assumed

Solid–Liquid Extraction 49

Figure 4 Architectural arrangement of cells of pickled cucumbers subject to differenttreatments (theoretical).

50 Aguilera

Figure 5 Resulting curves of the extent of extraction vs. time simulating salt diffusionin pickles with and without pretreatments to modify the microstructure.

impermeable, extraction would have been zero in this case for all times. Actualextraction of triglycerides from oilseeds having intact cells by organic solventsis extremely slow (1).

The fastest extraction is achieved in the situation of full rupture of the cellcomplex, and the slowest for intact cells. In the latter case, the curves start fromzero concentration (no washing) and the time to achieve 90% removal of soluteis 40% longer than in the previous case. Maintenance of intact cell membranesmay be beneficial to selectively extract small solutes from the cytoplasm keep-ing large molecules and debris within the piece to facilitate downstream separa-tion and purification (as in sucrose extraction from sugar beets).

The use of microscopy techniques to correlate the state of the microstruc-ture and its effect on extraction is presented in Aguilera and Garcıa (19) foraqueous extraction of protein from lupins; Aguilera and Lusas (20) to assess the

Solid–Liquid Extraction 51

extractability of oil from flakes, grits, and extrudates of high-oil corn usinghexane; Fan et al. (21) for the extraction of peanut oil; and Rastogi and Niranjan(22) for osmotic dehydration of pineapple tissue, among others. With the adventof nonintrusive microscopy techniques and real-time video microscopy, struc-tural features prior and during extraction may be observed and different architec-tural arrangements (presence of elements, phase ratios, geometrical parameters,etc.) characterized by image processing and analysis. Combination of micro-structural information, better data on diffusivities of individual phases, and ad-vanced computer techniques should lead to a more fundamental approach to thestudy of mass transfer phenomena in foods and other biological materials (23).

VI. USE OF MASS TRANSFER COEFFICIENTS

The problem with Eq. (1) is that the distance over which concentration changesoccur (δ) must be known to determine the gradient. This is not easy task forprocesses that occur inside process equipment. Moreover, we are in the presenceof interfacial mass transfer or transfer between two different phases (solid andliquid). Chemical engineers prefer to study interfacial mass transfer using masstransfer coefficients (individual or overall), which multiplied by a measurabledriving force give the rate of mass transfer. Thus, the distance problem becomeshidden in the coefficient, which also contains implicitly the diffusivity. A practi-cal expression for the rate of solute extraction takes the form:

Rate = mass transfer coefficient × driving force (16)

The driving force in extraction should be the difference between the chem-ical activity of the solute inside the solid and that in the bulk of the solution.For practical reasons, the rate is expressed as the product of the difference be-tween an external solute concentration (cout) and that in the interior of the solid(cin), and a mass transfer coefficient based on concentration. For this differenceto be meaningful it must be expressed in the same base; thus, cin is usually takenas that concentration of solute in the liquid phase that would be in equilibriumwith the concentration inside the solid c*. Then the driving force for extractionbecomes (c* − cs), where cs could be measured in the bulk of the solution andEq. (16) takes the form:

N1 = Kc(c* − cs) (17)

This simple equation states that the rate of extraction N1 depends on the differ-ence in a thermodynamic variable (expressed as concentration) and a globalmass transfer coefficient Kc that includes all physical and microstructural param-eters of the process. If the interfacial area for transfer (a) is unknown, the coeffi-

52 Aguilera

cient becomes Kca. If each phase is taken separately, individual mass transfercoefficients ki can be defined for transport between the interface and the bulkof the respective phase [see Cussler (3) for details].

If a solid is modeled as series of structures (i.e., a plant cell with proto-plasm, plasmalemma, cell walls, etc.), the observed global mass transfer coeffi-cient may be related to individual mass transfer coefficients inside each of thestructures as a sum of resistances in series:

1Kc

= Σ 1kci

(18)

and a rate-limiting step having the highest resistance (or the slowest flow) maybe identified. This structure then controls the mass transfer process and effortsshould be made to increase the rate within this phase. This approach can also beused when studying extraction as a series of sequential steps as shown in Fig. 1.

Theoretically, there is a relationship between D and Kc (e.g., Kc is propor-tional to D/δ, where δ is a thin film over which diffusion occurs) and, as pre-viously concluded, study of the influence of microstructure on extraction rate isreduced to the analysis of its effects on the mass transfer coefficient. Masstransfer coefficients are often correlated to dimensionless numbers, allowingpredictions to be made for other experimental conditions. Dimensionless num-bers are ratios of transport parameters and have physical meaning. For example,the most relevant of these numbers for solid–liquid extraction is the Sherwoodnumber (NSh), which is the ratio of mass transfer velocity (kL) to diffusion veloc-ity (D). A large NSh in solvent extraction means that the controlling step isdiffusion inside the solid, and reduction of particle size or restructuring into aporous material may prove effective in speeding extraction. NSh is in turn relatedto several other dimensionless numbers, e.g., to the Schmidt number represent-ing flow effects (momentum transfer) by the expression:

NSh =kcL

D= f�νD�

1/3

(19)

where ν is the kinematic viscosity.

VII. CONCLUSIONS

Food engineers can make a significant contribution to the study of extraction orleaching by introducing the microstructural variable into the problem. A goodarchitectural description of the solid being extracted together with fundamentaldata of diffusion coefficients in different structures or phases can be combinedwith modern computational methods to predict extraction rates from food solids.

Solid–Liquid Extraction 53

ACKNOWLEDGMENTS

This work was partly funded by project FONDEF D9712026.

NOMENCLATURE

a interfacial area of transfer (m2/m3)A area for diffusion (m2)Bn parameter in eq. 4c molar concentration (moles/m3)C concentration (g/L)D diffusion coefficient or diffusivity (m2/s)Deff , Dapp effective or apparent diffusion coefficient (m2/s)E/R extract-to-solid volume ratioFm correction factor in eq. 6J mass flux (moles/s)j flux (moles/s m2)k Boltzmann constantkc liquid phase mass transfer coefficientKc global mass transfer coefficientL characteristic length or slab thickness (m)M total amount of solute extracted (g)N0 Avogadro’s numberNSh Sherwood numberq mass of solute in the solid (g)qn parameter in eq. 4R gas constantr direction of flow or position inside the solidt time (s)T absolute temperature (K)X extent of extraction of solute (dimensionless)

Symbols

α aspect ratio (length/width)α m.E/Rδ film thicknessε porosity or void fractionφ volume fraction of platelets or spheresη viscosityτ tortuosity

54 Aguilera

λ correction factorν index (1 for infinite slabs, 2 for infinite cylinders, 3 for spheres)υ kinematic viscosity

REFERENCES

1. JM Aguilera, DW Stanley. Microstructural Principles of Food Processing and Engi-neering. Gaithersburg, MD: Aspen Publishers, 1999.

2. V Gekkas. Transport Phenomena of Foods and Biological Materials. Boca Raton,FL: CRC Press, 1992.

3. EL Cussler. Diffusion: Mass Transfer in Fluid Systems, 2nd ed. Cambridge, UK:Cambridge University Press, 1997.

4. RB Bird, WE Stewart, EN Lightfoot. Transport Phenomena. New York: Wiley,1960.

5. CJ King. Separation Processes, 2nd ed. New York: McGraw-Hill, 1981.6. J Crank. The Mathematics of Diffusion. London: Oxford University Press, 1975.7. HG Schwartzberg, RY Chao. Solute diffusivities in leaching processes. Food Tech-

nol 2(2):73–86, 1982.8. U Cogan, A Yaron, Z Berk, S Mizrahi. Isolation of soybean protein: Processing

conditions on yields and purity. JAOCS 44:321–324, 1967.9. HA Leninger, WA Beverloo. Food Process Engineering. Dordrecht, Netherlands:

D. Reidel, 1975, p. 502.10. M Moo-Young, G Andre. An appraisal of mass transfer phenomena in food pro-

cessing operations. In: P Linko, Y Malkki, J Olkku J Larinkari eds. Food Engineer-ing Progress. London: Applied Science Publishers, 1980, Vol. 1, pp. 342–353.

11. M Spiro, CM Page. The kinetics and mechanism of caffeine infusion from coffee:Hydrodynamic aspects. J Sci Food Agr 35:925–930, 1984.

12. G Karnofsky. The theory of solvent extraction. JAOCS 26:564–570, 1949.13. A Voilley, D Simatos. Modeling the solubilization process during coffee brewing.

J Food Proces Eng 3:185–198, 1980.14. R Anderson, R Osredkar, C Zagar. Diffusion properties of lipids: pure and in seeds.

JAOCS 54:487–489, 1977.15. MK Russell, GT Tsao. Protein removal from corn endosperm by solvent extraction.

AIChE Symp Series No. 218, 78:83–89, 1982.16. FH Schneider, U Rutte. Zur bindung der resthexans in raps-extraktionsschroten.

Fette Seifen Anstrichmittel 86:331–339, 1984.17. L Lebrun, GA Junter. Diffusion of sucrose and dextran through agar membranes.

Enz Microb Techn 15:1057–1062, 1993.18. J Crossley, JM Aguilera. Modelling the effect of microstructure on food extraction.

J Food Process Eng, 24:161–177, 2001.19. JM Aguilera, HD Garcıa. Protein extraction from lupin seeds: a mathematical

model. Int J Food Sci Technol 24:17–27, 1989.20. JM Aguilera, EW Lusas. Laboratory and pilot solvent extraction of extruded high

oil corn. JAOCS 63: 239–243, 1986.

Solid–Liquid Extraction 55

21. HP Fan, JC Morris, H Wakeham. Diffusion phenomena in solvent extraction ofpeanut oil: Effect of cellular structure. Ind Eng Chem 40(2):195–199, 1948.

22. NK Rastogi, K Niranjan. Enhanced mass transfer during osmotic dehydration ofhigh pressure treated pineapple. J Food Sci 63:508–511, 1998.

23. JM Aguilera, PJ Lillford. Microstructure as applied to food engineering. In: P Fito,E Ortega-Rodrıguez, GV Barbosa-Canovas eds. Food Engineering 2000. London:Chapman and Hall, 1996, pp. 20–36.

3Supercritical Fluid Extractionin Food Engineering

Nurhan Turgut DunfordOklahoma State UniversityStillwater, Oklahoma, U.S.A.

Jerry W. King and Gary R. ListU.S. Department of AgriculturePeoria, Illinois, U.S.A.

I. INTRODUCTION

The origins of supercritical fluid extraction (SFE) are firmly rooted in the phe-nomenon of solute solubility in gases as typified by the classic studies of Han-nay and Hogarth in the 1880s (1). However, this curious and somewhat unex-pected phenomenon, i.e., the dissolution of solids in such fluids as supercriticalethanol and water, was not exploited for any useful purpose until the midpartof the next century.

From a modern perspective, SFE was first exploited in Russia in the 1950s(2) and later in Germany (3) for the extraction and fractionation of componentsin natural products. However, it was the appearance of the Zosel patent (4),issued in Germany in the late 1960s, that firmly documented the myriad ofpossibilities for using supercritical fluids (SCFs) as processing agents. The U.S.version of this patent, entitled, “Process for the Separation of Mixtures of Sub-stances,” provided more than 60 examples of applications of super- and subcriti-cal ethylene or carbon dioxide to food processing and other applications. How-

Names are necessary to report factually on available data. However, the USDA neither guaranteesnor warrants the standard of the product, and use of the name by USDA implies no approval of theproducts and the exclusion of other that may also be suitable.

57

58 Dunford et al.

ever, this broad coverage did not deter other parties from similar patent filingsover the next 30 years or the industrial exploitation of critical fluids as versatileprocessing agents.

The classic, often cited applications of SCFs, especially supercritical car-bon dioxide (SC-CO2) or liquefied carbon dioxide, are for the decaffeinationof coffee (5) and the processing of hops (6). Thus, there is a historical prece-dent for using critical fluid media in food engineering, which continues up tothis present day. Since the above-cited applications, there has been a plethoraof additional applications of SFE to foodstuffs, agricultural raw materials, andassociated natural products (7). This has resulted in the development of 32processing plants worldwide employing SCFs. Currently, most of these facili-ties are centered in Germany, the United States, France, and Japan. Nationssuch as Great Britain, Australia, Canada, India, and Italy continue to developmore plant capacity for critical fluid processing; and other nations with a richlitany of natural products will undoubtedly enter the marketplace as users ofSCF technology in the future. It should be noted that not all of the new pro-cessing schemes employing SCFs will be wholly based on extraction, but mayinclude fractionation schemes or reactions conducted in critical fluid media.

Table 1 shows a list, which is not inclusive, of many users of critical fluidextraction for food and natural product processing throughout the world. Keyplayers in the various market segments are as follows: decaffeination—GeneralFoods, SKW Trostberg, Kaffee HAG, Hermsen; hops processing—HVG Barth(NATECO2), John Haas, Yakima Chief, Carlton United Breweries, SteinerHops, English Hops, SKW Trostberg; flavors/spices—Cultor, Quest, Flavex,

Table 1 Organizations Processing or Offering Critical Fluid–Derived Products

Flavex (Germany) Fuji Flavor (Japan)Hermsen (Germany) Kobe (Japan)HVG Barth (Germany) Mori Oil Mills (Japan)Kaffe HAG (Germany) Ogawa (Japan)SKW Trostberg (Germany) Takasago (Japan)KD-Pharma-IQA (Germany–Spain) Takeda (Japan)General Foods (United States) Cultor (France)John Haas (United States) HITEX (France)Praxair (United States) Norac (Canada)Yakima Chief (United States) Aroma Tech OY (Finland)Carlton United Breweries (United Kingdom) Quest (Holland)English Hops (United Kingdom) Wells Investment Ltd. (New Zealand)Steiner Hops (Germany–United Kingdom–

United States)

Supercritical Fluid Extraction 59

Norac, Ogawa, Fuji Flavor, Kobe, Mori Oil Mills, Takeda. Many of the proces-sors listed under the generic heading of flavors/spices also produce a variety ofother natural products such as specialty oils, natural pigments, and antioxidants.For example, Flavex is also involved in the processing of ginseng among othermoieties, and Norac in Canada processes rosemary antioxidants, saw palmetto,kava kava, and other botanicals (8).

Since the early 1990s, an awareness of the potential of critical fluid pro-cessing as a viable component for “green” processing has arisen (7). This cou-pled with an increasing consumer awareness of the identity and use of chemicalsolvents in food and natural products processing has provided further impetusfor the use of benign solvents as SC-CO2, ethanol, and water. Recently, the useof SC-CO2 for the processing of certain nutraceutical products (9) has providedadditional possibilities for exploiting SCFs. As governments worldwide are alsocurrently making regulations on the use of organic solvents even more strict(10), including the banning of some traditional solvents, environmentally benignSCFs such as SC-CO2 or non-ozone-depleting fluorocarbons will become evenmore attractive.

In this chapter, we have reviewed some of the basic fundamentals ofSCFs and their use for the extraction and fractionation of food-related materi-als. The relevant physical and chemical properties of SCFs, particularly SC-CO2, are initially discussed, followed by the key factors that contribute tosuccessful SFE, namely, the solubility of solutes in SCFs, phase equilibria,and mass transfer considerations. A discussion of the fundamentals requiredin effectively using SCFs, such as the type of extraction or fractionation, alongwith choice of solvents, entrainers, and condition of the matrix to be extracted,follows. The various application areas of SCF extraction are discussed atlength in following chapters.

II. PROPERTIES OF SUPERCRITICAL FLUIDS

The mathematical modeling, simulation, and design of an SFE process requireknowledge of physicochemical properties of compounds of interest. Further in-terpretation of physicochemical property values can be obtained by an understand-ing of molecular behavior. For example, the ideal gas law [Eq. (1)] is a primeexample of the correlation of properties.

PV = NRT (1)

Critical temperature and pressure, compressibility, density, heat capacity,dielectric constant, viscosity, diffusivity, and thermal conductivity are importantproperties, which are needed for the characterization of both solvent and solutesin an extraction process. Pure-component properties as well as mixture proper-

60 Dunford et al.

ties are used to describe individual solute and solvent characteristics and howthe solute and solvent behave as constituents of mixtures.

A. Critical Constants

The critical point of a pure substance is usually defined as the temperature andpressure at which the gas and liquid phases become indistinguishable. However,in the case of a solid and a liquid, these two phases do not become identicalwhen their densities are equal. Therefore, solid–liquid, solid–gas, and solid–solidequilibrium lines do not end by critical points, as do gas–liquid lines (Fig. 1).

When a substance is compressed and heated to its critical point it entersa phase referred to as “supercritical phase.” The matter that is in the supercriticalregion is called a supercritical fluid (SCF). Temperature, pressure, and molarvolume of a substance at the critical point are defined as the critical temperature(Tc), critical pressure (Pc), and critical molar volume (Vc), respectively. Theseparameters are collectively referred to as critical constants. Each substance hasa unique set of critical constants (Table 2). Applications of SFE in food engi-neering involve processing of many materials, which contain a number of com-ponents. Identification and characterization of natural material components areusually very difficult. Therefore, in the literature SFE often characterizes pro-cesses above the critical point of the solvent. In this case, mixture of solventand solute usually exists as two distinct, partially miscible phases below thecritical point of the mixture. Similarly, extraction processes just below the criti-cal point of the solvent are usually referred to as near-critical extraction basedon the pure solvent properties rather than that of the mixture.

Critical properties of small molecular weight compounds can be found instandard handbooks (12, 13). Critical properties of many biological materialscannot be determined experimentally due to the fact that these materials will bedecomposed before their critical point is reached. Usually, semiempirical meth-ods are used to estimate critical properties of natural products. The group contri-bution method is also widely used to estimate critical properties of biologicalmaterials. Reid et al. (13) reviewed Ambrose, Fedors, and Joback modificationof Lydersen’s method in detail. The Ambrose group contribution method iswidely used to estimate critical properties of organic materials. In this method,the critical properties are estimated using the following relations:

Tc = Tb[1 + (1.242 + Σ∆T)−1]; Pc = M(0.339 + Σ∆P)

−2; Vc = 40 + Σ∆ν (2)

The ∆ values for a number of compounds are given in Reid et al. (13).When the compound of interest contains functional groups for which ∆ parame-ters are not available, use of group contribution methods becomes difficult. Insuch cases, more empirical models are used (14, 15).

Supercritical Fluid Extraction 61

Figure 1 Phase diagram for a pure substance.

B. Physicochemical Properties

A unique feature of the SCF is the adjustability of their density in the supercriti-cal region by regulating the temperature and pressure of the system. An increasein temperature leads to a decrease in density in all cases. Density of a fluid isextremely sensitive to temperature and pressure near the critical point (Pr = 1,Tr = 1). Reduced density (ρr = ρ/ρc) of a pure compound at reduced pressure 1.0

62 Dunford et al.

Table 2 Critical Pressure and Temperatureof Common Fluids for SCFE

Crit. temp., Crit. pressure,Fluid Tc (°C) Pc (× 105 Pa)

Carbon dioxide 31.1 73.7Chlorotrifluoromethane 28.9 39.2Ethylene 9.3 50.3Ethane 32.3 48.8Propane 96.7 42.4Propylene 91.9 46.2Cyclohexane 280.3 40.7Isopropanol 235.2 47.6Benzene 289.0 48.9Toluene 318.6 41.1p-Xylene 343.1 35.2Trichlorofluoromethane 198.1 44.1Ammonia 132.5 112.7Water 374.2 220.4

can be changed from a value of about 0.1, a gas-like density, to about 2.0, aliquid-like density, by regulating reduced temperature in the range of 0.9–1.2(Fig. 2, Table 3). As the reduced densities become liquid-like, SCF begins toact like a liquid solvent. However, as the reduced temperature is raised to avalue of about 1.6, the fluid becomes gas-like due to the decreased density(expansion of fluid) with increasing temperature. Data on the physical propertiesof mixtures is scarcer than for the pure compounds. An interesting publicationof Magee (16) includes a list of references, which report density calculationsfor CO2-rich mixtures from various predictive models or theories.

At high pressures, gases deviate from ideal behavior due to enhancedphysical forces between ions, dipoles, induced dipoles, and higher poles, whichcontribute to the molecular interactions in the system. The potential energy ofinteraction, Ep, of point charges q1 and q2 can be defined as a function of thepermittivity of the medium, ε, and the distance between the charges, r [Eq. (3)].

Ep =q1q2

4πεr(3)

The relative permittivity, εr, also known as dielectric constant of the medium, isdefined as follows:

Supercritical Fluid Extraction 63

Figure 2 Variation of reduced density of a pure substance in the near-critical region(20).

Table 3 Comparison of Physical Properties of Gas, Liquid, and SCF (98)

SupercriticalGas fluid

(1 atm, LiquidPhysical property 15–30°C) Tc, Pc Tc, 4Pc 15–30°C

Diffusion coefficienta (cm2/s) 0.1–0.4 0.7 × 10−3 0.2 × 10−3 (0.2–2) × 10−5

Viscosity (g/cm.s) (1–3) × 10−4 (1–3) × 10−4 (3–9) × 10−4 (0.2–3) × 10−2

Density (g/mL) (0.6–2) × 10−3 0.2–0.5 0.4–0.9 0.6–1.6

aSelf-diffusion for gas and dense gas; binary mixture for liquid.

64 Dunford et al.

εr =εε0

(4)

where, ε0 is the permittivity in a vacuum.The static dielectric constant (DC) is a useful property to estimate the

solvent properties of the relatively polar fluids such as ethanol, methanol, andwater. DC is also a density-dependent property and can be changed significantlyby modest changes in temperature and pressure of the system. The DCs forSCFs become important because they are a measure of intermolecular forceenhancement through dipole–dipole interactions. For example, DC values forCO2 rise rapidly between 70 and 200 × 105 Pa at 40°C and reach liquid-likevalues around 200 × 105 Pa (Fig. 3) (17). This partially explains the greatersolvent power of SC-CO2 for low-volatility compounds at elevated pressures.

C. Transport Properties

1. Viscosity

Viscosity is an important property, which is needed for the characterization ofmomentum, mass, and energy transport within a system. The viscosity of a gasincreases with increasing temperature at a moderate pressure whereas that of anSCF decreases (18). This phenomenon is attributed to the higher molecular ve-locity and increasingly hindered “collision transfer” of momentum at high tem-perature and pressures. Viscosity of a substance changes by a factor of 3–5 inthe critical region rather than orders of magnitude (Table 3). Thus, when pres-sure is increased, gas viscosity approaches liquid-like value more slowly thandoes density (19). The viscosity of CO2 as a function of pressure is shown inFig. 4. The viscosity of CO2 is only 0.09 cP even at pressures as high as 300–400 atm, which is an order of magnitude lower than the typical viscosities ofliquid solvents (20). As another example, viscosity of supercritical water (Fig.5) is less than one-tenth of liquid water, and as a consequence its diffusioncoefficient and ion mobilities are one order of magnitude greater than those ofthe liquid water.

Viscosity measurement and relevant estimation techniques for pure andSCF mixtures have been discussed by Vesovic and Wakeham (21) in detail.Magee (16) also reviews the literature related to the viscosity for gas mixtures;however, this kind of information is virtually nonexistent for the SCF-naturalproduct mixtures.

2. Diffusivity

Knowledge of diffusion coefficients in SCF is important to the design and effi-cient operation of SFE processes. The diffusion coefficient is the proportionality

Supercritical Fluid Extraction 65

Figure 3 Density and dielectric constant of carbon dioxide as a function of pressureat 50°C (17).

constant between the molecular flux of a compound and its composition gradi-ent. Fick’s law defines it as follows:

J1 = −D12∂C1

∂Z

J2 = −D21∂C2

∂Z(5)

where C1, C2, and J1, J2 are the molar concentrations and fluxes of two compo-nents, respectively. Fick’s law holds for binary, isobaric mixtures and assumespresence of no external force field. The diffusivity of a pure SCF is orders ofmagnitude greater than that of a liquid, i.e., liquid CO2, which results in im-

66 Dunford et al.

Figure 4 Viscosity behavior of carbon dioxide (20).

Figure 5 Viscosity of water as a function of density and temperature (18).

Supercritical Fluid Extraction 67

proved mass transfer rates during SFE. In general, the binary diffusion coeffi-cient in SCF increases with temperatures and decreases with pressure at constantpressure and temperature, respectively (Fig. 6). Although the effect of composi-tion on diffusion coefficient is quite small at low pressures, it becomes signifi-cant at higher densities. Solute diffusion coefficients decrease with increasingsolute molecular mass. Furthermore, molecular mobility is hindered if the soluteand solvent molecules are significantly different in size.

The experimental methods used to determine diffusion coefficients in SCF

Figure 6 Diffusivity behavior of carbon dioxide (20).

68 Dunford et al.

systems, such as solid dissolution, capillary peak broadening, photon correlationspectroscopy, nuclear magnetic resonance, and radioactive tracer response tech-niques, have been reviewed by Giddings and Seager (22) and Liong et al. (23).Examples of measured diffusion coefficients reported in the literature are forC16 fatty acid esters (24), caffeine (25), and C18 unsaturated fatty acid methylesters (26).

3. Heat Capacity and Thermal Conductivity

Heat capacity and thermal conductivity data are needed for the characterizationof heat transfer behavior in a system. In the critical region, heat capacity atconstant pressure is large and goes through a maximum (Fig. 7) (27). However,heat capacity at constant volume shows only small changes in the critical region(Fig. 7). Angus et al. (28) have reviewed the heat capacity data published up to1976. Magee and Ely (29) have measured the heat capacity of CO2 at constantvolume, at a larger density range, 0.2–2.5 times the critical density. They alsocompared their experimental results with those calculated using an extendedBenedict-Webb-Rubin equation of state. In general, the measured and calculatedheat capacity values agreed well, except that relative errors were higher at thenear-critical region. Magee (16) also reviewed the heat capacity data for pureCO2 and CO2-rich mixtures.

Figure 7 Heat capacities for carbon dioxide at 320 K (27).

Supercritical Fluid Extraction 69

Thermal conductivity of fluids has been recognized as an important trans-port property, which is needed for the design of the SFE process equipment.Thermal conductivity “λ” is a proportionality constant between the heat flux,Q, and the temperature gradient that exists in the fluid. The Fourier’s law definesit as follows:

Q = λ �T (6)

For most SCFs thermal conductivity increases with increasing temperatureand density of the system. Table 4 shows the thermal conductivity data for waterand CO2 as a function of temperature at several pressures.

The Transient Hot-Wire Technique and The Coaxial Cylinder Method aretwo techniques that are used to measure thermal conductivity of SCFs over awide range of conditions (21). Although the Euken expression and correlationsbased on the work of Dymond had been used to estimate thermal conductivityof dense fluids, the calculated values did not agree well with the experimentaldata (21). Nieto de Castro (30) has reviewed the literature on the thermal con-ductivity in SCF.

III. EQUILIBRIUM PROPERTIES

A. Phase Equilibrium

Process modeling and operating conditions, as well as economic feasibility foran extraction process, require information on phase equilibria between the sol-ute(s) and solvent(s) involved. This information can be obtained experimentallyor via phase equilibrium calculations. Phase equilibrium calculations require anequation of state, which describes the mathematical relation between volume,pressure, temperature, and composition in the system. Deviations from ideal gaslaw are expressed in the equations of state such as van der Waals [Eq. (7)] andRedlich-Kwong (31) or Peng-Robinson equations (32).

Table 4 Thermal Conductivity [λ(mW/mK)] ofSome Fluids Used in SCF Technology (21)

T = Tc + 20 K T = Tc + 100 K

P = 0.1 P = 10 P = 0.1 P = 10MPa MPa MPa MPa Tc (K)

CO2 18.8 51.1 25.5 31.9 304.1H2O 54.0 67.8 63.7 73.0 647.1

70 Dunford et al.

�P + a

V2�(V − b) = RT (7)

where a and b are the constants specific for each gas. The van der Waals equa-tion of state assumes that molecules restricted to a volume (V − b) and repulsiveinteractions cause molecules to behave as small but impenetrable spheres. At-tractive forces between molecules reduce the pressure exerted by a real gas. Thecritical constants can be calculated from the van der Waals equation by settingthe first and second derivatives of Eq. (7) with respect to zero to obtain thefollowing relations:

Vc = 3b; Pc =a

27b2; Tc =

8a27Rb

(8)

Through algebraic manipulation, then, the critical compression factor, Zc, canbe given as:

Zc =PcVc

RTc

= 38

(9)

In a similar fashion, dimensionless reduced properties of a gas can be expressedas ratios of the respective critical properties, as:

Pr =P

Pc

; Vr =V

Vc

; Tr =T

Tc

(10)

The “law of corresponding states” (33) states that if reduced variables areused to define the state of gases, all gases show the same PVT behavior; inanother words, two different gases, each at the same Pr and Tr, will have thesame Vr. Using this analogy, van der Waals’ equation can also be written interms of reduced variables as:

Pr =8Tr

3Vr − 1− 3

V2r

(11)

This equation eliminates the constants a and b and provides a more directmethod of estimating physical properties. Other equations of state, their mixingrules (13, 34–36) along with PVT surface measurements, and their correlationsfor a number of fluids (37), are available in the literature.

Models describing equilibria between SCF and condensed phases are clas-sified in two groups: fluid–liquid and fluid–solid equilibria (31, 38–41). VanKonyenburg (42, 43) developed a system for classification of phase behaviorfor binary fluid mixtures. This system groups the phase behavior of fluid mix-tures into six general types, which are distinguished by the behavior of theircritical lines (20, 32, 44). Rizvi et al. (32) has reviewed the phase diagrams of

Supercritical Fluid Extraction 71

class I and class III mixtures for representative of typical biomaterials of interestto the food industry. Modeling of such phase equilibria shows highly nonidealbehavior due to the complexity of these natural products, which vary enormouslyin molecular size and chemical nature. Despite these complexities, equations ofstate have been developed to predict phase behavior of natural substances suchas fatty acid esters (45), vegetable oils (46), rapeseed oil (47), and limonene(48) in SCFs.

The equations of state calculations require the knowledge of acentric fac-tor, ω, which is defined as follows:

ω = −logPvpr − 1 (12)

where Pvpr is the reduced vapor pressure at Tr = 0.7.The acentric factor is a macroscopic property that measures the extent to

which the force field around a molecule deviates from spherical symmetry. Theacentric factor is practically zero for small, spherical, or highly symmetrical andnonpolar molecules.

Several experimental and predictive methods for the determination of theacentric factor of compounds have been reported in the literature (13). Araujoand Meireles (45) used Tu’s (49) indirect method and correlation of Vetere (50)to estimate the acentric factor for high and low molecular weight fatty acids,respectively. The study showed that acentric factor of the compounds increasedwith increasing carbon number and linearly decreased with increasing doublebond in the chemical structure.

B. Solute Solubility

The development of mathematical models describing the solubility behavior ofsolutes in SCFs requires an understanding of intermolecular interaction amongsolute and solvent, as well as solute and solute. Thus, the vapor pressure andthe solute–solvent intermolecular interactions are key properties that determinethe solubility of a solute in a supercritical solvent. The effect of temperature onsolubility manifests itself in the solute’s vapor pressure, the SCF’s density, andmolecular interactions in the supercritical phase. At low pressures, solubilitydecreases with increasing temperature due to the lower SCF density. However,at higher pressures, the SCF’s density is only slightly affected by temperature,so that solute solubility increases with temperature through its effect on solutevapor pressure.

In the highly compressible near-critical region, it is also known that thesolvent “clusters” around the solute due to the attractive intermolecular interac-tions (23). This effect is especially important in entrainer systems because ab-normally high local solvent density around the solute may preferentially attractentrainer to the solute. Unfortunately, many solubility models assume infinite

72 Dunford et al.

dilution of a solute in a solvent (SCF) and no solute–solute interactions. How-ever, there is experimental and computational evidence that strength of solute–solute interaction may be strong in very dilute solutions (14).

One measure of solvent power (capability of a solvent to dissolve varioussolutes) of an SCF can be described qualitatively by the solubility parameterapproach (51). The concept of solubility parameter was first defined for liquidsin the condensed phase by Hildebrand and Scott (51) as follows:

δ = �−Ev

Vl�1/2

(13)

where Ev and Vl are heat of vaporization and molar liquid volume, respectively.A liquid’s solubility parameter is theoretically calculated knowing the heat

of vaporization and molar volume of the liquid. Relating the solubility parameterto the energy of vaporization for dense gases is conceptually difficult becausethe heat of vaporization becomes zero at a solvent’s critical temperature andthe vaporization process cannot occur under supercritical conditions. However,Giddings (52) has combined the van der Waals equation with the theory ofcorresponding states, to yield and empirical relationship, where the solubilityparameter is defined by the following formula:

δ = 1.25P1/2c � ρρliq

� (14)

where Pc, ρ, and ρliq are critical pressure and density of gas and liquid, respec-tively.

When the solubility parameter defined by Eq. (14) is plotted against pres-sure, the relationship depicted in Fig. 8 resembles that of density vs. pressure(Fig. 2). At infinite compression, the SCF’s solubility parameter in Eq. (14)reduces to:

δliq = 1.25P1/2c (15)

where ρ = ρliq, approximating the solvent power of a gas in its liquid state.Returning to Eq. (14), the solubility parameter is a function of density,

which is a state variable. The contribution of δliq to δ is known as the “chemicaleffect” and is dependent on the choice of SCF, while the contribution of ρ/ρliq

is attributed to the “state effect” (53). Under supercritical conditions, a large δis obtained when pressure is high and temperature is slightly over the Tc. Thesolubility parameter of the fluid may vary from zero at low pressures up toliquid-like values �10 (cal/cm3)1/2 at ultrahigh pressures. SCFs have solubilityparameter values in the range of 0–10 (cal/cm3)1/2 (53). King and Friedrich (54)utilized the concept of reduced solubility parameter as a measure of the solute–

Supercritical Fluid Extraction 73

Figure 8 Solubility parameter vs. pressure plot for carbon dioxide (97).

solvent interactions to correlate solubility and distribution coefficients in super-critical and near-critical fluids.

The molecular structure of a solute influences its solubility in an SCF (54,55). Stahl and Quirin (56) were the first to report that the presence of polarfunctional groups such as OH, C=O, and COOH in a molecule significantlyreduced the solubility of a compound in SC-CO2. Solubility studies utilizingsterols indicated that increasing the number of carbon–carbon double bondsdecreased the solute’s solubility in SCF (55).

Substantial work has been carried on the measurement and prediction ofsolubility of lipid components in SCF, specifically SC-CO2. The literature onsolubility of seed oils in SC-CO2 has been reviewed by King (56). For example,Eisler and Friedrich (57) used the Fujishiro and Hildebrand equation (58) to

74 Dunford et al.

estimate vegetable oil solubility parameters in SC-CO2 and reported that calcu-lated values compared reasonably well with the experimental literature data.Similarly, Chastril (14) determined solubilities of a number of lipid componentssuch as stearic and oleic acid, tripalmitin, butyrin, and olein. Solubilities of α-tocopherol, cafestol, cholesterol, and water were also measured in pure CO2 inthe range of 40–80°C and 8–25 × 106 Pa (14). Chastril’s study (14) has relatedsolute solubilities directly to density of SCF rather than using the equations ofstate for the prediction. The solubility of relatively polar compounds such asfree amino acids, e.g., glycine, phenylalanine, tryptophan, and leucine, and sug-ars such as D-glucose and D-xylose in pure CO2 at 40°C and 5–20 × 107 Pa werealso reported, and solubilities of these compounds were very low, of the orderof 10−7–10−8 mole fraction (59).

C. Solvent Selectivity

The selectivity can be defined as the ability of a solvent to dissolve the desiredcompound to a greater extent than the other constituents of the mixture. Selectiv-ity of an SCF for component 2 vs. component 1 is defined as:

S = y2

y1

(16)

where y1 and y2 are the binary solute solubility of components 1 and 2, respec-tively.

The selectivity of an SCF can be tuned by regulating temperature andpressure of the fluid. This phenomenon allows the use of SCFs for selectiveextraction and fractionation processes.

Selectivity and solubility of a compound are primarily related to the solutevapor pressure and only secondarily to the intermolecular forces in the supercrit-ical phase (55). For example, although the vapor pressure of both cholesteroland ergosterol is negligibly small, selectivity of SC-CO2 for cholesterol is almosttwo orders of magnitude higher than that of the ergosterol due to the fact thatvapor pressure of ergosterol is lower (10−5–10−6 Pa) than that of cholesterol(10−3–10−4 Pa), showing that selectivity follows the ratio of vapor pressures (55).

D. Enhancement Factor

Frequently in SCF technology, a term called the enhancement factor, E, is uti-lized to describe the propensity of a solute to partition into a SCF. The enhance-ment factor normalizes the effect of vapor pressure, allowing an estimate to bemade of solute–SCF interactions in the supercritical phase, and is defined asfollows (44):

Supercritical Fluid Extraction 75

E = yactual

yideal

(17)

and

yideal =Ps

P(18)

where yactual = actual solute solubility in SCF, yideal = solubility in ideal gas, Ps =vapor pressure of the solid, and P = pressure of the system.

The solubility of a solute in a solvent changes rapidly near the criticalregion, and this is reflected in the enhancement factor. Enhancement factors ofthe order of 105–107 are common for solubilization of solutes in SCF. For exam-ple, enhancement factors are the same (�107) for cholesterol, stigmasterol, andergosterol in CO2 at 35°C and 200 atm, which indicates that the sterol-CO2

attractive forces are similar for these sterols (55). Therefore, as noted above,fractionation of the sterols via SFE is basically dependent on the vapor pressureof the components.

IV. MASS TRANSFER

A. Mass Transfer Principles

The economical scale-up and design of commercial SFE processes requires masstransfer data for the system. Even though SFEs have excellent mass transferproperties, such as gas-like diffusivity and viscosity, practically no surface ten-sion contribution at interface, liquid-like density, or pressure-dependent solventpower; SFE processes still have mass transfer limitations. For example, if therate-limiting step or the largest mass transfer resistance of a separation processis the transfer of the solute from the surface of a solid to the SCF phase, thegas-like diffusivity of a SCF will enhance diffusion and consequently speed upthe extraction. However, if the extraction is from a liquid phase into a SCFphase, mass transfer in the liquid phase will be the rate-determining step. Simi-larly, if the extraction is occurring from within a solid, then the internal solidphase diffusion will control the rate of mass transfer. It has been established bythe experimental measurements that initially at high solute concentrations in thesolid matrix, the mass transfer rate from a solid matrix into a SCF is constant.This region of the extraction curve is referred to as steady-state or solubility-controlled mass transfer region. However, after a certain amount of material isextracted, this rate starts to decline and the region is called diffusion-controlledmass transfer region (56). A typical SFE curve for oil production from a solidmatrix is shown in Fig. 9 (60).

Interfacial tension (IFT) is an important parameter that influences mass

76 Dunford et al.

Figure 9 A typical curve for an SCFE process (60).

transfer and solubility of compounds in SCFs. Packed columns have been usedfor fractionation of liquid mixtures utilizing SCF technology. The design ofsuch systems requires mass transfer data, including the wetting characteristicsof the packing surface by the fluid. Literature on the study of IFT in SCF isscarce and usually deals with model systems. However, Simoes et al. (61) re-ported the data on the IFT of crude and refined corn, coffee, and citrus oils inSC-CO2. It was shown that IFT of the oils decreased with increasing pressure(61). This phenomenon was explained by the breakup and eventually disappear-ance of the oil droplets on the packing material surface, with increasing pres-sure. Experiments carried out with corn oil demonstrated that the decrease inIFT was larger above 20 MPa, and at lower pressures only slight changes inIFT were recorded (61).

B. Mass Transfer Models

Fundamentals of SFE processes for natural materials and food systems are notwell defined due to the complex nature of the matrix and difficulty in obtaining

Supercritical Fluid Extraction 77

mass transfer data under high pressure. The majority of the SFE mass transfermodels published in the literature are based on extraction of oil (60, 62, 63). Anumber of researchers have attempted to simulate mass transfer rates in SCFsby using different approaches, such as empirical kinetic models, linear drivingforce approximations, the shrinking core leaching model, and local adsorption-desorption technique. For example, Brunner (64) used dimensionless numberssuch as Sherwood, Schmidt, and Reynolds numbers to derive correlations formass transfer coefficients in various solid fixed-bed SFE processes, i.e., coffeebeans and oil seeds. It was shown that extraction rate of whole coffee beanswas limited by the diffusion rate of caffeine within the solid matrix. Peker et al.(65) also studied a similar system and developed a mathematical model basedon a linear driving force approximation of mass transfer and partitioning ofcaffeine between the water and the SC-CO2. A similar approach was applied tospice extraction by Goto et al. (66). In this study, the SFE of peppermint oilwas described by a mathematical model that accounted for both local adsorptionof essential oil on lipids in the peppermint leaves and mass transfer from a solidmatrix to SCF.

Bulley et al. (67) and Lee et al. (68) used a one-dimensional, un-steady-state mathematical model to describe the SFE of a fixed bed of crushed canolaflakes. This method assumed plug flow within the seed bed and that axial disper-sion was negligible. They calculated canola oil concentration profiles in bothSC-CO2 and solid phases and determined overall volumetric mass transfer coef-ficients. The model adequately described the constant-rate period of the extrac-tion process. Cygnarowicz-Provost (60) used a similar model but included masstransfer coefficients that represented both constant-rate and diffusion-controlledregimes for the determination of caffeine extraction rates in SC-CO2. Likewise,the SFE of grape oil has been simulated using a plug flow model, allowing masstransfer coefficients in both solid and supercritical phases to be calculated (69).Another mass transfer model, which was applied to SFE of oils from herbaceousmatrices (70), assumed that diffusivity of the solute in the solid matrix was theonly variable in the system.

The shrinking core leaching model may be used when the solute to beextracted constitutes a large portion of the solid matrix and is held in the macro-pores by mechanical or capillary forces as a condensed phase. This model as-sumes that a sharp boundary exists between a core of material to be extracted,and an outer region, in which only partially saturated SCF exists in the pores.The shrinking core leaching model is valid where the diffusivity of the solutein SCF is much larger than that of the solvent in the solid. Simulation of sucha system requires mathematical description of a moving core boundary, whichshrinks nonlinearly due to the depleting solute in the pores during the extractionprocess. Therefore, the following assumptions may be made to simplify thecalculations; a pseudo-steady-state diffusion mechanism for extraction from theouter porous layer, negligible fluid velocity at the interface, and the radius of

78 Dunford et al.

the core can be determined by a simple mass balance (62). The shrinking coreleaching model has applied to ginger oil (71) and freeze-dried mackerel powder(72).

Another example of the mass transfer simulation efforts in food-relatedsystems was published by Dunford et al. (73). They developed a mathematicalmodel to describe the dynamic extraction behavior of oil and water from Atlan-tic mackerel at different moisture levels. Their model incorporated the interac-tions between oil and water in the SC-CO2. This model accurately simulated theexperimental data in the solubility-controlled region for the above extraction.

V. ENTRAINER EFFECT

Addition of an entrainer, which is also referred to as cosolvent, or modifier toa mixture containing a supercritical component can modify the extraction selec-tivity markedly. Entrainers, which are mostly liquids usually having solubilityparameters larger than the SCF component, can be used to increase solventpower of an SCF in addition to density adjustment of the extraction fluid. Hex-ane, benzene, chloroform, isopropanol, methanol, ethanol, acetone, and waterhave all been used as entrainers. Ethanol is the preferred entrainer for foodapplications because it is not toxic and is approved as GRAS (generally recog-nized as safe) status component. Entrainers are usually chosen to interact specif-ically with targeted solute, through hydrogen bonding, acid–base interactions,or strong dipole–dipole interactions. Entrainers also increase yields, then, bydecreasing the pressure and solvent requirements in SFE. Entrainers can affectthe extraction process in many ways: by increasing the volatility of the solutes,by increasing the density of an SCF, by potentially dilating a liquid condensedphase, or by enhancing the miscibility of the components. An entrainer normallyincreases the solubility of nonvolatile compounds in an SCF by shifting theequilibrium of the binary mixture and/or by shifting the critical point of thesolvent phase. Entrainers, which as noted previously are more polar compoundsthan SC-CO2, can form electron donor–acceptor complexes, i.e., hydrogen bonds,with polar solutes to increase solubilities and selectivities beyond what wouldbe expected based on solubility in the neat SCF. Supporting evidence for theabove trend, particularly increasing solute solubility in SCF, was recorded forthe addition of a modest amount of cosolvent (less than 10%). For example,Dobbs et al. (74) showed that the solubility of 2-aminobenzoic acid was in-creased by 600% by the addition of 3.5% methanol into SC-CO2. Several ther-modynamic models have been developed to correlate and in some cases to pre-dict the effects of entrainer on solute solubilities in SCF (3).

In a related phenomenon, when dry almonds are extracted with SC-CO2

at 40°C and 6 × 107 Pa, an oily extract containing the flavor components is

Supercritical Fluid Extraction 79

achieved (75). However, when the extraction is carried out in the presence ofethanol, two phases—an oil phase free of flavors and an ethanol phase contain-ing flavor components with a small amount of oil—were obtained (75). Influ-ence of an entrainer has also been shown for the extraction of caffeine fromcoffee beans, where neat SC-CO2 extraction of dry coffee beans cannot effi-ciently extract the caffeine as a moist coffee bean (5). Another study showedthat the pleasant flavor of tea could be extracted with SC-CO2 in the presenceof water; however, isolation of flavors by evaporating the water was not possiblesince most of the flavor components were removed azeotropically during evapo-ration (75).

An excellent example of where entrainer-based SCF separations are neces-sary is in the fractionation of polar solutes from nonpolar moieties using SC-CO2. Thus, substantial differences in the polarity of molecules, such as neutraloil components (triglycerides) and polar lipids (phospholipids), can be exploitedfor fractionation purposes with the aid of an entrainer such as a GRAS-approvedethanol. For example, Dunford and Temelli (76) and Montanari et al. (77) stud-ied the extraction of phospholipids from canola and soybean lipids, respectively,using SC-CO2 and ethanol mixtures. Initially, neutral lipids were extracted withSC-CO2 and then ethanol was added to SC-CO2 as an entrainer to enhance theextraction of the relatively polar phospholipids.

VI. CRITERIA FOR SOLVENT SELECTION

The critical temperature and pressure of a compound are two very importantphysical properties, when selecting an appropriate solvent for an SFE applica-tion. If fluids with high Pc are chosen as solvents, the extraction process will bepotentially more expensive due to the higher cost of equipment, which will needto be designed and manufactured for use at high pressures. If Tc of the solventis exceedingly high, then heat-sensitive materials could be affected adverselyduring the SFE process.

The chemical stability of the solvent under the processing conditions isanother important criterion that should be considered in solvent selection. Thechemical decomposition of a number of SCF–entrainer mixtures has been re-ported in literature (37). As an example, methanol can be very unstable in thepresence of stainless steel (37). SFCs should also be inert with respect to theprocessed raw material to avoid reactions during the extraction process. Low-boiling fluids should be preferred as supercritical solvents for ease of removalafter processing. The ideal solvent of choice should also be inexpensive, non-flammable, nonexplosive, noncorrosive, nontoxic, and readily available in highpurity. Carbon dioxide fulfills those requirements as a fluid for applications ofSFE technology for food processing, and it is preferentially used.

80 Dunford et al.

VII. EFFECT OF MATRIX COMPOSITION AND STRUCTUREON SFE YIELDS AND RATES

Extraction yield is an important parameter, which greatly affects feasibility of aproduction process. Stahl et al. (78) and Friedrich et al. (79) concurrently re-ported that the yield of an SFE process is influenced by the size and physicalstructure of the oilseeds. They showed that oilseeds had to be ground to assurecomplete extraction of the oil. It was also noted that the shape of the groundseeds as well as their size affected the SFE of oil. Similarly, Snyder et al.(80) illustrated that oil yields were quite low with cracked soybeans; however,theoretical yields were obtained from ground or thinly flaked seeds. Similarresults were reported with rapeseed (80) (Fig. 10) and canola (81).

Water is present in all biological materials at varying concentrations andplays an important role in the SFE and mass transfer kinetics. In some cases, amatrix’s moisture content determines the surface structure and/or activity of thecomponents, such as enzymes. High moisture content may provide an aqueousbarrier inhibiting diffusion of SC-CO2 into the matrix as well as diffusion of oilout of the matrix. An example of this phenomenon has been demonstrated byKing et al. (82). Here it was reported that the yield of fat extraction with SC-CO2 decreased with increasing moisture content of the beef samples.

The presence of water in the matrix may act as an entrainer and improvethe selectivity of SCF for a solute, i.e., caffeine extraction from coffee beans inthe presence of water, which was mentioned earlier in this chapter (5). During

Figure 10 SFE yields as affected by the size of the oilseeds (78).

Supercritical Fluid Extraction 81

essential oil extraction, polar component solubilities in SC-CO2 were improvedat higher moisture content in the sample matrix (83, 84). Swelling of plantmaterials in the presence of water may also be quite significant for extractionbecause it widens plant cell capillaries, thereby increasing the matrix porositywith subsequent improvement in solute diffusion (85). This phenomenon re-duces the extraction time or may even make an extraction possible. It is impor-tant to note that water present in the feed material may be coextracted alongwith the compounds of interest, affecting the quality and purity of the finalproduct. This effect was demonstrated for oil extraction from canola flakes andfish muscle using SC-CO2 by Dunford and Temelli (86, 87).

VIII. SCF PROCESSING SCHEMES

A. Extraction

A schematic diagram of a basic SFE process is shown in Fig. 11. As noted inthe figure, the major equipment components are the extractor, separators, heatexchangers, pumps, and compressors. If desired, more than one extractor can beconnected to the same system. The selection of equipment for a food processingsystem must ensure a sanitary and safe process. All surfaces, including sealsand gaskets, in contact with process fluids and solids must be suitable for foodprocessing and sterilization. Construction materials and fittings must be suitablefor the pressure and temperature conditions required by the process.

The solvent, i.e., CO2, is pumped into the system as a liquid or gas. If thesolvent is pumped as a liquid, it will be cooled in a reservoir or it will be keptin liquid phase by cooling the pump head. Depending on the process require-ments a system for entrainer addition can be incorporated into the system. Meth-ods for entrainer addition to an SCF system were reviewed by Dunford (88).The fluid is then pressurized and heated to the desired processing conditions. Asolvent preheater can be installed into the system to avoid temperature fluctua-tions due to the solvent being pumped into the system at a lower temperature.In the case of solid feed material processing, the matrix to be extracted will bepacked into the extraction cell in a mesh basket or between frits to prevent itsbeing carried from the cell during extraction. Following extraction, the pressureis reduced to precipitate the extract through a control valve. A high-pressuremetering valve can control the flow rate of the fluid. The pressure of the systemis controlled by the rate of pumping and a back-pressure valve setting. A com-puter system can be used to control the extraction conditions.

Choosing a continuous process rather than a batch or semicontinuous pro-cess may significantly improve the economics of an industrial scale process.Continuous transport of a large volume of solid feed, such as oilseeds, into andout of a high-pressure extractor is costly and difficult. However, advances in

82 Dunford et al.

Figure 11 Typical flow diagram for a SFE process.

high-pressure technology may allow continuous feed of solid materials. Todayan extractor design that allows intermittent loading and unloading of solid mate-rial through the lock-hopper vessels fitted below and above the extractor whileit is pressurized is used in the coffee-decaffeinating plant in Houston, Texas(20). A portion of the solid is discharged to the bottom hopper, while fresh feedis simultaneously charged to the extractor from the top hopper. In the meantime,SCF continuously passes through the extractor countercurrently relative to feed.Such an extractor design has several advantages. Sequencing these operationsminimizes raw material feeding and vessel unloading times. The compressioncosts are lowered since feed loading and unloading are carried out simultane-ously while maintaining pressure.

Simulated moving-bed technique has been developed (89) in an effort toapproach a countercurrent solid–fluid extraction process. In this method, a sin-gle fixed-bed column is subdivided into several zones. The apparent movementof the solid is achieved by switching between valves connected at the junctions

Supercritical Fluid Extraction 83

between zones. The solid and extract withdrawal ports are periodically shiftedin the direction of the solid movement and opposite to the fluid flow, thussimulating a countercurrent process. This technique has been applied to extrac-tion and fractionation of tocopherol from oleic acid (89) and fragrance and es-sential oil extraction (90).

B. Fractional Extraction

Fractionation of components of a mixture can be achieved by various modes offractional extraction. One fractional extraction method is to collect fractions asa function of time throughout a semicontinuous process. In such a case, theextract composition changes over time due to the changes in the composition ofresidual material in the extractor. Components that have higher solubility underthe initial processing conditions will be selectively extracted in the beginning.Then, as these components are depleted from the matrix, the extraction selectiv-ity will shift to other components. A good example of this behavior would beduring SFE of oilseeds. Friedrich and Pryde (91) showed that the amount ofphospholipids extracted from full-fat soybean flakes with SC-CO2 increased inthe fractions collected toward the end of the extraction process.

Fractional extraction can also be achieved by increasing solvent densityover time. This mode of extraction using incremental pressure programming hasbeen applied to fish oil ethyl esters (92), rice bran oil (93, 94), and phytosterolrecovery from vegetable oil deodorizer distillates (95). Fractional extraction us-ing an entrainer in the supercritical phase is another approach, which was dis-cussed earlier in this chapter (76, 77).

C. Column Fractionation

SFE from a liquid phase can be performed utilizing a vertical column. Thisprocess is also frequently referred to as supercritical fluid fractionation (SFF).The feed may be a liquid mixture or a solution containing solutes that are solidsunder extraction conditions or at slurry of solid particles. The column is usuallyfilled with a packing material or has an internal structure such as trays or bafflesto increase the contact area between fluid solvent and feed material and to en-hance mass transfer. The process can be carried out in semicontinuous or contin-uous mode. The feed and solvent could be in the batch and continuous modesduring the semicontinuous and continuous processes, respectively. Continuouscolumn operations tend to be more efficient and the feasibility of the operationcan be improved. Countercurrent operations are inherently more efficient thanconcurrent operations due to the larger concentration difference between solventand feed stream, which results in better mass transfer rates.

84 Dunford et al.

A schematic diagram of a typical countercurrent SFF process is shown inFig. 12. The feed is pumped into the column near the top, or at a point betweenthe column top and base, or from the bottom of the column. Because of theirrelative densities, the liquid phase descends while SCF phase rises through thecolumn during a countercurrent operation. The stream leaving the top of thecolumn contains the extract and the liquid leaving the bottom of the column isthe raffinate. The column can be operated at constant temperature, or a tempera-ture gradient can be imposed above the feed entry point. Thermal gradient im-posed on the column causes condensation of larger molecules in the extract

Figure 12 Flow diagram for a SFF process.

Supercritical Fluid Extraction 85

stream upon their encountering the lower solvent density at higher temperaturesupward in the column under isobaric conditions, thus creating an internal refluxin the column. This process is similar to liquid–liquid extraction with reflux andfractional distillation. Refluxing can improve the extract purity if the process isdesigned properly. Analogous to terms used in distillation and liquid–liquidextraction process with reflux, the lower and upper parts of the SFF column arereferred to as stripping and enrichment sections, respectively. The book by Clif-ford (27) is an excellent reference for a discussion of the fundamentals andmathematical modeling of SFE and SFF processes.

IX. EXTRACT AND SOLVENT RECOVERY

Recovery of extracted compounds is usually done by lowering the pressure.Hence, high molecular weight solutes in an SCF can usually be precipitated orphase separated at higher collection pressures than more volatile substances.Fractional separation of extracted solute mixtures can also be achieved in a seriesof collection vessels by successively lowering pressure on each vessel. Thismethod has been successfully applied to essential oils and spice extracts byReverchon (96) and Nguyen et al. (8), respectively.

Increasing the extract temperature at constant pressure may be anotheralternative for extract recovery. This method can be used if the solubility of acompound decreases with increasing temperature. This is what occurs mostlywith nonvolatile compounds. This method also improves the feasibility of anSFE process by eliminating recompression of the solvent for recycling.

Solute recovery by partial depressurization might not be very successfulif the solute is a very volatile compound. In such a case, adsorptive recoverymethods might be more effective. For example, the solute may be adsorbed onsilica, alumina, or active carbon and then separated from the adsorbent at ahigher pressure or by thermal desorption, among other options. Use of molecularfilters such as zeolites is another method for extract recovery from SCF. In sucha case, pore size of the filter media is chosen such that only fluid passes throughthe filter (permeate) and extracted solute is recovered as retentate (27).

Solvent separated from the solute by means of one of the methods men-tioned above can be either vented out to atmosphere or recycled to the systemdepending on the size of the operation. For small-scale equipment such as labo-ratory scale SFE units, solute-free gas is vented to the atmosphere due to thehigher cost of installing a recycling system as compared with the cheaper gascost. If the solvent is to be recycled, which is the case for pilot and industrialscale operations, it is usually passed through an adsorbent bed before it ispumped back into the extractor.

86 Dunford et al.

X. CONCLUDING REMARKS

As was noted in this chapter, SCF extraction offers considerable versatility forthe extraction of food-related products and raw materials. In general, the overallSFE process is simple, embodying many of the principles inherent in otherextraction processes. New approaches for the continuous handling of solid andliquid materials, in and out of extraction vessels, continues to evolve, and im-proved equipment design and operational dynamics are making SFE processesmuch more efficient and competitive. However, in some product applications,SCF technology faces competition from conventional techniques such as molec-ular (vacuum) distillation, a time-honored technique, although greater selectivityis potentially available from SFE using the described methods.

In closing, we would like to offer the following short list of “critical”references that will supplement the material discussed in this chapter. Althoughthis list is not inclusive of all available literature resources, it should assist thereader wishing to learn more about this novel technology and abets the formalreferences cited in this chapter.

NOMENCLATURE

P pressureV volumeN number of molesR gas constantT temperatureTc critical temperaturePc critical pressureVc critical volume∆T group contribution constant at constant temperature∆P group contribution constant at constant pressure∆V group contribution constant at constant volumePr reduced pressureTr reduced temperatureTb boiling temperatureρ densityρc critical densityρr reduced densitya van der Waals constantb van de Waals constantδ solubility parameterEv heat of vaporization

Supercritical Fluid Extraction 87

Ep potential energy of interactionε permittivity of the mediumεr relative permittivity, or dielectric constantε0 permittivity in a vacuumqi point charge for compound ir distance between the chargesJi molar flux for compound iCi molar concentration for compound iDij binary diffusion coefficient for component iVl molar liquid volumeρliq liquid densityδliq liquid solubility parameterS selectivityyi binary solubility of component i in the supercritical phaseE enhancement factorPs saturation pressure of solventM molecular weightS solubilityQ heat fluxω acentric factorPvpr reduced vapor pressureZc critical compression factorλ thermal conductivity�T temperature gradient

BIBLIOGRAPHY

G Brunner. Gas Extraction. Berlin: Springer-Verlag, 1997.BA Charpentier, MR Sevenants (eds.). Supercritical Fluid Extraction and Chromatography:

Techniques and Applications. Washington, DC: American Chemical Society, 1988.T Clifford. Fundamentals of Supercritical Fluids. New York: Oxford University Press,

1999.I.S.A.S.F. Proceedings of the 5th Meeting on Supercritical Fluids-Materials and Natural Prod-

ucts Processing. Nancy, France: Institut National Polytechnique de Lorraine, 1998.I.S.A.S.F. Proceedings of the 6th Meeting on Supercritical Fluids-Chemistry and Materi-

als. Nancy, France: Institut National Polytechnique de Lorraine, 1999.KP Johnston, JML Penninger (eds.). Supercritical Fluid Science and Technology. Wash-

ington, DC: American Chemical Society, 1989.JW King, GR List (eds.). Supercritical Fluid Technology in Oil and Lipid Chemistry.

Champaign, IL: AOCS Press, 1996.MB King, TR Bott (eds.). Extraction of Natural Products Using Near-Critical Solvents.

Glasgow: Blackie Academic, 1993.

88 Dunford et al.

E Kiran, JF Brennecke (eds.). Supercritical Fluid Engineering: Fundamentals and Appli-cations. Washington, DC: American Chemical Society, 1993.

E Kiran, JMH Levelt Sengers (eds.). Supercritical Fluids. Amsterdam: Kluwer, 1994.MA McHugh, VJ Krukonis. Supercritical Fluid Extraction, 2nd ed. Boston, MA: Butter-

worth-Heinemann, 1994.M Mukhopadhyay. Natural Extracts Using Supercritical Carbon Dioxide. Boca Raton,

FL: CRC Press, 2000.SH Rizvi (ed.). Supercritical Fluid Processing of Food and Biomaterials. London:

Blackie Academic, 1994.E Stahl, K-W Quirin, D Gerard. Dense Gases for Extraction and Refining. Berlin:

Springer-Verlag, 1987.

REFERENCES

1. JB Hannay, J Hogarth. On the solubility of solids in gases. Proc R Soc London 29:324, 1879.

2. PMF Paul, WS Wise. The Principles of Gas Extraction. London: Mills & BoonLtd., 1971.

3. GM Schneider, E Stahl, G Wilke. Extraction with Supercritical Gases. DeerfieldBeach, FL: Verlag-Chemie, 1980.

4. K Zosel. Process for the separation of mixtures of substances. U.S. Patent3,969,196 (1976).

5. K Zosel. Process for the decaffeination of coffee. U.S. Patent 4,260,639 (1981).6. DRJ Laws. Hop extracts-a review. J Inst Brew 87:24–29, 1981.7. MB King, TR. Bott. Extraction of Natural Products Using Near Critical Solvents.

Glasgow: Blackie Academic, 1993.8. U Nguyen. Supercritical fluid extraction applications in the nutraceutical industry.

Presented at the Annual Meeting of the Canadian Institute of Food Science andTechnology, June 6–9, Kelowna, BC, 1999.

9. PT Anastas, TC Williamson (eds.). Green Chemistry. Washington, DC: AmericanChemical Society, 1996.

10. F Temelli, JW King. Application of critical fluid technology to nutraceutical extrac-tion and refining. Presented at the Short Course on Nutraceuticals and FunctionalFoods. February 20–24, College Station, TX, 2000.

11. M McCoy. Industry intrigued by CO2 as solvent. Chem Eng News 77(24):11–13,1999.

12. RH Perry, CH Chilton (eds). Chemical Engineers’ Handbook. 5th ed. Japan: Mc-Graw-Hill Kogakushu, Ltd., 1973.

13. RC Reid, JM, Prausnitz, BE Poling. The Properties of Gasses and Liquids, 4th ed.New York: McGraw-Hill, 1987.

14. J Chrastil. Solubility of solids and liquids in supercritical gases. J Phys Chem 86:3016–3021, 1982.

15. JM del Valle, JM Aguilera. An improved equation for predicting the solubility ofvegetable oils in supercritical CO2. Ind Eng Chem Res 27:1551–1553, 1988.

16. JW Magee. Properties of carbon dioxide rich mixture. In: TJ Bruno, JF Ely (eds.).

Supercritical Fluid Extraction 89

Supercritical Fluid Technology: Reviews in Modern Theory and Applications. BocaRaton, FL: CRC Press, 1991, pp. 293–324.

17. P Hubert, OG Vitzthum. Fluid extraction of hops and tobacco with supercriticalgases. Agnew Chem Int Ed Engl 17:710–715, 1978.

18. EU Franck. Physicochemical properties of supercritical solvents. Ber BunsengesPhys Chem 88:820–825, 1984.

19. G Brunner, S Peter. State of art of extraction with compressed gases (gas extrac-tion). Ger Chem Eng 5:181–195, 1982.

20. MA McHugh, VJ Krukonis. Supercritical Fluid Extraction Principles and Practice,2nd ed. Boston, MA: Butterworth-Heinemann, 1994.

21. V Vesovic, WA Wakeham. Transport properties of supercritical fluids and fluidmixtures. In: TJ Bruno, JF Ely (eds.). Supercritical Fluid Technology: Reviews inModern Theory and Applications. Boca Raton, FL: CRC Press, 1991, pp. 245–289.

22. JC Giddings, SL Seagar. Method for rapid determination of diffusion coeffients.I&EC Fund 1:277–283, 1962.

23. KK Liong, PA Wells, NR Foster. Diffusion in supercritical fluids. J Supercrit Flu-ids 4(2):91–108, 1991.

24. KK Liong, PA Wells, NRJ Foster. Solubility of fatty acid esters in supercriticalcarbon dioxide. Ind Eng Chem Res 31:390–399, 1992.

25. R Feist, G Schneider. Determination of binary diffusion coefficients of benzene,phenol, naphthalene and caffeine in supercritical carbon dioxide between 308 and333 K in the pressure range 80-160 bar with supercritical fluid chromatography(SFC). Sep Sci Tech 17(1):261–270, 1982.

26. T Funazukuri, S Hachisu, N Wakao. Measurement of diffusion coefficients of C18unsaturated fatty acid methyl esters, naphthalene, and benzene in supercritical car-bon dioxide by a tracer response technique. Anal Chem 61:118–122, 1989.

27. T Clifford. Fundamentals of Supercritical Fluids. London: Oxford Press, 1999.28. S Angus, B Armstrong, KM de Reuck. Carbon Dioxide International Tables of the

Fluid State, Volume 3, Carbon Dioxide. Oxford: Pergamon Press, 1976.29. JW Magee, JF Ely. Specific heats (Cv) of saturated and compressed liquid and

vapor carbon dioxide. Int J Thermophys 7:1163–1182, 1986.30. Nieto de Castro. Thermal conductivity and diffusivity in supercritical fluids. In: TJ

Bruno, JF Ely, eds. Supercritical Fluid Technology: Reviews in Modern Theoryand Applications. Boca Raton, FL: CRC Press, 1991, pp. 335–363.

31. JM Praustnitz. Molecular Thermodynamics of Fluid-Phase Equilibria. EnglewoodCliffs, New Jersey: Prentice-Hall, 1969.

32. SSH Rizvi, AL Benado, JA Zollweg, JA Daniels. Supercritical fluid extraction:fundamental principles and modeling methods. Food Techn 6:55–61, 1986.

33. KSJ Pitzer. Corresponding states for perfect liquids. Chem Phys 7:583–590, 1939.34. J Vidal. Equation of state-reworking the old forms. Fluid Phase Equil 13:15–33, 1983.35. J Vidal. Phase equilibria and density calculations for mixtures in the critical range

with simple equations of state. Ber Bunsenges Phys Chem 88(9):784–791, 1984.36. KE Gubbins. Equations of state. New theories. Fluid Phase Equil 13:35, 1983.37. TJ Bruno. Thermophysical property data for supercritical extraction design. In: TJ

Bruno, JF Ely (eds.). Supercritical Fluid Technology: Reviews in Modern Theoryand Applications. Boca Raton, FL: CRC Press, 1991, pp. 293-334.

90 Dunford et al.

38. M Gitterman, I Procaccia. Quantitative theory of solubility in supercritical fluids. JChem Phys 78(5):2648–2654, 1983.

39. I Procaccia, M Gitterman. Quantitative analysis of supercritical extraction. AIChEJ 29(4):686–687, 1983.

40. RT Kurnik, RC Reid. Solubility extrema in solid-fluid equlibria. AIChE J 27(5):861–863, 1981.

41. RT Kurnik, RC Reid. Solubility of solid mixtures in supercritical fluids. FluidPhase Equil 8(1):93–105, 1982.

42. PH van Konyenburg, R Scott. Critical lines and phase equilibria in van der Waalsmixtures. Phil Trans R Soc (London) A. 298:495–540, 1980.

43. R Scott, PH van Konyenburg. Static properties of solutions. Part 2. Van der Waalsand related models for hydrocarbon mixtures. Discuss Faraday Soc 49:76–87, 1970.

44. JM Prausnitz, RN Lichtenthaler, EG de Azevedo. Molecular Thermodynamics ofFluid-Phase Equilibrium, 3rd ed. Englewood Cliffs, New Jersey: Prentice Hall, 1999.

45. ME Araujo, MAA Meireles. Improving phase equilibrium calculation with Peng-Robinson EOS for fats and oils related compounds/supercritical CO2 systems. FluidPhase Equil 169:49–64, 2000.

46. E Batista, S Monnerat, L Stragevitch, CG Pina, CB Goncalves, AJA Meirelles.Prediction of liquid–liquid equilibrium for systems of vegetable oils, fatty acidsand ethanol. J Chem Eng Data. 44:1365–1369, 1999.

47. L Coniglio, Kobayashi, T, R Gani. Model prediction of supercritical fluid-liquidequilibria for carbon dioxide and fish oil related compound. Ind Eng Chem Res 34:2473–2484, 1995.

48. Y Iwai, T Morotomi, K Sakamoto, Y Koga, Y Arai. High pressure vapor-liquidequilibria for carbon dioxide+limonene. J Chem Eng Data 41:951–952, 1996.

49. A Vetere. Predicting the vapor pressures of pure compounds by using Wagnerequation. Fluid Phase Equil 62:1–10, 1991.

50. CH Tu. Group-contribution method for the estimation of vapor pressures. FluidPhase Equil 99:105–120, 1994.

51. JH Hildebrand, RL Scott. The Solubility of Nonelectrolytes, 3rd ed. New York:Reinhold Publishing Corp., 1950.

52. JC Giddings, MN Myers, JW King. Dense gas chromatography at pressures to 2000atmospheres. J Chromatogr Sci 7:276–283, 1969.

53. JC Giddings, MN Myers, L McLaren, RA Keller. High pressure gas chromatogra-phy of nonvolatile species. Science 162(10):67–73, 1968.

54. JW King, JP Friedrich. Quantitative correlations between solute molecular structureand solubility in supercritical fluids. J Chromatogr 517:449–458, 1990.

55. JM Wong, KP Johnston. Solubilization of biomolecules in carbon dioxide basedsupercritical fluids. Biotechnol Prog 2:29–39, 1986.

56. E Stahl, KW Quirin, D Gerard. Dense Gases for Extraction and Refining. Berlin:Springer-Verlag, 1987.

57. RL Eissler, JP Friedrich. Estimation of supercritical fluid-liquid solubility parame-ter differences for vegetable oils and other liquids from data taken with a stirredautoclave. J Am Oil Chem Soc 65:764–767, 1988.

58. R Fujishiro, JH Hildebrand. The liquid–liquid solubility of cyclohexane and perflu-orotributylamine at 25°C. J Phys Chem 66:573–574, 1962.

Supercritical Fluid Extraction 91

59. E Stahl, W Schilz. Solubility behavior of polar natural substances in supercriticalgases in the pressure range up to 2500 Bar. Chem-Ing-Tech 50:535–537, 1978.

60. M Cygnarowicz-Provost. Design and economic analysis of supercritical fluid ex-traction process. In: JW King, GR List (eds.). Supercritical Fluid Technology inOil and Lipid Chemistry. Champaign, IL: AOCS Press, 1996, pp. 155–179.

61. PC Simoes, R Eggers, PT Jaeger. Interfacial tension of edible oils in supercriticalcarbon dioxide. Eur J Lipid Sci Technol 263–265, 2000.

62. MC Jones. Mass transfer in supercritical extraction from solid matrices. In: TJBruno, JF Ely (eds.). Supercritical Fluid Technology: Reviews in Modern Theoryand Applications. Boca Raton, FL: CRC Press, 1991, pp. 365–381.

63. M Poletto, E Reverchon. Comparison of models for supercritical fluid extractionof seed and essential oils in relation to the mass-transfer rate. Ind Eng Chem Res35:3680–3686, 1996.

64. G Brunner. Mass transfer from solid material in gas extraction. Ber BunsengesPhys Chem 88:887–891, 1984.

65. H Peker, MP Srinivasan, JM Smith, BJ McCoy. Caffeine extraction rates fromcoffee beans with supercritical carbon dioxide. AIChE J 38(5):761–770, 1992.

66. M Goto, M Sato, T Hirose. Extraction of peppermint oil by supercritical carbondioxide. J Chem Eng Jpn 26:401–407, 1993.

67. NR Bulley, M Fattori. Supercritical fluid extraction of vegetable oil seeds. J AmOil Chem Soc 61:1362–1365, 1984.

68. AKK Lee, NR Bulley, M Fattori, A Meisen. Modeling of supercritical carbon dioxideextraction of canola oilseed in fixed beds. J Am Oil Chem Soc 63:921–925, 1986.

69. H Sovova, J Kucera, J Jez. Rate of the vegetable oil extraction with supercriticalCO2. II. Extraction of grape oil. Chem Eng Sci 49:415–420, 1994.

70. E Reverchon, G Donsi, OL Sesti. Modeling of supercritical extraction from herba-ceous matrices. Ind Eng Chem Res 32:2721–2726, 1993.

71. M Goto, BC Roy, T Hirose. Shrinking core leaching model for supercritical fluidextraction. J Supercrit Fluids 9:128–131, 1996.

72. Y Ikushima, N Saito, K Hatakeda, S Ito, T Asano, T Goto. A supercritical carbondioxide extraction from mackerel (Scomber japonicus) powder: Experiment andmodeling. Bull Chem Soc Jpn. 59:3709–3713, 1986.

73. NT Dunford, M Goto, F Temelli. Modeling of extraction with supercritical carbondioxide from Atlantic mackerel (Scomber scombrus) at different moisture contents.J Supercrit Fluids 13:303–309, 1998.

74. JM Dobbs, JM Wong, KP Johnston. Nonpolar co-solvents for solubility enhance-ment in supercritical fluid carbon dioxide. J Chem Eng Data 31(3):303–308, 1986.

75. JP Calame, R Steiner. Extraction in flavor and perfumery industries. Chem Ind 10:399–403, 1982.

76. NT Dunford, F Temelli. Extraction of phospholipids from canola with supercriticalcarbon dioxide and ethanol. J Am Oil Chem Soc 72(9):1009–1015, 1995.

77. L Montanari, P Fantozzi, JM Snyder, JW King. Selective extraction of phospholip-ids from soybeans with supercritical carbon dioxide and ethanol. J Supercrit Fluids14:87–93, 1999.

78. E Stahl, E Shutz, HK Mangold. Extraction of seed oils with liquid and supercriticalcarbon dioxide. J Agric Food Chem 28:1153–1157, 1980.

92 Dunford et al.

79. JP Friedrich, GR List. Characterization of soybean oil extracted by supercriticalcarbon dioxide and hexane. J Am Oil Chem Soc 30:192–193, 1982.

80. JM Snyder, JP Friedrich, DD Christianson. Effect of moisture and particle size onthe extractability of oils from seeds with supercritical CO2. J Am Oil Chem Soc61:1851–1856, 1984.

81. M Fattori, NR Bulley, A Meisen. Carbon dioxide extraction of canola seed: Oilsolubility and effect of seed treatment. J Am Oil Chem Soc 65:968–974,1988.

82. JW King, HJ Johnson, JP Friedrich. Extraction of fat tissue from meat productswith supercritical carbon dioxide. J Agric Food Chem 37:951–954, 1989.

83. E Stahl, D Gerard. Solubility behavior and fractionation of essential oils in densecarbon dioxide. Perfum Flav 12:29–37, 1985.

84. N Gopalakrishnan, CS Narayanan. Supercritical carbon dioxide extraction of carda-mom. J Agric Food Chem 39:1976–1978, 1991.

85. R von Eggers, W Stein. Hochdruck-extraktion von olsaates. Fette Seifen Anstrich86:10–16, 1984.

86. NT Dunford, F Temelli. Extraction conditions and moisture content of canola flakesas related to lipid composition of supercritical CO2 extracts. J Food Sci 62:155–159,1996.

87. NT Dunford, F Temelli, E LeBlanc. Supercritical CO2 extraction of oil and residualproteins from Atlantic mackerel (Scomber scombrus) as affected by moisture con-tent. J Food Sci 62:289–294, 1997.

88. NT Dunford. Use of SC-CO2 for edible oil processing. PhD dissertation. Universityof Alberta, Edmonton, AB, Canada, 1995.

89. A Bertucco, F Sanmartin, G Storti. Simulated moving bed technology for continu-ous, countercurrent solid-fluid supercritical extraction. J Supercrit Fluids 8:138–148, 1995.

90. M Bork, JP Korner. Description of an industrial-size supercritical gas extractionplant for semicontinious operation. Proceedings of 2nd International Symposiumon Supercritical Fluids, Boston, MA, May 20–22, 1991, pp. 368–369.

91. JP Friedrich, EH Pryde. Supercritical CO2 extraction of lipid-bearing materials andcharacterization of the products. J Am Oil Chem Soc 61:223–228, 1984.

92. WB Nilsson, EJ Gauglitz, JK Hudson. Supercritical fluid fractionation of fish oilesters using incremental pressure programming and a temperature gradient. J AmOil Chem Soc 66:1596–1600, 1989.

93. NT Dunford, JW King. Using a supercritical carbon dioxide fractionation techniquefor phytosterol enrichment in rice bran oil. J Food Sci, 65(8):1395–1399.

94. NT Dunford, JW King. Supercritical carbon dioxide fractionation of crude rice branoil using a packed column with characterization of the resultant fractions. Proceed-ings of the 5th International Symposium on Supercritical Fluids (ISSF) (proceed-ings on CD). Atlanta, Georgia, April 8–12, 2000.

95. NT Dunford, JW King. Phytosterol-enriched trygliceride fractions from vegetableoil deodorizer distillate using supercritical fluid fractionation technology. Separa-tion Science and Technology 37(2):1–2, 2002.

96. E Reverchon. Supercritical fluid extraction and fractionation of essential oils andrelated products. J Supercrit Fluids 10:1–38, 1997.

Supercritical Fluid Extraction 93

97. KP Johnston. New directions in supercritical fluid science and technology. In: KPJohnston, JML Penninger, eds. Supercritical Fluid Science and Technology. Wash-ington, DC: American Chemical Society, 1989.

98. LG Randall. Present status of dense (supercritical) gas extraction and dense gaschromatography: Impetus for DGS/MS development. Sep Sci Technol 17(1):1–7.

99. G Brunner, S Peter. Zum Stand der extraction mit komprimierten gasen. Chem-Ing-Tech 53(7):529–542, 1981.

4Extraction Systems

Rudolf Eggers and Philip T. JaegerTechnische Universitat Hamburg-HarburgHamburg, Germany

This chapter deals with different methods and equipment used for solvent ex-traction of food components from natural sources. A main distinction is madebetween systems using common liquid solvents that are industrially applied atgreater scale since the first decades of the 20th century and an alternative extrac-tion method using compressed gases, which is called supercritical fluid extrac-tion (SFE). SFE started gaining importance in the mid-1970s, especially as aresult of rising consciousness with respect to environmental aspects and therecent development of appropriate high-pressure equipment. In the first section,conventional extraction systems using liquid solvents are discussed, includingequipment necessary for pretreatment and solvent recovery. The second sectionis concerned with selection and design of equipment required for SFE.

I. CONVENTIONAL EXTRACTION SYSTEMS

Design of extraction systems and detailed selection of suitable equipment de-pends on the objective of the process and physical properties of the material tobe extracted as well as of the obtained product. The solubilizing ability andselectivity of liquid solvents that are mainly based on water, hydrocarbons likehexane, or alcohol are used to leach or extract certain desired components fromthe applied source material, which is a naturally obtained solid in this case. Thissection first focuses on the extraction process itself describing different process-ing principles and giving an overview of the required equipment. Product qualityand extraction efficiency depend very much on conditioning of the solid feedmaterial ahead of solvent extraction, which will also be discussed. Posterior to

95

96 Eggers and Jaeger

extraction, the solvent has to be recovered from the product and the exhaustedmeal, taking care of the product quality and increasing environmental demands.Finally, an example of a complete plant used for hops extraction by alcohol isdescribed.

A. Classification

In the past, extraction or leaching was often divided into percolation and immer-sion methods, referring to whether the solid is completely submerged or thesolvent is just trickled through a solid bed (1). In general, classification hasbecome clearer since technical development has concentrated on a few differenttypes of extractors, most of them working continuously as percolators (2). Onthe other hand, immersion leaching of solid-solvent slurries using agitators orscrews has largely lost industrial relevance. Even though many of the formerextractor types (3) have nearly disappeared, they are still worth mentioning be-cause older equipment may still be perfect for certain applications. Therefore, itis useful to start out by listing all possible operation modes and the relativeworking principles in order to document technical improvements in the pastdecades but finally focusing on the actual state of the art.

1. Operation Mode

Batch Extraction. Within batch processing, extraction is carried out invessels that are filled with the solid matter to be extracted. Afterward solvent iseither percolated through the solid bed or added to the vessel until the solid iscompletely submerged. In the latter case, the solvent-solid mixture may bestirred so as to enhance mass transfer. After a designated holding time, thesolvent-extract mixture, called miscella, is drawn from the vessel and the solidmatter is discharged. This way of processing has almost completely disappearedbecause of the required interruptions of the operation for charging and discharg-ing and high amounts of required solvent. Only for some special applicationswith small product rates, such as extraction of flavors, is batch extraction stillused (e.g., extraction of lavender).

Operating several batch extractors in parallel with cross-current solventflow allows achievement of higher product rates but does not make use of theentire solvent capacity.

Quasi-continuous Extraction. In order to increase extractor efficiency,several batch extractors can be operated in series using the solvent that is loadedin one extractor for passing it through another solid bed that still contains higheramounts of extractable substances. In this way, solute concentration is enhanced,gradually approaching the total solvent capacity. One or two extractors are al-ways off-stream for discharging and charging. After a certain interval, input and

Extraction Systems 97

output ports are changed so that the solid bed extracted to the highest extent isshut off for discharging and a freshly charged extractor is switched in-line.Usually, extraction is performed countercurrently for maintaining continuously asufficient concentration gradient of the solute in the solid matter and the solvent.Therefore, the freshly entering solvent is contacted with the solid extracted tothe highest extent. Afterward, the solvent is passed through the extractors in theorder of decreasing bed age until the solvent is almost completely loaded and isbrought into contact with the latest charged solid.

Continuous Extraction. For complete continuous operation, the solidmatter has to be charged and discharged continuously to and from the extractor.Different types of conveying systems exist depending on whether percolation orimmersion extraction is applied. These are presented in the following. In thecase of extraction performed under conditions other than atmospheric pressure,either adequate sluice systems for the solid matter are required or quasi-continu-ous operation must be applied. If, for instance, proper extraction temperaturesare above boiling temperature at atmospheric pressure, extraction is to be per-formed at elevated pressures. With the use of flammable organic solvents theirescape through leakages can be prevented by operating under slight vacuum.Table 1 gives an overview of different types of extractor systems, includingtheir field of application with some specific examples.

2. Working Principle

Single-Stage Extraction. The simplest apparatus for liquid solvent ex-traction is a single-stage vessel used for immersion extraction. In order to con-fine the extraction period, the vessel may be stirred. Therefore, the solvent-solidmixture should form a flowable slurry. The extraction time has to be optimizedbecause after a while extraction kinetics becomes quite slow and additionalleaching becomes time consuming. The loaded solvent is retrieved and replacedby fresh solvent. This procedure is repeated several times until the solid matteris extracted to an acceptable extent.

Multistage Extraction. In multistage static-bed extraction using a coun-tercurrent operation mode, the fresh solvent enters the vessel containing themost exhausted solid. The extract being retrieved from this vessel is succes-sively passed through a battery of extractors until arriving at the vessel mostrecently loaded. The extract retrieved from this one is discharged for furtherprocessing, i.e., desolventizing. The purpose of this operation is to increase thesolute load of the solvent to a maximal value for posterior product separationand for solvent recovery to be as economical as possible. This method of quasi-continuous processing can be carried out with immersed solid as well as a solidbed exposed to percolating solvent. A typical application is the extraction of

98 Eggers and Jaeger

Table1

Ext

ract

ion

Syst

ems:

Cha

ract

eris

tics

and

App

licat

ions

Ope

ratio

nW

orki

ngpr

inci

ple

Ext

ract

ion

syst

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eld

ofap

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xam

ples

Bat

chIm

mer

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extr

actio

nSt

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dve

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Phar

mac

yA

lkal

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Stat

icbe

dpe

rcol

atio

nSi

ngle

-sta

gepe

rcol

ator

Spic

esPe

pper

Stat

icbe

dcr

oss-

curr

ent

Mul

ti-st

age

perc

olat

orpe

rcol

atio

nQ

uasi

-con

tinuo

usSt

atio

nary

bed,

coun

terc

urre

ntM

ultis

tage

perc

olat

orba

ttery

Inst

ant

mat

eria

l,su

gar

Inst

ant

coff

ee,

suga

rfr

ompe

rcol

atio

nbe

ets

Con

tinuo

usR

otat

ing

cells

,co

unte

rcur

rent

Rot

ocel

Veg

etab

leoi

lpe

rcol

atio

nR

otat

ing

bed,

coun

terc

urre

ntC

arro

usel

Veg

etab

leoi

l,sp

ices

,So

ybea

noi

l,pa

prik

a,pe

rcol

atio

n,st

atio

nary

siev

ein

stan

tm

ater

ial

pepp

er,

hops

tray

botto

mSt

atio

nary

bed,

coun

terc

urre

ntSt

atio

nary

bask

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Extraction Systems 99

coffee for instant coffee production by a battery of percolators. At temperaturesof up to 180°C and pressures of up to 20 atm, around 40–60% of water-solublematerial is extracted from coffee grounds by water in an upstream or down-stream fashion. At temperatures above 140°C hydrolysis of polysaccharidestakes place enhancing their solubility, which increases the total extraction yield.These sugars are also capable of retaining flavors. A series of four to eightpercolators allows the establishment of temperature gradients along the extrac-tion line for taking influence on product yield and quality due to differing solu-bilities of the various substances present in roasted coffee (4). Figure 1 showsa battery of percolators for instant-coffee extraction. Vessel diameters rangefrom 0.25 to 0.75 m. Typical capacities are around 1 t resulting in bed heightsof 4.5–6 m. Applying vacuum to the vessel facilitates loading of the percolators.The ball valve at the top is opened and the grinded coffee enters the vessel.Grind sizes range from 3 to 5 mm, the result of a compromise between favoringmass transfer and limiting pressure drop. After being sufficiently leached, thespent coffee ground is discharged by “shooting” through the ball valve at thebottom of the vessel. Holdup is maintained at 30–35 min showing a tendencyto shorter times.

An extractor system developed by NIRO (Denmark) for coffee extraction,called FIC (fast instant coffee), comprises seven percolators arranged in a circleas a compact unit (Fig. 2). The percolators themselves are about half the size ofconventional percolators reducing residence time to half. By their relativelycompact design, high liquid velocities are reached that enhance mass transferfrom the ground coffee particles. Operation is carried out in two steps based onwater temperatures of 100°C for aroma extraction and 180°C for hydrolysis ofpolysaccharides. The process works with high efficiency although concentrationof the produced liquid extract is somewhat lower than in the case of normalpercolators. The complete extraction cycle takes about 1.5 h from the freshwater entering the unit to withdrawing the concentrated coffee solution.

For extraction of sugar from sugar beets 10–16 columns are used, eachhaving a diameter of around 1.6 m and a height of 2.6–3.2 m, resulting involumes of 3–4 m3. The scalded beets (cossettes), resembling shoestring pota-toes of 3–7 mm width and 5–8 cm length, are loaded to the vessel by means ofhinged covers. Extraction is carried out using water at 40–75°C. Holdup is keptbetween 50 and 120 min. Different apparatus alternatives also exist such asversatile, chain belt, and ring extractors.

Moving-Bed Extraction. Extractors with continuously moving beds aredistinguished according to the mechanism of transporting the solid matter andcontacting the solvent. Moving-bed extractors can be divided into percolatorsand immersion extractors. In percolation extraction, the solvent phase is passedthrough the solid bed, which is mainly stationary with respect to the solid-

100 Eggers and Jaeger

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Extraction Systems 101

Figure 2 FIC extractor for instant-coffee production. (Courtesy of Niro, Denmark.)

containing cell. Most extractors that make use of the percolation principle haveconveyer elements such as conveyer belts, frames, cells, or baskets that carrythe solid matter through the extractor. Depending on the actual position, eitherfresh solvent or miscella is sprayed on the solid passing by. Commonly, freshsolvent is sprayed on the most exhausted solid. The miscella leaving here isrecirculated and used for extraction of freshly entering solid. Recirculation isalso used for enhancing contact time by submerging the bed to a greater extent.As much as possible countercurrent flow of solid with respect to solvent must

102 Eggers and Jaeger

be achieved. Accounting for the flow direction of the draining miscella, thegeneral flow pattern is a cross-countercurrent flow. Nevertheless, when makinguse of gravity for miscella flow, as in basket elevators or loop extractors, someplaces of cocurrent flow are unavoidable. If the static solid bed is mixed up atleast once during extraction, e.g., by dumping the content of one basket intoanother, a slightly higher yield may be achieved. To a certain extent, solid mix-ing already occurs in loop extractors. Cross-current operation of a moving bedmeans that fresh solvent is used throughout the entire extractor, resulting inrapid leaching of the solid but also causing a high consumption of solvent.

An intensive solid–liquid contact, which includes continuous mixing ofthe solid, is carried out in immersion extraction using screws for conveying thesolid through the extractor that is filled with solvent. The extractor may evenconsist of several different compartments dividing zones of different miscellaconcentrations. In this case, solid transport is achieved by screws from onetrough to the other throughout the whole extractor. The solid is submergedpartly within the solvent realizing countercurrent flow between the solid and thesolvent. Immersion extraction is keener in facilitating entrainment of small parti-cles than percolation where there is always some sort of self-filtration. Thus,solid content in the miscella after immersion extraction may easily rise to 1–10%, whereas there are only several parts per million after percolation.

B. Industrial Extractors

Some general trends concerning efforts for improving extraction efficiency andperformance can be observed. Development of an extractor implies a compromisebetween simplicity with respect to the number of moving parts and transportingsystems, such as solvent pumps and solid-conveying systems, and efficiency ofrecirculation pattern for the miscella. Extractors containing compartments, as op-posed to simple conveyer belts, facilitate submersion of the solid, which usuallyleads to higher extraction efficiencies. Shallow beds that are not divided intoseparated stages are in danger of forming lakes of solvent freely flowing on thebed surface and mixing miscella of different concentrations. Instead, in deepbeds the absolute residence time is often enhanced so that the liquid (miscellaor solvent) is sprayed on such a moving bed and dissolving solute on its way,then is drained from the solid in a subsequent section that does not correspondto its solute concentration. For example, full miscella drained from the descend-ing leg of a basket elevator might be collected in the half-miscella tank belowthe rising leg.

In the following, different types of extractors working with a moving bedare presented. Most of these extractor types have existed for several decades.Technical improvements are mostly confined to some details concerning theposition of inlet/outlet streams and recirculation of miscella. The extractors are

Extraction Systems 103

subdivided into those containing rotating parts and those in which longitudinalmovements are carried out. The latter machines make use of conveying elementssuch as conveyor belts or chains. In general, capital costs are lower than in caseof the rotating principle. Moreover, they can be delivered preassembled to agreat extent. Since they mostly work with a shallow bed, extraction results maydiffer from the previously mentioned rotating deep-bed extractors depending onthe material being extracted. In general, the range of application of shallow-bedextractors is somewhat confined due to the extractability and permeability ofthe meal.

The Rotocel extractor is a well-established extraction system developedby Blonox in the early 1960s (5). Then the successor Dravo closed down andfusion of licensees Krupp and Extraktionstechnik in 1990 resulted in productionof the Carrousel extractor. The Rotocel consists of up to 18 cells located in acircle, each of them containing perforated bottoms that are periodically openedwhen the cells pass the discharging section during their rotation. One rotationlasts more than 1 h depending on the extractability of the solid. The rotationalenergy requirement is fairly low because in spite of large diameters there is littlefrictional force to be overcome. The Rotocel is a so-called “deep-bed” extractor,with a bed height of 1.8–3 m. The flakes, which may also be introduced as aslurry using miscella, are not moved relative to their neighbors. In this wayfragile solid structures can be handled, but there is no additional mixing effectthat otherwise could enhance extraction yields. Miscella drawn from the bottomof the cells is collected and used for leaching solid of less bed age. This recircu-lation system consists of up to 10 miscella pumps.

The Carrousel extractor is similar to the Rotocel, except that only theframe of the extraction compartments rotates atop a static sieve tray, the struc-ture of which is shown in Fig. 3. In this way, friction of particles scrappingalong the sieve tray bottom needs to be overcome; but on the other hand, parti-cles are kept in movement relative to each other, which enhances mass transfer.

Figure 3 Segment of the sieve tray bottom of a carrousel extractor. Original (concen-tric) (a) and actual (b) orientation of sieve gaps.

104 Eggers and Jaeger

One rotation takes about 1 h, which means that at a diameter of 8 m the highestrelative velocities of particles with respect to the sieve tray bottom do not exceed7 mm/s. After being exposed to different extraction stages during one cycle,each compartment arrives at the solid outlet position where the complete solidbed of the respective compartment is discharged through the open bottom. Theseparation walls of the compartments have a conical cross-section (Fig. 4) thathelps in discharging the exhausted meal downward.

Carrousel extractors have diameters of up to 15.3 m. At a bed height of 2m an extractor of 8 m in diameter has a capacity of 75 m3. Double-deck carrou-sels containing diameters of up to 8 m are also in use (Fig. 4). After passingone cycle in the upper deck, the solid drops onto the lower level for extractionduring another rotational period. Such an extractor is capable of extracting oilfrom 2000 t/d soybeans containing 18% of oil using hexane; the residual oilcontent amounts to 0.8%. Triple-deck carrousels have also been constructed, butpoor accessibility to the middle deck for cleaning and repairing is a disadvan-tage. Figure 5 gives an overview of solid and liquid flow, also indicating therespective extract concentrations in countercurrent extractors like the Carrouselextractor.

The weight of rotating parts can be reduced by using a stationary basketextractor originally fabricated by French Oil Mill Machinery Co. In such anextractor, the compartments filled with solid are stationary and only feed spoutsand the positions of the solid discharging facility and draw-off of miscella ro-tate. Usually, 12–20 cells are applied, each of them 1.8–3 m deep. In the mean-time, the French Company switched from the stationary basket principle to car-rousel-type extractors.

Next to rotating extractors also the conveyer belt principle is applied tooil extraction. DeSmet uses sieve tray belts for transporting the meal in theirbelt extractor. Either fresh solvent or miscella provided by a system of miscellarecirculation is sprayed onto the shallow solid bed usually containing heightslower than 0.6 m. The solvent containing the freshly extracted solute drops fromthe belt into collecting trays. Before leaving the extractor, the solid is treatedonce more with pure solvent (benzene). A slight vacuum within the extractor isobtained by Venturi nozzles. Belt extractors have the advantage of low appara-tus costs and fairly uncomplicated installation. Extraction results depend on aprecise adjustment of transporting velocity, amount of liquid sprayed on thesolid, and solid bed permeability. Following efforts to submerge the solid bedto the highest extent possible, lakes may be formed on top of the solid movingbed freely flowing to all sides and mixing half and full miscella with one another.

Similar to rotational extraction systems, horizontal/vertical working ex-tractors offer different ways of moving the solid for passing it through differentstages of extraction. In general, recent development is leaning toward stationaryscreen plates or sieve trays on which the solid is pushed by cells or frames fixed

Extraction Systems 105

Figure 4 Interior of a double floor carrousel extractor. (Courtesy of Krupp, Hamburg.)

to chain conveyors, as in the case of the Crown loop extractor (Fig. 6). Theloop extractor is quite compact with one cocurrent and two countercurrent ex-traction zones.

In the sliding cell extractor from Lurgi (Fig. 7), U-shaped cells run onroller tracks pushing the solid material over stationary screen plates. The screenplates consist of rods aligned in a flow direction. The cross-section of the rods

106 Eggers and Jaeger

Figure 5 Concentration profiles of countercurrent extraction, e.g., Carrousel extractor.

Figure 6 Crown loop extractor.

Extraction Systems 107

Figure 7 Lurgi sliding cell extractor.

is V shaped to prevent clogging. The feed material is introduced into the cellsof the upper belt by a filling device. After approximately half of the extractiontime, the screen plate ends and the feed is dumped through the open cell bottominto the cell, just arriving at the lower screen plate in the feed transfer stage, asindicated in Fig. 7. Before being discharged, the feed is passed through a finaldrainage zone. The solvent passes through the feed material countercurrently byspraying the collected miscella onto feed at a previous position; it becomesenriched with extract until finally leaving the extractor.

Recently, a sliding bed extractor was developed by Krupp containing achain-frame conveying system on top of a static sieve tray passing two levels(Fig. 8). Miscella collection is also divided into two levels with a sophisticatedrecycling system. Full miscella is liberated from fines by integrated hydrocy-clones. Bed height is adjusted at 0.5–1.3 m.

Baskets fixed to a conveyer belt allow either for construction of tall ex-tractors that take little floor space or for horizontal extractors in case one-flooroperation is required. A variety of methods concerning collection and recircula-tion of the miscella and solvent-solid feed positions exist, always intending tomaximize the number of regions of countercurrent flow while confining thenumber of pumps required. The basket elevator (Fig. 9), also known as theBollman extractor (6), contains a rising and a descending leg, each of whichcontains around 15 baskets that continuously descend and rise undergoing dif-ferent stages of charging/discharging of solid, fresh solvent, and circulated ex-

108 Eggers and Jaeger

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Extraction Systems 109

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tract. The solid bed formed in the baskets is rather shallow taking heights be-tween 0.5 and 0.7 m. The solid is fed to the top basket of the descending leg.At this place, half miscella originating from drainage through the more ex-hausted solid beds of the rising leg is introduced draining downward throughthe descending baskets in a cocurrent fashion. The miscella draining from thebottom basket of the descending leg is collected in a sump and drawn off forfurther processing. Fresh solvent is sprayed on the top basket of the rising legdraining downward through the rising baskets in a countercurrent fashion, whileextract concentration is enhanced resulting in the formerly mentioned half mis-cella, which is collected in a sump at the bottom. After arriving at the top andbeing contacted to fresh solvent, the exhausted solid is discharged by invertingthe basket.

A couple of extractor types make use of the screw transporting principle,such as the vertical screw tower from Buckau Wolf, Germany, the Hildebrandtextractor formerly applied to soybeans, and the horizontal helix from Raffinerie

110 Eggers and Jaeger

Tirlemontoise used for sugar beets, all of which have not been in operation toa great extent (5). Two helicoidal screws transport the solid through the double-screw conveyor called Contex, offered at present by Niro, Denmark. The extrac-tion liquid flows through the solids as a submerged stream by means of gravitydue to a slight inclination of the vessel. Complications during operation arecaused by disintegration of solid particles and flow conditions that depend onsolid compacting.

Multiplate tower extractors like the Bonotto extractor make use of rotatingplates or paddles for transporting oilseed flakes until they fall through openingsin the plates to the floor below. Solvent and miscella are transported upward ina countercurrent fashion. Such problems as bypassing, fines entrainment, andback mixing of miscella inhibited commercial application.

C. Safety Aspects

Working with volatile and flammable solvents implies risks. During normaloperation, a number of measures, such as the use of explosion-protected equip-ment, working at slight vacuum and continuous control of escaping gases byignition detectors can for the most part guarantee safe handling. Thus, mostaccidents involving ignition of solvents or even explosions occur as a result ofequipment failure (7). When the plant is shut down and vessels are opened forrepair, strict safety guidelines might be missing and residual solvent vaporsmight come into contact with air, producing flammable mixtures. The U.S. Na-tional Fire Protection Association has formed a committee dedicated to safetyof solvent extraction plants, which issued the following statement given in part:

NFPA 36 Committee on Solvent Extraction Plants:Par. 5–8.3: Extractors, Desolventizers, Toasters, Dryers, Spent

Flake Conveyers shall be of a design that minimizes the possibilityof ignition of product deposits. Such equipment shall be protectedby extinguishing systems using inert gas, steam, or a combinationof the two, controlled from a safe remote location.

Par. 5–8.1.7: The extractor shall be provided with means toremove solvent vapors so that the concentration of vapors inside theunit in the area where work is required can be maintained at orbelow 25% of the lower flammable limit, e.g., by a purge fan sizedso that it changes the empty air volume of the vessel once every 3 min.

D. Conditioning

1. Mechanical Pretreatment

Extraction efficiency is highly contingent on preparation of the solid matterthat undergoes extraction. Small particles are advantageous to small diffusion

Extraction Systems 111

resistance within the particles. On the other hand, powders of very small particlesize require great efforts of milling or grinding. While preparing the solid bed,reagglomeration may even occur and during operation as a trickle bed extractorchannels might be formed, resulting in insufficient extraction yields. If the bed’spermeability decreases pressure drop might increase and lakes may be formedon top of the solid bed. Drainage is retarded, which may result in undesiredback mixing in case the solid bed is moved through successive extraction zoneswithin the extractor. Furthermore, the so-called fines of very small particle sizesare at risk of being entrained. In general, the content of particles of diameterbelow 0.5 mm should not exceed 5–10%.

Oilseed processing preparation methods may be distinguished dependingon whether they are combined with mechanical deoiling or are applied for ob-taining solid material of defined size and structure. In the case of oil contentabove 25 wt%, mechanical deoiling by expression is suitable for economicalreasons. Rapeseeds and sunflower seeds both containing more than 40% andcorn germs with about 50% oil always undergo previous pressing. Cottonseedscontaining about 25% are the limiting case for previous mechanical deoiling.Corn and soy, containing less oil, are usually directly extracted after condition-ing. Mechanical deoiling implies changed characteristics of the feed material forextraction, e.g., moisture and cell structures differ from their natural state dueto high pressing temperatures. Even agglomerates may be formed that have tobe crushed prior to further processing. Furthermore, different equipment formechanical pretreatment exist due to the type of force applied. The fluted rollingmill (Fig. 10) mainly cuts larger particles into pieces. A hammer mill (Fig. 11)inserts kinetic energy in particles, which is converted to surface energy by in-creasing surface area during impact with the agitator or the wall. In this case,the fines content is higher than that of the formerly described cutting principle.

A roller mill works by pressing two cylinders against each other. In casethere is no differential circumferential speed, the seeds are flaked only by com-

Figure 10 Working principle of a fluted breaker rolling mill.

112 Eggers and Jaeger

Figure 11 Working principle of a hammer mill.

pressing. A slight differential speed gives rise to shear forces, and seeds mayalso be ruptured. Extruders being high-shear devices are also capable of ruptur-ing oil cells prior to solvent extraction or mechanical pressing. For processingof high oil content material the extruder may be provided with a drainage cagesimilar to those used for screw presses described below. In order to obtain parti-cles of defined size, pelletizing or granulating may be applied by pressing, cut-ting, or adding moisture. The adequate pretreatment depends on the individualfeed material and must be determined by experience.

As already mentioned, if the content of extractable substances is fairlyhigh, prepressing is carried out prior to extraction. Commonly, the screw pressesused easily reduce the oil content in solids to 10% or less. Following solventextraction requires less solvent because of the reduced amount of extractablesubstances. In addition, cell structures are destroyed, mechanically liberatingenclosed substances. Using a pressure measurement technique described by Eg-gers et al. (8), the screw geometry may be adjusted to obtain an adequate pres-sure profile for achieving an optimal pressing result. Presses of the EP seriesmanufactured by Krupp contain a shaft with a rising outer diameter in flowdirection at a constant inner diameter of the strainer cage (Fig. 12) in order toachieve adequate compacting. Recent developments tend to obtain the completefinal product only by mechanical treatment (full or final pressing; see section

Extraction Systems 113

Figure 12 Screw press. (Courtesy of Krupp, Hamburg.)

below). Therefore, different steps of pressing according to different operatingtemperatures may be applied for obtaining products of specified quality andquantity. Usually, elevated operating temperatures result from frictional forces.In order to avoid temperature rising, cooling of the press is needed. On the otherhand, rising solid temperature before feeding may help performance and en-hance product yield. A press recently developed by Krupp contains differenttemperature zones within one machine. Here prepressing is carried out at lowertemperatures resulting in higher product quality but low yield. Final pressing atelevated temperatures has the objective of enhancing the product yield. Mon-forts has developed a compact double-acting screw press called Komet havinga broad range of applicable source material (9) but without any special facilityfor cooling. Screws of various slopes are used for solids of different hardness,but the shafts always have the same constant outer diameter. Compacting is addi-tionally regulated by use of different nozzles at the cake discharge spout. Veryhard solids should be broken or crushed before being fed to the press. If theresidual press cake still contains a considerable amount of extractable sub-stances, solvent extraction follows pressing but the compacted solid cake shouldpass through a cake breaker first.

Presses manufactured by Anderson International under the name Expellerare in use for prepressing and full pressing of a variety of oil containing materialsuch as cottonseed, peanuts, corn germ, and sesame seed. After conditioning forcareful adjustment of its temperature, the feed material enters the downspoutwhere it receives a first pressing by a vertical screw (Fig. 13). Leaving thissection, the solid arrives at the horizontal barrel containing a second screw forfinal pressing. The compacting pressure of the solid cake is regulated by a hy-draulic cone choke at the position of solid discharge. A couple of different

114 Eggers and Jaeger

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Extraction Systems 115

Expeller types are offered to meet specific demands. The Expeller 33 prepressleaves 15% residual oil content in the cake for further solvent extraction work-ing at rates of up to 90 t/d feed material. The 33 duplex press was developed tohandle especially hard fibrous and high oil content materials such as copra andpalm kernels. The residual solid contains about 7% oil. The high-capacity Model55 allows processing of more than 25 t/d with residual oil contents of 4–5%.Commonly, Anderson expellers are combined with thermal conditioning likecooking and drying of the feed prior to pressing.

2. Thermal Pretreatment

Materials that are destined for oil production and contain a large amount ofproteins, such as cottonseeds, soybeans, flaxseeds, sesame seeds, and peanuts,must be cooked prior to pressing in order to coagulate proteins and allow effi-cient recovery of the liberated oil. Therefore, the raw material is maintained forabout 20 min at a moisture content of around 10% and temperatures of 90–95°C. Afterward the material must be dried in a separate step to approximately3% moisture before entering mechanical pressing so as to stiffen the particles.Materials containing small amounts of proteins only undergo the drying proce-dure. Drying time strongly depends on the physical properties of the solid parti-cles, e.g., size, moisture, and porosity.

Cooking of proteins and starches is combined to adjustment of moistureand porosity by the expander technology. Vapor is introduced into the solidstructure during extrusion in a screw press–like device containing steam inletnozzles at the circumference (Fig. 14). The solid-vapor mixture is expanded atthe outlet gap resulting in increased porosity due to explosive-like evaporationfrom the solid cells. Amandus Kahl offers a ring gap expander disposing of anadjustable ring-formed gap that keeps a defined pressure of the solid movingbed. The resulting material is homogeneous having a high porosity at an in-creased bed density (compacting). Penetration of liquid in posterior solvent ex-traction is facilitated while the resulting homogeneous structure and high densityfacilitate enhanced solid throughput and safe performance of percolators. Theexpander-extruder-cooker fabricated by Anderson also allows blending of waterinto the feed material for cooking at a prescribed moisture level.

An alternative method of preparing source material for extraction is so-calledelectropermeabilization. High-intensity electric field pulses cause cell damage andrelease of liquid contained in the cells (10). Because of its high viscosity, oil cannotbe obtained by this technique but it may be applied to fruit juice extraction.

E. Solvent Recovery

The extracted cake and the product need to be liberated from any solvent residu-als by stripping or distillation procedures. Therefore, the miscella leaving the

116 Eggers and Jaeger

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Extraction Systems 117

extractor is passed through a distillation step for stripping the solvent and ob-taining a solvent-free product. This product is ready for further treatment, suchas liquid extraction of undesired components, enrichment of desired ones, orchemical modification.

The exhausted meal is discharged from the extractor and transported tothe desolventizing step by conveyer belts or screws. Here the solvent is evapo-rated in order to obtain a solvent-free solid residual that can be used, e.g., asanimal food. Usually, the desolventized meal is toasted to increase its nutritionalvalue as food material. Desolventizing and toasting is usually carried out in twoseparate steps. Lurgi offers a toaster including so-called pre-desolventizer stagesat the top end of the same tower. Steam is passed from below through vaporduct trays holding the meal at various heights of the tower. Double-arm agitatorsmove the meal atop these trays until it drops through outlet holes to the traybelow. Within the trays there are also steam-heated sections for indirect heating.Using solvents that are partly miscible with water (e.g., alcohol, acetone), steamcannot be applied in view of posterior separation and solvent recovery. In thiscase, the so-called flash desolventizing process can be applied contacting themeal with superheated solvent.

If volatile flammable organic solvents are used, operation of extraction,distillation, and desolventizing is carried out at slight vacuum to prevent the riskof explosion in case of leakage. If less volatile solvents are used, e.g., water,high temperatures must be applied, e.g., in spray drying of the extract-solventmixture. Often, high temperatures are disadvantageous with respect to valuablecompounds such as flavors and fragrances. Since the 1930s freeze drying isincreasingly applied in order to evaporate water, e.g., from the extracted solu-bles of coffee. Here temperatures of 60–85°C are applied for sublimation ofwater at 50 Pa from coffee that is frozen at temperatures below −30°C. Thehighly concentrated frozen coffee is either filled into trays that are moved ontoheated plates through a vacuum tunnel or directly scrapped over heated platesby agitators. In the former case, heat transfer is worse but the latter methodleads to abrasion of the solid particles.

In general, environmental protection and consumer demands afford devel-opment of solvent recovery systems that provide adequate cleaning of exhaustair and careful treatment of conserving valuable product components.

F. Final Pressing Systems

For production of small amounts of edible oil, mere mechanical pressing maybe suitable. Physical limits of the minimal residual oil content exist due to strongadhesive forces. In case of rapeseed, a content of 7% of oil remaining in thesolid matrix can be achieved only by pressing. Further treatment using solventsis not economic. Accounting for the loss in residual oil compared with a content

118 Eggers and Jaeger

of 1.5% after combined prepressing and solvent extraction and high costs ofmaintenance in the case of full pressing, production costs of oil coming fromeither process are usually equal at production rates above 500 t/a.

Fruit juices are commonly produced only by mechanical treatment. Beltfilter presses, e.g., manufactured by Flottweg or Zentrifuges from Westphalia,both in Germany, are used to mechanically extract juice, such as from applesand cherries. In processing of citrus juice, crushing of the fruit must be avoidedsince oil and bitter compounds originating from the peel have to be kept low inthe juice. The FoodTech Extractor System from FMC, Florida, extracts juiceand oil simultaneously by scratching off the peel using two approaching cupswhile the peeled fruit moves into a strainer tube. Here the juice is separatedfrom the seeds and the rest of the fruit. In general, around half of the weightremains as solid residue subsequently processed to cattle feed supplement. Fur-thermore, about 0.4% is formed by citrus oil situated in the peel. Next to press-ing of the peel, water is used for rinsing the oil, which on its turn requiresseparation of the obtained emulsion by centrifugation. Quality of peel oil de-pends on the amount of water used. While the oil obtained directly from press-ing may be sold as cold-pressed citrus oil, the oil obtained from a subsequentaroma recovery step using multistage evaporation and distillation, and thuslower in quality is applied for production of soaps and detergents. Within thejuice extractor offered by Brown International Corp., the peel oil is obtainedbefore juice extraction. The peel is cut and rinsed with water. Centrifuges carryout separation of the resulting emulsion.

G. Complete Extraction Plants

Figure 15 shows a flow diagram of a hop extraction plant using ethanol assolvent. Cone hops is fed to a double-deck Rotocel extractor with 16 compart-ments after being dried. The miscella drawn from the extractor is concentratedby passing a four-stage vacuum evaporator at gentle temperatures. Completeelimination of the alcohol is carried out in a posterior separation step. The spenthops discharged from the extractor is desolventized in a dryer and subsequentlypelletized for animal feed application. Recovered ethanol-water mixture is ad-justed to the desired composition by rectification and recycled. Compared toalternative CO2 extraction (discussed in the next section), ethanol possesses littleselectivity resulting in a product that contains almost the natural composition ofextractables given by the feed material.

II. SUPERCRITICAL FLUID EXTRACTION (SFE)

Extraction from solid material using supercritical fluid extraction (SFE), espe-cially carbon dioxide (CO2) extraction, is established on an industrial scale for

Extraction Systems 119

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120 Eggers and Jaeger

a wide range of applications. Besides some large plants used for industrial pro-duction of decaffeinated coffee and hops extract, there are a number of smallermultipurpose plants that obtain extracts from a variety of natural materials, suchas spices, herbs, and valuable vegetable/essential oils (11).

A. Extraction System

In Fig. 16 a general flow sheet of a supercritical fluid extraction is shown. Sucha processing line mainly consists of a pressurizing device, a pressure vessel forextraction, one for separation (i.e., solvent recovery), and a couple of heat ex-changers.

If separation is performed in the most common way by pressure release,the fluid must be vented through a butterfly valve before entering the separator.Separation is carried out at lower pressure than extraction. The type of pressuriz-ing device used to recirculate the fluid by rising pressure back to extractionconditions depends on the state of the solvent fluid coming from the separator.If the pressure in the separator is high enough for the fluid to be liquified bychilling in a reasonable temperature range, piston pumps may be applied. Onthe other hand, gas compressors working with much lower volumetric efficien-cies are needed if the fluid at the pump inlet is in a gaseous state. Nevertheless,the corresponding low separation pressures might be of interest for highly vola-tile components that are well solubilized by the compressed fluid. Pressure mustbe reduced considerably for precipitating these solutes. Alternative methods ofsolvent recovery also exist for a complete isobaric solvent cycle. The solute can

Figure 16 General flow sheet of supercritical fluid extraction.

Extraction Systems 121

either be absorbed by an additional liquid, e.g., water, or adsorbed on a fixedbed, e.g., of activated carbon. Therefore, the pump is just needed for maintainingfluid flow and overcoming relatively low-pressure drops along the processingline. For arriving at the operating pressure and compensating solvent losses, arelatively small additional pump has to be installed. The solvent cycle is com-monly represented by a T-S diagram from which energy balances may be drawnthat are needed for heat exchanger design. For a usual SC-carbon dioxide (CO2)extraction with liquid CO2 at the pump inlet, Fig. 17 shows this clockwise turn-ing cycle in such a T-S diagram.

After being liquified in the chiller C1 the fluid is compressed by the pumpP1. Pressure increase is supposed to take place nearly reversibly (isentropically).HE1 heats up the fluid to extraction temperature. Adiabatic expansion (no sig-nificant change in enthalpy) occurs in the butterfly valve RV1. Due to the Joule-Thompson effect, the carbon dioxide is cooled down arriving at saturation con-ditions. For better separation, the loaded fluid is completely evaporated justbefore entering the separator in HE2.

Besides some large-scale plants for extraction of α-acids from hops (12),there are quite a number of smaller, multipurpose plants for obtaining extractsof a variety of natural materials such as spices, herbs, and solids containing flavorsor fragrances, all of them using similar setups as depicted in Fig. 16 (13). For

Figure 17 CO2 cycle, depicted in a T-S diagram.

122 Eggers and Jaeger

decaffeination of green coffee beans, either the beans are moistened or moistcarbon dioxide is used (14). Separation and regeneration of the supercriticalsolvent after decaffeination is usually performed by isobaric adsorption on acti-vated carbon or absorption by water (15). In a similar way, caffeine-free tea canbe produced (16). An alternative procedure has been proposed by Buse GmbHfor decaffeination of green coffee beans using water saturated with carbon diox-ide at pressures of up to 30 MPa (17).

B. Batch and Continuous Processing

Up to now, supercritical extraction of solids at industrial scale is mostly per-formed in a discontinuous manner due to the lack of reliable sluice systems forcontinuous inflow and outflow of solids to and from high-pressure vessels. Inorder to save expensive operation time, two to four extractors are often operatedin turns. Figure 18 schematically shows a plant for solid extraction containingtwo extractors and an optional second separator in case stepwise pressure reduc-tion is necessary. Figure 19 shows a time schedule of discontinuous supercriticalextraction including charging and discharging of four vessels in turns. One ves-sel is always off-line for depressurizing, discharging, charging, purging, and pres-surizing.

Different methods have recently been proposed to achieve continuouscharging and discharging of solids to and from high-pressure vessels. By usingscrew conveyers or extruders, the solid is compacted entering into the extractor(18, 19). The resulting pressure drop along the moving solid bed prevents pres-sure loss while charging the extractor. An alternative multistage sluice systemcontaining various compartments on an axially moving frame has problems ofscaling up due to lack of appropriate sealing systems. In case a slurry is formed,continuous processing can be carried out using common piston and membranepumps (20). The solid–liquid dispersion is introduced into rather tall and thinextractors or columns where the supercritical fluid is directed either co- or coun-tercurrently with respect to the liquid flow. Anyway, using a liquid “carrier”enhances mass transfer resistance from the solid particles into the supercriticalfluid phase, which can partly be overcome by finely dispersing the liquid withinthe supercritical solvent.

Finally, discharging of solid powders formed inside the extractor by dis-solving the original carrier phase completely within the supercritical fluid maybe realized by simply opening a discharge valve at the bottom and blowing outthe solid particles together with the supercritical solvent. For pressure release ofvessels containing, for example, pressurized carbon dioxide, one must also takeinto account temperature drops toward the vapor–liquid line or even surpassingthe triple point that is situated at a slightly elevated pressure. As a consequence,outflow from the vessel partly takes place as a two-phase flow and blocking by

Extraction Systems 123

Figure18

Flow

diag

ram

ofan

extr

actio

npl

ant

with

two

extr

acto

rsan

dfr

actio

nate

dse

para

tion.

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Figure 19 Time schedule of supercritical–solid batch extraction using four extractionvessels in turns, 120 kg per batch, 1200 kg CO2/h at 250 atm.

dry ice (solidified carbon dioxide) may occur (21). The interior of the vessel iseasily cooled down to −30°C (Fig. 20).

C. Recovery Systems

As mentioned above, different principles of solvent (supercritical fluid) recoveryhave been proposed that are also applied at industrial scale. Next to the energy-consuming method of depressurizing the supercritical solvent, adsorption andabsorption methods are used especially in case high amounts of supercritical

Extraction Systems 125

Figure 20 Pressure and temperature decrease during depressurizing of moist carbondioxide from a 50-L vessel.

fluid must be circulated and the substances dissolved by this solvent are destinedfor elimination. If the objective is to obtain a valuable product as the extractphase, separation by pressure reduction is usually most suitable. Nevertheless,caffeine that is dissolved by supercritical carbon dioxide for decaffeination ofgreen coffee beans may be absorbed by water in some type of high-pressureliquid–fluid extraction plant such as a cocurrent spray tower, countercurrentpacked column, or mixer-settler (22), concentrating the caffeine in a posteriormembrane module and obtaining a product of elevated purity (23). The high-pressure countercurrent principle is described by Jaeger focusing on the wettingbehavior of packing materials at elevated pressures (24).

At industrial scale, adsorption of caffeine on activated carbon dominates,but up to now the caffeine is burned for thermal recovery of the adsorbent. Eachrecovery implies a 10% loss of activated carbon. Recent developments areaimed at obtaining the adsorbate without being destroyed during regeneration.The use of alternative adsorbents, such as ion exchangers (25), is also proposedto enhance processing rates or others with hydrophilic properties in order tofacilitate their regeneration by water (26). For dimensioning an adsorber, onehas to consider a sufficient length in order to establish plug flow within theadsorbent bed. Therefore, the minimal length-to-diameter ratio is about 10. Therequired length further depends on the amount of substance to be adsorbed.

126 Eggers and Jaeger

Adsorption capacities range between 25 and 80 g/kg adsorbent. Back mixingshould be prevented by keeping fluid velocities at values below a few centime-ters per second by choosing an adequate inner diameter.

D. Pretreatment

Similar to conventional liquid extraction, the solid feed material has to be condi-tioned properly. Mass transfer should be favored by maintaining small particles,eventually removing skins by pealing or dehulling. On the other hand, the struc-ture of the solid bed should be porous and as homogeneous as possible. Largeparticles result in high void fraction, but at the same time the mass transfer frominside the particles into the bulk fluid is slowed down. Solid particles that tendto stick to each other (e.g., at high moisture content) often form channels by thefluid or even complete blocking. As a consequence, the solid material is ex-tracted heterogeneously and to a low extent. Figure 21 shows the influence ofpretreatment on the kinetics of supercritical extraction of sunflower oil. Extrac-tion of dehulled and flaked sunflower seeds proceeds faster. In general, flaking

Figure 21 Kinetics of oil extraction from sunflower seeds at 50 MPa, 60°C. DHF =dehulled, flaked; UDHM = undehulled, milled; UDHP = undehulled, pelletized.

Extraction Systems 127

appears to be an appropriate approach to pretreatment since the thin layers mini-mize transport resistance while leaving a firm and porous structure to the bed.

E. Vessel Design

In supercritical extraction, pressure vessels are needed for supply and recoveryof the solvent, the extraction (loading of the solvent) itself, and a number ofheat exchangers. According to their dimensions and way of operation, differenttypes of construction and factoring of these vessels are applied (27). Figure 22summarizes the most important methods of pressure vessel construction. In gen-eral, vessels with solid walls and those containing compound (layered) walls aredistinguished. Solid-walled vessels are normally produced as single forgings.Using this method, the diameter is limited to about 1 m and the wall thicknessto about 150 mm. Greater heights may be obtained by joining several cylindersby circumferential welding. Wall thickness in excess of 200 mm is achieved byjoining two half shells by longitudinal welding.

The technological limitations of weight and size imposed on the men-tioned methods for solid-walled vessels are overcome to a large extent by theuse of vessels with laminated walls. The presence of such multiple layers whilenormally beneficial, can produce complications; the insertion of an adapter or anozzle in a layered wall requires very careful design.

Working with corrosive substances, the multilayer principle has the advan-tage of separating the corrosion problem from the required pressure resistance.The inner layer is made of a corrosion-resistant material with less tensile strength,whereas the outer layers resist the high pressure. A disadvantage of this type ofvessel is reduced thermal conductivity. Usually, contact between the layers isnot ideal leaving small gaps that give rise to enhanced thermal resistance. How-ever, at high pressure, this effect is partially reduced due to additional pressingof the layers toward each other. Calculation of wall thickness is carried outaccording to national codes, such as the Boiler and Pressure Vessel Code of theAmerican Society of Mechanical Engineers (ASME).

Especially for design of extractor vessels, rules concerning some construc-tion principles are based on experience gained in the past two decades of indus-trial application of SFE. The ratio of inner length to inner diameter influencesthe performance of extraction and should therefore be carefully chosen. At toolow an inner diameter, wall effects become noticeable. Although axial disper-sion may be relatively low, back mixing becomes relevant within a tall extractor.On the other hand, high inner diameters may result in heterogeneous extractionwith respect to the radial position. In most cases, solid extraction is performedupflow. The fluid enters the bottom of the extractor at its center. The extractorcontains a so-called product basket that has a sieve tray bottom or, even better,

128 Eggers and Jaeger

Figure22

Met

hods

ofpr

essu

reve

ssel

cons

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tion.

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a sinter metal bottom. At its circumference, the basket must be sealed towardsthe inner wall of the extractor to prevent bypassing of the fluid. Since the basketenters the extractor by opening the extractor top, this top must be removable.Therefore, any fixed tubing at the top would have to be replaced for openingthe extractor. Therefore, the outlet is positioned just below the top to one sideof the extractor. This has to be considered for a homogeneous flow within theextractor because having the outlet on one side results in asymmetrical flow.

In the case of discontinuous charging and discharging of solids, the vesselmust contain quick-acting closures. Furthermore, cleaning must be facilitated ifthere is a risk of accumulating precipitates.

F. Heat Exchangers

The specific heat transport properties of the respective supercritical fluid andtheir variation depending on operating conditions must be taken into accountfor heat exchanger design. The rate of heat to be transferred within the fluidcycle, e.g., CO2, may be deduced from the T-S diagram taking the respectivepoints of specific enthalpy. Coming from the pump (Fig. 16) the supercriticalfluid, e.g., CO2, enters the heater HE1 at, say, 15°C and 30 MPa. The corre-sponding enthalpy of CO2 amounts to about −284 kJ/kg. Raising the temperatureto 100°C, which gives an enthalpy of −116 kJ/kg, 168 kJ/kg of heat needs to betransferred. At a CO2 mass flow of 500 kg/h the transferred heat flux comes outto be 23 kW.

For cleaning purposes, heat exchangers should be constructed as tube bun-dles or double tubes. In particular, the evaporator following the throttling valveis at risk of being blocked by precipitated extract. Vertical orientation of thisheat exchanger helps downflow of precipitating liquid. For the condenser posi-tion, different possibilities are discussed leaving some doubts for the best solu-tion. In the course of cooling, a condensate film is formed on the walls increas-ing heat transfer resistance to the gaseous phase. A diagonal orientation downtoward the fluid outlet allows the freshly formed liquid to drop off the tubewalls right away, leaving only a thin film of condensate. For laboratory-scaleplants, heat exchangers may also be constructed as a coil of high-pressure tubeplaced in a thermostat bath to keep dimensions small despite relatively highheat transfer area.

G. Pumps and Compressors

In general, pumps are used to transport liquids through pipes. Every pump hascertain operating characteristics due to which mass flow and pressure increaseare related for the installed piping. Figure 23 qualitatively shows operating char-acteristics for centrifugal and piston pumps. Centrifugal pumps that are usually

130 Eggers and Jaeger

Figure 23 Characteristic curves of different types of pumps.

not applied to high-pressure technology show a reciprocal behavior of pressurewith respect to mass flow: at higher pressure mass flow diminishes. The pumpis actually operated at the point of intersection of the operating line and thecharacteristic line of the piping due to pressure losses caused by friction. Onthe other hand, piston pumps that are commonly used for high-pressure purposesideally maintain a constant mass flow no matter the pressure at the outlet, as-suming a noncompressible liquid. Cavitations, i.e., evaporation due to pressureloss at the pump inlet, must be avoided in the case of both centrifugal and pistonpumps. So-called net pressure suction height (NPSH) is a measure of the mini-mal inlet pressure that ensures complete liquid pumping, usually based on prop-erties of water.

Since piston pumps have a certain constant volume per stroke, it is verydangerous to shut valves right behind the pump. Pressure will increase untileither tubes burst or major damages occurs, e.g., at the pump transmission.Therefore, security devices such as security valves and rupture discs are obliga-tory.

If the fluid to be circulated is highly compressible and the pressure is to beincreased, compressors with multistage working principle need to be used. Pres-sure is increased stepwise with an intermediate cooling step. In each step pressuremay be raised by a factor of 3–4. Compressors have a much lower volumetricefficiency than pumps but can work with almost any suction pressure.

Supercritical fluid extraction works with fluids under conditions rangingfrom the need of compressors to the possibility of applying pumps. Usually, the

Extraction Systems 131

circulating fluid enters the pump at elevated pressure. Whether or not a liquidstate can be assured at this point depends on conditions of the previous separa-tion step and the chilling capacity. In the following we will assume that a pumpis used. The two main different types of reciprocating pumps used for supercriti-cal fluids are plunger (piston) and membrane pumps. Membrane pumps havecertain advantages, especially concerning sealing. While piston pumps need spe-cial dynamic sealing packing to guarantee hermetic operation, the membraneitself separates the pressure chamber from the pressure-transmitting liquid andthe transmission of the pump (Fig. 24). For food and pharmaceutical applica-tions sealing toward, say, lubricants may be decisive for pump selection. On theother hand, the fact that the efficiency of piston pumps is higher than that ofmembrane pumps is important for realizing high mass flows.

For circulating fluids on a high-pressure level but relatively small pressuredrops in the cycle, canned centrifugal pumps (hermetic) may be applied.

1. Pump Efficiency (Volumetric Efficiency)

Multiplying the stroke volume and the pumping frequency gives a theoreticalvalue of the volume flow of the pump. This volume flow is never reachedbecause of various factors that are quantified by using the so-called pump effi-ciency, ηp. ηp is composed of a systematic efficiency accounting for back flowthrough valves, fluid losses, and so forth, as well as an elastic efficiency takinginto account elasticity of the pumping head, stagnant volumes, and compressibil-ity of the fluid. The elastic efficiency that usually comes close to the total pumpefficiency is defined by (28):

ηE = 1 − (εTκ + λE)∆pH

h

with H/h denominating the stroke ratio. In case of a nonelastic piston (λE = 0)and a normal stagnant volume (εT = 1), the pump efficiency for a single actingpiston pump comes out as:

ηE = 1 − ∆V/V0

where ∆V/V0 is the relative compressibility, which can also be defined as ∆V/V0 = κ ∆p, κ being the compressibility coefficient. In Fig. 25, the relative com-pressibility is depicted as a function of pressure for CO2.

The compressibility is not only important to the pump efficiency but alsoto the pulsation of the pump. The volume flow characteristics of a double-actingplunger pump can be seen in Fig. 26. In the case of incompressible fluids onepeak is directly followed by the other. For compressible fluids the first part ofthe peak is cut away and the flow becomes similar to that of a single-actingpump.

132 Eggers and Jaeger

Figure24

Sche

mat

icof

api

ston

(a)

and

am

embr

ane

(b)

pum

p.

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Figure 25 Relative compressibility of CO2.

Figure 26 Volume flow characteristics of a double-acting pump.

134 Eggers and Jaeger

2. Dimensioning of Pumps

For solid extraction using supercritical CO2, velocity with respect to the emptycross-section of the extractor should amount to a few millimeters per second. Inmany applications the resulting mass flow (in kilograms per hour) comes out asabout 20 times the content of the extractor (in kilograms), of course still depend-ing on the solubilities in detail. Whether or not a sufficient mass flow can beachieved depends strongly on conditions at the pump inlet and on the desiredextraction pressure. Taking into account conditions on the suction and the high-pressure side of the pump, an estimation may be carried out using the volumetricefficiency described above.

3. Formation of Gas Hydrates

In general, low temperatures are required for assuring liquid state of the fluid atthe pump inlet. For many gases, including CO2, there are limits given in casewater is present in the cycle. In spite of moist CO2 containing only around0.02–0.2 wt% water, gas-hydrate crystals may be formed in or near the con-denser, growing on the inner walls and possibly blocking tubes and pumps.These hydrates were found to be formed below 10°C and be quite stable, eventu-ally lasting for days after stopping operation (31).

H. Industrial Plants

In the meantime, several applications for SFE have been established. Two ofthe most important industrial scale applications are extraction of α acids fromhops and caffeine from green coffee beans. The purpose of using supercriticalfluids is mainly to obtain a solvent-free product of high quality, carefully treatedwith respect to temperature and to save costs for environmental protection. Aselection of industrial operated plants is shown in Table 2.

The product value depends on whether the objective of the process is toobtain the fluid-soluble extract or the remaining nonsoluble substances. Becauseof rather low solubilities within supercritical CO2 in most cases, product costsare much higher when aiming for the extract. In detail, the costs depend on thesolubility of the desired component in the solvent phase; on the possibilities ofcontinuous, quasi-continuous, or semicontinuous operation; and on the type ofregeneration of the solvent. Since the used supercritical solvent may have anincreased selectivity in comparison with conventional liquid solvents, the finalcomposition of the extract mostly differs from its natural composition withinthe solid matrix. This fact has to be taken into account considering productstability since many natural components disintegrate rapidly after being isolatedor simply when nonsoluble natural stabilizing agents or antioxidants are absentin the obtained extract.

Extraction Systems 135

Table 2 Industrial Applications of SCF-Extraction

Company/Country Capacity (L) Products

Yasuma (Japan) 100 Spices, pigments, food additives (19)Fuji Flavor (Japan) 300 Tobacco, flavorsSago Koryo (Japan) 300 Flavors, pharmaceutical compounds (19)Mori Seiya (Japan) 500 Flavors, pigments (19)Idemitsu Sekiyu (Japan) 1000 Flavors (19)Takeda Seiyaku (Japan) 1200 Pharmaceutical compounds (19)Essences (Italy) 1200 Essential oils (20)CUB (Australia) 1000 Flavors (21)Universal Flavors Ltd. (UK) 1000 Flavors (21)HAG (Germany) 45.000 Decaffeinated coffee (21)Barth (Germany) 12.000 Hops (22)SKW (Germany) 18.000 Hops (22)Sumitomo Seika (Japan) 100 Coffee extract (19)

REFERENCES

1. FA Norris. Extraction of fats and oils. In: Bailey’s Industrial Oil and Fat Products,Vol. 2, 4th ed. New York: John Wiley & Sons, 1982, pp. 230–237.

2. MA Williams. Extraction of lipids from natural sources. In: FD Gunstone, FB Pad-ley eds. Lipid Technologies and Applications. New York: Marcel Dekker, 1997,pp. 127–135.

3. M Bockisch. Nahrungsfette und -Ole. Stuttgart: Ulmer, 1993, pp. 319–333.4. General Foods Corp.: Verfahren zur Herstellung von Kaffee-extrakt. German Patent

2125545, May 22, 1971.5. HG Schwartzberg. Continuous counter-current extraction in the food industry. CEP:

67–85, 1980.6. RE Treybal. Mass-Transfer Operations, 3rd ed. New York: McGraw-Hill, 1980,

pp. 740–744.7. CL Kingsbaker: Extractor failure: Safety procedures. Inform 10:1142–1147, 1999.8. R Eggers, H Boeck, W Stein. Druck- und Temperaturprofile beim Abpressen von

olsaaten in seiherschneckenpressen. Fette Seifen Anstrichmittel 87:494–499, 1985.9. L Falla, J Ospina, E Posada. Determinacion de las condiciones de extraccion de

aceite de cafe por extrusion. Proceeding of the 13th ASIC Colloquium, Paipa, 1989,pp. 232–235.

10. A Angersbach, MN Eshtiaghi, D Knorr. Impact of high intensity electric fieldpulses on plant cells and tissues. In: G Barbosa-Canovas (ed.), COFE 97. NewYork: AICE, 1997, pp. 11, 18–21.

11. E Stahl, K-W Quirin, D Gerard. Dense Gases for Extraction and Refining. Berlin:Springer Verlag, 1987.

12. Brewing Research Foundation. Hop extraction with carbon dioxide. US Patent4,218,491, Aug. 19, 1980.

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13. Norac Technologies Inc. Process for the supercritical extraction and fractionationof spices. European Patent 0511673 A1, Apr. 30, 1992.

14. HAG AG. Method for the decaffeination of raw coffee. US Patent 3,879,569, Apr.22, 1975.

15. Studiengesellschaft Kohle mbH. Process for recovering caffeine. US Patent3,806,619, Apr. 23, 1974.

16. O Vitzthum, P Hubert. Method for the manufacture of caffeine free black tea. USPatent 4,167,589, Sept. 11, 1979.

17. Buse GmbH. Verfahren zur Entcoffeinierung von Rohkaffee. European Patent90122400.6, Nov. 23, 1990.

18. R Eggers. Extraktion von Fettrohstoffen mit Uberkritischem CO2. Fat Sci Technol96:513–518, 1994.

19. G Kaschel. Die Schneckenmaschine mit Gassperre als Ein- und Austragsystem furdie Hochdruckextraktion von Pfanzlichen Naturstoffen. Dissertation, TechnischeUniversitat Hamburg-Harburg, 1992.

20. Nestle. Decaffeination process, European Patent 0010637, Oct. 3, 1979.21. A Fredenhagen, R Eggers. Vessel release of pressurized carbon dioxide containing

nitrogen or water. Two-Phase Flow Modeling Exp 1:623–627, 1999.22. A Pietsch. Die Gleichstromverspruhung mit Uberkritischem Kohlendioxid an den

Beispielen Hochdruckentcoffeinierung und Carotinoidaufkonzentrierung. Disserta-tion, Technische Universitat Hamburg-Harburg, 2000.

23. A Pietsch, R Eggers. The mixer-settler principle as a separation unit in supercriticalfluid processes. J Supercrit Fluids 14:163–171, 1999.

24. PT Jaeger: Grenzflachen und Stofftransport in Verfahrenstechnischen Prozessen amBeispiel der Hochdruck—Gegenstromfraktionierung mit uberkritischem Kohlendi-oxid. Dissertation, Technische Universitat Hamburg-Harburg, 1998.

25. HAG AG. Verfahren zur Entcoffeinierung von Rostkaffee. German Patent DE3303679 A1, Aug. 16, 1984.

26. D.E.J. International Research. Process for extracting caffeine from solutions thereofin carbon dioxide under high pressure. US Patent 4,364,965, Dec. 21, 1982.

27. R Eggers. Design and operation of the pressure vessels used in near-critical extrac-tion processes. In: MB King, TR Bott (eds.). Extraction of Natural Products UsingNear-Critical Solvents. Glasgow: Blackie Academic, 1993, chapter 8.

28. R Eggers, G Fischer. Zum Einfluss der Kompressibilitat verdichteter Fluide aufden Fordervorgang in Verfahren zur Extraktion mit Gasen. Pumpentagung Karls-ruhe Oct 6th–8th, 1992, B4-02.

29. V Hung, MA Unger: Applications of supercritical fluid extraction in the Japanesefood industry. Food Aust 46:235–239, 1994.

30. E Reverchon. Supercritical fluid extraction and fractionation of essential oils andrelated materials. J Supercrit Fluids 10:1–38, 1997.

31. MV Palmer, ST Ting. Application for supercritical fluid technology in food pro-cessing. Food Chem 52:345–352, 1995.

5Optimization

Constantina TziaNational Technical University of AthensAthens, Greece

I. INTRODUCTION

Optimization is the act of obtaining the best result possible or the effort forachieving the optimal solution under a given set of circumstances (1). In design,development, processing operation, and maintenance of engineering systems,common goals are either to minimize the cost or maximize the desired profitsas product quality and operation yield. A systematic and efficient way to meetthese goals is to place the emphasis on design and process optimization formanufacturability/performance (or yield), quality, and cost (2, 3). Such opera-tions should be made efficient by applying the relevant optimization methodsand taking the appropriate technological and managerial decisions among allpossible alternatives.

Optimization can be defined as the process of finding the conditions thatgive the optimum (maximum or minimum) value of a function of certain deci-sion variables subject to restrictions or constraints that are imposed (2). Optimi-zation may be the process of maximizing a desired quantity or minimizing anundesired one. The conditions (values of the processing variables) that producethe desired optimum value are called optimum conditions while the best of allfeasible designs is called optimal design. In its most general meaning, optimiza-tion is the effort and process of making a decision, a design, or a system asperfect, effective, or functional as possible.

Optimization for a system may mean the design of system parametersor the modification of its structure to minimize the total cost of the system’sproducts under boundary conditions associated with available materials, finan-

137

138 Tzia

cial resources, protection of the environment, and governmental regulation, tak-ing into account the safety, operability, reliability, availability, and maintainabil-ity of the system. Optimizers or decision makers use optimization in the designof systems and processes, in the production and in systems operation. Someexamples of the optimization use are: selection of processes or size of equip-ment, equipment items and their arrangement, operation conditions (tempera-ture, pressure, flow rate, chemical composition of each stream in the system),equipment combination in specific processes to increase the overall systemavailability, etc.

Formal optimization theory encompasses the special methodology, tech-niques, and procedures used to decide on the one specific solution in a definedset of possible alternatives that will best satisfy a selected criterion or function.The application of scientific methods and techniques to decision-making prob-lems based on mathematical programming techniques may achieve the optimumof the operation result—the maximization or minimization of the criterion orfunction.

A number of optimization methods have been developed for solvingdifferent types of optimization problems and hence various methods of effi-cient experimentation and simulation are available for performing the optimi-zation. In many applications, the process to be optimized can be formulatedas a mathematical model and the optimization experiments may be contactedor simulated in software. Today, even very large and complex systems canbe modeled by means of computers, and optimization can yield substantiallyimproved benefits.

Optimization methods have been found effective in many areas of en-gineering design and operation or even in business systems. In chemicalprocesses, optimization methods were developed in an effort to producehigh-quality products under normal manufacturing and working condi-tions, reducing their functional variations under real manufacturing circum-stances (low quality of raw materials, poor manufacturing equipment, etc.)and further lowering the cost of making the product (including developmentand manufacturing). A wide variety of problems in the design, construction,operation, and analysis of chemical plants or industrial processes can besolved by optimization. These methods have particular economical and tech-nological interest for food engineers, as they are widely used in food pro-cesses to achieve high efficiency in design, development and manufacturing.Now optimization methods are used routinely in industrial processes offoods.

In this chapter, an overview of the principles involved in design and pro-cess optimization is presented, the appropriate methods of optimization are de-scribed, and the applications of optimization in food engineering, especially infood extraction processing, are reviewed.

Optimization 139

II. OPTIMIZATION THEORY

A. Quality Engineering—The Design Optimization Problem

The quality of a product or a manufacturing process can be quantified in termsof the deviation from the target performance due to undesirable effects by fac-tors external to the product, manufacturing imperfection or deterioration/chang-ing of product performance characteristics (3).

In a representative design optimization problem of a product or a processthe common elements must be considered (Fig. 1). The response (y) is influ-enced by various factors; the dependence of the response y on the factors canbe denoted by a function (f) or a mathematical model. Input or signal factors(or pre-assigned parameters) (s) are selected by the engineering knowledge andare set by the operator to attain the intended output (target performance). Con-trol factors (or design or decision variables) (x) are the product or process pa-rameters specification, and their best values (levels) are determined by the de-signer using a number of criteria. Noise factors (z) are the uncontrollable factorsand their levels change from the environmental conditions of manufacture.

One approach to reduce the variation of a product or process is to limit oreliminate the noise factors. The role of design optimization is first to center thedesign parameters specifying the levels of control factors in order to minimizesensitivity to all noise factors. Product or process design can make the product orprocess robust against all factors, whereas manufacturing process design and actualmanufacturing can reduce only the variation due to manufacturing imperfection.

A typical engineering problem can be posed as a process represented bysome equations or by experimental data, while a performance criterion as mini-mum cost, maximum yield, etc. The process or model and the performancecriterion comprise the optimization problem. The goal of optimization is to find

Figure 1 Block diagram of a product or a process in an optimization problem.

140 Tzia

the values of the variables in the process that yield the best (maximum or mini-mum) value of the performance criterion while satisfying the constraints—thisis the optimization of a given problem.

B. Classification of Design Problems

Design problems can be broadly classified into static and dynamic problemsdepending upon the absence or presence of signal factors, respectively (3). Staticrefer to problems in which the values of the dependent variables remain constantwith respect to time, while dynamic problems represent the ones where theprocess-dependent variables change with time (2).

Static problems are also classified depending whether the response is con-tinuous or discrete. The desired value may be the maximum or the minimumvalue or be categorized into ordered categories. Examples of static problems arethe maximum yield of a desired constituent during an isolation process, theminimum product loss during a manufacturing process, or the best sensory qual-ity and acceptability of a food.

Dynamic problems are classified depending on the nature of the signalfactor and the response variable (continuous or discrete). Examples of dynamicproblems are fermentations in food processing in which the response (i.e., yieldas product weight or concentration of a constituent, etc.), results by continuousmonitoring of a process (viscosity increasing during curd formation in yogurtproduction, ethanol content increasing during wine fermentation).

C. Elements of Optimization Problems—Definitions

Optimization determines the values of independent variables that result in anoptimal value of a dependent variable. This involves a process for finding theunique set of process conditions that produces the best results, usually afterestablishing a mathematical model of the product or process. The theory, meth-ods, and techniques used to attain an optimum and to locate the optimum operat-ing conditions are the subject of optimization. Following are definitions of thebasic elements and terms that are common to all optimization problems (2, 4,5). The representation of the relationship of the elements in an optimizationproblem and solution is in Fig. 2.

Performance function or objective function or response or criterion orcriterion function or performance measure: This is a function of the designvariables that quantifies the desired result. It is the measured variable that deter-mines and reflects the performance of the system or process; it is the quantityto be optimized (maximized or minimized). It is often the profit, cost, productyield, energy consumption, and particularly for food systems the shelf life, con-sumer preference, quality characteristics, physical properties, etc.

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Figure 2 Elements of an optimization problem.

Decision (independent) variables or parameters or (control) factors:These are the parameters in the process that affect the value of the objectivefunction and can be adjusted to improve performance. They are free and inde-pendent variables that must be specified in the process model. In food systems,these commonly include temperature, pressure, flow rates, pH, concentration,moisture content, concentration of ingredients, etc.

Constraints: These are the constraints on the allowed solutions that mayhave a problem to be optimized. The constraints, equality or inequality, limitthe values of the dependent variables, the decision variables or even the per-formance functions of the process. Equality constraints are the fixed val-ues that variables will have in the boundaries. Inequality constraints refer toupper or lower values of the variables that the system may approach at bound-aries.

Mathematical model: The model is the mathematical representation of theprocess that determines the performance functions in terms of the decision vari-ables or other independent variables subject to constraint. Modeling of a processis the application of engineering sciences and knowledge to describe mathemati-cally (using mathematics, statistics, numerical analysis, computer software) aprocess and its performance. Simulation is the use of the model to assess differ-ent scenarios. The simulation of the process with a mathematical model facili-tates the process optimization against to costly experiments predicting processresults for any one set of decision variables. Furthermore, simulation helps togain understanding, to evaluate alternatives, and to answer specific questions.Optimization is described as the simulation performed aiming to maximize (orminimize) a certain process objective; the search for the desired optimum isusually done using mathematical algorithms.

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Optimization method or technique or procedure: This is the method ofsearching to find the optimum combination of decision variables possible withinthe boundary of the constraints. It can be as simple as selecting the combinationamong all possible combinations that produces the best results from the objec-tive functions in the mathematical model. However, in complex problems wherethere are many decision variables with a wide range of values, a structuredtechnique based on mathematical algorithms must be followed for solving of theproblem. Computers and associated software make the computations involved inthe selection (solution) feasible and cost-effective.

Feasible solutions: These are values of the design variables that satisfy allthe constraints.

D. Problem Optimization Procedure

In a general case of optimization, the problem must first be formulated and thenthe system to be optimized must be defined. Following, a function (mathemati-cal model) that describes the system must be constructed and then the system’soptimum solution must be found. The simulation of the process by a mathemati-cal model as well as the selection of the appropriate optimization method isfundamental to a successful process optimization. In all optimization problems,the above elements must be considered (6). A typical optimization problemprocedure including the individual optimization steps and the relationships be-tween the important optimization elements are presented in Fig. 3.

1. Optimization Problem Formulation

Formulation of problem optimization requires identifying the essential elementsof a given system and organizing them into a mathematical form, namely: (a)the objective function (criterion) and (b) the process model and constraints (2).It means that the variables that affect the performance of the system and thevariables that determine and reflect the system’s behavior must be specified.The objective function represents profit in terms of the (key) variables of theprocess being analyzed and the process model and constraints describe the inter-relationships of the (key) variables. A systematic approach for assembling thephysical and empirical relations and data involved in an optimization problemand procedures are recommended.

The need for formulation of the problem has been illustrated by examplesof relative optimization problems. Problem formulation and establishment of asatisfactory function that describes the behavior of the criterion as a function ofthe independent parameters is the most critical aspect in a problem optimizationand usually the most difficult step of a successful optimization study; artistry,judgment, and experience is required during the problem formulation step ofoptimization.

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Figure 3 Problem optimization procedure.

For the formulation of problem optimization the following steps must becarried out:

1. Description of the system to be optimized. The system boundariesmust be clearly defined so system parameters become independent of externalparameters. All the subsystems that significantly affect the performance of thesystem should be included in the optimization problem. When dealing with com-plex systems, they are usually divided to subsystems that are optimized individ-ually (suboptimization); similarly, during suboptimization the subsystem bound-aries must be carefully selected.

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2. Definition of a criterion (parameter). A single dependent parameter isdefined, that serves as the overall evaluation criterion for the specific optimiza-tion problem. It will be maximized or minimized to satisfy the objectives andmeasures the degree the solution satisfies the desired objectives of the problem.Often, only one primary criterion is used as an optimization performance mea-sure and all other criteria are treated as problem constraints or parameters. How-ever, in complex problems, where the specification of the optimum solution iscomplicated by conflicting criteria, a single parameter must be established foroptimization needs, taking into account the relative importance of all relevantcriteria. In practice, if it is desirable to develop a design that is “best” withrespect to many, usually competitive, criteria, advanced techniques for solvingthe optimization problem must be applied, i.e., simultaneously minimizing costand environmental impact, while maximizing efficiency and reliability.

The selection of criteria on which the system design will be evaluated andoptimized is a key element in formulating an optimization problem. Optimiza-tion criteria may be economical (total capital investment, total annual costs,annual net profit, return on investment), or technological (profitability evalua-tion criteria) (thermodynamic efficiency, production time, production rate, reli-ability, total weight, etc.), and environmental (e.g. emitted pollutants). An opti-mized design is characterized by a minimum or maximum value, as appropriate,for each selected criterion.

3. Definition of controlled or independent parameters. These are parame-ters that affect the performance of the process or system and can be adjusted,changed, and controlled. Their values determine the criterion parameter valueand must lie within the boundaries of the defined problem. The most criticalelement in optimization problems is the selection of all the appropriate variablesthat contribute to the decision-making process of attaining the optimum. Thevariables must include all the important variables that affect the performanceand cost effectiveness of the system, while not including details or variables ofminor importance.

Functional and regional constraints: Functional constraints representphysical or functional interrelationships that exist among the independent pa-rameters, while regional constraints limit the range over which the independentparameters can vary; they must be considered in addition to the parameters inoptimization problems.

Other parameters: Formal optimization depends on a clear definition ofthe system to be optimized. It only works for the specific system describedwhere all criteria and parameters are defined within the system and are isolatedand independent from other parameters outside the boundaries of the system.Therefore, formal optimization theory should be used only to find optimumparameters in well-defined systems with unique, quantitative-dependent and-independent parameters, criterion function, and functional and regional con-

Optimization 145

straints. So, the clearer the description of a given system, the more sure will bein obtaining the best solution. Furthermore, after the optimum set of parametershas been determined, other parameters, not included in the original problem,may be used and then compared with the others.

Suboptimization: It is usually applied to complex systems, where the opti-mization of the entire system may not be feasible, either due to the problemformulation or the inadequate optimization techniques. It can be also applied foreconomic and practical reasons. Then, the optimization of one subsystem as partof the whole problem is done, ignoring some variables that affect the objectivefunction or other subsystems; it does not however ensure the optimization ofthe overall system.

2. Development of Mathematical Model—Designand Evaluation

The further procedures for formal optimization and solution techniques includethe development of a model, model design, and application. That means theforming of the model, establishing and treating constraints, determining feasiblesolutions, and assigning of performance measures (6). In most cases of optimiza-tion, models are used instead of trial-and-error experiments and statistic tech-niques are applied.

Design includes general description and specification of the programmingtechnique and algorithms, formulation of the mathematical description, and sim-ulation of the model. In order to obtain a mathematical description of the processor system to be optimized, a mathematical model of the process or system isformed (7, 8).

A mathematical model is a description in terms of mathematical relationsof the functions of a physical system; the way variables are interrelated and theindependent variables affect the performance criterion are described by mathe-matical expressions. Models should closely represent the system and allow thedetermination of the performance functions in terms of the decision variables;consequently, development of a model requires a good understanding of thesystem.

The optimizer has to select the specific mathematical representation to beused in the model, as well as the assumptions and limitations of the model. Allthe important aspects of the problem should be included, so that they will betaken into account in the solution (specific values of the variables, assignedvariables that are functions of time, other independent variables, satisfaction ofconstraints or certain goals, etc.). Care must be taken not to omit any significantfactor to save time. The degree of accuracy needed in the model must be deter-mined. A successfully developed model helps to solve of the optimization prob-lem. Process design simulations and flow-sheeting software are also very usefulduring modeling. Model building is completed by evaluation and application; it

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is an iterative process as improved models can be produced by feedback ofinformation.

The mathematical model for an optimization problem consists of:

1. Objective function to be maximized or minimizedf(x) objective function (a)

2. Equality and inequality constraintsh(x) = 0 equality constraints (b)g(x) ≥ 0 inequality constraints (c)

where x is a vector of n variables (x1, x2, . . . , xn), h(x) is a vector of m1 equa-tions, and g(x) is a vector of m2 inequalities. The total number of constraints ism = (m1 + m2).

1. The objective function expresses the optimization criterion (C) as afunction of the dependent and independent variables (x1, x2, . . . , xn) and maybe represented by:

C = f(x1, . . . , xn) optimization criterion function (d)

The equality and inequality constraints are provided by appropriate models aswell as by appropriate boundary conditions. These models usually may be acost function, a benefit or profit function, or functions associated with materialand energy balance with engineering design. The models also contain equationsand inequalities that specify the allowable operating ranges, the maximum orminimum performance requirements, and the bounds on the availability of re-sources.

System models may be linear or nonlinear (set of algebraic, differential,or partial differential equations), static (focused on steady-state operation), ordynamic (focused on transient state), deterministic (not allowing variations), orstochastic (or probabilistic with time-varying parameters). Mathematical modelsexist for some food processes; for the rest they must be developed based onrelevant engineering and scientific knowledge. Their complexity varies depend-ing on the application, so in some food systems model development may bedifficult or impossible. Then, an approximating function is developed as closelyas possible to the system by performing an appropriate experimental design withlimited number of experiments and by model-fitting techniques.

2. The constraints, which in most optimization problems may limit thevalues of dependent or independent variables and consequently the region ofallowable solutions, must be identified. Constraints may be imposed by the par-ticular characteristic of the system or represent external restrictions (equipment,legal, etc.). The system model must account for all the imposed constraints.

Constraints restrict the values of the system variables. Constraints oftenare classified as being either equality or inequality constraints. The types ofconstraints involved in any given problem are determined by the physical nature

Optimization 147

of the problem and by the level of complexity used in forming the mathematicalmodel.

Constraints may be rigid, referred to as physical variables (restricted tonon-negative), government regulations, or customer requirements, etc., or maybe soft-negotiable to some degree (soft constraints). The former are viewed asabsolute goals, while the latter as goals associated with target values.

Functional constraints: These are physical principles of operation, whichgovern the relationship among the various independent parameters (x1, x2, . . . ,xn) of the problem and are represented by equations:

fci = fi(x1, x2, . . . , xn) (e)

Regional Constraints: These are practical limits on the range in whicheach parameter (xi) or function of the parameters can be varied and are ex-pressed as inequalities:

l1 ≤ ri(x1, x2, . . . , xn) ≤ l2 (f)

In an optimization problem, functional constraints can be used to eliminate thenumber of independent parameters. Normally, the number of functional con-straints must be less than the number of independent parameters, while therecan be any number of regional constraints.

Feasible solutions: After the constraints are established, the existence offeasible solutions for the given problem (points or region) that simultaneouslysatisfy all of the constraints must be examined, whereas the soft constraints maybe relaxed in order to minimize the deviations from goals.

Evaluation of the model: This is carried out according to the evaluationcriteria established in the problem definition and sensitivity testing of the modelinputs and parameters. Use actual data, which may entail statistical analysis ofthe fitted parameters, in the model when possible.

Fitting functions to empirical data: A model relates the output, i.e., thedependent variable(s) to the independent variable(s). Each equation in the modelusually includes one or more coefficients that are presumed constant. The termparameter as used here will mean coefficient and possibly input or initial condi-tion. With the help of experimental data, the form of the model can be deter-mined, and subsequently (or simultaneously) the value of some or all of theparameters in the model estimated.

Model validation: This consists of validation of model assumptions andmodel behavior by comparison with historical input–output, literature, and per-formance data and simulation. In general, data used in formulating a modelshould not be used to validate it. No single validation procedure is appropriatefor all models. The model should predict the desired features of the processperformance with suitable accuracy.

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3. Performing of Optimization—Attaining the Optimum

During the final step of the problem optimization process—attaining the opti-mum—the optimization must be performed by the appropriate selected method(5). Optimization methods have as scope to adjust, re-adjust, and locate thevalues of the decision variables that optimize (maximize or minimize) the per-formance function, while insuring that all variables (dependent or independent)satisfy the constraints of the system.

The already developed model of the system usually dictates the approachof optimal solution. Such approaches are: unconstrained optimization, con-strained optimization with a single performance measure or multiple perfor-mance measures, and optimization of systems that evolve over time (dynamicoptimization).

Assigning of performance measures: The performance measures that areto be optimized must be assigned (6). The selection of meaningful performancemeasures is critical for the optimization results. Usually, one of the performancemeasures is assigned as a target and the remaining are converted into softconstraints. Performance measures are incorporated into the optimizationprocess either by contacting actual experiments or using numerical searchingtechniques.

Solution of optimization problems: The reliability of the solutions ofoptimization problems depends on well-defining the problems. In complex prob-lem optimization, initial considerations may be primarily based on the optimiz-er’s judgments of the relevant parameters until a clearer configuration of theproblem parts can be defined, together with the relevant criteria, parameters,and constraints. Procedures for defining competing systems or alternate strate-gies and formulating the problem in order to apply formal optimization tech-niques are also common to operations research, systems analysis, and systemsdesign.

The general formal optimization problem is formulated in terms of thethree previously referred equations: the criterion function, and the functionaland regional constraints. The techniques used for solving the optimization prob-lem depend on the complexity of the optimization equations. Detailed treatmentof the various techniques for solving of equations represent the literature aboutoptimization.

Consequently, the formulation and solution of an optimization probleminvolves the establishment of an evaluation criterion based on the problem ob-jectives, followed by the determination of the optimum values of the controlla-ble or independent parameters that will best satisfy the evaluation criterion.It is accomplished either by performance of measures or analytically. In mostoptimization problems where there are conflicting criteria a compromise mustbe done weighting their relative value.

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III. OPTIMIZATION METHODS

A mathematical theory of optimization has been developed and has found appli-cation in a variety of engineering situations (1, 2, 5, 9, 10). The developmentof the digital computer allowing rapid numerical calculations has made the utili-zation of optimization procedures practical in many design situations.

Mathematical programming techniques are useful in finding the minimumof a function of several variables under a prescribed set of constraints. Stochas-tic process techniques can be used to analyze problems described by a set ofrandom variables having known probability distributions. Statistical methodsenable one to analyze the experimental data and build empirical models to ob-tain the most accurate representation of the physical situation.

An optimization or a mathematical programming problem can be statedas follows:

find X = �x1

x2

�xn

� which minimizes or maximizes f(X)

in unconstrained optimization problems or

find X = �x1

x2

�xn

� which minimizes or maximizes f(X) under the constraints:

hj(X) ≤ 0, j = 1, 2, . . . . . , mgj(X) = 0, j = 1, 2, . . . . . , p

in constrained optimization problems, where X is an n-dimensional vector calledthe design vector and f(X) is termed the objective function; in the case of con-strained problems hj(X) and gj(X) are known as inequality and equality con-straints, respectively. The minimizing/maximizing point or minimizer/maxi-mizer is denoted by x*.

A. Basic Theoretical Background (1, 2, 9, 10)

In any engineering system the design vector, defined during the design processby a set of variables, represents the selected design variables as X:

X = �x1

x2

�xn

� where xi are the design variables i = 1, 2, . . . , n.

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A vector is also represented by x and usually refers to a column vector.A matrix is referred as X and its elements as Xij or xij. A column vector of nvariables is also represented in n-dimensional space Rn by: x = [x1, x2, . . . , xn]

T,where the superscript T signifies the transpose of a row vector to form thecolumn vector x.

In the n-dimensional space (Rn), each coordinate axis represents a designspace (or design variable space). A point x in n-dimensional space is the vector(x1, x2, . . . , xn)

T, where xi is the component in the i-coordinate direction. For agiven function f(x) of n variables, points at which f(x) assumes local maximumand local minimum values are of interest. Each point in the n-dimensional de-sign space, called design point, represents either a possible or an impossiblesolution to the design problem. Optimization methods by iterative processesgenerate a sequence of points x(k) or {x(k)}(where k is the iteration number)intending to find x*, the solution of the problem.

In general, it is assumed that the problem functions are smooth, continu-ous and continuously differentiable (C1). Therefore for a function f(x) at anypoint x, there is a vector of first partial derivatives, or gradient vector.

�f(x) = �∂f/∂x1

∂f/∂x2

�∂f/∂xn

�where � denotes the gradient operator (∂f/∂x1, ∂f/∂x2, . . . . . . , ∂f/∂xn)

T. If f(x)is twice continuously differentiable (C2), then there is a matrix of second partialderivatives �2f(x) with elements ∂2f/(∂xi ∂xj) that is called Hessian matrix anddenoted by H(x). This matrix is square and symmetric. Since any column is�(∂f/∂xj), the matrix can be written as �(�fT).

Special cases of many variable functions include the general linear func-tion, which can be written as:

l(x) = ∑n

i=1

aixi + bi = aTx + b

where a and b are constants. The general quadratic function, can be written as:

q(x) = 1⁄2xTGx + bTx + c

where G, b, and c are constant and G is symmetric.The basis for all optimization methods is the classic theory of maxima

and minima. Mathematically, the theory concerned with finding the minimumor maximum (extreme points) of an unconstrained function of n variables f(x1,x2, . . . , xn) that can be interpreted geometrically as finding the point in an n-dimension space at which the function has an extremum. An optimal point x*

Optimization 151

is completely specified by satisfying the necessary and sufficient conditions foroptimality.

Following the desirable features of functions, as well as the necessarysufficient conditions to guarantee the defined extremum (minimum or maxi-mum), are presented. The knowledge of the basic properties of objective func-tions and constraints is considered necessary.

It is preferable and more convenient to work with continuous functions ofone or more variables as well as with functions having continuous derivatives.Then the extreme points may lie at the stationary points and consequently anecessary condition for a minimum or maximum is:

f′(x) = 0 or �f(x) = 0 (except in case of saddle points).

Hence, in order to locate the points where the partial derivatives are zero, thesolving of n-algebraic equations should be done: ∂f/∂xj (x1, x2, . . . , xn) = 0 (j =1, . . . , n). The equations can be solved directly or in some cases an approximatesolution can be obtained by minimizing the sum of squares of the residuals(least squares method).

It is also better to work with a unimodal function that has a single extre-mum (minimum or maximum); that is, a stationary point. Multimodal functionhas two or more extrema (maximum or minimum); these are multiple stationarypoints. The global extremum is the biggest or smallest among a set of extrema;local extrema may be significant in practical optimization problems involvingnonlinear functions. Sufficient conditions for a unique (isolated) or extremumto exist are:

f″(x) = �2f (x) = H(x) > 0 for unique (global) minimumf″(x) = �2f (x) = H(x) < 0 for unique (global) maximumf″(x) =�2f (x) = H(x) = 0 neither maximum nor minimum (inflection point).

The determination of convexity or concavity is helpful to establish if a localoptimal solution is also a global optimal solution. A convex region is useful inoptimization involving constraints. Equality constraints limit the feasible set ofpoints on hyper-surfaces, curves, or even a single point, while inequality con-straints specify a feasible region comprised of the set of points that are feasible.

To simplify the above, the case of a function of one variable x is assumed.A point x* is called local minimum if f(x*) ≤ f(x + h) for all sufficiently smallpositive and negative values of h. Similarly, a point x* is called a local maxi-mum if f(x*) ≥ f(x + h) for all values of h close to zero. A point x* will be aglobal minimum at x* if f(x*) ≤ f(x) or a global maximum of f(x) if f(x*) ≥ f(x)respectively for all x, and not just for all x close to x*, in the domain overwhich f(x) is defined.

Examples of local and global optimum points are presented in Fig. 4.Local minimum or maximum can be located at the boundaries or at points at

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Figure 4 Types of stationary points of a function. A, inflection point (scalar equivalentto a saddle point); B, global maximum; C, local minimum; D, local maximum.

which the first derivative (f′) is zero or discontinuous (stationary points). Oneof the local minimum/maximum can be a global minimum/maximum. Pointswhose first derivative is zero, may be neither a maximum nor a minimum, buta mini-max, or better known as a saddle point and movement away form it willresult in an increase or decrease on f(x) depending on the direction of the move-ment. Difficulties are caused when f(x) is a non-smooth function as its minimado not satisfy the same conditions as smooth minima.

B. Basic Optimization Methods

Many methods have been developed for the efficient solution of optimizationproblems; there is no general mathematical method for conducting the searchfor the optimal value.

The optimization methods can be categorized according to: (a) the natureof the objective function; (b) the constraints; and (c) the decision variables in-volved.

The objective function may:

1. Contain only a single decision variable (single or one-dimensional opti-mization) or many decision variables (multidimensional optimization)

2. Be continuous or contain discontinuities, and3. Be linear or nonlinear (linear programming—LP or nonlinear pro-

gramming—NLP)4. Be one or more than one (single or multi-objective programming)

Constraints may or may not exist in a problem and constrained or uncon-strained optimization are the two main categories. The constraints may be ex-pressed as linear or nonlinear equations or inequalities.

Optimization 153

The decision variables may be continuous, integers, or a combination.Depending on the nature of the design variables static or dynamic optimizationmethods may exist. Also, based on the deterministic nature of the variables,deterministic, or stochastic programming is classified.

Graphical methods involve a procedure of finding a maximum or mini-mum point of the objective function by the graphical plotting of the objectivefunction values. These methods are elementary, have reduced accuracy and areusually applied in the case of one or two-variable functions.

Methods may be indirect or direct. Indirect methods determine extremum(a) by using derivatives and values of the objective function. A necessary condi-tion for an extremum point of a differentiable objective function is that the pointis a stationary point. Direct methods search for an extremum by directly compar-ing function values at a sequence of trial points without involving derivatives.These methods generally can more easily treat problems involving objectivefunctions with discontinuities, points of inflection, and boundary points, whilecomputers have enabled their application.

Numerical methods are search techniques that have been developed todetermine maxima and minima of functions. The objective function is computedat a starting set of the independent variables. A second set is then selected andthe comparison of the new value of the objective function with the initial oneindicates if the objective function is improving toward an optimum. Searchesare simultaneous, when all sets of evaluation values are preselected, or are se-quential when new sets of data are selected based on information from theprevious sets of data. In applying numerical optimization methods, their effi-ciency should be taken into account before selecting the appropriate optimiza-tion method.

When the objective function is continuous and continuously differentiableand not near the region limits, the optimization may be done analytically. Thisimplies solving a set of differential equations of the first derivatives of theobjective function with respect to each independent variable and following theusual mathematical procedures for maxima and minima. Additional mathemati-cal procedures include the use of Lagrange multipliers and variational calculus.

Given that a function and its derivatives are continuous, a method thatexhibit quadratic convergence may be best; such methods can locate the exactmaximum/minimum of a quadratic function (assuming it has a well-defined op-timum) in a finite number of calculations.

Gradient-based methods are search techniques that use derivatives; thegradient vector �f(x) is central to these methods. At each point the gradientvector is perpendicular to the contour of the function and points in the directionof the greatest incremental increase in f(x). The gradient search works as fol-lows: (a) evaluate a search direction r = �f(x) at the current best x = x0; (b) letx = x0 + νr, where ν is a real scalar; (c) search for a maximum of f with respectto values of ν ≥ 0; and (d) reassign the current best x and return to step (a). This

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method is simple, however is inefficient when the contours are elongated orirregular in any way.

Following, the usual cases of a single-variable function or a multivariablefunction with no constraints, and a multivariable function with equality and in-equality constraints are presented. Also, some commonly used techniques applica-ble to these types of optimization problems are briefly discussed (1, 2, 5, 9–12).

1. Unconstrained Optimization

Unconstrained optimization methods are applicable when searching for a mini-mum or maximum of a function that is not subject to any constraints. Uncon-strained optimization may be single or one-dimensional or multi-dimensionaloptimization.

Single or One-Dimensional Unconstrained Optimization. This is themost elementary type of optimization problem; the function f(x) has only oneindependent variable. The techniques applicable to this type of problem areimportant because some techniques applicable to multivariable functions involverepeated use of a single-variable search.

To develop the necessary and sufficient conditions for a minimum or max-imum of a function f(x), a Taylor series expansion (with n terms) about thepresumed extremum x* can be performed.

f(x* + h) = f(x*) + hf′(x*) + h2

2!f″(x*) + . . . . +

hn-1

(n-1)!f (n-1)(x*) + hn

n!f (n)(x* + θh) for 0 < θ < 1

(1)

If f′(x*) = f″(x*) = . . . . = f(n-1) (x*) = 0, but f (n) (x*) ≠ 0then

f (x* + h) − f(x*) = hn

n!f (n)(x* + θh)

(2)

f(x*) is a minimum if f(n)(x*) > 0 and n is even, f(x*) is a maximum if f(n)(x*) < 0and n is even, and neither a minimum nor a maximum (inflection point) if n is odd.

For example when n = 2, given that f′(x*) = 0 at the stationary point, thehigher order terms are negligible compared to the second-order terms and henceequation becomes:

f(x* + h) = f(x*) + h2

2!f″(x*)

(3)

Then, the nature of f(x*) depends on the value of f″(x*) as already referred.That is at x* exists a minimum if f″(x*) > 0, a maximum if f″(x*) < 0 or aninflection point if f″(x*) is indefinite.

Optimization 155

Multidimensional Unconstrained Optimization. The theory for the mini-mum or maximum applied in one-dimensional optimization is generalized andextended in the case of a function of n independent variables. Therefore, for thenecessary and sufficient conditions for minimum or maximum of an uncon-strained function of several variables, the Taylor’s series expansion of a multi-variable function about a point X* is:

f(X) = f(X*) + d f (X*) + 12!

d2 f (X*) + . . . . +

1N!

dNf(X*) + RN (X*, h)(4)

where the last term, called the remainder, is given by:

RN (X*, h) = 1(N + 1)!

dN+1f(X* + θ h)(5)

where 0 < θ < 1 and h = X − X*.It must he noted that the r th differential of f(x) at X* (partial derivative

of r order) is the polynomial:

drf(X*) = ∑n

i=1∑

n

j=1

� � �∑n

k=1

hihj . . . hk∂rf(X*)

∂xi∂xj � � � ∂xk

(6)

r summations

The necessary condition for f(X) having an extreme point (maximum orminimum) at X = X* is the first partial derivatives of f(X) to exist at X* and tobe 0.

∂f∂x1

(X*) = ∂f∂x2

(X*) = . . . . = ∂f∂xn

(X*) = 0(7)

A sufficient condition for a stationary point X* to be an extreme pointdepends on the nature of the matrix of second partial derivatives (Hessian ma-

trix) of f(X*). This results to the quantity Q (given that∂f∂xi

(X*) = 0 for

i = 1, . . . , n and second partial derivatives of∂2f∂xixj

(X*) are continuous in the

vicinity of X*):

Q = ∑n

i=1∑

n

j=1

hihj∂2f(X*)∂xi∂xj

= hT J h* x=x* (8)

where Jx = x* = � ∂2f(∂xi ∂xj)x = x*� is the Hessian matrix of f(X). Consequently, if

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J*x = x* > 0 then X* is a relative minimum point, and if J*x = x* < 0 then X* is arelative maximum point. In case that the Hessian matrix is semidefinite (but notdefinite), the stationary points should be investigated for sufficiency in actualpractice.

However, in the general case where the partial derivatives of f of all ordersup to the order k ≥ 2 are continuous in the vicinity of a stationary point X*, and

dk-2*x = x* = 0dk*x = x* ≠ 0 (the first nonvanishing higher-order differential of f at X*)

then, if k is even, when dk*x = x* > 0, X* is a relative minimum point, whendk*x =x* < 0, X* is a relative maximum point, and when dk*x = x* is semidefinite(but not definite), no general conclusion can be drawn (X* is not an extremepoint, if k is odd).

In the case of a function of two variables f(x,y), the Hessian matrix may

be neither positive nor negative definite at a point (x*,y*) at which∂f∂x

= ∂f∂y

= 0.

That point is a saddle point.Unconstrained nonlinear multivariable optimization. The unconstrained

nonlinear programming methods used for multivariable optimization are itera-tive procedures in which the following two steps are repeated: (a) starting froma given point choose a search direction and (b) minimize or maximize in thatdirection to find a new point. These methods mainly differ in how they generatethe search directions.

Direct/indirect methods. Direct methods for single variable functions in-clude the region elimination methods, the two-point equal interval search, thebisecting method (or dichotomous search), the Fibonacci method, and the goldensection method; the last two methods are considered the most efficient. Thepoint estimation methods, or polynomial approximation methods usually involvea quadratic or cubic approximation of the objective function and for this Pow-ell’s method is considered the most efficient.

For multivariable functions, direct methods include the random search, thegrid search, the univariate search, the sequential Simplex method, the Hooke-Jeeves pattern search method, and Powell’s conjugate direction method. Thesemethods are relatively simple to understand and execute. However, they are notas efficient and robust as many of the indirect methods.

The indirect methods include the steepest descent/ascent gradient method(or Cauchy’s method), the conjugate gradient methods, Newton’s method, Mar-quardt’s method, the Secant methods, and the Broyden-Fletcher-Goldfarb-Shanno (BFGS) method.

2. Constrained Optimization

Constrained optimization methods are applicable to locate stationary points ofa function, but the solutions are subject to equality or inequality constraints.

Optimization 157

Optimization of the function must be carried out over a restricted domain of theindependent variables.

Optimization of Systems with Equality Constraints. Constrained optimi-zation of a continuous function f(x1, x2, . . . , xn) of n independent variables sub-jected to m equality constraints gi(x1, x2, . . . . , xn) (i = 1, 2, . . . , m) will beconsidered. The problem is defined when m ≤ n.

Three methods are mainly used for the solution of such problems: directsubstitution, constrained variation, and Lagrange multipliers.

Direct substitution methods involve the substitution of the m constraintequations directly into the objective function. The resulted objective function isnot subject to any constraint and hence its optimum can be found by applyingthe unconstrained optimization techniques. This method is suitable for solvingsimpler problems, but cannot be applied to many practical problems due tononlinearity of the constraints equations.

Constrained variation methods in the general case of n variables with mconstraints scope in finding an expression for the first-order differential of f(df) at all points where the constraints are satisfied. Each constraint equation isexpressed as a linear equation of the variations dxi, i = 1, 2, . . . , n (m equationsof n variations). Each one of m variations as well as df are expressed by theremaining n-m variations. The optimum points are obtained when df = 0.

df = ∂f∂x1

dx1 + . . . . . . . + ∂f∂xn

dxn = 0

dg1 =∂g1

∂x1

dx1 + . . . . . . + ∂g1

∂xn

dxn = 0(9)

. . . . . . . . . . . .

dgm = ∂gm

∂x1

dx1 + . . . . . . + ∂gm

∂xn

dxn = 0

Hence, the necessary conditions for the extremum of f(X) are given by the n-mequations (Jacobian determinants):

J� f, g1, g2, . . . , gm

xk, x1, x2, . . . , xm� =

∂f∂xk

∂f∂x1

∂f∂x2

. . .∂f∂xm

∂g1

∂xk

∂g1

∂x1

∂g1

∂x2

. . .∂g1

∂xm

∂g2

∂xk

∂g2

∂x1

∂g2

∂x2

. . .∂g2

∂xm

� � � O �∂gm

∂xk

∂gm

∂x1

∂gm

∂x2

. . .∂gm

∂xm

= 0

(10)

where k = m + 1, m + 2, . . . . . , n.

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Similarly, in the case of two independent variables x1, x2 where the func-tion f(x1,x2) is subject to the constraint g(x1,x2), a necessary condition for f tohave an extreme point at (x1*,x2*) is:

df = ∂f∂x1

dx1 +∂f∂x2

dx2 = 0(11)

dg = ∂g∂x1

dx1 +∂g∂x2

dx2 = 0(12)

or by combining the equations:

� ∂f∂x1

∂g∂x2

− ∂f∂x2

∂g∂x1� (x1*, x2*)

= 0 (13)

It is noted that the variations dx1, dx2 about the point (x1*,x2*) are called admissi-ble variations.

A sufficient condition for X* to be a constrained relative extremum isresulted by the Taylor series expansion of f, in terms of n-m variables (xm + 1,xm + 2, . . . , xn) about the extremum point X*:

f(X* + dX) f(X*) + ∑n

i=m+1� ∂f∂xi�

g

dxi +12! ∑

n

i=m+1∑

n

j=m+1� ∂2f∂xi∂xj

�g

dxidxj (14)

where � ∂f∂xi�

g

and � ∂2f∂xi∂xj

�g

are the first and second partial derivatives of f with

respect to xi and xi xj (holding all the n-m variables constant) respectively, whenx1, x2, . . . , xm are allowed to change so that the constraints gj (X* + dX) = 0,j = 1, 2, . . . , m, are satisfied.

The quadratic form Q defined by:

Q = ∑n

i=m+1∑

n

j=m+1� ∂2f∂xi∂xj

�g

dxidxj (15)

or the matrix:

� ∂2f

∂x2m+1�

g

� ∂2f∂xm+1∂xm+2

�g

� � � � ∂2f∂xm+1∂xn

�g

� � O �

� ∂2f∂xn∂xm+1

�g

� ∂2f∂xn∂xm+2

�g

� � � �∂2f

∂x2n�

g

� (16)

depending on their positive or negative value for all nonvanishing variations

Optimization 159

dxi are determinant for the optimum point X*. This method involves difficultcomputations as the constraints are more than two.

Lagrange Multipliers method is the most commonly used method in con-strained optimization problems with equality constraints. The Lagrange functionL, for a problem of n variables and m equality constraints is defined for eachconstraint gj (X) as:

L (x1, x2, . . . , xn, λ1, λ2, . . . , λm) = f (X) + λ1 g1 (X) + λ2 g2 (X) +. . . . . . + λm gm (X) (17)

where the quantities λj are called Lagrange multipliers.The necessary conditions for the extremum of L are produced by the solu-

tion of a system of n+m equations in terms of the unknowns, xi and λj, and aregiven by:

∂L∂xi

= ∂f∂xi

+ ∑m

j=1

λj∂gj

∂xi

= 0, i = 1, 2, . . . , n(18)

∂L∂λj

= gj (X) = 0, j = 1, 2, . . . , m(19)

From the solution, the relative constrained extremum X* and the λ* result. Itmust be noted that the vector λ* of Lagrange multipliers is a sensitivity factorthat indicates how tightly the constraint is binding at the optimum point.

A sufficient condition for f(X) to have a relative extremum at X* is deter-mined by the sign of the quadratic form Q:

Q = ∑n

i=1∑

n

j=1

∂2L∂xi∂xj

(X*,λ*) dxidxj (20)

or by the sign of the roots of the polynomial zi:

zi = L11−z L12 � � � L1n g11 g21 � � � gm1

L21−z L22 � � � L2n g21 g22 � � � gm2

� � O � � � O �Ln1−z Ln2 � � � Lnn−z g1n g2n � � � gmn

g11 g12 � � � g1n 0 0 � � � 0g21 g22 � � � g2n 0 0 � � � 0� � O � � � O �

gm1 gm2 � � � gmn 0 0 0 0 = 0

(21)

where Lij =∂2L∂xi xj

(X*, λ*), gij =∂gi

∂xj

(X*)(22)

So, if Q or each root of zi is positive or negative for all values of dX, X* willbe a constrained minimum or maximum respectively. It must be noted that the

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point X* is not an extreme point if all of the roots of zi have not the same sign.The solution of a simple two-dimensional problem by Lagrange multipli-

ers method is:

L(x1, x2, λ) = f(x1, x2) + λ g(x1, x2) (23)

Then, the necessary conditions for the extremum X* (x1, x2) are given by:

∂L∂x1

(x1, x2) =∂f∂x1

(x1, x2) + λ ∂g∂x1

(x1, x2) = 0(24)

∂L∂x2

(x1, x2) =∂f∂x2

(x1, x2) + λ ∂g∂x2

(x1, x2) = 0(25)

∂L∂λ

(x1, x2, λ) = g (x1, x2) = 0(26)

Optimization of Systems with Inequality Constraints. Constrained opti-mization subject to inequality constraints is treated as in the case of equalityconstrained problems after transformation of inequality constraints to equalityones by introducing the slack variables and solving the so formed constrainedequations to define the feasible region and find the optimum solution. A usualproblem of this category is the linear multivariable optimization with inequalityconstraints.

For the general case of a function f (X) of n independent variables subjectto m constraints gj(X) ≤ 0 (j = 1, 2, . . . , m), the constraints are transformed toequality ones as:

gj(X) + yj2 = 0 (j = 1, 2, . . . , m) (27)

where yi are non-negative slack variables.Hence, the problem becomes the finding of the extremum of f(X) subject

to:

Gj(X, Y) = gj(X) + yj2 = 0 (j = 1, 2, . . . , m) (28)

where Y is the vector of slack variables and can be solved by using the pre-viously referred Lagrange multipliers method as:

L(X, Y, λ) = f(X) + ∑m

j=1

λjGj(X,Y) (29)

where λ is the Lagrange multipliers vector.Then, applying the necessary conditions for the stationary points, the fol-

lowing (n + 2m) equations of (n + 2m) unknowns result:

Optimization 161

∂L∂xi

(X, Y, λ) = ∂f∂xi

(X) + ∑m

j=1

λj∂gj

∂xi

(X) = 0, i = 1, 2, . . . , n (30)

∂L∂λj

(X, Y, λ) = Gj(X, Y) = gj(X) + yj2 = 0, j = 1, 2, . . . , m

(31)

∂L∂yj

(X, Y, λ) = 2 λj yj2 = 0, j = 1, 2, . . . , m

(32)

3. Optimization of Dynamic Systems

A dynamic system is characterized by time- or space-dependent behavior andvariety by time or distance. The problems are called sequential decision prob-lems or multistage decision problems as the decisions are to be made sequen-tially or at a number of stages respectively. Many dynamic systems are encoun-tered in chemical engineering and food engineering (fermentations or thermalprocesses of foods).

For the optimization of multistage decision problems, dynamic program-ming techniques are used. Dynamic programming methods decompose an opti-mization problem into a sequence of subproblems that can be solved serially.Each of the subproblems may contain one or few decision variables.

A serial multistage decision process of n stages in decreasing order isrepresented schematically in Fig. 5. Each single-stage i decision process can becharacterized by an input (input state parameter) (Si+1) which is the output ofi + 1 stage, a decision variable (Xi), an output (output state parameter) (Si) anda return or objective function (Ri). The output is obtained as a result of makingthe decision, while the return depends on the effectiveness of the decision madeand the output that results from the decision. The state transformation functions

Figure 5 Dynamic process of n stages.

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or design equations relate input to output and the return functions are repre-sented as:

Si = ti(Si+1, xi) (33)

Ri = ri(Si+1, xi) (34)

where xi is the vector of decision variables at stage i.The objective of dynamic optimization is, given the values of variables of

the initial state or final state or both of the stages (boundary problem), to findthe continuous decision control variable(s) that optimize some function of theindividual stage returns. That is to find x1, x2, . . . , xn that optimize the objectivefunction f(R1, R2, . . . , Rn), mathematically expressed by:

fn(Sn) = opt [Rn(Sn, Xn) + fn-1(Sn-1)] (35)The functions of a system may differ in nature (i.e., differential). In order for amultistage problem to be solved by dynamic programming applying the decom-position technique, the objective function must be monotonic and separable,satisfying the requirements:

f = ∑n

i=1

Ri = ∑n

i=1

Ri(xi,Si+1)

f =Πn

i=1Ri =Π

n

i=1Ri(xi,Si+1) (36)

where xi are real and non-negative.

4. Programming Optimization

Linear programming (LP) is an optimization method applicable to problems inwhich the objective function and the constraints (equalities or inequalities) ap-pear as linear functions of the design/decision variables. Simplex method ofDantzig is the most efficient and popular method for solving general LP prob-lems, although other superior methods as Karmarkar’s have been developed. Itis considered suitable for complex problems solution. In case there is a largenumber of variables and/or constraints, the decomposition principle can be usedto solve the problem, while Karmarkar’s method has been shown more efficientthan the simplex method in this case.

Quadratic programming deals with problems that have a quadratic objec-tive function, linear constraints and can be solved by suitably modifying thelinear programming techniques.

If the objective function and the constraints are fairly simple, expressedin terms of the design/decision variables, the classical methods of optimizationcan be used to solve the problem. But classical analytical methods can not be

Optimization 163

used if the expressions are not stated as explicit functions or they are too compli-cated to manipulate.

Several methods are available for solving an unconstrained nonlinear opti-mization problem. The direct search (nongradient) methods require only theobjective function values but not partial derivatives of the function in findingthe optimum. The direct search (zero-order) methods use zero-order derivativesof the function. Direct methods are most suitable for simple problems involvingsmall number of variables. The descent (gradient) techniques require, in additionto the function values, only first derivatives of the function (first-order methods)or both first and second derivatives of the function (second-order methods).Descent methods are generally more efficient than direct search techniques.

There are many techniques available for the solution of a constrained non-linear programming problem. In the direct methods, the constraints are handledin an explicit manner, whereas in most of the indirect methods, the constrainedproblem is solved as a sequence of unconstrained optimization problems.

Geometric programming (GP) is a method of solving of nonlinear pro-gramming problems, where the functions and the constraints are in the form ofpolynomials. It places emphasis on the relative magnitudes of the terms of theobjective function rather than the variables; instead of finding optimal values ofthe design/decision variables first, it first finds the optimal value of the objectivefunction. Geometric programming is especially advantageous in cases that theoptimal value of the objective function is of interest, as well as in optimizationof a complicated problem reducing it to one involving a set of linear algebraicequations.

Many problems in plant design and operation involve variables that are notcontinuous but instead have integer, discrete, or fractional values. If an integersolution is desired, it is possible to use any of the common techniques and roundoff the optimum values of the design variables to the nearest integer values.

Integer programming (IP) refers to problems where all of the design/deci-sion variables are restricted to be integers. A special case of IP is binary integerprogramming (BIP) (or zero-one programming), where all variables are either 0or 1. Discrete programming refers to cases of problems where the variables arerestricted to take only discrete values. In mixed integer programming (MIP),some of the variables are restricted to be integers while others may be continu-ous (fractional values). Many IP problems are linear in the objective functionand constraints, hence are subject to solution by linear programming (MILP).

Stochastic or probabilistic programming deals with situations, where someor all optimization problem parameters are described by stochastic (or randomor probabilistic) variables. The stochastic problem is converted into an equiva-lent deterministic one that can be solved by familiar techniques (linear, geomet-ric, dynamic, and nonlinear programming).

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5. Experimental Optimization—Response SurfaceOptimization (RSM)

In some problems when the behavior of the system or process is not known anda model cannot be developed or the model developed is very complex, experi-mental optimization methods are used. Experiments are designed with appro-priate methods and empirical models based on the experimental data result. Theoptimum is then approached by using statistical techniques, the most known isthe response surface method (RSM), which is widely used in food applications.Assuming that the response of n independent variables is a function of the levelsand combinations of these variables, response surfaces that result provide insightinto the overall behavior and show the existence of optimal regions.

IV. OPTIMIZATION IN FOOD EXTRACTION PROCESSING

A. Application of Optimization in Food Engineering

Optimization has been applied in several areas in food engineering, and variousoptimization methods have been used both in processing and manufacturing.Areas of such applications are individual equipment or total system design, pro-cess design, as well as design, layout, control, management, and operation ofthe total plant. Following, more details for all these application areas of optimi-zation approaches are presented.

A selection of practical engineering perspectives referring to plant opera-tion and optimization has been edited (13). Mathematical programming (linear,nonlinear, integer, combinatorial) and modeling techniques for various industrialapplications relating to manufacturing, operation, decision-making, productionscheduling, or management have been also presented (14).

Especially, optimization in design and control of chemical engineeringprocesses approached by global optimization methods (deterministic or probabi-listic) and relative applications have advanced (15). Flow-sheeting optimizationduring the early stages of design, applying global optimization to modular pro-cess design approaches provides many advantages (16). Genetic algorithms(GA) or evolutionary ones (EA) have been investigated in chemical engineeringproblems and have been proved to be powerful and robust optimizing techniqueseither for total processes and/or particularly for extraction processes (17, 18).Optimization resources and specific information about optimization are avail-able on the Internet in the NEOS—Network Enabled Optimization TechnologyCenter (OTC)—which is a joint enterprise of Argonne National Laboratory andNorthwestern University (19).

The design and optimization of thermal systems is of primary interest forfood engineering as is strongly related to cost as well as to food quality. Thermal

Optimization 165

processing is commonly applied in chemical processing industries and hencethermal systems are often encountered into manufacturing plants. Thermal sys-tems are necessary in food processing plants either in thermal processes of food(pasteurization, sterilization, blanching, cooking, cooling, freezing, etc.) or in othercomplementary operations as storage under cooling or freezing. Consequently,thermoeconomic optimization involving thermodynamic optimization and modi-fication of the structure and design parameters of a thermal system, allows mini-mization of the total cost (optimization criterion) of the system satisfying tech-nological or environmental constraints (20).

As far as individual equipment is concerned, the design and optimizationof a falling-film evaporator and the construction of a mathematical model forits operation, aiming at both minimizing the overall operating cost (by minimiz-ing water removal cost) and maintaining product quality of a large-scale produc-tion of concentrated apple juices has been cited (21).

Optimization has found application in various processes widely used infoods, as in dehydration, heat processing, and formulation of foods. Examplesof such areas where optimization techniques have been successfully used arepresented.

In thermal processing of foods, the optimization problem generally con-sists in the determination of processing conditions, the combination of processtime and retort temperature, ensuring the constraint of required safety level butsimultaneously minimizing the associated quality degradation (or maximizingproduct quality) (22). In classic problems of thermal processed canned foods,kinetic models both of bacteria spores’ inactivation and nutrients (i.e., thiamine)degradation have been proposed as basis of optimization. Optimization methodsrequire studies of time–temperature dependent kinetics for each of these factors,or studies for finding the optimum temperature profile or the optimum containergeometry of the thermally processed foods. Direct search by trial-and-error,mathematical algorithms and dynamic programming approach the optimizationsolution. A proper optimization technique is based on the Pontryagin’s maxi-mum principle that searches out the optimum surface temperature as a smooth,continuous function of time, which maximizes the objective function (4). Fur-thermore for the design and optimization of thermal processing of foods, power-ful dynamic models and techniques have been developed suitable and efficientfor real-time industrial applications (optimization and control), i.e., for thermalsterilization (23, 24).

As far as the food dehydration is concerned, it is related to dynamic opti-mization problem and the scope is to find an optimum temperature profile thatwould result in maximum nutrients retention (i.e., ascorbic acid) achieving thedesired moisture content, given a specific drying time, while constraints for thefinal moisture or nutrient content of the dehydrated product may exist. Mathe-matical models describing the drying process of foods in accordance with the

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equipment used have been developed, while optimization techniques may besimilar of those applied in thermal processing optimization that consider eithercontinuous (Pontryagin maximum principle) or discrete-step optimum tempera-ture profile (4).

Optimization is usually applied in food products formulation, as a formu-lation process must meet a set of specifications for nutrient levels, be cost-effective, and is subject to constraints, i.e., for combined weight or certain nutri-ent levels. Such problems may be solved by simple mathematical optimizationtechnique as linear programming (25).

Optimization of the nutritional quality of formulated foods in respect todifferent nutrients proportions has been investigated by evaluating the effect ofthe component proportion in the final product nutritional quality. Therefore, itis feasible to maximize the nutritional quality of mixtures by obtaining optimumformulation or by substituting a component with another in the product formula.A usual problem is the formulation of protein mixtures intended to satisfy hu-man requirements with respect to essential amino acids. The optimization innutritional experiments is very significant and particularly in protein mixturesproblems, because the protein quality is evaluated in vivo and it is possible toexpress synergism effects due to complementarities of essential amino acids.Hence, the maximum nutritional quality is desired during product development,either by changing the concentrations of the protein ingredients (soybean proteinproducts, milk proteins, whey proteins, etc.) or by the substitution of an animalorigin protein by another (vegetable or less costly) in a formulated mixture ofknown nutritional quality (26). In nutritional evaluation experiments the mixtureresponse surface methodology proved to be an extremely useful tool for theoptimization of the mixture’s protein quality.

Moreover, the estimation of the nutrient values in commercial food prod-ucts is very important for the human diet. They must be taken daily in certainquantities, some of them are essential for human health (amino acids, linoleicacid), others are functional components (dietary fibers). Many missing nutrientsvalues for nutrients of interest are encountered in widely used foods’ composi-tion data due to lack of accepted analytical methods for some nutrients, highcost of chemical analysis, and proliferation of commercial foods. The missingnutrient value estimation is performed by various methods of different accuracy.Mathematical optimization proves more accurate to estimate nutrient values offood product than other techniques used (i.e., trial-and-error); linear program-ming as well as quadratic programming are comparably efficient in accuracy(27).

Sensory optimization of a food product is another application area whereoptimization methods have been used. The product ingredients affect many attri-butes of the product that determine its acceptability by the consumers. Hence,the best product formulation is found by evaluating multiple sensory attributes

Optimization 167

or responses of food, carrying different weights in the perception of the product,using graphical optimization methods (28). In product optimization problemsspecial (classical or novel) sensory methodologies and techniques are used (29).Mathematical models with linear or quadratic equations created by experimentaldesigns, show the relationships between independent variables (food formulas,processing conditions) and the responses (rating attributes). These models areuseful for the prediction of the likely attributes, the maximization of the accep-tance, or for optimization problems under certain constraints of formulation (30,31). An example of optimizing acceptability is the case of low-calorie productscontaining alternative sweeteners. Following a central composite design, re-sponse surface methodology (RSM) (32) can select the optimum product formu-lation (optimal concentration of sweetener) that optimizes the sensory quality ofthe product (maximum acceptability). Also during development of new foodproducts, combining experimental design of both product and concept generatesoptimal products and concept within constraints, such as cost, desired sensoryprofile limits, or targeted population (33).

The quality optimization of thermally processed foods (i.e., cooked) andthe relative optimization methods employed for these processes are of interestin food engineering. Commercial processes try to meet the requirements formicrobiological safety, nutritive value, and sensory acceptance of foods. Scopeof the methods is to determine the suitable processing conditions, especially thetemperature profile (temperature–time of processing), which maximize qualityfactors of foods undergoing thermal processing (i.e., retention or degradation ofnutrients, microbial inactivation, etc.) (34).

Dried potato cubes are an example of thermally processed food, the qual-ity of which is optimized. The process includes potato blanching in water con-taining pectin and polydextrose, followed by a two-step drying procedure by abatch-type high-temperature fluidized bed drier (HTFB) at varying conditions,and a tunnel drier at standard conditions. The objective is to find biopolymersconcentration, time of blanching, as well as temperature and time of drying inHTFB that obtain the maximum of rehydration ratio, puffing and water holdingcapacity, and minimum of nonenzymatic browning (7). In such problems, RSMis usually the method used for manipulation of experimental data. Using thesame technique, the extrusion of a high-protein snack has also been optimized.Maximum value of expansion ratio and sensory rating of extrudates and mini-mum value of shear strength was obtained by controlling the feed moisture andtemperature at the central zone of the extruder barrel (7).

The optimization framework of heat-sensitive foods’ drying process, beingalso essential for a food engineer, has been developed by combining the equip-ment and material models to attain the cost-effective product drying (with effi-cient energy saving), and minimizing its quality degradation. The potato slice issuch a material that suffers from loss of quality relating to color, nutrients, taste

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and/or texture and therefore the optimal operation of the dryer equipment mayboth maximize heat recovery and minimize quality degradation (i.e., loss ofascorbic acid, nonenzymatic browning), satisfying simultaneously the imposingconstraints (drying time, food moisture content, ascorbic acid content, and enzy-matic browning degradation) (35).

Frying of foods is another thermal process in which the frying temperaturesignificantly affects the quality characteristics (expansion volume, expansionratio, color, texture parameters) and the sensory ones (appearance, texture score,acceptance score) of the fried product. As continuous process is commonly usedinvolving certain consecutive deep frying steps, the corresponding optimumtemperature values can be approached based on the quality and sensory productcharacteristics. A popular and effective optimization method for such multivari-ate problems is response surface methodology (RSM) (36).

Enzymatic synthesis processes of certain valuable compounds have in-creased interest for investigation of parameters affecting the enzyme-catalyzedsynthesis. Enzymatic produced compounds are preferred over the chemical-syn-thesized products and are widely used in foods and beverages with many indus-trial applications, i.e., low-molecular-weight esters by lipase-catalyzed esterifi-cation are flavor compounds of commercial importance. Consequently theoptimization of such enzymatic processes for economical syntheses can be at-tained by central composite rotatable experimental design (CCRD) using re-sponse surface methodology (RSM) to predict best performance conditions (37).

Modeling, simulation, and optimization are nowadays particularly valu-able due to widespread use of computers and are important approaches to anumber of problems in the food industry. Linear programming can be used forsimple problems such as the optimization of product quality determining theconcentrations of certain components that affect its quality. Examples of indus-trial processes in which linear programming has been applied are referred.

LP has been applied to optimize the fixed process arrangement in an applejuice concentration plant which includes: apple crushing, juice extraction, fur-ther pressing of the resulting pomace after rehydration, mixing of juice streams,aroma stripping (for aroma recovery), juice clarification and concentration. Opti-mal plant operation can maximize the net profit by arranging the operating flowrates and taking into account the effects of costs, prices, apple varieties, and theoperational capacity constraints for equipment. Also, LP was utilized for qualityoptimization in aseptic processing, where considering the microbial and enzymeinactivation, cooking quality and component quality retention (thiamin and chlo-rophyll), the optimal process (holding time and temperature) was derived. An-other example of optimization using LP was in potato drying, where maximizingthe rate of drying and minimizing undesirable side reactions (browning, ascorbicacid oxidation) with negative effect on finished product quality, the optimaldrying profile was defined. (8). In addition, LP was used to quality optimization

Optimization 169

of potato chips, where given the quality as a function of moisture content, oilcontent and color characteristics of chips, the optimum frying conditions (tem-perature and time) were determined considering the constraints for temperature,oil, and moisture content of chip, and temperature and time of frying (7).

Optimization and control by automatic systems of various food processeshas been cited both for product quality and economic reasons, i.e., white winefermentation, sugar crystallization, extrusion cooking, pasta goods production,or for other complementary industrial operations, such as collection of raw milk,reduction of product losses, and costs of water purification during cleaning indairy industries. Optimal design and control of individual food processingequipment, i.e., climbing film evaporator for tomato juice, multistage hyperfil-tration unit for apple juice is also cited (38).

Optimization of industrial operations attempts to exploit cost, time, and qual-ity profits. For example, a methodology is proposed for optimization of a vegetableoil hydrogenation unit. A mathematical model based on data from a hydrogenationplant is developed to select the optimum operating conditions of temperature andhydrogen pressure that provide the desired product in minimum time (39).

On the other hand, optimization of the overall efficiency, total operatingcost, yield, total return, quality of output, and throughput of the whole foodmanufacturing plant is necessary when changing the raw materials or the prod-uct properties. Of course, it can be applied to large food processing plants; it isevident that routine application of optimization methods during operation ispracticed only in certain parts of industrial plants. However, with the increaseduse of computers and computer control in food manufacturing integrated optimi-zation may be applied to every modern processing plant. The new approach forfood process and plant optimization is optimization throughout the food chain.The optimization of a complete food chain consists of product, process, and rawmaterials optimization, and by analyzing the system efficiency (losses, wastes)leading to optimum plant, process, and equipment design and operation (40).

Finally, optimization is also applied in management and control of foodprocessing plants. Application of optimization in management of food process-ing and food service industries has been presented.

In addition to the above applications of optimization in food processing,similar techniques are used in optimizing food analysis methods of high accu-racy such as gas and liquid chromatographic methods (41). This has increasedsignificance when the constituents of interest are related to food safety specifica-tions. Also, optimization of instrumental measurements related to foods havebeen investigated, i.e., methodologies for extracting features from sensors outputresponses of instrument analyses (electronic nose, near infrared analyzer) (42,43).

In conclusion, it is clear that the increased use of optimization in foodmanufacturing enables the efficiency, productivity, and quality to be increased

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and the energy use, product loss, and environmental pollution to be reduced.Furthermore, the optimization philosophy, ideas, and techniques should beadopted throughout the food handling system—from the harvesting of raw ma-terials to the transportation, processing, packaging, distribution, and consump-tion of the final products.

B. Optimization of Extraction Processes

Extraction is applied to recover or remove constituents and consequently theproduct of interest may be the extracted materials or the extracts. The designand optimization of an extraction process entails the knowledge and the role ofthe technological parameters of the process.

Conventional solvent extraction involves the removal of certain constit-uents from a mixture of solids (leaching or solid extraction) or liquids (liquidextraction) by means of a solvent in which exhibit different solubility. Supercrit-ical fluid extraction (SFE) involves the separating of a mixture by contacting itwith a fluid under conditions of temperature and pressure above its critical point.The separation of solvent from the extracted solute in conventional extractionis carried out by distillation or evaporation, while in SFE separation is accom-plished either by reducing the pressure at constant temperature or by raising thetemperature at constant pressure (44). Following, the most important parametersthat significantly affect the extraction efficiency and determine the process de-sign and success are presented.

In leaching, the important parameters are:

1. The preparation of solid material by size reduction (crushing, grind-ing, flaking or cutting into pieces or cosettes); solid size must besuitable (surface area per unit volume) to make the solute more acces-sible to the solvent and to favor the extraction but not very fine tocause packing of solids and impede free flow of solvent

2. The selection of solvent for extraction based on a number of charac-teristics (capacity, selectivity, chemical inertness, thermophysicalproperties, flammability, toxicity, cost, availability)

3. The selection of operating temperature—temperature must be highenough to give higher solubility of solute in solvents, but not veryhigh to cause solvent losses, extraction of undesirable constituents ordamage of sensitive components

4. The equipment depending on the mode of operations (batch or continu-ous), the solids handled (fixed bed, percolation, full immersion, inter-mittent drainage or dispersed/moving contact) or the performing ar-rangement (one stage or multistage)

In liquid extraction the important parameters are:

Optimization 171

1. The selection of solvent as previously referred, mainly based on highsolute capacity and selectivity; particularly, solvent interfacial tensionmust have low value to get a good dispersion with high interfacialarea for extraction, but not very low to cause emulsion formation andcreate problems in separation

2. The equipment depending on the contacting of the phases (by gravityor by centrifugal force)

In supercritical fluid extraction the important parameters are:

1. The capacity and selectivity of the extracting supercritical fluid andtheir dependence on temperature and pressure

2. The ratio of solvent mass flow rate to the mass of solids treated;knowledge of the solvent, raw material and extract physical propertiesis necessary

3. The operating pressure and temperature, which must not cause de-composition of the raw material

4. Bulk density of the solid feed depending on the density of the solids,the form and consistency of the material and its moisture content

5. The mechanical treatment of the raw material depending on the natureof raw material (raw material purification or extract recovery); me-chanical treatment should be avoided in the case of solids recovery,whereas in the production of a soluble extract may be desirable

6. The time of extraction in respect to solvents, raw material feed andoperating conditions

The technical and economical feasibility of an extraction process is deter-mined knowing the equipment size, operating conditions, solvent flow rates andextraction yields. Extraction optimization is involved both in the design andthe operating conditions for the equipment (separation allocation, differences inphysical and/or chemical properties for separation, equipment type and sequenceseparators, fixation of separated phases, entire process operating conditions).

Optimization of liquid–liquid extraction processes has been investigatedand examples for staged and continuous models of extraction appeared in theliterature (45). In staged processes treated as an integer variable, the optimiza-tion is more difficult, while in continuous processes for either cocurrent or coun-tercurrent flow, integer variables are avoided and optimization can be carriedout by other techniques. Steady-state continuous countercurrent liquid extractionhas been modeled; a plug flow model was proved sufficiently accurate for acontinuous pilot-scale extraction column. It can be used to determine the maxi-mum extraction rate to various applications.

A common problem in optimization of extraction processes is that opera-tion time for the lowest cost is usually different from the time that gives the

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best yield (effectiveness), so that it is impossible to have both optimal cost andyield simultaneously. In this case the operation time becomes the independentparameter whose value determines the value of the criteria: minimum cost andmaximum yield. The relative importance of the two conflicting criteria must bejudged in order to obtain an optimum extraction time. So, if lowering of theoperation cost is of interest, the time of extraction will be optimized, while theyield will be the primary criterion in an isolation of desired components orremoval of toxic substances from food sources. Suboptimum with respect tooperation cost and yield or optimum with respect to the combined criteria ofcost and yield may be found. An overall criterion function can be establishedapplying a more formalized optimization. Regional constraints on time mightbe imposed; a very large time above a certain value is not cost effective or theyield is not accepted at time below a certain value. After the optimum operationtime has been determined other parameters not included in the original problem(extraction medium, extraction system) may be used, thus decreasing the opera-tion cost and increasing yield by formal optimization techniques. It must benoted that formal optimization theory should be used only to find optimumparameters in well-defined systems with quantitative-dependent and indepen-dent parameters, criterion function, and functional and regional constraints.

C. Application of Optimization in FoodExtraction Processing

Extraction is a separation process widely used in food processing industry forvarious applications, while recently with supercritical fluid extraction is possibleto improve the quality of food products. Extraction may be a basic step in pro-cessing of many food products (i.e. sugar, edible oils, etc.), used for the recoveryof active constituents or for the removal of undesirable constituents from rawmaterials, while important components may be separated from natural productswith many applications to food industries (flavors, antioxidants, etc.).

Optimization of protein isolation process by optimizing the protein extrac-tion step has been studied from oilseeds or beans as: tomato seed, pigeon pea,peanut, and flaxseed (46, 47, 48, 49). The protein isolation process from vegeta-ble sources includes mainly extraction or leaching of proteins by aqueous solu-tions and isoeletric precipitation; therefore extraction of proteins, evaluated byprotein extraction yield, is determinant for the total protein yield of the wholeprotein isolation process. The optimization of this process should attain the max-imum both of protein extraction yield or total protein yield as well as of theprotein quality of the isolated product. It must be mentioned that the conditionsoptimizing the above responses are not the same; usually maximizing the proteinextraction yield or the total protein yield decreases protein isolate content. Thus,the basic factors affecting protein solubilization during extraction such as: parti-

Optimization 173

cle size of proteinaceous material, temperature, extracting medium (water ordiluted salt solution), solid to liquid ratio, pH (neutral or alkaline region) havebeen studied by proper experimental design and using response surface method-ology (RSM) or steepest-ascent gradient method. Mathematical models for theprotein extraction yield or total protein yield as well as for the protein contentof protein isolate were obtained, while the extraction conditions that lead to themaximization of the responses were determined.

Surimi usually used as raw material in processed foods (fish sausage, fishcake, or kamaboko) is water-leached, minced flesh of deboned fish. Surimiprocessing starts with leaching and additional processes follow, such as grind-ing, setting, heating, and frozen storage. Because the processing conditions sig-nificantly affect the quality characteristics of surimi products (gel strength,whiteness) the optimization of surimi processing should be conducted. An opti-mization study has been presented that uses a surface response method witha central composite design of surimi processing experimentation. Among theprocessing conditions the leaching parameters (leaching water in each leachingcycle, number of leaching cycles) were examined as well as parameters of thefollowing processing steps. The optimal processing conditions that maximizegel strength and minimize whiteness of surimi products by canonical analysisand ridge analysis as well as fitting of a suitable model was investigated (50).

Optimization has been widely applied to oil processing industry, in oilsolvent extraction as well as in oil expression or mechanical extraction.

Mechanical extraction is an alternative method for oil expression by crush-ing the seeds in a mechanical screw press. In this case, both oil yield and energypressing cost are affected by the preprocessing conditions of the seed (dehulling,preheating, flaking or grinding, seed moisture content, seed temperature prior ofpressing) as well as by the design and operation characteristics of the press(pressure, temperature, pressing time) and their interactions. Mathematical mod-els based on expression conditions have been developed to predict the maximumsunflower oil recovery (51).

Olive oil is characterized as a “natural food product” because it is theonly oil produced by mechanical method that can be consumed without furtherprocessing. In addition, due to its nutritive and health properties, olive oil isrecognized as a functional food product. This oil, virgin olive oil, should meetcertain specifications according to regulation EC No. 2568/91 related to qualitycharacteristics (oil acidity or oxidative status, etc.). For the above reasons, spe-cial mechanical methods have been developed. The classical method of extrac-tion of olive oil is based on the mechanical pressing of olives. An alternativemethod also has been developed, which involves centrifugal separation of oliveoil using horizontal decanters. Special quality levels can be attained in virginolive oil production when all operations (olive collection and storage, extractionprocessing, packaging and storage of end product) are carefully carried out.

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Efficiency of expression is usually evaluated by extraction yield expressed aspercent of oil extracted to the percent of oil in the olives. The expression param-eters affecting oil yield have been examined: pressure compaction rate, cakethickness, rigid solid (pits) content in the olive paste, and the optimization ofolive paste expression process has been studied experimentally (52). Conse-quently, the simultaneous variation of the parameters must be investigated toobtain the maximum olive oil extraction yield and the optimal operation ofindustrial olive pressing systems to be determined. The pressure raise increasesoil yield, however it is important for olive oil quality the pressure to be keptunder constrained values. Therefore, the effect on olive oil quality of parame-ters, such as olive ripening and storage prior to processing and olive paste mix-ing time and temperature, should be studied so extraction can be optimized foroil quality requirements (53).

In recent years, extraction of vegetable oils using supercritical fluid extrac-tion (SFE) as an alternative to the current industrial processes (expeller pressing,solvent extraction) and its optimization has been investigated (54, 55). Carbondioxide, which is the most commonly used solvent to SFE processes, is alsoused in oil extraction; supercritical pentane has also been cited (54). The effi-ciency of SFE of oils depends on the variables: temperature, pressure, particlesize of seeds, contact time between the extracting fluid and the oil-bearing mate-rial, ability of the fluid to penetrate the oil-bearing material, and solubility ofthe oil in the extracting fluid. Optimization of SFE of various vegetable oilssuch as soybean oil, canola oil, and sunflower oil, has been studied and theconditions that maximize oil recovery have been determined. In particular, theeffect of extraction variables on oil recovery has been estimated by factorialdesigns and the optimum extraction conditions have been found using modifiedsimplex method for the case of soybean oil. Simulation and modeling of a sun-flower oil SFE plant and the optimization of the continuous process operation(extraction and CO2 recovery) was carried out and the optimal operating condi-tions found (55). Mathematical model of a fixed-bed extractor of canola oil hasbeen presented giving the concentration of oil on both the solid and fluid phasesand determining the overall volumetric mass transfer coefficients; a one-dimen-sional unsteady state model was used and the predicted results were in goodagreement with the respective experimental results (56).

Extractive processes used in alcoholic food/beverage products have beenstudied and optimized. The design, optimization, and control of extractive alco-holic fermentation in continuous mode are preferred. Combined with selectiveethanol extraction, fermentation is enhanced; the combination of fermentationand selective ethanol extraction enables efficient control of the process. Thedynamic behavior of the process was studied by factorial design simulatingindustrial process, and a dynamic matrix control (DMC) algorithm was used to

Optimization 175

control the extractive process. Modeling and simulation with response surfaceanalysis was used to determine the operational conditions that maximize alcoholyield and productivity (57). Also, a countercurrent supercritical fluid extractionprocess on a pilot plant of distilled alcoholic drinks has been studied to optimizethe efficiency of ethanol extraction yield. Thus, by factorial experimental designthe extraction and fractionation conditions determined the ethanol percentageand the total yield of extracted ethanol of the residual extracted drink and ofeach isolated fraction, allowing the production of concentrated extracts with arich aroma and low ethanol content (58).

Other analytical extractive processes that have been optimized are the sep-aration of certain amino acids (i.e., DL-tryptophan), which are important foodcomponents from aqueous solution using an emulsion liquid membrane. Thecritical extraction parameters: carrier concentration in the membrane system,pH, initial amino acids concentration were studied and using response surfacemethodology (RSM) the optimum separation conditions were found where themaximum amino acid extraction yield was attained (59).

Extraction (or acid leaching) is a special step or pretreatment in manycases of analytical procedures of some determinations in food matrixes: vita-mins, trace elements, pesticides residues, etc., in foods, industrial products,water, or soil. The analytes should be isolated and/or preconcentrated withoutchanging their original chemical forms and subsequently analyzed using analyti-cal or instrumental methods (i.e., atomic absorption spectrometry in traceelements, chromatographic methods such as gas chromatography (GC), high-performance liquid chromatography (HPLC) in pesticides or vitamins, spectro-photometry in tannins, etc.). The optimization of an analytical procedure ofteninvolves the study of the effects of experimental conditions particularly of timeconsuming and expensive ones as the extraction pretreatment processes (60–66).Critical variables in such extraction (or leaching) procedures are: type of solvent(organic solvent, saponification reagent, acid/oxidant reagent type and concentra-tion), leaching volume, time, temperature, particle size of ground food product(matrix), extraction method. The particular variables relative to the extractionmethod are: (a) soxhlet: reflux frequency; (b) solid-phase extraction (SPE): typeof sorbent, eluting solvent flow rate, sample pH, sample volume, elution volume,addition of modifier, water sample flow rate; (c) supercritical fluid extraction(SFE): CO2 pressure, temperature, static, and dynamic time; and (d) ultrasound ormicrowave assisted: sonication, frequency of ultrasound energy or microwavepower, exposure time, temperature (67, 68). Experimental (central composite,Plackett-Burman—PBD, orthogonal array—OAD) design and simplex optimiza-tion method or response surface methodology (RSM) are effectively used to studythe effects of extraction variables and find the optimum values of extraction vari-ables in which maximum percent recovery of the extracted components is attained.

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NOMENCLATURE

xi(i = 1, 2, . . . n) independent parameters, variables, de-sign variables in optimization prob-lem

x, (x1, x2, . . . . . , xn) vector of n variables, n-dimensionalvector, design vector

X matrixXij or xij elements of matrix Xx* extremum (minimizer/ maximizer)x = [x1, x2, . . . . . . , xn]

T column vectorx(k), {x(k)} k = 1, 2, . . . iterates in an iterative methodf(x), f(X) objective functionh(x), hj(X) vector of equality constraintsg(x), gj(X) vector of inequality constraintsC, f(x1, . . . , xn) optimization criterion functionfci = fi(x1, x2, . . . . . . . . , xn) functional constraintsl1 ≤ ri(x1, x2, . . . . . . . . , xn) ≤ l2 regional constraintsf′(x), dx first derivative of functionf″(x) second derivative of functiondr f(x) rth differential of f(x) (partial deriva-

tive of r order)�f(x) first partial derivative or gradient

vector�, (∂f/∂x1, ∂f/∂x2, . . . . . . , ∂f/∂xn)

T first derivative operator (elements∂xi/∂xj)

�2f(x) second partial derivative

�2 second derivative operator (elements

∂2/ ∂xi ∂xj)H(x), �(∂f/∂xj), �(�fT) Hessian matrixJ*x=x* = [∂2f/(∂xi ∂xj)* x=x*] Hessian matrix of f(X)dk* x=x* k-order differential of f at X*

Jacobian determinantsJ � f, g1, g2, . . . , gm

xk, x1, x2, . . . , xm�

L Lagrange functionλj Lagrange multipliersRn n-dimensional spaceBIP binary integer programmingCCD central composite designCCRD central composite rotatable designGMP geometric programming

Optimization 177

IP integer programmingLP linear programmingMILP mixed integer linear programmingMIP mixed integer programmingNLP nonlinear programmingOAD orthogonal array designPBD Plackett-Burman designRSM response surface optimizationSFE supercritical fluid extractionSPE solid-phase extraction

REFERENCES

1. SS Rao. Engineering Optimization: Theory and Practice, 3rd ed. New York: JohnWiley & Sons, Inc. 1996.

2. TF Edgar, DM Himmelblau. Optimization of Chemical Processes. Singapore: Mc-Graw-Hill Book Co., 1989, Chapters 1–9, p. 3–438.

3. G Taguchi, MS Phadke. Quality engineering through design optimization. In: D Khos-row, ed. Quality Control, Robust Design, and the Taguchi Method. Pacific Grove,CA: Wadsworth & Brooks/Cole, Advanced Books & Software, 1989, pp. 77–96.

4. AA Teixeira, CF Shoemaker. Process optimization. In: Computerized Food Pro-cessing Operations. New York: AVI, Van Nostrand Reinhold, 1989, pp. 169–195.

5. J Floros. Optimization methods in food processing and engineering. In: YH Hui,ed. Encyclopedia of Food Science and Technology, Vol. 3. Toronto: John Wiley &Sons, Inc., 1992, pp. 1952–1965.

6. DA Pierre. Optimization. In: McGraw-Hill Encyclopedia of Science & Technology,Vol. 12, 9th ed. New York: McGraw-Hill, Inc., 2002, pp. 479–485.

7. E Rotstein, J Chu, IS Saguy. Simulation and optimization. In: KJ Valentas,E Rotstein, RP Sing, eds. Handbook of Food Engineering Practice. CRC Press,1997, pp. 559–580.

8. I Saguy, L Levine, S Symes, E Rotstein. Integration of computers in food process-ing. In: HG Schwartberg, MA Rao eds. Biotechnology and Food Process Engineer-ing. New York: Marcel Dekker, 1990, pp. 415–441.

9. JS Arora. Introduction to Optimum Design. Singapore: McGraw-Hill, 1989.10. R Fletcher. Practical Methods of Optimization, 2nd ed. Chichester, UK: John

Wiley & Sons, 1987.11. G Dieter. Optimization. In: Engineering Design, 2nd ed. New York: McGraw-Hill,

1991, pp. 199–230.12. WD Baasel. Optimization techniques. In: Preliminary Chemical Engineering Plant

Design, 2nd ed. New York: Van Nostrand Reinhold, 1982, pp. 450–469.13. GF Nalven. Plant Operation and Optimization. Practical Engineering Perspectives.

New York: American Institute of Chemical Engineers, 1996.14. TA Ciriani, RC Leachman. Optimization in Industry. Mathematical Programming

and Modeling Techniques in Practice. Chichester, UK: John Wiley & Sons, 1993.

178 Tzia

15. CA Floudas. Global optimization in design and control of chemical process sys-tems. J Process Control 10:125–134, 2000.

16. RP Byrne, IDL Bogle. Global optimization of modular process flowsheets. Ind.Eng. Chem. Res. 39:4296–4301, 2000.

17. B Gross, P Roosen. Total process optimization in chemical engineering with evolu-tionary algorithms. Comp Chem Eng 22:S229-S236, 1998.

18. MF Yeo, EO Agyei. Optimizing engineering problems using genetic algorithms.Eng Comp 15(2):268–280, 1998.

19. http://www.mcs.anl.gov/home/otc20. A Bejan, G Tsatsaronis, M Moran. Thermoeconomic optimization. In: Thermal De-

sign & Optimization. Toronto, Canada: John Wiley & Sons, Inc., 1996, pp. 463–513.21. M Moresi. Design and optimization of falling-film evaporators. In: S Thorne, ed.

Developments in Food Preservation—3. Essex, UK: Elsevier Applied Science Pub-lishers, 1985, pp. 183–244.

22. A Van Loey, M Hendrickx, C Smout, T Haentjens, P Tobback. Recent advancesin process assessment and optimization. Meat Sci 43:S81-S98, 1996.

23. E Balsa-Canto, AA Alonso, JR Banga. A novel, efficient and reliable method for ther-mal process design and optimization. Part I: Theory. J Food Eng 52:227–234, 2002.

24. E Balsa-Canto, AA Alonso, JR Banga. A novel, efficient and reliable method forthermal process design and optimization. Part II: Applications. J Food Eng 52:235–247, 2002.

25. SK Sharma, SJ Mulvaney, SS Rizvi. Product formulation and process optimizationusing linear programming. In: Food Process Engineering, Theory and LaboratoryExperiments. New York: John Wiley & Sons, Inc., 2000, pp. 1–19.

26. IA Castro, J Tirapegui, RSSF Silva. Protein mixtures and their nutritional propertiesoptimized by response surface methodology. Nutr Res 20(9):1341–1353, 2000.

27. BJ Westrich, IM Buzzard, LC Gatewood, PG McGovern. Accuracy and efficiencyof estimating nutrient values in commercial food products using mathematical opti-mization. J Food Comp Anal 7:223–239, 1994.

28. MC Gacula, Jr. Product optimization. In: Design and Analysis of Sensory Optimi-zation. Trumbull, CT: Food and Nutrition Press, 1993, pp. 105–229.

29. C Stampanoni-Koeferli, P Piccinali-Schwegler, D Hong-Chen. Application of clas-sical and novel sensory techniques in product optimization. Lebensm.-Wiss.u.-Technol. 31:407–417, 1998.

30. H Moskowitz. Optimizing consumer product acceptance and perceived sensoryquality. In: E Graf, IS Saguy, eds. Food Product Development: From Concept tothe Marketplace. New York: AVI, Chapman & Hall, 1991, pp. 157–187.

31. JL Sidel, H Stone, HA Thomas. Hitting the target: sensory and product optimiza-tion. Cereal Food World 39(11/12):826–830, 1994.

32. MV Pastor, E Costell, L Izquierdo, L Duran. Optimizing acceptability of a highfruit-low sugar peach nectar using aspartame and guar gum. J Food Sci 61:852–855, 1996.

33. H Moskowitz. Sensory segmentation and the simultaneous optimization of productsand concepts for development and marketing of new foods. In: E Graf, IS Saguy,eds. Food Product Development: From Concept to the Marketplace. New York:AVI, Chapman & Hall, 1991, pp. 311–326.

Optimization 179

34. SD Holdsworth. Quality optimization. In: Thermal Processing of Packaged Foods.London: Blackie Academic & Professional, 1997, pp. 183–201.

35. JC Ho, SK Chou, AS Mujumdar, MNA Hawlader, KJ Chua. An optimization frame-work for drying of heat-sensitive products. Appl Thermal Eng 21:1779–1798, 2001.

36. C-S Chen, J-J Chen, T-P Wu, C-Y Chang. Optimizing the frying temperature ofgluten balls using response surface methodology. J Sci Food Agric 77:64–70, 1998.

37. S Hari Krishna, AP Sattur, NG Karanth. Lipase-catalyzed synthesis of isoamylisobutyrate—optimization using a central composite rotatable design. Proc Bio-chem 37:9–16, 2001.

38. M Renard, JJ Bimbenet. Automatic control and optimization of food processes.Proceedings of the International Symposium on the Automatic Control and Optimi-zation of Food Processes (ACOFP), Paris, France, 1986. Essex, UK: Elsevier Sci-ence Publishers, 1988, pp. 417–557.

39. AS Moharir, S Bhatia, DN Saraf. Optimization of industrial vegetable oil hydrogen-ators. JAOCS 60(8):1543–1547, 1983.

40. P Filka. Optimization of food processing plants. In: JJ Bimbenet, ed. AutomaticControl and Optimization of Food Processes. Proceedings of the International Sym-posium on the Automatic Control and Optimization of Food Processes (ACOFP),Paris, France, 1986. Essex, UK: Elsevier Science Publishers, 1988, pp. 405–415.

41. R Romero, D Gazquez, MG Bagur, M Sanchez-Vinas. Optimization of chromato-graphic parameters for the determination of biogenic amines in wines by reversed-phase high-performance liquid chromatography. J Chrom A 871:75–83, 2000.

42. S Roussel, G Forsberg, V Steinmetz, P Grenier, V Bellon-Maurel. Optimization ofelectronic nose measurements. Part I: Methodology of output feature selection.J Food Eng 37:207–222, 1998.

43. T Naes, H Martens. Principal component regression in NIR analysis. J Chemom 2:155–157, 1988.

44. KK Tiwari. Extraction technologies related to food processing. In: AG Gaonkar, ed.Food Processing: Recent Developments. New York: Elsevier, 1995, pp 269–302.

45. TF Edgar, DM Himmelblau. Optimization of Chemical Processes. Singapore: Mc-Graw-Hill, 1989, pp. 474–480.

46. GN Liadakis, C Tzia, V Oreopoulou, CD Thomopoulos. Protein isolation fromtomato seed meal, extraction optimization. J Food Sci 60:477–482, 1995.

47. IY Mizubuti, OB Junior, LW de Oliveira Souza, RS dos Santos Ferreira da Silva,EI Ida. Response surface methodology for extraction optimization of pigeon peaprotein. Food Chem 70:259–265, 2000.

48. IYS Rustom, MH Lopez—Leiva, BM Nair. Optimization of extraction of peanut pro-teins with water by response surface methodology. J Food Sci 56:1660–1663, 1991.

49. PKJPD Wanasundara, F Shahidi. Optimization of hexamrtaphosphate-assisted ex-traction of flaxseed proteins using response surface methodology. J Food Sci 61:604–607, 1996.

50. SY Hsu. Optimization of the surimi processing system with a central compositedesign method. J Food Eng 24:101–111, 1995.

51. MS Singh, A Farsaie, LE Stewart, LW Douglass. Development of mathematicalmodels to predict sunflower oil expression. American Society of Agricultural Engi-neers. St. Joseph, MI: USA Transactions of the ASAE, 1984, pp. 1190–1194.

180 Tzia

52. E Pagliarini, C Peri, B Zanoni, M Ghizzardi. Optimization of olive paste expres-sion: Maximizing yield by central composite design approach. J Sci Food Agric71:470–474, 1996.

53. E Monteleone, G Caporale, A Carlucci, E Pagliarini. Optimization of extra virginolive oil quality. J Sci Food Agric 77:31–37, 1998.

54. FM Lancas, MEC Queiroz, ICE Silva. Simplex optimization of extraction of soy-bean oil by supercritical pentane. Chrom 40(7/8):421–424, 1995

55. M Bravi, R. Rubbico, F Manna, N Verdone. Process optimization in sunflower oilextraction by supercritical CO2. Chem Eng Sci 57:2753–2764, 2002.

56. AKK Lee, NR Bulley, M Fattori, A Meisen. Modelling of supercritical carbondioxide extraction of canola oilseed in fixed beds. JAOCS 63(7):921–925, 1986.

57. AC Costa, DIP Atala, F Maugeri, R Maciel. Factorial design and simulation for theoptimization and determintation of control structures for an extractive alcoholicfermentation. Process Biochem 37:125–137, 2001.

58. FJ Senorans, A Ruiz-Rodrıguez, E Ibanez, J Tabera, G Reglero. Optimization ofcountercurrent supercritical fluid extraction conditions for spirits fractionation.J Super Fluids 21:41–49, 2001.

59. E Bayraktar. Response surface optimization of the separation of DL-tryptophan us-ing an emulsion liquid membrane. Process Biochem 37:169–175, 2001.

60. H Bagheri, M Saraji, M Chitsazan, SR Mousavi, M Naderi. Mixed-level orthogonalarray design for the optimization of solid-phase extraction of some pesticides fromsurface water. J Chrom A 888:197–208, 2000.

61. P Bermejo-Barrera, O Muniz-Naveiro, A Moreda-Pineiro, A Barmejo-Barrera. Themultivariate optimization of ultrasonic bath-induced acid leaching for the determi-nation of trace elements in seafood products by atomic absorption spectrometry.Anal Clin Acta 439:211–227, 2001.

62. MC Garrigos, F Reche, ML Marın, K Pernıas, A Jimenez. Optimization of theextraction of azo colorants used in toy products. J Chrom A 963:427–433, 2002.

63. Z Jokai, J Hegoczki, P Fodor. Stability and optimization of extraction of four arse-nic species. Microch J 59:117–124, 1998.

64. J Lee, L Ye, WO Landen, Jr., RR Eitenmiller. Optimization of an extraction proce-dure for the quantification of vitamin E in tomato and broccoli using responsesurface methodology. J Food Comp Anal 13,45–57, 2000.

65. B Martınez, F Rincon, MV Ibanez. Optimization of tannin extraction from infantfoods. J Agric Food Chem 48:2097–2100, 2000.

66. CC Nascentes, M Korn, MAZ Arruda. A fast ultrasound-assisted extraction of Ca,Mg, Mn and Zn from vegetables. Microch J 69:37–43, 2001.

67. GA Csiktusnadi-Kiss, E Forgacs, T Cserhati, T Mota, H Morais, A Ramos. Optimi-zation of the microwave-assisted extraction of pigments from paprika (Capsiumannuum L.) powders. J Chrom A 889:41–49, 2000.

68. P Larrayoz, M Carbonell, F Ibanez, P Torre, Y Barcina. Optimization of indirectparameters which affect the extractability of volatile aroma compounds from Idia-zabal cheese using analytical supercritical fluid extractions (SFE). Food Chem 64:123–127, 1999.

6Fats and Oils from Plant Materials

Lei Xu and Levente L. DiosadyUniversity of TorontoToronto, Ontario, Canada

I. INTRODUCTION

Although “extraction” often describes the mass transfer process between twoliquid phases, in the following the term “extraction” refers to the preferentialdissolution of one or more constituents of a solid by contact with a liquid sol-vent. It is one of the oldest and most commonly used unit operations in the foodindustry. “Leaching” is another term often used to describe the operation, al-though originally referring only to percolation of a liquid solvent through afixed bed of the solid. Sometimes the term “washing” is employed, as it isintimately associated with extraction as a complementary step. The fundamentalpurpose of extraction is the removal of a soluble fraction from an insoluble,permeable solid phase with which it is associated. The soluble fraction may besolid or liquid, and may be physically held within, chemically bound with, ad-sorbed on, or mechanically held in the pore structure of the insoluble matrix.

Although the mining and metallurgical industries are the largest users ofextraction technology, extraction is also a critical unit operation in the foodindustry. Many important foods and food ingredients are obtained by extractionfrom materials of life origin, including microbes, plants, and animals. For in-stance, sugar is extracted from sugar beets with hot water, tannin is dissolvedout of various tree barks, and beverages such as tea and coffee are prepared bothdomestically and industrially by leaching operations. In terms of technologicalsophistication and production scale, vegetable oils are among the most importantproducts recovered from seeds such as soybeans and rapeseed by extraction;therefore, these are discussed in greater detail in this chapter. Although in somecases mechanical pressing methods can be used, solvent extraction is a more

181

182 Xu and Diosady

efficient method for the recovery of oil from oil-bearing materials. It is the onlyefficient technique to recover oil from seeds or other materials low in oil (1).Since heat treatment can be minimized, oil produced by solvent extraction is ofhigh quality, and thermal damage to meal, the residue after oil removal, is alsominimal.

The effectiveness and efficiency of extraction depend on many factors,including the solvent used, solid preparation, extraction temperature, modes ofoperation, and equipment. The theories of extraction have been well developedand are presented in detail in texts such as Treybal’s Mass Transfer Operations(2). Commercial processes of edible oil extraction are extensively reviewed inBailey’s Industrial Oil and Fat Products (3), while essential oils are reviewedin Guenther’s The Essential Oils (4).

The choice of solvent for extraction is largely determined by the solubilitycharacteristics of the constituent of concern and the solid. Ideally the smallercomponent of interest should be highly soluble in the selected solvent, whereasthe matrix or other major components should have little or no solubility in it.“Infinite” selectivity is never achieved in practice, and the process must bemanipulated to obtain high yield and high purity of the desired components withminimal coextraction of undesired impurities. In addition to solubility, cost andsafety are also taken into consideration when a solvent is chosen for extraction.To achieve complete extraction, a combination of several unit operations, suchas successive extractions with two or more solvents, may be required.

The pretreatment given to the oil-bearing solid frequently plays a veryimportant role in the efficiency of the extraction process. In many instances ofextraction, small particles of the soluble material are completely surrounded bya matrix of insoluble materials. The solvent must then diffuse into the mass,and the resulting solution must diffuse out before a separation can result. Particlesize reduction by crushing and grinding of such solids greatly accelerates theextraction action, since the soluble portions are then made more accessible tothe solvent. The extraction process must find a suitable compromise betweenthe increased extraction rate obtained by reducing the particle size and increaseddifficulty in separating the small solid particles from the liquid solvent. Plants,microbes, animals, and their parts are cellular in structure, and the desired com-ponents are usually found in the cells. If the cell walls remain intact upon expo-sure to a solvent, the extraction depends solely on the diffusion of the solutethrough the cell walls, often controlled by osmotic pressure. Pretreatment tech-niques can be used to disrupt cell walls chemically or thermally. Alternatively,the aspect ratio is changed in a mechanical process. This allows the reductionof the penetration distance of the solvent while maintaining sufficiently largeparticle size to permit efficient separation from the solvent. As an example,cellular material may be cut into thin slices before extraction so as to reducethe penetration distance required for the solvent to reach the individual plant

Fats and Oils from Plant Materials 183

cells. In oil extraction, oilseeds are usually rolled to form flattened flakes withthickness ranging from 0.15 to 0.5 mm, while retaining major dimensions of upto 15 mm. The rolling and flaking evidently crushes some of the cell walls andopens up passageways for penetration of the solvent by capillary action (5, 6).It is impractical and sometimes even undesirable to grind the material smallenough to release the contents of individual cells because of the difficultiesexcessively fine particles may cause in subsequent separation. Seed preparationincludes heat and moisture adjustment to facilitate cell disruption and solventpenetration.

It is usually desirable to extract at as high a temperature as possible be-cause higher temperatures result in higher solubility of the solute in the solvent;thus, higher concentrations in the extraction liquor are possible. Moreover, theviscosity of most liquids is lower and the diffusivity greater at higher tempera-tures, leading to increased rates of extraction. Higher operating temperaturesalso reduce energy requirements in solvent recovery. However, the solvent se-lection and temperature must take into account that in some natural productshigh temperatures may cause deterioration and denaturation.

Extraction operations can be conducted in batch (unsteady state) or contin-uous (steady state) mode. In each case, both stage-wise and continuous-contacttypes of equipment can be used. Extraction equipment uses two techniques forsolid–liquid contact. The liquid can be sprayed and percolated over the solid,or, alternatively, the solid can be entirely immersed in the solvent. The choiceof equipment depends on the physical and chemical characteristics of the matrix,the value of the products, and the desired throughput.

II. THEORIES OF EXTRACTION

A. Rate of Extraction

The performance of an extraction process is governed by both mass transfer andequilibrium phenomena. It can be affected by many physical and chemical fac-tors, some of which are difficult to evaluate quantitatively. It is, therefore, nota simple matter to theoretically predict extraction rates, and quite frequently,such data have to be determined experimentally. If extraction only involvessimple washing of a solute from the surface of a solid, it may be very fast,comprising merely the blending of solution and solvent, and its efficiency is thusdetermined almost entirely by the effectiveness of the mechanical separation ofliquid from solid. Extracting a solute from the internal parts of a solid, on theother hand, will be much slower, in which case solids are made of a skeletalstructure of insoluble substances with pores impregnated with the solute. Diffu-sion theory can be used to describe extraction processes that fall into this cate-gory. Beginning with Fick’s law, the following equation is derived (7):

184 Xu and Diosady

NA = NA

NA + NB

DAB

z � ρM�av

lnNA/(NA + NB) − xA2

NA/(NA + NB) − xA1

where NA and NB are molar fluxes of components A and B, respectively, DAB isthe diffusivity of A in B in solution, ρ and M are the solution density andmolecular weight, respectively, xA1 and xA2 are mole-fraction concentrations ofcomponent A at the beginning and end of the diffusion path, respectively, andz is the distance in the direction of diffusion. For most common cases wherecomponent A diffuses through nondiffusing B, NB = 0, where

NA = DAB

z � ρM�av

(xA1 − xA2)

The diffusivity varies significantly among liquids. In the absence of exper-imental data, it can be estimated by the following empirical correlation for non-electrolytes:

DAB = (117.3 × 10−18(ϕMB)0.5T

µν0.6A

where DAB is diffusivity of A in a very dilute solution in solvent B, MB themolecular weight of solvent, T the temperature, µ the solution viscosity, νA thesolute molar volume at normal boiling point (0.0756 m3/kmol for water), and ϕthe association factor for solvent (2.26 for water, 1.9 for methanol, 1.5 for etha-nol, 1.0 for unassociated solvents such as benzene and hexane).

The complexity of the structures of natural products may make the appli-cation of these models difficult. In addition to the above attributive factors, therate of diffusion can also be affected by the degree of cell rupture, the diversityof structures in the matrix, and the rate of dissolution of each constituent. Allof these phenomena make it impossible to apply a single simple equation tomodel extraction. Although methods of dealing with solvent extraction of vege-table oils have been suggested (8, 9), very little has been published on modelingthe extraction of other natural products.

In oil extraction, the term “miscella” refers to the mixture of oil and sol-vent outside seed particles, and “raffinate” the mixture of the two within theparticles. The design of large-scale solvent extraction apparatus for oilseeds ex-traction requires the knowledge of the rate at which equilibrium is attained be-tween the miscella and raffinate. In most oilseeds, oil is stored in small, 1- to10-µm oil bodies, which are enclosed by protein-embedded membranes. Themembrane protein oleosin has high affinity for both oil and water on oppositeends of the molecule. The extraction proceeds first by washing out of free oil,which is held in large intercellular spaces and is not bound to the flake structure;it is followed by a diffusion process. Assuming that the cell walls are broken,

Fats and Oils from Plant Materials 185

the oil must still diffuse through the oleosin membrane of the oil droplets. Thenext step is the release of bound oil into the solvent, which may involve thebreaking of hydrogen bonds or hydrophobic attractive forces. The solution thenmust diffuse back out through the solid matrix to the bulk miscella, which in-volves diffusion first through the solid itself, then through the laminar boundarylayer on the surface of the solid. Therefore, oil extraction may be quite slow,particularly when the oil content of the seed is reduced to low levels (≤1.0%).

Studies have shown that the oil extraction rate is influenced by a numberof factors, including the thickness, size, shape, and internal structure of the seedparticles; the intrinsic capacity for diffusion of solvent and oil, which is deter-mined primarily by the viscosities of both; and, especially at low oil levels, therate of extraction of other substances that are less readily soluble than glycer-ides, which are the primary components of interest in vegetable oils (1). Amongthese factors, the effect of the thickness of oilseed flakes is most evident.Othmer and Agarwar (5) correlated the rate of hexane extraction of soybean oilto flake thickness and remaining oil concentration as follows:

− dCdt

= kF −3.97C

3.5

where C is the concentration of oil in the flake, t the time, k the proportionalityconstant, and F the flake thickness. Since F has an exponent of almost 4, asmall increase in flake thickness would result in a great decrease in the oilextraction rate.

When using a homogeneous oil-impregnated material made of thin plate-lets of uniform thickness, approach of estimation of extraction rate becomessimpler as the application of diffusion theories is possible. The calculation hasbeen thus developed by Boucher et al. (8) as follows:

E =8

π 2 ∑n=∞

n=0

1

(2n + 1)exp�−(2n + 1)2�π2�

2�DtR

2��where E is the fraction of total oil unextracted at the end of time t, R is one–halfthe platelet thickness, and D is the diffusivity of the oil within the platelets.Except for the initial stage where the free oil is washed out quickly, the aboveequation can be reduced to:

E =8

π 2exp�− π2

Dt

4R2 �or

logE = −0.091 − 1.07Dt

R2

186 Xu and Diosady

Thus, when the extraction proceeds past its initial stage, a plot of log E againstt gives a straight line. The slope is a function of the diffusivity of the oil andthe platelet thickness. The turbulence in the solvent, indicated by the Reynoldsnumber, was found to have no significant effect on the extraction rate, showingthat the limiting step in extraction is diffusion through the cell wall within theplatelets, and the liquid-film resistance to the transfer of oil to the solvent wasinconsequential (8, 10). The rapid release of oil during the initial stage of extrac-tion was thought to be caused by the rupture of some oil bodies in slicing theseeds, plus the occurrence of void spaces in the seeds after drying (9). Thediffusivity of the oil in the platelets was correlated to the viscosities of bothsolvent and oil by Boucher et al. (8) under their test conditions:

D = 12.96 × 10−6(µ 0µ s)−0.46

where µ o and µ s refer to the viscosities of oil and solvent, respectively. Thenumerical values in the formula are obviously specific to the structure of theplatelets and therefore are applicable only to the lot used in their tests.

B. Calculations for Steady-State Operation

In steady-state operation, it is assumed that the solid is contacted with morethan enough solvent to dissolve all the soluble solute and there is no preferentialadsorption of either solvent or solute by the solid. Adequate contact time permit-ting, all the solute will be dissolved, and the oil concentration in the miscellawill be equal to that in the solvent inside the seed matrix; thus, there is anequilibrium between the effluent solution and the solid. The insoluble solid isthen separated by settling, filtration, drainage, or centrifugation. This is usuallynot achieved in practice as the solute may be incompletely dissolved due toinadequate contact time and some solute is usually adsorbed by the solid. There-fore, without corrections for these effects, the actual stage efficiencies are al-ways lower than the calculated ones.

Similarly to other unit operations, two typical calculations are performed inextraction: characterization and design. In the former case the initial solute contentof the solid, and the number and amount of washings are known, and the finalamount or concentration of solute in the extract is sought. This yields an estimateof the extent to which an extraction process proceeds. In case of design calcula-tions, the final solute content of the solid is specified, with a known amount andconcentration of solute in the solvent; whereas the number of washings, or thenumber of stages, is calculated in order to design an extraction process.

Steady-state operation can be achieved in either a single-stage or a multi-stage manner. Single-stage extraction involves only one cycle of mixing thesolid with the solvent and separating the resulting insoluble solid by physicalmeans. Figure 1 shows all the streams involved in a complete process of single-

Fats and Oils from Plant Materials 187

Figure 1 Single-stage extraction (11).

stage extraction. Mass balance is performed for all components in the system togive (11):

FyyF + R0x0 = E1y1 + R1x1 for solute C

F(1 − yF) + R0(1 − x0) = E1(1 − y1) + R1(1 − x1) for solvent A

F + R0 = E1 + R1 = M1 for solution A + C

M1 is the mass of the mixture, or the slurry, on an insoluble solid–free basis.The solid and solute concentrations in the slurry are, therefore:

NM1 =B

F + R0

= B

M1

yM1 =yFF + R0x0

F + R0

respectively.By continuously contacting the solids with fresh batches of solvent, more

solute can be extracted than a single batch extraction. This is called multistagecross-current extraction. All of the above equations are applicable to the calcu-lations for addition stages, in which the already extracted solids from any stageare the feed solids to the next; thus, the procedure for a single stage is repeatedwith the changes in the subscripts to indicate different stages. Multistage cross-current extraction can be used to extract additional solute from the solids, but amuch larger amount of solvent is required for this operation, and as a result,dilute solutions of the solute will inevitably be produced as it proceeds to thelast stages of extraction. Although this is uneconomical use of solvent, the pro-cedure allows experimental determination of the performance of countercurrentsystems, using simple laboratory equipment.

Countercurrent extraction minimizes solvent use and operating costs. In

188 Xu and Diosady

this approach, the final solution is withdrawn from contact with the fresh solidand the fresh solvent is mixed with the solid with most of the solute alreadyextracted, keeping the driving force at the highest level throughout the process.A flowsheet of multistage countercurrent extraction is shown in Fig. 2. Theprocess can be operated with any number of extraction and separation steps. Anarray of these steps is called an extraction battery.

Like in single-stage extraction, calculations of multistage cross-current ex-traction are based on stage-wise mass balance.

The solution (A + C) balance gives:

F + RNp+1 = R1 + ENp= M

The solute (C) balance gives:

FyF + RNp+1xNp+1 = R1x1 + ENpyNp

= MyM

where M is total mass of the mixture (slurry) on an insoluble solid–free basis.The slurry is produced by mixing the solids and solvent such that:

NM =B

F + RNp+1

yM =FyF + RNp+1xNp+1

F + RNp+1

where NM and yM are, respectively, the concentrations of solid and solute in theslurry on insoluble solid–free basis.

The extraction process can be graphically modeled using a diagram asshown in Fig. 3, to indicate the approach to equilibrium obtained theoreticallyor experimentally, and the number of theoretical stages required for the extrac-tion. The number of stages is dependent on the average stage efficiencies, whichare affected by the mass transfer properties of each stage. In the special casewhere there is a constant value of N for slurry of all stages, the system approxi-mates a constant R/E. When, in addition, the equilibrium between the soluteconcentration in the effluent solution, x, and that in the solid (insoluble solid-free basis), y, is of a linear relationship, i.e., m = y/x = constant, the final concen-tration in the solid, yNP

can be calculated by the following equation:

yF − yNp

yF − mxNp+1

=� RmE�

Np+1

− R

mE

� RmE�Np+1

− 1

Fats and Oils from Plant Materials 189

Figure2

Mul

tista

geco

unte

rcur

rent

extr

actio

n(1

1).

190 Xu and Diosady

Figure 3 Stage construction diagram (11).

Similarly, the number of stages, Np, can be estimated when the final concentra-tion is specified. If no preferential adsorption of the solute occurs, the soluteconcentration is the same in both the solution and the solid, so that m = 1.

To calculate the steady-state performance of a multistage countercurrentextraction, the concentrations in the intermediate stages can be experimentallyestimated through batch simulation, using cross-current procedure as indicatedearlier. The scheme for such a simulation was proposed by Scheibel (12) andTreybal (13). Fig. 4 shows an example of batch simulation of two-stage counter-current extraction. Each circle in the figure represents a batch extraction. Start-ing from stage A, F amount of feed solid is contacted with S amount of freshsolvent. The mixture is then separated into a clear solution R′ and a solid EA.At stage B, the solid from stage A, EA, is again contacted with the same amountof solvent S before the separation to yield a solid E′. The whole simulationprocess is followed through as illustrated in Fig. 3. It consists of several runs.

Fats and Oils from Plant Materials 191

Figure 4 Batch simulation of a countercurrent cascade (13).

From each run R amount of solution and E amount of solid are produced. Al-though R and E at the beginning are clearly different in solute concentrationfrom those produced by a countercurrent extraction, after several runs in thismanner, they approach very closely the values obtained in a truly continuousmultistage countercurrent extraction.

III. SOLVENTS FOR EXTRACTION

For oil extraction, nonpolar solvents are usually used, as triglyceride-based oilsare typically miscible with these solvents. The most common solvents in solvent

192 Xu and Diosady

extraction of oil are light paraffinic petroleum fractions. Both hexane (bp 66–69°C) and heptane (bp 89–98°C) mixtures are widely used; sometimes cyclichydrocarbons, such as cyclohexane (bp 71–85°C), are also used (16). Whilethese solvents are efficient in oil extraction, the major disadvantage to their useis their flammability. Strict safety measures must be taken to avoid fire andreduce explosion hazard in the plants where they are used (17). Much effort hasbeen taken to find alternative solvents, particularly since the Clean Air Act of1990 designated hexane as a hazardous air pollutant. A methylpentane-typenaphtha (bp 55–61°C) is now commercially available (18). Trichloroethylene(bp 86.7°C), due to its nonflammability, is a much safer solvent to handle interms of prevention of fires and explosions (19, 20), but all chlorinated organicsolvents are increasingly perceived as too toxic to be used in the production offood materials. There is pressure on the industry to switch to solvents that areperceived to be more benign: alcohols and even aqueous solutions. Both isopro-panol (21) and ethanol (22, 23) have been used commercially to extract soybeanand cottonseed oil. Recently, high-concentration isopropanol (�96%), recoveredby pervaporation techniques, has been used instead of the commonly distilledazeotropic isopropanol (87.8%) for oil extraction in order to increase the oilsolubility (24). Both cottonseed and soybean oils thus extracted, after refining,bleaching, and deodorization, well met commercial standards. However, al-though improving the flavor, the alcohols tend to lower functionality of soybeanprotein products, and due to low oil solubility and higher latent heat than hex-ane, they require significantly more energy for solvent recovery. Supercriticalfluids have been tested for oil extraction (25, 26), but due to the extremely highpressures required, most equipment has low capacity, which currently limits theapplication of this technology to the production of high-value products such ascoffee, hops, and flavor concentrates.

IV. SOLVENT EXTRACTORS

Two types of commercial units are currently used for extraction: immersion andpercolation. In the immersion type of extractor, solids to be extracted are con-veyed through a pool of solvent. The particle size of the solid material must bereduced as much as possible to facilitate maximal contact between the solventand the solid. However, in this method the separation of fine particles and themiscella is difficult, thus limiting its use to low-volume operations. Most large-volume units are percolation-type extractors that are considerably more efficientin terms of energy consumption and space requirements.

In the percolation unit, the solid material (flakes or prepressed cake) formsa fixed bed, and the solvent is pumped and sprayed over the material, anddrained through the bed, washing the oil down with it. Clearly the flakes or

Fats and Oils from Plant Materials 193

pieces of prepressed cake should be large enough to permit a reasonable flowrate of solvent through the solid material. Fine material can prevent the drainage,causing the bed to be flooded with solvent and resulting in improper washingand poor extraction.

One of the first designs based on percolation was the basket-type extractor(Fig. 5), wherein the seed flakes are carried in baskets supported on endlesschains. The baskets are loaded at the top of the descending leg, and the flakesare sprayed with half miscella that percolate through the beds of the flakes inthese baskets as they go down. The miscella collected at bottom on the descend-ing side is full miscella and thus is removed for oil and solvent recovery. Onthe ascent, the flakes are contacted with another stream of liquid that starts aspure solvent; thus, oil in the flakes is extracted in a countercurrent manner asthe baskets move up. A short drainage time is provided at the top before thebaskets are automatically flipped to unload the extracted and drained flakes intoa discharging hopper, and they are then conveyed to the desolventizer.

This early type of solvent extractor is bulky and requires intensive mainte-nance due to frequent chain breakdown. It has been largely superseded by rotaryand horizontal extractors. A rotary extractor is, in fact, a vertical cylindricalshell with radially divided chambers called baskets or cells, either rotating orstationary, around a central shaft. Figure 6 shows a cut-away view of a rotaryextractor manufactured by De Smet Process and Technology, Inc., the reflexextractor. Its liquid spraying nozzles and flakes hopper are fixed while the bas-kets in the shell rotate. The material is mixed with miscella as it enters theextractor and is thus slurry-fed to the rotating baskets. The miscella leaves thebasket through the perforated bottom, and is collected and pumped to the previ-ous basket. Thus, a countercurrent flow is set up between the moving seedbeds and the stationary miscella or solvent headers. The most exhausted mealis contacted with fresh solvent, while the fresh seed is first contacted with nearlyoil-saturated miscella. The totally sealed basket dividers ensure that each mis-cella stage flows vertically through the basket of material, and these dividersalso allow the entire bed of material in the extraction zone to be thoroughlysoaked in miscella. It is claimed that, since the miscella has more contact timeto penetrate the flakes than in shallow-bed designs, even thicker flakes can beused to achieve desired residual oil content. After the extraction process is com-plete, a basket passes over an open portion of the screen, allowing the materialto discharge into the dump hopper at the base of the extractor by gravity. Anumber of similar designs were developed based on the earlier Rotocel extractorby the Dravo Engineers and Contractors. Another variant of this design, devel-oped by the French Oil Mill Machinery Company, called the stationary basketextractor, maintains the seed-holding baskets or cells immobile while rotatingthe solvent headers and bottom connector systems.

Horizontal extractors feature the horizontal movement of the baskets while

194 Xu and Diosady

Figure 5 Basket solvent extractor (1).

Fats and Oils from Plant Materials 195

Figure 6 Reflex solvent extractor. (Courtesy of De Smet Process & Technology, Inc.)

the solvent is percolated through the moving seed bed. One typical example ofthis type is the Crown Model III continuous loop extractor made by the CrownIron Works Company (Fig. 7), in which the solid material is deposited on vee-bar screen as a shallow bed (0.8 m) and travels a distance about 50 times thebed depth in a closed chamber shaped like a loop. The fresh material is fed tothe extractor through an inlet hopper on the top, and moves through four stageson the upper level and another three on the lower level. In the first stage on theupper level it is washed with already concentrated miscella. The solids thenmove to the second stage concurrent to the miscella. The rich miscella drainedfrom the second stage is pumped through a hydrocyclone before leaving theextractor as full miscella containing approximately 25% oil. The next five stages

196 Xu and Diosady

Figure7

Cro

wn

loop

extr

acto

rM

odel

III.

(Cou

rtes

yof

Cro

wn

Iron

Wor

ksC

ompa

ny.)

Fats and Oils from Plant Materials 197

are all operated in countercurrent manner. As the solids move from the upperlevel to the lower level between stages 4 and 5, the bed is completely turnedover in the “tail” section so that both sides of the solids are washed with themiscella. Before being discharged, the extracted solids are drained of residualmiscella in a drainage section. The mechanical features of this type of extractorpromote solvent contact, rapid drainage, and complete extraction with a varietyof products and allow utilization of fragile flakes with high content of fines.Extractors with capacities of 50–4000 t/d are in commercial service.

Crown Iron Works Company also developed a Model IV extractor forcountercurrent extraction of granular materials that are heavier than solvent (Fig.8). The material is carried through a pool of solvent by a series of inclined dragconveyors. The solids pulled out of the solvent pool by the last conveyor issprayed with fresh solvent and then are drained of residual miscella before beingdischarged. Full miscella is drawn off where solids are fed to the extractor.During the transfer of solids from one conveyor to another, the beds are com-pletely turned over, thus ensuring good solvent–product contact.

V. EXTRACTION OF OILSEEDS

The recovery of oil from oil-bearing plant materials has been a vital industryfor thousands of years. Hence, its technology evolved from primitive manualoperations in the early times to continuous processing by automated machineryin the modern days. This evolution was obviously driven by the need to maxi-mize the oil yields, improve the oil quality, and produce a valuable residue afteroil extraction. Oils from plants are usually contained in seeds, such as soybeans,sunflower seed, cottonseed, and rapeseed/canola, or fruits such as palm (Table1). They contain 15–50% of oil and are the major source of vegetable oils forhuman consumption (Table 2). The oil extraction process is often followed byfurther processing to yield more refined oil products.

Many processes have been developed to recover oil from oilseeds, butthe most common ones are hydraulic pressing, expeller pressing, and solventextraction. Developed in late 1700s, hydraulic pressing, also known as batchpressing, applies pressure using a hydraulic ram to batches of oilseeds confinedin a barrel that allows oil to escape while retaining solids. Because it is a labor-intensive, batch operation, its industrial use is uneconomical and thus declineddrastically over the last two decades. Currently continuous expellers or screwpresses are used for the mechanical extraction of oilseeds. These machines areable to create the pressures generated by a hydraulic press ram but require mini-mal labor because they operate continuously. The reduced labor cost and in-creased throughput and yield more than make up for their higher power require-ment and maintenance cost (27).

Figure8

Mod

elIV

Ext

ract

or.

(Cou

rtes

yof

Cro

wn

Iron

Wor

ksC

ompa

ny.)

198

Fats and Oils from Plant Materials 199

Table 1 World Production of MajorOil-Bearing Seeds, Legumes, and Fruitsin 1998–1999 Crop Year (in 106t)

Soybeans 154.70Rapeseed/canola 35.78Cottonseed 33.63Sunflower seed 27.26Groundnuts, shelled 20.88Palm 16.71Linseed 2.62Sesame seed 2.59

Source: Ref. 14.

It is impossible to obtain complete oil removal by mechanical pressing,since the solids retain significant quantities of oil inside the matrix. In addition,as the oil content is reduced, the friction between particles escalates rapidly, andthe heat generated by expeller causes very high temperatures that can degradeboth the oil and the residual meal. To recover the last 20% of the oil, solventextraction is the preferred operation. Solvent extraction as a batch process firstappeared in 1870. Continuous solvent extraction was developed shortly afterWorld War I. Since then, its use has expanded rapidly in oilseeds processingdue to its high efficiency in oil removal and recovery. As solvent extraction isenergy intensive, in oilseeds with high oil contents much of the easily removableoil is recovered by expelling prior to solvent extraction. Thus sunflower, rape-seed, and cottonseed are processed by a two-stage technique consisting of aprepress followed by solvent extraction, resulting in more complete removal ofoil with a lower usage of solvent.

Table 2 Oil Content of a Numberof Oil-Bearing Vegetable Materials

Cottonseed 18–20Palm kernels 45–50Peanuts 45–50Rapeseed/canola 45–50Sesame seed 50–55Soybeans 18–20Sunflower seed 35–45

Source: Ref. 15.

200 Xu and Diosady

A. Canola (Rapeseed)

The technology to process canola or rapeseed is typical of high-oil-content seedprocessing. Although expeller pressing or direct solvent extraction may be used,these techniques either leave a fairly high residual oil content in the meal or arenot energy efficient. The prevailing process in developed countries is prepress-solvent extraction, which is a combination of mechanical and solvent extraction,with the final portion of oil being removed with a solvent, usually hexane. Inthis process, much less residual oil is left in the meal.

1. Pretreatment and Prepressing of Oilseeds

A typical oil extraction process begins with seed preparation (Fig. 9). The seedsusually carry foreign materials such as unwanted seeds, sticks, leaves, and evenmetal shreds. They are typically cleaned by passing through a magnetic separa-tor and seed cleaners using a combination of air aspiration, agitation, andscreens.

In rapeseed, the enzyme myrosinase catalyzes hydrolysis of glucosino-lates. Among the hydrolysis products are undesirable isothiocyanates, oxazolidi-nethiones, and nitriles (28), which are soluble in oil. In order to maintain the oilquality, it is necessary to keep the glucosinolates intact by rapidly inactivatingmyrosinase by “cooking” in a multistage cooker. The seed is first cracked toincrease the area available for heat transfer. To achieve rapid myrosinase de-struction and to minimize the time spent at the active temperatures of the en-zyme (40–65°C), the cracked seed is contacted with live steam, which quicklyraises the temperature by releasing the latent heat, and also adds water to aid inthe denaturation of myrosinase. The excess water is removed by indirect heatingin subsequent stages of the cooker employing stacked steam-jacketed trays withbottoms heated by steam. Since canola has much lower levels of glucosinolatesthan conventional rapeseed varieties, the cooking temperature for canola hasbeen decreased from �120°C to less than 100°C.

The cooked seeds are then immediately pressed to separate oil by screwpresses. A typical screw press consists of a hard-surfaced, horizontal worm shaftwithin a barrel composed of cages containing spaced lining bars. As the wormshaft rotates inside the cage assembly, it moves the seed through and out. Pres-sure on the seed increases as it moves through the barrel, expelling the oilbetween the lining bars. The pressure at the discharge end of the shaft formsthe partially extracted seed into a cake. Important operating factors in prepress-ing are the speed of shaft rotation, the shaft assembly configuration, and settingof the choking mechanism at the discharge.

Pressing cannot remove all of the oil without significant thermal damageto the meal. Typical residual oil content in fully pressed cake is 6–10%, which

Fats and Oils from Plant Materials 201

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makes this approach noncompetitive. Thus, in modern plants seeds are pressedonly to the point where the meal protein is not seriously damaged, leaving 15–22% oil in the prepressed material. This removes two-thirds of the oil, usesmuch lower pressures and considerably less energy than the equivalent full-press operation, and results in better oil and meal quality. As the oil in the seedis contained in structures shaped like fibrous capillaries, application of too highpressures may seal these channels again and make further extraction difficult.Too much pressure may also cause the cake to become hard and impermeableto the solvent. It is, therefore, critical to control the pressure in the press andlimit the amount of oil removed from the cake.

To further free the oil, and to stabilize and strengthen the seed, anotherthermal step, usually referred to as “conditioning,” is required after prepressing.Conditioning involves heating the seed to reduce the viscosity of the oil and tomake it easier for the oil to separate from the ruptured seed cell. As the seed isheated by the expeller, external heating may not be needed for this step. Duringconditioning, it is important to control the moisture content of the seed, as wellas the temperature, because once the seed is crushed and flaked, microbial dete-rioration can take place rapidly, particularly if the material has a high moisturecontent or is of poor quality. Therefore, it is crucial to have a continuous andrapid flow of material from one piece of equipment to another. The flaked seedsare usually conditioned to a moisture content of 10–12% at a temperature of80–90°C.

As indicated earlier, oilseeds consist of oil bodies that contain liquid oilenclosed by a protein membrane. A typical oil body is 2–10 µm in diameter.The walls of oil bodies are essentially nonpermeable; therefore, for subsequentsolvent extraction these walls must be broken to free the oil and facilitate thepenetration of solvent into the cells. This is usually accomplished by flaking theprepressed seed in a roller mill after carefully adjusting the temperature andhumidity. Flakes of 0.15–0.5 mm in thickness and up to 10 mm in diameterallow rapid solvent penetration with high bed permeability.

2. Solvent Extraction

A simplified flow sheet of a typical solvent extraction process is shown in Fig.10. The seed flakes after preparation or the cakes from prepressing are conveyedto the solvent extractor. As described in detail in the section II.D, every extractorperforms two primary functions. First, it must provide retention time requiredto allow large volumes of solvent to penetrate and wash the solid material(flakes or cake). Second, it must separate the solids from the miscella to mini-mize solvent carry-over in the solids to the next phase of the operation, whichis desolventizing of the extracted solid material.

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3. Posttreatment

After oil extraction, the solid material (meal) is transported in a closed conveyersystem to the desolventizer-toaster (DT), where the residual solvent is removedby heating the meal enough to evaporate the solvent, but not so much as todeteriorate its nutritional value (Fig. 11). This unit is made of steam-heated traysmounted, one on top of the other, with a vertical shaft and sweep arms attachedto move the meal around and down to the level through automatic openings inthe trays. The top four trays make up the desolventizer and the bottom two trays

Figure 11 Desolventizer-toaster. (Courtesy of De Smet Process & Technology, Inc.)

Fats and Oils from Plant Materials 205

the toaster. The desolventizer may be divided into a pre-desolventizing and adesolventizing zone. By the time the meal reaches the toaster, most of the sol-vent has been evaporated and moved out the top of the desolventizer. Somemoisture addition through steam injection is required to facilitate further evapo-ration by dissociation of bonding between the meal protein and hexane throughhydrophobic interactions. The meal discharged from the DT may contain consid-erable moisture; thus, it must be further dried for final moisture adjustment andcooled. In some cases, all of these steps are accomplished in a single unit,dubbed DTDC for desolventizer-toaster-dryer-cooler. The hot solvent vaporfrom the desolventizer is used in most instances to heat other evaporation equip-ment before being condensed and sent back to the solvent tank for reuse. Fine-meal dust is recovered prior to condensation and solvent recovery.

To recover the crude oil from miscella, the solvent must be stripped fromthe oil in typical distillation columns. As mentioned above, the hot desolven-tized vapor is used to indirectly heat the vessel to evaporate some of the solvent.An additional stripping unit indirectly heated by steam is used to remove mostof the remaining solvent and water. The final traces of solvent are removed bydirect steam injection. The solvent vapors are then condensed in either water-cooled or air-cooled condensers. During solvent extraction, all equipment ismaintained under slightly negative pressure to minimize solvent losses. The airleaked into the system is passed through a vent condenser and a mineral oilabsorption column before being vented to the atmosphere, both for explosionsafety and solvent economy.

After the solvent is stripped from miscella, the crude oil can be furtherprocessed before storage, to remove entrained compounds such as phospholipidsor gums.

B. Sunflower Seed

Sunflower seed processing is similar to that of other high oil content oilseedsincluding canola and rapeseed. Therefore, this section, concentrates on differ-ences in the operations.

1. Dehulling

Sunflower seed contains 23–25% fibrous hull, which can interfere with the ex-traction process. Thus, the processor must decide whether to dehull prior toprocessing, which in turn determines the nature of the meal produced. Process-ing with the hulls produces a meal with a fiber level of about 18–20% andprotein content of 28–30%. Removal of about 75% of the hulls produces a mealwith a fiber level of 11–13% and a protein level of 40–42%. The market deter-mines if dehulling is required.

206 Xu and Diosady

However, if the hulls are not removed, they tend to reduce the total yieldof oil by absorbing and retaining oil in the press cake. To accommodate theadditional volume of the hulls, larger equipment is required to process the extratonnage. Moreover, wear on the prepress lining bars and worm shafts is in-creased, thus increasing the cost of the already expensive prepressing operation.Therefore, dehulling is often desirable. Two types of dehulling equipment areusually used: knife and impact, although a disk type is also available. Suchmachines break the hull fraction away from the seed, and the mixture is thenseparated using shaking screens and aspiration. The dehulling process must befinely tuned to leave a minimum of meats in the hull fraction to minimize oillosses.

2. Preparation

The preparation of sunflower seed is similar to that of other high oil contentseeds such as rapeseed. The seeds, or meats, are rolled to rupture and exposethe oil cells. They are then conditioned, by heating, to reduce the viscosity ofthe oil and to make it easier to separate the oil from the meats or seeds. Propertemperature and moisture adjustments are critical to maximize the effectivenessof the prepressing stage that follows. This adjustment is usually termed as “tem-pering.” Cooking at high temperature is not required for sunflower seed.

3. Prepressing and Solvent Extraction

These steps vary little from canola and rapeseed processing except for the ad-justments made on the prepressing equipment including the shaft arrangement,spacing of the bars in the cage surrounding the shaft, the rotation speed, and thechoking mechanism. After prepressing, the cake containing 16–18% oil is bro-ken and granulated. To make sure that the residual oil content is less than 2%after solvent extraction, some manufacturers condition and flake the broken cakepieces again before conveying them to the solvent extractor. Others simply con-vey the material directly to the solvent extraction process. In both cases, produc-tion of fine particles must be minimized because large amounts of fine materialmay cause the solvent to flood the extractor bed instead of percolating throughthe material for proper extraction.

C. Soybean

Oil removal from soybeans is normally carried out by solvent extraction alonesince soybeans have a relatively low oil content of 18–20%, as compared withthat of rapeseed or sunflower seed. The clean, dry, and tempered beans arebroken into four to eight pieces in cracking mills, usually composed of two pairs

Fats and Oils from Plant Materials 207

of fluted rolls turning at different speeds, cutting beans as they are fed throughthe two passes. The hulls of the dry beans are then easily separated by aspira-tion.

The cracked and dehulled beans are conditioned by raising their tempera-ture to 65°C and adjusting their moisture content using live steam. Conditionersare normally rotating drums with an internal steam coil. Conditioning enhancesthe performance of the subsequent flaking and extraction operations. The condi-tioned beans are fed to flaking mills to produce consistent flakes with an idealthickness of 0.3 mm. Good flaking is essential to maximize oil extraction in thesolvent extractor.

Extruders are becoming the preferred means of pretreating soybeans. Wa-ter is added to the cracked seed, which is then heated and pressurized in theextruder. As the pressure is released at the exit, the absorbed water is rapidlyvaporized, resulting in an expanded, porous structure where many of the cellsare ruptured. This porous microstructure improves oil and solvent mobility.

The solvent extraction of soybean oil is similar to that of other seeds (Fig.12). During the process, the heat reduces the natural trypsin inhibitor levels insoybeans, thus increasing the nutritive value of the meal.

D. Corn

Since the corn kernel contains only about 5% oil, it is uneconomical to processcorn simply to obtain the oil. Corn oil is obtained by processing the corn germ,which is a byproduct of the wet or dry milling of corn, a major industry. Corngerm contains about 50% oil. This oil is recovered by expelling or direct solventextraction (dry-milled germ), or by their combination in prepress-solvent extrac-tion (wet-milled germ). The actual processes involved are almost identical tothose described above for other oilseeds.

VI. EXTRACTION OF ESSENTIAL OILS

While edible oils are triglyceride-based oils mainly from oilseeds, essential oilsare the volatile chemical components from various parts of plants, which areresponsible for the characteristic aromas of these plants. To name a few, oilsextracted from bitter almond, lemon and orange peels, rose and jasmine flowers,peppermint, and lovage roots are all essential oils. Although lipid-soluble, essen-tial oils do not contain triglycerides, and they are complex mixtures of com-pounds that usually fall into the following four classes (29):

1. Terpenes2. Straight-chain hydrocarbons and their oxygen derivatives

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3. Benzene derivatives4. Volatile sulfur and nitrogen compounds

Essential oils are widely used in the food and fragrance industries. Theiractive components contribute a significant portion of the flavor or aroma of aspice. They are often diluted with oil or emulsified to make them dispersiblebefore addition to a flavor or fragrance system.

To recover essential oils, distillation has always been most commonlyused to take advantage of their volatility (30). The components in essential oils,however, have much higher boiling points than water; therefore, they are actu-ally codistilled with steam or water to avoid thermal damage. Codistillation withwater or steam distillation takes advantage of the fact that in the vapor phasethe oil, water, and air do not interact. The composition of the vapor phase isdirectly proportional to the vapor pressures of the vapor components, i.e., water/steam and the volatile oils. The steam acts as a carrier and removes the oilvapors, which have been evaporated well below their boiling point. This is espe-cially important because many of the essential oil components have high boilingpoints and would break down thermally well below their normal boiling points.After condensation, the oils and water are immiscible and thus are easily sepa-rated.

Like oilseeds processing, the production of essential oils also begins withthe size reduction of the raw material, the comminution of the plant parts. Asthe essential oils are enclosed in “oil glands” or “oil cells” of the plant, the rateof oil vaporization will be entirely determined by the rate of hydrodiffusion ifthe plant parts are left intact, which is obviously a very slow process. Conse-quently, the plant material must be cut to some extent to rupture enough oilglands and meanwhile reduce the thickness of the material for the steam todiffuse, thus increasing the vaporization rate of the essential oils. The extent ofcomminution required varies with different plant parts. Seeds or fruits must bethoroughly crushed in order to disrupt as many of the cell wall as possible soas to make the oil directly accessible to the steam. This can be achieved bypassing them through a roller as in oilseeds processing. Root, stalks, and allwoody materials should be cut into small pieces to expose a large number ofoil glands. They can be processed by a hay or ensilage cutter, which reducesthe long natural parts of the plant to short lengths to be more easily handled fordistillation. On the other hand, flowers, leaves, and other thin parts of the plantcan be distilled without comminution since the structures in these parts are thinand permeable enough to allow rapid evaporation of the oil.

The crushed or chopped material is ready for distillation. Three types ofhydrodistillation are used in the essential oil industry: water distillation, waterand steam distillation, and direct steam distillation. In water distillation, thematerial makes direct contact with boiling water by floating on the water or

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being immersed. The water can be brought to boil by fire, steam jacket, or coil,or even perforated steam coil. Some plant materials, such as almonds and roseflowers, can only be distilled in this manner since upon direct contact withsteam they tend to agglomerate and form compact lumps, making it hard for thesteam to penetrate. In water and steam distillation, the plant material is sup-ported on a perforated screen placed above the boiling water, so that it is con-tacted by the steam instead of water. The steam remains saturated in this system(Fig. 13). In steam distillation, live steam (saturated or superheated) is appliedthrough open or perforated steam coil. The choice among these distillation meth-

Figure 13 Retort for water and steam distillation of essential oils (30).

Fats and Oils from Plant Materials 211

ods is usually based on both their efficiency and their effect on the essentialoils. Although higher steam pressures result in increased vaporization rate ofthe oils, the higher temperatures of the pressurized steam may cause thermaldecomposition of the oils.

The equipment for distillation of essential oils consists of three parts: theretort, the condenser, and the oil separator. The retort serves as a vessel wherethe steam or boiling water contacts the plant material to vaporize its essentialoils. It can be as simple as a cylindrical container with a removable cover thatcan be clamped on the cylindrical section. A pipe is attached to the top of theretort to lead the vapor to the condenser. In the case of steam distillation, traysare placed close to the bottom of the container, and the steam is introducedthrough a open steam line. In multitray retorts, each tray is only filled with arelatively shallow layer of the material to ensure a uniform distribution of boththe material and the steam. The height of a typical retort is 1.5–1.8 m. It isinsulated to minimize heat loss.

The oil vapor is converted to liquid and cooled down using a condenser.The most common condensers, used in the production of essential oils are coilcondensers with the oil vapor and the steam passing inside the coil while coldwater enters from the bottom of the condenser and flows outside the coil againstthe steam and oil vapor as shown in Fig. 14.

The third important part of the distillation equipment is the oil separator,which, as indicated by its name, separates the essential oil from the condensedwater. Many separators are built based on the principle of the ancient Florentineflask (Fig. 15). The condensate flows from the condenser into the separatorwhere the water and the oil separate into two layers since they are immiscibleand different in specific gravity. The oil is usually lighter and floats on top ofthe water. Both layers are then removed continuously from the separator throughoutlets at different levels.

In addition to distillation, essential oils can also be processed by extrac-tion. Extraction with cold fat is practiced with jasmine and tuberose flowers fortheir essential oils. Although this method gives a much greater yield than othermethods, it is a lengthy and labor-intensive process. Extraction with volatilesolvents is considered as the most technically advanced process, producing con-centrates and alcohol soluble “absolutes,” the aromas of which truly representthe oils in their natural forms. Citrus oils, for instance, are often extracted withalcohol, from which two layers are obtained: one hydrophobic layer containingthe terpene hydrocarbons and one aqueous alcohol layer with the polar com-pounds. They are separated and the polar compounds recovered.

The essential oils may be used as collected or subjected to further purifica-tion, concentration, or separation. Vacuum distillation is the preferred methodfor refining most essential oils. With this technique the oil components can be

212 Xu and Diosady

Figure 14 Coil condenser (30).

fractionated according to their boiling points. Rectified, folded, and terpenelessoils are made through the selection of the various fractions obtained from redis-tillation.

Extraction with supercritical carbon dioxide or propane is now practicedon small scales for highly valued components of plants in order to produceeither flavor or aroma concentrates in the form of oils or resins.

Fats and Oils from Plant Materials 213

Figure 15 Florentine flask for oils lighter than water (30).

REFERENCES

1. FA Norris. Extraction of fats and oils. In: D Swern, ed. Bailey’s Industrial Oil andFat Products, 4th ed., Vol. 2. New York: John Wiley & Sons, 1979, pp. 175–252.

2. RE Treybal. Mass Transfer Operations, 3rd ed. New York: McGraw-Hill, 1980.3. YH Hui, ed. Bailey’s Industrial Oil and Fat Products, 5th ed., Vol. 4. New York:

John Wiley & Sons, 1996.4. E Guenther. The Essential Oils, Vol. 1. New York: D. Van Nostrand, 1948.5. DF Othmer, JC Agarwal. Extraction of soybeans—theory and mechanism. Chem

Eng Progr 51:372–378, 1955.6. DF Othmer, WA Jaatinen. Extraction of soybeans. Mechanism with various sol-

vents. Ind Eng Chem 51:543–546, 1959.7. RE Treybal. Mass Transfer Operations, 2nd ed. New York: McGraw-Hill, 1968,

pp. 15–37.

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8. DF Boucher, JC Brier, JO Osburn. Extraction of oil from a porous solid. Trans AmInst Chem Eng 38:967–993, 1942.

9. HP Fan, JC Morris, H Wakeham. Diffusion phenomena in solvent extraction ofpeanut oil. Effect of cellular structure. Ind Eng Chem 40:195–199, 1948.

10. JO Osburn, DL Katz. Structure as a variable in the application of diffusion theoryto extraction. Trans Am Inst Chem Eng 40:533–556, 1944.

11. RE Treybal. Mass Transfer Operations. 2nd ed. New York: McGraw-Hill, 1968,pp. 628–675.

12. EG Shreibel. Calculation of liquid-liquid extraction process. Ind Eng Chem 46:16–24, 1954.

13. RE Treybal. Mass Transfer Operations. 2nd ed. New York: McGraw-Hill, 1968,pp. 408–488.

14. ISTA. Oil World Statistics Update. Hamburg, Germany: Mielke, 1998.15. NOV Sonntag. Sources, utilization, and classification of oils and fats. In: D Swern,

ed. Bailey’s Industrial Oil and Fat Products, 4th ed., Vol. 1. New York: John Wi-ley & Sons, 1979, pp. 271–288.

16. AL Ayers, JJ Dooley. Laboratory extraction of cottonseed with various petroleumhydrocarbons. J Am Oil Chem Soc 25:372–379, 1948.

17. NFPA 36. Solvent Extraction Plants. Quincy, MA: National Fire Protection Associ-ation, 1993.

18. MA Williams, RJ Hron Sr. Obtaining oils and fats from source materials. In: YHHui, ed. Bailey’s Industrial Oil and Fat Products, 5th ed., Vol. 4. New York: JohnWiley & Sons, 1996, pp. 61–157.

19. IJ Duncan. Commercial extraction of vegetable oils by means of trichloroethylene.J Am Oil Chem Soc 25:277–278, 1948.

20. OR Sweeney, LK Arnold. A new solvent extraction process for soybean oil. J AmOil Chem Soc 26:697–700, 1949.

21. FC Magne, EL Skau. Phase equilibrium data pertaining to the extraction of cotton-seed oil with ethanol and 2-propanol. J Am Oil Chem Soc 30:288–291, 1953.

22. RK Rao, LK Arnold. Alcoholic extraction of vegetable oils. IV. Solubilities ofvegetable oils in aqueous 2-propanol. J Am Oil Chem Soc 34:401–404, 1957.

23. RK Rao, LK Arnold. Alcoholic extraction of vegetable oils. V. Pilot-plant extrac-tion of cottonseed by aqueous ethanol. J Am Oil Chem Soc 35:277–281, 1958.

24. EW Lusas, LR Watkins, SS Koseoglu. Research in replacing hexane with isopropylalcohol as an extraction solvent. Annual Meeting of American Oil Chemists’ Soci-ety, Chicago, 1991.

25. GR List, JP Friedrich, DD Christianson. Properties and processing of corn oilsobtained by extraction with supercritical carbon dioxide. J Am Oil Chem Soc 61:1849–1851, 1984.

26. GR List, JP Friedrich. Processing characteristics and oxidative stability of soybeanoil extracted with supercritical carbon dioxide at 50°C and 8,000 psi. J Am OilChem Soc 62:82–84, 1985.

27. GC Mustakas. Recovery of oil from soybeans. In: DR Erickson, EH Pryde, OLBrekke, TL Mounts, RA Falb, eds. Handbook of Soy Oil Processing and Utiliza-tion. St. Louis: American Soybean Association, 1980, pp. 49–66.

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28. CH VanEtten, ME Daxenbichler, IA Wolff. Natural glucosinolates in foods andfeeds. J Agric Food Chem 17:483–491, 1969.

29. AJ Haagen-Smit. The chemistry, origin, and function of essential oils in plant life.In: E Guenther, ed. The Essential Oils, Vol. 1. New York: D. Van Nostrand, 1948,pp. 15–83.

30. E Guenther. A. Distillation of essential oils. In: E Guenther, ed. The Essential Oils,Vol. 1. New York: D. Van Nostrand, 1948, pp. 85–187.

7Proteins from Plant Materials

Lei Xu and Levente L. DiosadyUniversity of TorontoToronto, Ontario, Canada

I. INTRODUCTION

As the world population continues to grow exponentially, the demand for foodproteins will inevitably increase substantially. It has been forecast that withinthe next decade, in order to just maintain the present level of human nutrition,the supply of vegetable proteins must be doubled together with a fourfold in-crease in the production of animal-based proteins (1). Since such a dramaticgrowth in meat production is hard to achieve, human diet will likely rely moreon vegetables and cereals as protein sources. In addition to continuing to sustainof human life as they have done for thousands of years, plant-based proteins (orvegetable proteins) offer great potential as functional food ingredients. In thepast decades, many processes have been developed to produce proteins to beused as meat extenders in processed meat products or as protein-enrichingagents in beverages. In the processing of vegetable proteins, much attention ispaid to the preservation or improvement of their nutritional value as well asorganoleptic and functional properties.

Storage proteins in oilseeds, legumes, and cereals have been the focus ofprocess and product development for food and industrial protein products. Pro-tein products can be produced by the preferential extraction of nonprotein seedcomponents, as is the case with soy protein concentrate, or by the dissolution,purification, and recovery of proteins in the form of protein isolates.

For protein isolate production a number of aqueous and some nonaqueousorganic solvents have been investigated (1). The storage proteins include a num-ber of albumins, globulins, prolamines, and glutelins with a variety of molecularweights and a range of solubility characteristics. Albumins are typically soluble

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in water (pH 6–8), globulins in dilute salt solutions, prolamines in ethanol, andglutelins in dilute acid or alkali. Most of these proteins are highly soluble instrong alkali, although their functional properties may be altered by partial orcomplete denaturation at these extreme conditions.

Soy proteins have been used as traditional foods in the orient. Accord-ingly, they have been studied extensively, and there are numerous commercialprocesses for the production of a wide range of commercial soy protein prod-ucts. Many other protein sources were investigated in attempts to find alterna-tives to soy proteins. This was driven by the functional, nutritional, and aller-genic problems of soy proteins, as well as the desire to develop value-addedproducts from a number of underutilized plants or plant products, such as oilseedmeals after oil extraction. Significant research advances were made in the pro-duction of wheat gluten, corn zein, and a number of oilseed proteins from pea-nuts, cottonseed, canola, and others.

In cereal grains the main storage proteins are usually the alcohol solubleprolamines, while the salt-soluble globulins are abundant in oilseeds and le-gumes (2, 3). Regardless of their origins, these globulins all belong to twoclasses of different molecular sizes, usually denoted in the literature by theirsedimentation coefficients as 7S and 11S (1).

Most protein extraction processes are based on aqueous extraction usingsingle- or multistage batch operation. The latest advances made in the methodsfor protein extraction feature the combinations of solvents with optimized ex-traction parameters and advanced separation technologies such as membraneprocessing to increase both the yield and quality of the protein products.

In addition to their main use as functional ingredients in many food sys-tems, proteins from vegetable sources are also used in some industrial processesas size, adhesive, or dispersive agents.

II. SOY PROTEINS

The processing of soybean proteins has evolved since ancient times. For nearly15 centuries people in Asia have combined the use of rice and the products ofsoybeans to produce an inexpensive diet that is reasonably well balanced interms of essential amino acids. They learned that the greatest nutritional valuefrom soybeans is obtained through a water extraction followed by coagulationof the resulting milky liquid into a curd. Crude as this ancient method is, it hasprovided the basis for the development of many modern processing methods.

Nutritional studies show that soy proteins are an excellent source of foodproteins. The amino acid composition of soy protein isolate nearly meets theessential amino acid pattern set by FAO/WHO/UNU as shown in Table 1 (3,4). In combination with rice, the low methionine level of soy and the low lysine

Proteins from Plant Materials 219

Table 1 Comparison of Essential Amino Acid Content of Soy Protein Isolatesto WHO/FAO/UNU Requirements

WHO/FAO/UNU suggestedEssential amino acidpattern (mg/g protein)

content of isolated soyEssential amino acid 2–5 years 10–12 years Adult protein (mg/g protein)

Histidine 19 19 16 26Isoleucine 28 28 13 49Leucine 66 44 19 82Lysine 58 44 16 63Methionine and cysteine 25 22 17 26Phenylalanine and tyrosine 63 22 19 90Threonine 34 28 9 38Tryptophan 11 9 5 14Valine 35 25 13 50

Source: From Ref. 3.

level of rice are both overcome. The high digestibility of soy proteins has beendemonstrated by extensive nutritional studies on both animals and humans. Cur-rently, four major classes of soy protein products are in commercial production:flour, protein concentrates, protein isolates, and textured products. Their totalproduction exceeds one million tonnes.

Soy flour can be made from either full-fat dehulled soybeans or dehulleddefatted flakes. Full-fat soy flour contains fat in excess of 18%, while the oilcontent of defatted flour is usually less than 1%; thus, the protein content ismuch higher than that of full-fat flour. To process full-fat flour, high-quality,sound, clean, yellow soybeans are selected. Conventionally, these beans are firstconditioned to a desirable moisture level and then cracked in roll mills. Thecracked beans are dehulled by aspiration and screening. The meats are cookedat temperatures above 93°C by steam in another conditioner to inactivate en-zymes such as lipoxygenase. After being dried, they are ground in two stepsusing a hammer mill to particle sizes small enough to pass through a 0.149-mmscreen (No. 100 U.S. Standard Screen) (5).

The production of defatted soy flour is directly linked to oil extractionfrom soybeans, which involves solvent extraction and desolventizing. The defat-ted and desolventized soy flakes with an appropriate moisture content are thenground in a hammer mill until 97% passes through a 0.149-mm screen (No. 100U.S. Standard Screen).

The term protein concentrate refers to products containing at least 70%protein produced by alcoholic extraction of carbohydrates, leaving behind the

220 Xu and Diosady

insoluble proteins, whereas protein isolate is usually used to describe productswith more than 90% protein produced by the extraction, purification, and recov-ery of soy proteins. Although both protein concentrates and isolates can be madefrom full-fat soybeans, the products find limited use due to their appreciable fatcontent. Therefore, manufacturing of these protein products usually start withdefatted soy flour after the grinding of the defatted flakes to facilitate subsequentextraction. The dehulled flour has a protein content of about 50%.

Protein concentrates are produced by washing the meal to remove solublecarbohydrates with dilute acid, aqueous ethanol, or hot water, as none of thesedissolves significant amounts of soy protein. With dilute acid, the pH of theextraction solution is adjusted to 4.5, the observed isoelectric point of soy pro-teins, where the solubility of soy proteins is minimal (6). The insoluble materialcontaining most protein is thus separated from the soluble impurities in a batchoperation that is repeated several times. The effect of ethanol is similar as mostproteins are insoluble in 60–80% aqueous ethanol; thus, soluble oligosaccha-rides are extracted and separated (7). Aqueous alcoholic extraction also causesdenaturation of the trypsin inhibitors present in the seed and removes some ofthe undesirable phenolic components, thus improving the quality of the product.The ethanol is removed from the protein concentrate and recovered by flashdesolventizing. For hot water extraction, the starting material is first toasted todenature the proteins and decrease their nitrogen solubility index (NSI), andthus decrease protein solubility, while maximizing carbohydrate dissolution (8).In the treatment, the soy flakes are extracted with hot water at 66–93°C in eithera batch or a continuous countercurrent manner, and pH is maintained in therange of 5.3–7.5.

The isolation of soy proteins takes an approach opposite to protein con-centration (5, 9–11), wherein the protein is dissolved while most of the impuri-ties are left behind in the solids. A generic process is outlined in Fig. 1. Theextraction is carried out with dilute alkali, pH 8–10, usually at elevated tempera-tures of 50–55°C. Although more protein could be extracted in more alkalinesolutions, protein tends to deteriorate faster at high pH values. Extraction condi-tions, including pH, temperature, liquid-to-solids ratio, and additional reagents,vary among different manufacturers, and information about most of the detailedoperating conditions remains proprietary. After extraction, separation of the aque-ous extract from the solids is achieved by screening, centrifugation or decanting,and polish-filtration. Optionally, the extract may be further clarified by chemicalpurification by adsorbents, depending on the process adopted by the manufac-turer. A food grade acid is then used to acidify the extract containing the solubleprotein portion to pH 4–5. The most commonly used acids are acetic, hydrochlo-ric, phosphoric, and sulfuric. Upon acidification a protein precipitate is formed,collected by centrifugation or filtration, and washed. The precipitate is usuallyneutralized with dilute food grade alkali to form sodium proteinates, which can

Proteins from Plant Materials 221

Figure 1 Commercial production of soy protein isolates (42).

be modified by jet cooking at 141–160°C, followed by treatment with a proteo-lytic enzyme for 1–30 min to improve the wettability and dispersibility of theproduct (10). Although spray drying is the preferred drying method, the productmay be dried by other means such as a forced draft oven or a range drier. Inthe end, about one-third of the starting mass is recovered as a protein isolatecontaining more than 90% protein on a moisture-free basis (N × 6.25), one-thirdremaining as the insoluble residue, and one-third in the whey solution. Afterdrum drying, the insoluble residue is used as a feed ingredient. The whey solu-tion contains several undesired flavor components and trypsin inhibitors thatmust be heat inactivated before it can be used even for feed.

Despite the generally desirable nutritional properties of soy protein con-centrates and isolates, these products are typically powdery and do not exhibitthe consistency required for palatable foods. In order to expand their use, novelprocesses have been developed to obtain soy protein products with the moredesirable meat-like textures. Technologies involved in these processes includeextrusion and spinning. In the 1960s, an extrusion cooking process was devel-oped for the production of a fully toasted, full-fat, and texturized soy flour (12).The extrusion of soy protein is similar to other food extrusion processes. Theextruders for texturizing are designed to handle high-moisture materials. In theprocess, the dehulled flakes are first cooked with steam at 93–100°C and pre-conditioned to a moisture content of 18%. The cooked material is then passed

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through a high-speed mixer where more steam is mixed with the flakes beforeentering a Wenger extruder, wherein the material is pushed along inside a barrelby a rotating screw while being heated by friction or indirect steam. High tem-peratures of 121–143°C are reached for 1–1.5 min within the barrel. At thesehigh temperatures some hydrogen bonds break, unfolding the structured pro-teins, which are then elongated and aligned by shear forces. When the melt isforced through the die at the end of the barrel, it is rapidly expanded by therelease of steam upon leaving the extruder, producing a porous matrix. As theproteins cool, new hydrogen bonds form to give the product a fibrous, meat-like texture. The cooled product is ground in a pin mill to desirable particlesizes and used in foods such as “bacon bits.”

The spinning process was originally developed in the synthetic textile in-dustry for the production of nylon fibers. It has been adapted to the commercialproduction of soy protein fibers (10, 11). A typical process for soy proteinspinning is shown in Fig. 2. A soy protein isolate is first dissolved in strongalkali to make a “dope,” which is a solution with a protein concentration ofabout 20% at a pH of 12–13. After filtration, the dope is pumped through aspinneret made of a platinum plate with thousands of small holes less than 1mm in diameter into a coagulating bath. As the dope contacts the acid and saltin the coagulating bath, the protein precipitates, forming protein fibers. Phos-phoric acid and NaCl are usually used as the precipitating agents in the bath.To continue the texture-forming process, the fibers are combined to make towsabout 0.5 cm in diameter to be stretched. The stretched tows are then washedto remove the acid and salt, and heated to be hardened. Binders such as eggalbumin are added to improve the cohesiveness of the product, followed by theaddition of fat, flavor, color, and supplementary nutrients in order to obtain apalatable food. Because the final product has a texture similar to that of pro-cessed meat, it is used as a meat analogue.

The functional properties of these soy protein products play a critical rolein determining their use in food systems. These include solubility, hydration andwater absorption, viscosity, gelation, emulsification, foaming properties, and or-ganoleptic properties such as color and flavor. Commercially available soy pro-teins exhibit a wide range of solubilities at pH 7 from 25% to 95% (13), andthe solubility is dependent on pH and salt concentration. While higher pH resultsin high solubility, the effect of elevated salt level is to reduce solubility. Com-mercial soy proteins also have a wide range of water binding ability (14). Insolu-ble protein granules bind much more water than soluble proteins. Slurries ofsoy proteins have relatively low apparent viscosity and do not form strong gelnetworks; however, heat or alkali treatments can increase viscosity and improvegelation functionality (15). As with other functional properties, commercial soyproteins display a range of emulsion and foam properties. Kolar et al. (10)reported a range from 10 to 40 mL oil/100 mg protein for oil binding. Unlike

Proteins from Plant Materials 223

Figure 2 Spinning process for production of soy protein fibers (41).

other functional properties, color and flavor are primarily due to small nonpro-tein compounds and their interactions with proteins. A number of phenolic com-pounds and their oxidization products have been linked to the yellowish browncolor of soy protein products (16). The products of lipid oxidation catalyzed bylipoxygenase are responsible for the undesirable beany flavor of most commercialsoy proteins (17). Once formed, these compounds bind to proteins and thus arehard to remove. However, for commercial food applications these organolepticproblems can be avoided through careful control of food processing conditions.

Presently a large variety of soy protein products are manufactured com-mercially worldwide. These are used in processed meat products, imitationcheese, whipped toppings, soy milk, nutritional beverages, and baked products.There is also a significant market for the industrial use of soy proteins in paperproducts, adhesives, and polymers.

224 Xu and Diosady

III. GLUTEN

“Gluten” refers to the insoluble portion of proteins in cereal grains, first identi-fied more than 200 years ago in dough from wheat flour, and isolated by simplywashing the dough with water. Much research has been done on gluten and theflour proteins ever since.

The wet impure gluten carries 65–70% water, has a creamy color, andfeels like rubber. When dried, the crude gluten contains 75–80% protein and5–15% carbohydrates, the remaining being lipids. Gluten can be divided intotwo fractions. One fraction is soluble in 60% aqueous alcohol, known as gliadinor prolamine. The other fraction, although insoluble in neutral water, is solublein acidic or alkaline solutions, and is known as glutelin (18).

The nutritional and functional properties of gluten have been well docu-mented. Gluten has acceptable levels of most essential amino acids but is lowin lysine. Its functional properties can be changed by reducing and oxidizingagents, and heat treatment. Oxidizing agents make gluten lose its extensibilityand become brittle, whereas reducing agents increase its extensibility. Glutenmay lose some or all of its “vitality” when subjected to heat treatment.

In early industrial processes gluten was only a byproduct of wheat starchprocessing. The Alsatian method (19) is among the first methods to recovergluten, wherein whole wheat is steeped in water for 1–2 days. The softenedwheat grain is then placed in bags of mesh to permit dissolution of starch whileretaining the gluten. The bags are passed between a series of rolls with progres-sively smaller gaps, thus squeezing out starch but leaving gluten in the bags.The further separation of gluten from the hulls is long, and the yield of glutenis usually low. The Martin process uses wheat flour to make both starch andgluten (20). It starts with making a stiff dough containing about 40% water.After hydration for 1 h, the dough is rolled between fluted rolls under a sprayof water to wash away the starch and leave the gluten in a single, coherentmass. This method produces good-quality gluten with high yield. Another pro-cess, called the batter process, involves making a batter, which is broken upwhile water is added to wash away the starch from the gluten (21). The glutenin this method is recovered as fine curds with more than 80% protein. Althoughdeveloped decades ago, both the Martin and the batter processes are still usedin separating starch and gluten from wheat flour. Figure 3 shows the major stepsinvolved in either method. The gluten is separated from starch milk by a seriesof centrifuges. In the refining step, the lighter low-grade starch is separated fromthe heavier prime starch. The gluten is either flash or spray dried. The massdistribution among products is 59% prime starch, 8% tailing starch, and 15%gluten with a protein content of 80%. Based on the batter process, a continuousprocess has been developed (22). First a slack dough is made by mixing flourwith water at a ratio of 0.7 : 1. Then the dough is heated to 48–57°C, followedby breaking up of the dough in the presence of additional water. The starch is

Proteins from Plant Materials 225

Figure 3 Manufacture of wheat starch (41).

thus largely washed out. The gluten is recovered as curds and separated fromthe starch by screening the slurry.

Other processes include the alkali process (23), in which the wheat proteinis dissolved in 0.03 N NaOH solution by mixing 1 part of flour with 18 partsof the alkaline solution, and after centrifugation to separate starch, the superna-tant is acidified with H2SO4 to produce a precipitate containing 70–90% protein,which has no vitality. This method was modified with NH4OH to replace NaOH(24). As a result, the gluten is recovered with vitality.

The vital gluten is used extensively as an ingredient in yeast-raised bakedgoods, particularly bread. Besides increasing the final protein content in thebread, it also results in a greater volume of better crumb texture and a longershelf life of the bread. Canada has been a major producer and exporter of wheatgluten and contributed significantly to the commercial development of glutenextraction processes.

IV. RAPESEED AND CANOLA PROTEIN PREPARATION

Although its oil has been consumed as food by humans for thousands of years,the idea of using rapeseed protein in food is relatively recent. One obvious

226 Xu and Diosady

reason for the delay is directly related to the organoleptic characteristics ofrapeseed meal after oil removal. It has an unappealing dark green color and anunpleasant taste that may be characterized by both bitterness and astringency.According to modern studies, both problems are caused by phenolic compoundsin rapeseed (16). Nevertheless, due to increasing demand for vegetable oils, theproduction of rapeseed has grown drastically in the last three decades. It is nowranked second in world production, exceeded only by soybeans (24a). The mealis used in animal feed but commands a significantly lower price than soy meal.Nutritional studies have shown that rapeseed proteins have well-balanced aminoacid composition (25, 26), and are particularly high in lysine (27), which is alimiting amino acid in most cereal and oilseed proteins. Due to its abundanceand high-quality proteins, rapeseed should play an important role in meeting theneed of the world’s fast increasing population for food proteins. Consequently,there has been continued research interest in extracting rapeseed proteins forfood.

Rapeseed meal has a protein content of up to 40% after oil extraction;however, its direct use as a food protein source is prevented by its undesirablecomponents. In addition to phenolic compounds, it contains high levels of glu-cosinolates (more than 100 µmol/g defatted meal), which upon hydrolysis re-lease toxic compounds such as isothiocyanates and oxazolidinethiones (27a).Although the genetically improved “canola” varieties have much lower amountsof glucosinolates (20–30 µmol/g defatted meal), their meals are still unaccept-able for incorporation into any food formulations. Canola meal also has a muchhigher phytate content of 3–5% than many other cereals and oilseeds (28, 29).Phytates are undesirable in foods as they tend to strongly bind minerals such asFe and Zn, and make them unavailable for metabolism (30, 31). It is obviousthat before rapeseed or canola protein can be used for food, much or all of thesecompounds must be removed.

Although much process research has been directed to treating the meal, avariety of methods have been developed to isolate high-quality canola proteins.Most of the published methods were based on soy protein extraction technology:aqueous extraction followed by protein precipitation. Unlike soybeans, canolacontains a wide range of proteins, with a broad band of isoelectric points and awide range of molecular weights.

For rapeseed protein extraction, alkaline aqueous solutions have been sug-gested as solvent. While over 80% extractabilities with dilute NaOH solutionshave been reported by several researchers (32–34), these values cannot be at-tained using commercial meal as starting meal. To improve extraction, research-ers used consecutive extractions at differing pH values. Blaicher et al. (35) ex-tracted rapeseed protein at pH 9.5 and 12.0 in two consecutive stages, respectively,and achieved a 92% extractability. Although higher pH results in increased pro-tein extractability, it could cause chemical modification of the proteins such as

Proteins from Plant Materials 227

the formation of harmful lysinoalanine (36) under extremely alkaline conditions(pH > 12). Since rapeseed proteins are predominantly globulins (37), dilute so-dium chloride solution has been explored as a protein solvent. NaCl concentra-tions from 0.2 N to 2 N have been reported (25, 38–40). Extractabilities ob-tained with NaCl solutions were typically lower than those obtained with NaOHmedia. The recovery of salt-extracted protein is complicated by the need toremove the salt by dialysis.

Following the example of soy technology, rapeseed proteins are usuallyrecovered by isoelectric precipitation. The precipitate, which is usually washedand dried, constitutes the protein isolate. A wide range of isoelectric points frompH 2.6 to 10.0 are observed with rapeseed proteins (32, 34, 41, 42), due to theircomplex compositions as well as varietal difference among rapeseed strains.Consequently, protein recovery of single-step isoelectric precipitation is low,and the highest ever reported is 65.7% of the amount of protein extracted at pH11, achieved at pH 3.6 by El Nockrashy et al. (43). Multistage precipitation atdifferent pH values does not significantly increase the yield of protein precipi-tate (34, 44).

Despite extensive studies in the past, no process has been commercializeddue to low protein recovery, poor product quality, and high cost of the process-ing methods. To reduce the product losses and improve product quality, a mem-brane-based process was developed for rapeseed and canola protein isolationstarting with defatted meals (45, 46). It consists of five main steps: alkalineextraction, isoelectric precipitation, ultrafiltration followed by diafiltration, anddrying (Fig. 4). Two protein isolates are produced: soluble and precipitated,with a combined protein recovery of more than 70% of the protein in the meal.Both products are high in protein (>90%), low in phytates (<1%), essentiallyfree of glucosinolates (<2 µmol/g), and have desirable functional propertiescomparable to those of soy protein. This process is simple, economical, andseems commercially viable. Recently, it has been modified with additionalmembrane processing to remove phenolic compounds (47). As a result, bothcolor and taste of the products have been significantly improved, thus makingthe process more attractive. The high quality of both canola protein isolatesgenerated much commercial interest, but commercial production has not yetbeen initiated.

V. OTHER EDIBLE PROTEIN SOURCES AND PROCESSES

A. Cottonseed

Cottonseed kernels contain up to 50% protein. The presence of toxic gossypolin the pigment glands of cottonseeds makes the protein unacceptable for fooduse. Hence, all of the processes for production of cottonseed protein for human

228 Xu and Diosady

Figure 4 Membrane-based canola protein isolation process (66).

consumption involve removal of the pigment glands. A mixed solvent extractionprocess was developed for the simultaneous removal of the oil and pigment(48). In this process, cottonseeds are dehulled and flaked to a thickness of 0.07–0.24 mm in the conventional manner. The moisture content is adjusted to 7–15% by drying of the flakes at 60°C. The pigment glands in the meats remainintact under these conditions, so the gossypol generally does not mix with theprotein of the meats. The flakes are then extracted and washed with a solventmixture made of 53% acetone, 44% hexane or petroleum ether, and 3% water(v/v) at ambient temperature. Both oil and pigments are dissolved in the solventmixture, and the extracted meats could have a protein content up to 90% orhigher, and are essentially free of gossypol.

Proteins from Plant Materials 229

High protein concentrates for human consumption are also produced by aliquid cyclone process (49). Preparation of the seeds entails drying, flaking,disintegrating, and separating by screen and gravity. The cottonseed flakes aremixed with hexane to form a slurry fed to a liquid cyclone for classification andseparation. The underflow from the cyclone contains essentially all of the intactpigment glands whereas the overflow carries the fine solids with the desirableprotein. After filtration and drying, a protein concentrate with a protein contentof 73% is obtained. There has been limited commercial development of thisprocess.

B. Sunflower Seed

Defatted sunflower meal can also be a valuable source of protein for incorpora-tion into food products such as breakfast cereals, processed meat products, andsnack foods, as both a protein supplement and a functional ingredient. Processesfor the isolation of sunflower protein have also been based on the soy proteinisolation process, involving alkaline extraction of the meal to extract protein atpH 9–11, followed by acidic precipitation of protein from the extract at pH3.5–6 (50). The recovered precipitates contain more than 90% protein. How-ever, the products thus obtained readily turn to an undesirable green color. Thisis caused by chlorogenic acid, a phenolic compound in sunflower meal. Once itappears, the green color cannot be removed from the product by any conven-tional technique. It is, therefore, necessary to remove the color-causing com-pound prior to protein isolation. This can be achieved by multistage acid wash-ing of the meal with water adjusted to acidic pH close to that for precipitation(51). Another method is ultrafiltration of the alkaline extract of sunflower mealto remove chlorogenic acid before acid precipitation of protein (52). Both treat-ments lead to a much lighter colored protein isolate, but the complexity and ex-pense of these processes have thus far prevented commercialization of sunflowerprotein isolates.

C. Sesame Seed

Sesame seed is an excellent ancient food source. Its oil content is greater than50%. After oil removal the meal contains about 60% protein, and its dehulledflour is even richer in protein. In commercial processing, sesame oil is extractedby expelling and/or solvent extraction before the recovery of the protein materi-als. However, in a patented aqueous process both sesame oil and protein wererecovered simultaneously (53). The dehulled cracked seed was extracted withcalcium hydroxide solution. The mixture of oil and protein was then either (a)sent to a precipitation tank where the pH was brought down to 4–5, and theresulting precipitate was separated by centrifuge and spray dried; or (b) passed

230 Xu and Diosady

to an oil separator where the oil was removed by centrifuge, and the dissolvedprotein was then recovered as calcium proteinate by spray drying.

D. Peanuts

In peanut processing, it is essential to use the dehulled kernels in good conditionin order to avoid aflatoxin contamination. Skins may also be removed by blanch-ing followed by air aspiration to produce clean cotyledons. Prepress solventextraction is commercially used for oil removal (54). Peanut kernels are groundin a hammer mill, and conditioned to desirable moisture content in a stackcooker. The cooked meats are expelled to prepress the oil. The prepressed mealis ground again and reconditioned before being extracted with hexane. The de-fatted flakes are desolventized in steam-jacketed tubes at temperatures increas-ing from 65°C to 107°C during the process. The desolventized flakes are cooledand ground to a flour that contains about 60% protein.

Processes for making full-fat and partially defatted flakes from peanutkernels involve first grinding the low-moisture kernels to a flour consistency andmixing the flour with water to form an emulsion-suspension, which is heated tohigh temperatures between 93°C and 116°C to inactivate lipoxygenase. Thesolids are drum dried as flakes. Full-fat flakes contain 50% oil and 30% protein,whereas partially defatted flakes have 30% oil and 40% protein (55–57).

E. Lupin

Lupin is a legume with a high protein content ranging from 25% to 44%, de-pending on the variety. In the past its use has been limited to the feed forruminant animals due to the presence of toxic alkaloids. The emergence of ge-netically improved “sweet” varieties low in alkaloids are now making it possibleto use lupin as a protein source in food. Lupin is attractive particularly in areaswhere soybeans do not grow well whereas lupin is abundant, such as in northernEurope. Like soy and rapeseed proteins, lupin protein can also be isolated by aprocess based on alkaline extraction followed by acidic precipitation (58, 59).The protein in defatted lupin flour is approximately 70% soluble under slightlyalkaline conditions such as pH 8.6. Lowering the pH to 4–5 permits precipita-tion of about 80% of the soluble protein. The obtained precipitates contain morethan 90% protein. Although studies show that the protein efficiency of lupinprotein alone is low, it is significantly improved when used in combination withadditional proteins or amino acids (58). Due to their high solubility, lupin pro-tein isolates can be used in protein-enriched drink formulations. In Chile, lupinprotein products have been incorporated into milk-based formulas for schoolbreakfast and lunch programs since the mid-1980s (58). Extensive developmentwork is proceeding in Denmark and Germany. However, there are several hur-dles due to the high toxicity of the untreated seed.

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REFERENCES

1 . MF Marcone. Biochemical and biophysical properties of plant storage proteins: acurrent understanding with emphasis on 11S seed globulins. Food Res Int 32:79–92, 1999.

2. VA Portyanko, NR Sharopova, AA Sozinov. Genetic control and polymorphismof some subunits of storage globulin in cultivated oat. Cytol Gene 31:20–23, 1997.

3. DH Waggle, FH Steinke, JL Shen. Isolated soy proteins. In: RH Matthews, ed.Legumes: Chemistry, Technology and Human Nutrition. New York: Marcel Dek-ker, 1989, pp. 99–138.

4. WHO/FAO/UNU. Energy and protein requirements. Report of a joint WHO/FAO/UNU meeting. Technical Report Series 724. Geneva: WHO, 1985.

5. SJ Circle, AK Smith. Processing soy flours, protein concentrates, and protein iso-lates. In: SJ Circle, AK Smith, eds. Soybeans: Chemistry and Technology, Vol. 1,Proteins. Westport, CT: AVI, 1972, pp. 294–338.

6. L Sair. Proteinaceous soy composition and method of preparing. US Patent2,881,076 (1959).

7. GC Mustakas, LD Kirk, EL Griffin Jr. Flash desolventizing defatted soybeanmeals washed with aqueous alcohols to yield a high protein product. J Am OilChem Soc 39:222–226, 1962.

8. FA Norris. Protein recovery from seed meal. Canadian Patent 697,264 (1964).9. CW Kolar, SH Richert, CD Decker, FH Steinke, RJ Vander Zanden. Isolated soy

protein. In: AM Altschul, HL Wilcke, eds. New Protein Foods, Vol. 5, Seed Stor-age Proteins. New York: Academic Press, 1985, pp. 260–299.

10. WJ Wolf, JC Cowan. Soybeans as A Food Source. Boca Raton, FL: CRC Press,1975.

11. HE Snyder, TW Kwon. Soybean Utilization. New York: Van Nostrand Reinhold,1987, pp. 74–144.

12. GC Mustakas, EL Griffin Jr, VE Sohns. Full fat soybean flours by continuousextrusion cooking. World Protein Resources. Am Chem Soc Adv Chem Ser 57:101–108, 1966.

13. AM Nash, WJ Wolf. Solubility and ultracentrifugal studies on soybean globulins.Cereal Chem 44:183–192, 1967.

14. CW Hutton, AM Campell. Water and fat absorption. In: JP Cherry, ed. ProteinFunctionality in Foods. Washington, DC: American Chemical Society, 1972.

15. N Catsimpoolas, EW Meyer. Gelation phenomena of soybean globulins. I. Pro-tein–protein interactions. Cereal Chem 47:559–570, 1970.

16. FW Sosulski. Organoleptic and nutritional effects of phenolic compounds on oil-seeds protein products: a review. J Am Oil Chem Soc 56:711–714, 1979.

17. JE Kinsella. Functional properties of soy proteins. J Am Oil Chem Soc 56:242–258, 1979.

18. JW Knight. Wheat Starch and Gluten. London: Leonard Hill, 1965, pp. 84–105.19. DA Fellers. Fractionation of wheat into major components. In: Y. Pomeranz, ed.

Industrial Uses of Cereals. St. Paul, MN: American Association of Cereal Chem-ists, 1973, pp. 207–228.

20. RA Anderson. Manufacture of wheat starch. In: RL Whistler, EF Paschall, eds.Starch: Chemistry and Technology. New York: Academic Press, 1967, pp. 53–63.

232 Xu and Diosady

21. CE Rist. Batter process makes wheat flour a source of gluten and starch. Sugar J11(9):26–27, 1949.

22. RA Anderson, VF Pfeifer, EB Lancaster, C Vojnovich, EL Griffin, Jr. Pilot-plantstudies on the continuous batter process to recover gluten from wheat flour. CerealChem 37:180–188, 1960.

23. RJ Dimler, HA Davis, CE Rist, GE Hilbert. Production of starch from wheat andother cereal flours. Cereal Chem 21:430–446, 1944.

24. KL Phillips, HR Sallans. Ammonia process for separating starch and gluten fromwheat flour. Cereal Sci Today 11(2):61–64, 1966.

24a. ISTA. Oil World Statistics Update. Hamburg: Mielke, 1998.25. MT Lo, DC Hill. Evaluation of protein concentrates prepared from rapeseed meal.

J Sci Food Agric 22:128–130, 1971.26. FW Solsulski, G Sarwar. Amino acid composition of oilseed meals and protein

isolates. Can Inst Food Sci Technol J 6:1–5, 1973.27. R Ohlson, K Anjou. Rapeseed protein products. J Am Oil Chem Soc 56:431–437,

1979.27a. CH VanEtten, ME Daxenbichler, IA Wolff. Natural glucosinolates in foods and

feeds. J Agric Food Chem 17:483–491, 1969.28. B Uppstrom, R Svensson. Determination of phytic acid in rapeseed meal. J Sci

Food Agric 31:651–656, 1980.29. M Naczk, LL Diosady, LJ Rubin. The phytate and complex phenol content of

meals produced by alkanol- ammonia extraction of canola. Lebensm Wiss Technol19:13–16, 1986.

30. A Rutkowski, H Kozlowska. Chemical constituents and protein food processingof rapeseed. J Am Oil Chem Soc 56:475–477, 1979.

31. M Cheryan. Phytic acid interaction in food systems. CRC Crit Rev Food Sci Nutr15:297–335, 1980.

32. A Girault. The study of some properties of rapeseed protein with a view to proteinconcentrate production. J Sci Food Agric 24:509–518, 1973.

33. AJ Finlayson, RS Bhatty, SL McKenzie. The effects of extraction solvents on theyields and structures of Brassica sp. meal proteins. Can Inst Food Sci Technol J9:212–215, 1976.

34. L Gillberg, B Tornell. Preparation of rapeseed protein isolates: Dissolution andprecipitation behavior of rapeseed proteins. J Food Sci 41:1063–1069, 1976.

35. FM Blaicher, F Elstner, W Stein, KD Mukherjee. Rapeseed protein isolates: effectof processing on yield and composition of protein. J Agric Food Chem 31:358–362, 1983.

36. P Pfaender. Lysinoalanine—a toxic compound in processed proteinaceous food.World Rev Nutr Diet 41:97–109, 1983.

37. RS Bhatty, SL McKenzie, AJ Finlayson. The proteins of rapeseed (Brassica napusL) soluble in salt solutions. Can J Biochem 46:1191–1197, 1968.

38. DF Owen. Detoxification and isolation of rapeseed protein by aqueous saline ex-traction and isoelectric protein precipitation. US Patent 3,758,452 (1973).

39. RD Siy, FDF Talbot. Preparation of low-phytate rapeseed protein by ultrafiltra-tion: II. Membrane development and testing. J Am Oil Chem Soc 58:1021–1023,1981.

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40. TJA Finnigan, MJ Lewis. Nitrogen extraction from defatted rapeseed, with partic-ular reference to United Kingdom commercial rapeseed meal. J Sci Food Agric36:520–530, 1985.

41. FW Sosulski, A Bakal. Isolated proteins from rapeseed, flax and sunflower meals.Can Inst Food Sci Technol J 2:28–32, 1969.

42. E Keshavarz, RK Cheung, RCM Lui, S Nakai. Adaptation of the three stage ex-traction process to rapeseed meal for preparation of colorless protein extracts. CanInst Food Sci Technol J 10:73–77, 1977.

43. AS El Nochrashy, KD Mukherjee, HK Mangold. Rapeseed protein isolates bycountercurrent extraction and isoelectric precipitation. J Agric Food Chem 25:193–197, 1977.

44. LP Kodagoda, S Nakia, WD Powrie. Preparation of protein isolates from rapeseedflour. Can Inst Food Sci Technol J 6:135–141, 1973.

45. YM Tzeng, LL Diosady, LJ Rubin. Production of canola protein materials byalkaline extraction, precipitation, and membrane processing. J Food Sci 55:1147–1151, 1990.

46. L Xu, LL Diosady. The production of Chinese rapeseed protein isolates by mem-brane processing. J Am Oil Chem Soc 71:935–939, 1994.

47. L Xu, LL Diosady. Removal of phenolic compounds in the production of high-quality canola protein isolates. Food Res Int 35:23–30, 2002.

48. WH King, VL Frampton. Cottonseed meal of improved quality. US Patent2,950,198 (1960).

49. EA Gastrock, EL D’Aquin, PH Eaves. High-protein oilseed, particularly cot-tonseed, concentrate production by a continuous process. US Patent 3,615,657(1971).

50. LP Hanson. Vegetable Protein Processing. Park Ridge, NJ: Noyes Data, 1974, pp.186–214.

51. DE O’Connor. Light-colored protein isolate from sunflower meal by acid washingprior to alkaline extraction. US Patent 3,586,662 (1971).

52. DE O’Connor. Isolating light-colored protein from sunflower meal by alkali ex-traction under an inert gas blanket followed by membrane ultrafiltration. US Pa-tent 3,622,556 (1971).

53. JJ Liggett. Recovery of protein and oil from oilseeds with a saturated calciumhydroxide solution. US Patent 3,476,739 (1969).

54. JG Endres. Legume protein flour and concentrates. In: RH Matthews, ed. Le-gumes: Chemistry, Technology, and Human Nutrition. New York: Marcel Dekker,1989, pp. 139–160.

55. JH Mitchell. Process for making peanut flakes. US Patent 3,689,287 (1972).56. JH Mitchell. Peanut flakes. US Patent 3,800,056 (1974).57. JH Mitchell. Process for making flavorless food extenders derived from peanuts,

and a method of recovering peanut oil. US Patent 3,947,599 (1976).58. J King, C Aguirre, S de Pablo. Functional properties of lupin protein isolates

(Lupinu albus cv Multolupa). J Food Sci 50:82–87, 1985.59. FS Taha, SS Mohamed, AS, El-Nockrashy. The use of soya bean, sunflower and

lupin seeds in the preparation of protein bases for nutritious beverages. J Sci FoodAgric 37:1209–1216, 1986.

8Sugars and Carbohydrates

Pascal ChristodoulouHellenic Sugar Industry S.A.Thessaloniki, Greece

I. INTRODUCTION: THE AIM OF EXTRACTION

Solid–liquid extraction is one of the unit operations of process engineering, theobjective of which is to effect the migration of a substance enclosed in a solidinsoluble matrix to a surrounding solvent, analogous to a desorption. In the caseof sugar beet extraction (for sugar cane extraction see Secs. X–XV, and forstarch extraction see Sec. XVI and XVII), sucrose is present in the form of anaqueous solution (juice) in the cellular structure of the sugar beet the solventwithin the solid matrix is identical to the external extraction medium. To allowthe sucrose to pass through the tissue, the semipermeability of the cell mem-branes must be overcome by thermal denaturation. The exit surface for the diffu-sion is enlarged by slicing the beet. The transport of substance by diffusion(release of sugar and entry of water into the cossettes) results from the concen-tration gradient existing in the cossettes. The driving force of diffusion appearsto be not only the concentration gradient but rather the chemical potential. Asthe extraction proceeds, not only does this gradient diminish, but the concentra-tion of nonsugar substances in the extract increases, so that the whole processmust be stopped at the appropriate point. Extraction is counted as one of themajor stages of manufacturing process in the sugar industry (Fig. 1).

The change over time of the mean sucrose concentration in the cossettesis determined by a number of process parameters. To a large extent these canbe influenced by operational measures (draft, temperature, shape and surface ofthe cossettes, as well as load factor and design of the diffuser). Uncontrolled,on the other hand, are properties inherent in the cellular structure of the beet,which depend on the variety, time of harvesting and storage conditions, as wellas possible diseases and frost damage.

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Figure 1 Block diagram of process units from beet or cane.

236

Sugars and Carbohydrates 237

Over the years numerous researchers have contributed to investigate thetheoretical understanding of the course of cossette extraction and to analyze theeffect of changes in process engineering and design. Pursuit of the theory ofsugar extraction ultimately aims at allowing the most exact simulation of theprocess possible within the context of process control and to achieve its optimi-zation, not only in terms of extraction yield and juice purity, but also in thedesign of the installations (1).

During the extraction of sugar from the sugar beet or sugar cane the physi-cal processes are prevailing. The soluble substances of beet cossettes and caneparticles are extracted, and remaining are the exhausted pulp and bagasse, re-spectively. The aim of extraction should be to win an extract that contains aslow as possible of nonsugars, has a high concentration, and can be further pro-cessed in an economical way. During extraction the destruction of sugar throughthermal or microbiological activity should be avoided (2). The operating param-eters of extraction should ensure good pressability of the pulp (3). The issue ofoptimization of sugar extraction is a dynamic problem. The subject is not simplebecause with the parameters numbering more than four their interdependence isquite complex (4). More information about optimization of beet extraction isgiven in Sec. VIII.

In the technical application of sugar extraction (raw material sugar beetor sugar cane) and of starch and carbohydrates (raw material corn or potatoes)there are some similarities and some differences. One similarity is that for theextraction of sugars warm water of pH 5.5–6.0 (50–55°C in the case of starchextraction from corn, 70–80°C in the case of sugar extraction from sugar beetor sugar cane) in countercurrent direction to the high concentration is used. Adifference is that the solid insoluble matrix varies in every case and the tech-niques of extraction (diffusion, milling, wet milling) are modified accordingly.

The similarities and the differences in the process of extraction are mir-rored also in the technological equipment applied in each case. In the case ofsugar beet and sugar cane, the equipment used for the preparation of the rawmaterial before the main process of extraction is different, i.e., slicing machinesin the case of sugar beet (Fig. 9) and shredders in the case of sugar cane (Fig.28), but the main extractors are similar (Figs. 16, 20, 31).

The equipment used for the extraction of starch from corn presents somesimilarities to the old discontinuous Robert diffusion batteries, which were usedin the beet sugar industry until the end of the 1950s and are now obsolete.

II. COMPOSITION OF SUGAR BEET AND CHEMICALBEHAVIOR OF CONSTITUENTS DURING EXTRACTION

Advances in analytical chemistry in recent decades, particularly the develop-ment of chromatographic and enzymatic methods, have considerably expanded

238 Christodoulou

the understanding of the nonsugar substances in sugar beet. A typical chemicalcomposition of sugar beet is presented in Table 1.

By the marc content of sugar beet is meant the total beet componentsremaining after complete aqueous extraction of the soluble constituents underindustrial processing conditions of temperature, pH value, and duration (6). Themarc content affects the amount of pressed pulp produced and thus is relevantto the mass balance of extraction. Knowledge of the marc content or the volumeof juice derived from it is necessary for the analytical determination of the sugarcontent in beet. Cellulose 0.9–1.2%, hemicellulose 1.1–1.5%, pectin substances0.9–2.4%, and lignin 0.1–0.3% are the principal constituents of sugar beet marcand of the skeletal substance of exhausted pulp.

However, not to be overlooked are significant quantities of proteins (0.1–0.4%), lipids (0.05–0.1%), saponins (0.05–0.1%), and ash constituents (0.1%).More information about the behavior of the beet constituents during extractionis given in the literature (5, 7).

Invert sugar, apart from respiration of sugar, normally accumulates in thebeet during storage at a rate of 10–20 g/(t.d) beet. Invert sugar can be consideredas a quality criterion of beet handling and sucrose loss, i.e., treatment of thebeet in the period after harvesting and during storage. It is important to notethat the organic acids increase in the thin juice owing to invert sugar degradationduring juice purification (8).

A. Morphology of Sugar Beet

Sugar beets, as grown for sugar production, are the storage organs of the bien-nial Beta vulgaris saccharifera in its first vegetative year. The tap roots arespindle shaped or coniform, and have white flesh with a white or bone-colored

Table 1 Chemical Composition of Sugar Beet (5)

Component g/100 g beet

Water 73.0–76.5Dry substance 23.5–27.0Sucrose 14.0–20.0Nonsucrose substances 7.0–9.5Water-insoluble compounds (marc) 4.5–5.0Soluble compounds �2.5Nitrogen-free organic compounds 0.9–1.1Nitrogenous compounds 1.0–1.2Inorganic compounds 0.4–0.5

Sugars and Carbohydrates 239

rind. All sugar beets currently grown in the world have their origins in the so-called white Silesian beet, selected as new cultivar out of a multitude of folderbeet strains by Franz Achard at the end of 18th century. The body of the beet,after elimination of the leaves, is divided into three zones: the epicotyl or crown,the hypocotyl or neck, and the tap root (Fig. 2) (9).

B. Ultrastructure of the Native Sugar Beet Root

The inner structure of the sugar beet tissue is similar to that of other dicot plants.The following cell types can be distinguished among others:

- Meristem cells are young cells that divide.- Parenchyma cells are capable of dividing only in a limited way, with an

average diameter of 40–60 µm. Their main function is sucrose storage,and they account for about two-thirds of all cells in the sugar beet root.

- Phloem cells are involved in the transport of dissolved organic sub-stances.

Figure 2 Beet root showing morphological and technologically important zones (40).

240 Christodoulou

- Treachery elements, of 20–40 µm diameter, serve for the transport of waterand dissolved ions; dead cells, the walls of which are stiffened by ligninincrustations, together form a tubular system, the xylem (Fig. 3).

- Epidermal cells cover the outer beet surface.

The vacuole accounts for about 95% of the total volume of the parenchy-matous cells in sugarbeet. Bounded by a membrane, the vacuole contains almostexclusively dissolved sucrose. The remaining 5% of the inner cell space is takenup by the cytoplasm that contains the cell nucleus and other organelles (Fig. 4).

Sucrose is synthesized in the leaves of the living, unharvested sugar beetplant and transported via the vascular system along the phloem (Fig. 3) to thestorage tissue of the root. Whereas the diffusion of sucrose from the cell wallthrough the adjacent cell membrane into the cytoplasm is only moderately im-peded, the vacuolar membrane presents a barrier of extremely low permeabilityfor sucrose. In the living cell, this barrier is overcome by active, energy-consum-ing transport, which produces the high sucrose concentration of about 0.5 mol/L in the vacuole. In contrast, the sucrose concentration in the cell wall amountsto only about 0.06 mol/L. The resulting osmotic pressure of 0.4–0.8 MPa, de-pending on the metabolic activity, pushes the cell walls together and gives themtheir mechanical stability (10).

In Figs. 37–40 are given computer simulations, showing the electrostaticpotential profiles (Fig. 38) and the hydrophilic and hydrophobic regions of thesucrose molecule. In Fig. 41 is given the hydrophobic topographies for the amy-lose fraction of starch (46, 47). These computer simulations contain a tremen-dous amount of information. The complexity of the molecule surface of sucroseand amylose can explain the complexity of their extraction process.

C. Physical Properties of Beet

The technological value of the beet is affected not only by its components butalso by its morphological and physical characteristics (11). The elastic and plas-tic properties of beet affect slicing and the behavior of the cossettes during theextraction process. These properties are identified above all by the elasticitymodulus, the breaking stress, and the bending capacity (12).

D. Physical Properties of Beet After Denaturation

Denaturation means killing the living plant tissue. This can be done by heat,freezing, chemical action, electroplasmolysis, or ultrasound. The technologicalobjective is to improve the material transport of the substances immobilized inthe living cell through the tissue and into the extraction liquid. In industrialprocessing, denaturation takes place by heat (70–78°C).

Sugars and Carbohydrates 241

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242 Christodoulou

Figure 4 Parenchyma cell from a young sugar beet (4 months old), 1200 times en-larged (40). A, Amyloplast; C, cytoplasm; I, intercellular space; L, lipid vesicle; M,mitochondrion; N, cell nucleus; T, pit; Pl, plasmalemma; V, vacuole.

In principle this leads to continuous alteration of the cell tissue. Not onlyare the cell membranes destroyed but the cell walls also change their innerchemical structure through hydrolytic degradation reactions (molecular chainbreakage and detachment of polysaccharide fragments). Despite the chemicalchanges caused by temperature, the cell retains its physical integrity. Thus, itcontinues to be a barrier to mass transport throughout the subsequent pressingprocess. Following denaturation the tissue has lost its strength and the cell liquidcan be squeezed out because the osmotic pressure exerted on the cell wallthrough the membrane is absent (13).

Conventional sugar technology uses the minimal temperature stability ofthe cell membranes during the heating of the sugar beet cossettes to greater than70°C in the extraction apparatus. Only remaining is the cell wall with a thick-ness of 2 µm, which can be considered from its porous texture as an ultrafiltra-tion of this physical membrane. Figure 5 shows the inner structure of this physi-cal membrane. In this scheme it is possible to recognize the cellulose fibrils andother soft polymers, between them also the pectin, where these fibrils combinein chain formations. In an electron microscope it is possible to see clearly thepores that are formed through the cellulose fibrils (Fig. 6) (14).

E. Fractal Structure of the Beet Tissue

Examination of Figs. 3, 5, and 6 clearly reveals the self-similarity in differentscales that is the fractal structure of the beet tissue. The majority of the biological

Sugars and Carbohydrates 243

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244 Christodoulou

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Sugars and Carbohydrates 245

tissues have a fractal structure, and beet tissue belongs in this category (15). In abiological tissue the fractal chaotic structure ensures the following advantages:

- In very small volumes huge surfaces are found.- In an usual Euclidean solid or surface the diffusion grade of one sub-

stance under the influence of random fluctuations is directly propor-tional to time. In the fractal structure the diffusion grade increases veryquickly at a power of time t n (n > 1). This phenomenon is called over-diffusivity.

- One fractal structure can be constructed in a short time from a simplealgorithm, which is repeated in different scales without significantchanges (16).

The phenomenon of overdiffusivity of beet tissue during extraction by hot watercan explain the observed acceleration of the diffusion rate, especially in thebeginning of the extraction process.

III. MATHEMATICS OF EXTRACTION

A considerable advance was made in the mathematical theory of extraction bySilin (17) when he developed equations relating the various factors affecting ex-traction. He started with Fick’s law, a special case of Fourier’s general diffusionlaw:

ds = D � A �dc

dr� dt (1)

where ds is the weight of the dissolved substance diffusing through the area Ain time dt and dc/dr is the concentration gradient of the dissolved substance.

D is the diffusion coefficient, which depends on temperature according tothe Einstein correlation

D =k � T

η(2)

where k is a constant for the dissolved substance, T the absolute temperature,and η the viscosity of the solution (18). The diffusion coefficient D is a measureof the mass transport velocity in the cossettes. In Table 2 diffusion coefficientsof sucrose from beet tissues are given according to different authors. It must benoted, however, that the values for sugar beet shown in Table 2 do not refer toidentical material, as would be desirable (13).

Schliephake and Wolf (19) investigated the mechanism of sugar extrac-tion, especially during the initial stages. They defined three basic phases: osmo-

246 Christodoulou

Table 2 Diffusion Coefficients of Sucrose fromBeet Tissues (40)

Diffusion coefficient [cm2/(s.10−5)] Temp. (°C)

0.6–1.13 751.0–1.17 750.5–1.0 750.8–1.2 700.82–0.92 700.8–0.88 750.91 65–751.0 75

sis, denaturation, and diffusion (Fig. 33). During the initial phase, water entersthe cell by osmosis. Denaturation increases the permeability of the cell wall,and the juice–water mixture is forced to flow out of the cell by the suddenchange of the cell pressure. Only after the completion of those two initial reac-tions can diffusion be considered the rate-determining process. The duration ofthe initial phase depends on the extraction temperature and corresponds to thetime needed for denaturation. During that time, less than 5 min under normaloperating conditions, more than 20% of the sugar is extracted.

Figure 7 shows the three basic phases, as indicated by the change ofthe relative concentration of sugar and water in the beet slices, during theextraction with water, at two temperatures. The straight line represents thetheoretical extraction, during which the water and sugar concentration shouldremain nearly constant relative to each other throughout the process, if diffu-sion is the only rate-determining factor. The maxima and the minima of thecurves indicate the completion of the first two stages: osmosis and denatur-ation (19).

Only in the ideal case does the extraction liquid pass by the cossettes inlaminar flow. In reality, there is some turbulence that is superimposed on thedesired direction of the material transport along the extraction route and is char-acterized by the axial dispersion coefficient Dax,2. Superimposed on this micro-mixing of the liquid phase is a macromixing caused by the churning of theessentially uniformly moving cossettes and which, analogous to the behavior ofthe liquid, can be represented by an axial dispersion coefficient Dax,1. Accordingto Buttersack and Schliephake (1), the extraction is described by the followingpartial differential equation:

∂c2

∂t= ∂c1

∂t+ u � Φ �

∂c1

∂z− Dax � Φ2 �

∂2c1

∂z2(3)

Sugars and Carbohydrates 247

Figure 7 Change of water, with change of sugar concentration in the beet duringextraction, with water at 60°C and 70°C (18, 19).

where c1 and c2, respectively, represent the sucrose concentration in the cossettes andin the liquid along the extraction route z, while Dax is the axial dispersion coefficientcomprising the effective contribution of macro and micromixing (Dax,1 or Dax,2, re-spectively). The mean cossette velocity is u1 and Φ is the ratio of the differencevelocity u (between cossettes and liquid) and the mean cossette velocity u1.

Φ =u

u1

(4)

Formula (3), being a partial differential equation, has only chaotic solutions (15,45). Buttersack and Schliephake (1) and Christodoulou (15) emphasized thecomplexity of the extraction process.

Other theoretical handling of extraction in beet sugar factory is given byClaasen (6), Dubourg (24), Bruniche Olsen (25), Silin (17), Ebell and Storz(18), and Schneider and Reinefeld (22). The principles of extraction was thepriority subject of the 15th General Assembly of the International Commissionof Sugar Technology (CITS) in Vienna 1975 (26).

248 Christodoulou

IV. TECHNICAL EXTRACTION OF BEET

In all extraction systems, fresh cossettes are heated and subsequently ex-tracted in a countercurrent of liquid and solid phases. The extraction equip-ment is fed at one end with fresh cossettes, and exhausted cossettes are re-moved at the other end. At this end, feed water is introduced and passesthrough the extractor countercurrent to the cossettes. An extraction plant ismade up of (Fig. 8):

- Cossette production, i.e., slicing- Cossettes heating (denaturation or scalding)- Transport of cossettes to the extractor and removal- A countercurrent exchange section- Fresh water supply and separation of exhausted cossettes- Pulp-pressing equipment- Press water return (20)

V. SLICING MACHINES

Only two types of machine out of those which came into being more than100 years ago were successful, namely, the disk- and drum-slicing ma-chines.

A. Disk-Slicing Machines

The disk-slicing machine is characterized by its vertical configuration and hori-zontally rotating slicing disk. The diameter of the disk has been increased overthe years to 2780 mm. With 32 knife blocks in total and a slicing length of 400mm per knife box, the slicing capacity can be raised to about 4000 t/d dependingon the knives used and the cossette thickness.

B. Drum-Slicing Machines

The drum-slicing machine consists of a horizontally rotating drum in which theknife blocks are carried on supporting members (Fig. 9). The knife blocks aredepending on the manufacturer, fitted with two or three row boxes. The boxlength is 600 mm, which means that there are three knives per row, groupedside by side. With a drum diameter of 2200 mm and 22 knife blocks eachwith 3 rows, the maximum output of the machine is 8000 t/d, according to themanufacturer’s claim.

Sugars and Carbohydrates 249

Figure 8 Block diagram of beet extraction process (40).

C. Control of Throughput

The slicing capacity of the machine is proportional to the rotational speed ofthe disk or drum. Control of slicing output is achieved via a belt-weighing scalethat is situated in the conveyor leading to the extractor. This picks up the devia-tion from the preset value and transmits a signal to the rotational speed control-ler on the slicing machine that raises or lowers the slicing rate to the correctlevel. Drum and disk slicers are fitted with frequency inverters. The optimalslicing speed lies in the range of 2.0–5.0 m/s, which corresponds to a drumrotational speed of 25–50 min−1.

D. Cleaning

Both disk- and drum-slicing machine are fitted with special cleaning arrange-ments. These consist of a blow-out device using compressed air, alone or incombination with rotating brushes or high-pressure steam. These arrangements

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Sugars and Carbohydrates 251

should ensure that while the machine is operating, fibers and other small frag-ments are removed from the face of the knives. In this way, the slicing through-out can be maintained over a longer period and the quality of the cossettes canbe kept consistent.

E. Comparison of Types of Slicing Machines

Considering the situation today, it can be said with certainty not only that thedrum-slicing machine yields a greater output but that its technological advan-tages enhance its future prospects.

F. Knives

The quality and shape of cossettes will be determined by the choice ofthe beet knife. Of the many types of beet knives the Konigsfelder knifehas been successful, mainly as a semi-cutting knife. Based on the require-ments of the extraction, beet knives with different numbers of divisionsare used as a corrugated splitter chevron or square cossettes as wanted;then A or B knives synchronized or offset are installed in the knife blocks(Fig. 10).

The same beet knives are suitable for disk-slicing machines and drum-slicing machines. The length of the knife can be 137, 167, or 200 mm (Fig. 11).New slicing machines, independent of the manufacturer, are presently beingequipped with 200-mm knives. There are four steps in the preparation of theknives:

- Cleaning and dismounting of the knife blocks- Straightening and dressing of the knives- Routing of the knives and finishing by filing- Reassembly and adjustment of the knife blocks

The first of these steps will be mainly manual. The other steps are performedeither semiautomatically or automatically. The consumption of the knives de-pends principally on the preparation of the beet and the treatment of the knivesthemselves. The consumption should be of the order of 3–5 knives per 1000 tof beet processed.

G. Ventilation of Slicing Machine Operating Position

Inadequate space can cause aerosols to appear in the vicinity of the machine.Adequate ventilation should be introduced to avoid impairing the health of theoperator (20).

252 Christodoulou

Figure 10 Cutting sequence for corrugated splitting knives (40). Top: synchronized.Bottom: offset.

VI. PROCESS PARAMETERS OF EXTRACTION

A. Quality of Cossettes

The size and physical condition of the cossettes are of great importance forextractor performance. Since time is required for sugar to diffuse through agiven distance, advantage is gained by shortening the distance. The Silin numberand the Swedish number serve as indicators of cossette quality.

1. Silin Number

Length of 100 g of cossettes in meters, thus giving a standard for the fitnessand surface area of the cossettes. This parameter depends on the number of

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254 Christodoulou

knife divisions, type of knife, and clearance to the fore layer; it ranges between10 and 18.

2. Swedish Number

Standard for the permeability of the cossettes’ bed. Measured by the quotient:

Mass of cossettes >5 cm long

mass of cossettes <1 cm long

The ratio of the portion of the cossettes >5 cm long to that <1 cm long shouldbe greater than 10.

3. Mush Content

The mush content is the mass of cossettes <1 cm long in relation to the totalcossettes’ mass. This should not exceed 5%.

4. Draft

The ratio between the beet processed and the raw juice extracted expressed inpercentage, either in terms of mass or volume (normally not used), is referredto as the “draft” (21).

Draft =weight of diffusion juice

weight of beets× 100

Drafts used vary between 100 and 135; sugar lost in pulp and draft varies in-versely. Steam cost and evaporator capacity have to be balanced against the addi-tional sugar obtained (18). Their quality must be adapted to the extractor in use.

Percolation rate across a cylinder packed with cossettes decreases rapidlyif the Swedish number falls below 10. Maximal percolation rates are obtainedat values >15. The rigidity (turgidity) of cossettes has a considerable influenceon cossette permeability. This depends on the quality of beet and can deteriorateas a result of storage or frost.

Process parameters that can modify cossette rigidity are the extractiontemperature and the pH value, as well as the feed water pH value and the cationcomposition. Foaming of the liquid phase and the presence of fines can alsoreduce the permeability (21).

5. pH Value

The cell juice of sound beets varies between 6.3 and 6.6. In the absence of adiffuser infection the pH of the diffusion juice is about 0.1 unit lower than thepH of the cell juice. Within the diffuser a lower pH may have beneficial effects.At a pH of 5.5 the diffusion of impurities appears to be at a lower level. This

Sugars and Carbohydrates 255

might be a result of degradation of the pectins at higher pH. The exhausted pulpis firmer and permits more efficient removal of the water in the pulp presses.Considering corrosion, a pH of 6.0 or slightly below 6.0 within the diffuserappears to be a good compromise. To obtain this, not only the supply water butalso the press water may have to be acidified (18).

Acids used for pH control in extraction are sulfuric, sulfurous, or rarelyhydrochloric acid (a highly corrosion-resistant static mixer is needed); 95% ofthe anions of these acids are found in the raw juice. Sulfuric and sulfurous acidsare precipitated to an extent of about 50% in the juice purification (21).

6. Temperature

The temperature levels in the extractor are generally between 68°C and 75°C.The target temperature in extraction is dictated by several variables:

- Denaturation of the beet cells.- Protection against microbiological development, which is achieved

above 73°C.- Rate of extraction.- Limiting of the thermal degradation of the beet marc, especially the

cellulopectic skeleton. To denature cossettes should be heated asquickly as possible to 70–75°C.

Extraction accelerates as temperature rises because the diffusion rates increasewith increasing temperature; for example, a fall in the sucrose losses from 0.25 to0.20 g/100 g beet at equal draft can be mathematically forecasted for a rise intemperature from 69°C to 73°C. Sugar losses from microbiological activities alsodecline with rising temperatures. Extraction above 75°C is impossible because ofthe inherently labile nature of beet at elevated temperature. This is due to thebreakdown of the cell wall substances, especially through pectin degradation.

Targeted average temperatures for normal beet material are about 70–73°C. Under technical conditions, taking into account the extraction losses, me-chanical properties of the cossettes during their transport through the extractor,pressability of the cossettes, as well as the filtration properties in juice purifica-tion, one will gradually approach the optimal operating temperature. Excessiveoperating temperatures affect the processing of healthy beet negatively. Muchmore attention has to be paid to the operational temperatures in the case ofdeteriorated beet (21). Schneider and Reinefeld proposed a temperature profileas displayed in Fig. 12 for healthy mature sugar beet (22).

7. Time

The time during which the cossettes are in contact with the surrounding liquidis considered the extraction time. The average extraction time should not exceed

256 Christodoulou

Figure 12 Optimal temperature profile for juice extraction using normal, healthy beet(40).

75 min. The breakdown of cell wall substances (e.g., pectin) increases withincreasing average extraction time and poor retention time distribution. Thisleads to deteriorating juice and pulp quality. Short extraction times lead to im-proved utilization of the equipment (21).

8. Bacterial Control

The very broad range of sugar losses due to microbial activity found in practice,i.e., from 0.05% to 0.2% on beet, is certainly due to operating conditions. Mini-mization of sucrose losses or formation of acids, improved pulp dewatering, andavoidance of the addition of bacteriostats are arguments either in favor or notin favor of bacterial control. Bacterial control in extraction equipment is com-mon practice with formaldehyde application as a standard procedure in coun-tries, where its use is allowed. However, other substances, such as SO2, carba-mates, quaternary ammonium compounds, and hydrogen peroxide, have alsobeen tested and applied. Avoiding the use of any bacteriostat has been practicein the Japanese and Austrian sugar industries. With deteriorated beet that hasundergone freezing, significant activity by mesophilic microorganisms can oc-cur. Notably, lactic acid bacteria grow under such conditions when no appro-priate control is possible (23).

Sugars and Carbohydrates 257

Hein and Pollach proposed the use of products from hops extracts to in-hibit thermophilic microorganisms while avoiding the potential health hazardsusually associated with the application of bacteriostats (44).

9. Extraction Water

Historically the first Roberts discontinuous batteries were fed with fresh purewater. All the extracted water from pulp straining and pressing was discardedas waste water. Nowadays for environmental as well as economic reasons thepress water is reintroduced into the extraction system. The mass balance in Fig.21 gives an example of the press water and fresh water flows. The more effec-tive the pressing of the pulp, the greater is the mass of press water and the lowerthe necessary complement of fresh water. When the dry substance content ofthe pulp increases, its mass on beet decreases. With increasing pressed pulp drysubstance content, for the same sugar losses on beet, the sugar contents of thepressed pulp and the press water increase. It is possible to introduce fresh waterand press water separately. The fresh water is fed at the tail (top) of the extrac-tion equipment and the press water at the point of the extractor where the liquidphase in the extraction equipment has the same sugar content as the press water.It is also possible to mix the two water feeds and to introduce the mixture atthe tail (top) of the extraction equipment.

Condensates, used as fresh water, are alkaline and contain ammonia. It isnecessary to acidify them so that the pH value at the tail (top) of the extractoris below 6. The desired temperature is obtained by mixing cold and warm con-densates. In the case of feeding the extraction with mixed water, it is possibleto control the temperature of the mixture by adjusting the amount of the differentkinds of water without reheating (21).

10. Pulp Press Water Treatment

Pulp press water contains colloids and small pulp particles, which in some casesare eliminated by a pulp screen before passing a heat exchanger and reintroduc-tion into the extractor. Because of the heat losses during pulp transport andpressing, press water is generally obtained at below the extraction temperature.At press water temperatures thermophilic bacteria can develop. To fight thiscause of infection, heating the press water before its reintroduction into theextraction equipment is desirable. Complete pasteurization requires heating toabout 90°C for the required time. In fact, this is rarely done, but some heatingis always done (21).

11. Pressing Aids

To improve the pulp pressing, pressing aids are used. Generally these pressingaids are di- or trivalent cations—Ca2+ or Al3+—which are mostly added as sul-

258 Christodoulou

fates. These polyvalent cations increase the rigidity of the pulp. Aluminum sul-fate is added as a water solution. Calcium sulfate can be produced in the factoryby reacting sulfuric acid with milk of lime or carbonation lime. Natural gypsumis also used. The effect on the dry substance content of the pulp is maximal fora consumption of aluminum sulfate of about 350 g/t beet (i.e., 6 eq Al3+ /t beet)and for a consumption of calcium sulfate of about 1000 g/t beet (i.e., 13 eqCa2+ /t beet).

The addition of such pressing aids modifies the ionic composition of theliquid phase by yielding higher concentration of Ca2+ or Al3+ throughout theextractor. Fixation of these ions begins when the cossettes mix with the juiceso that the rigidity of the cossettes as well as the permeability of their mass areboth improved (21).

12. Antifoaming Agents

The beet root contains surfactive components that pass into the liquid phase inthe extractor. Production of persistent foam occurs by air inclusion within thecossettes’ mass. Foam inhibits circulation of the liquid in the extractor, so thatit is necessary to use antifoaming agents. There are many kinds of antifoamingagents, including fatty acids, that are used in the extraction equipment at levelsbetween 0 and 150 g/t of beet (21).

13. Presence of Oxygen During Extraction

Extractor design has an influence on the aeration of the juice–cossette mixture.In this respect there are essential differences between the drum (RT) and belt(De Smet) extractors, as well as between trough (DDS) and tower extractionequipment. Oxidation of polyphenols to melanins, may be different in differentextraction equipment. Polyphenols are readily oxidized to melanins, which areremoved in juice purification. The total polyphenol contents in DDS and RTextractors were about 11 mg/L. The oxygen content of the cossette–juice mix-ture influences the composition of the metabolites in the event of developingbacterial activity (21).

14. Extractor Operation with Deteriorated Beet

The impact of alternating freeze and thaw cycles can lead to considerable degra-dation of the cell structure in sugar beet. Such sugar beets are not suitable fornormal extractor operations. The extraction procedures must therefore be adaptedto the substantially altered beet material even if extraction loss increases. Deteri-orated sugar beets are unsuitable for producing desirable cossettes because themush content increases. Percolation of the extraction fluids through cossetteshaving a high mush content is considerably impaired. Operation with coarsercossettes is therefore necessary to avoid mechanical problems (plugged screens,

Sugars and Carbohydrates 259

extractor plugs). Extractor temperatures are generally lowered (70°C maximum)to minimize further breakdown of the cell wall substances. It could be desirableto reduce the retention time of the cossettes in the extraction fluid, consequentlyreducing the average extraction time. Operating capacity is substantially re-duced; 5% of the beet material is partly degraded. Periodic shock treatmentwith bacteriostats (formaldehyde among others, depending on approval for thisapplication) is recommended to reduce the bacteriological activity. Without suchmeasures being taken, the microorganisms present in the deteriorated beet, beingassociated with the decomposition, will increase substantially (21).

15. Diffusion Juice

The juice drawn from the extractor contains a considerable amount of colloidalmatter besides a large number of fine pulp particles, which are difficult to re-move by screening. It is desirable to eliminate most of the suspended matterbefore the juice is sent to the purification process. The color of the juice isusually gray and changes to a dark gray or almost black on contract with theair. This darkening is caused by enzymatic reactions and is accelerated in thepresence of iron. However, the color of the diffusion juice is of little importanceas it is removed without difficulty during purification. The nonsucrose contentof the diffusion juice is related to the quality of the beets and the conditionsunder which the sugar is extracted in the factory. In some beet-growing areas,storage after harvesting has more effect on the quality of the diffusion juice thannormal variations of the growing conditions as experienced from year to year.

The following percentages of the mineral components are extracted fromthe beet: 80–90% of K and Na, 10–30% of Ca, 60–80% of Mg, and about 80%PO3

4− and SO2

4−. Amino acids and betaine are almost completely extracted, and

the protein and pectin content of the juice is a function of pH and temperature.At higher temperatures more pectins are dissolved, but the protein content ap-pears to be at its lowest level at about 70°C.

The following organic non–amino acids are present in larger quantities:citric, oxalic, malic, acetic, and lactic acid. There are only minute amounts oflactic acid found in healthy beets’ acid; its presence in the diffusion juice usuallyindicates a thermophilic infection in the extractor. The invert sugar content inthe juice normally ranges from 0.4% to 0.8% on solids. Higher concentrationsare found during processing of stored beets, up to 2.1% on solids (18).

VII. BEET EXTRACTORS

In the first sugar beet factories, the sugar juice was obtained from the beet bypressing (the procedure followed for centuries with sugar cane). While the struc-ture of sugar cane is extremely fibrous and therefore suitable for pressing, beet

260 Christodoulou

tissue is soft, and not particularly permeable, which makes difficult even a mod-erate exhaustion of the sugar juice contained in the beets by pressing. In theefforts of finding a method other than the uneconomical one of pressing, thefirst to succeed were Florent Robert and his son Julius Robert. After a longperiod of experiment, they succeeded for the first time, in the 1864 campaignat the Seelowitz factory in Mahren, in producing juice on a factory scale bywhat they called “osmotic maceration” (25).

Historically, the Robert battery, also called a “batch diffuser” and nowobsolete, first realized the countercurrent principle with fixed cells where thesolid phase was enclosed and transport of the liquid phases was controlledthrough pipes connected to the cells (21).

Schneider and Reinefeld categorized the continuous extraction systems,which are all working countercurrently, in three classes (22):

1. Controlled transport of both juice and cossettes: cell–extractor (RTextractor), cross-flow filtering belt conveyor extractor (De Smet ex-tractor).

2. Controlled transport of cossettes and uncontrolled transport of juice.This class comprises several chain-type extractors: Silver chain-type,Olier, Oppermann & Deichmann, J-diffuser, and Oliver–Morton.

3. Uncontrolled transport of juice and cossettes: tower extractors [BMA,Buckau Wolf, sloping through extractors (DDS and Silver slopetype)].

A. RT extractors (RT from Raffinerie Tirlemontoise, Belgium)

RT extractors are large revolving drums, separated into “cells” by a helixattached to the interior surface. As the drum with its helix revolves, the juice,which stays at the bottom, is transported from the tail to the head end.

Thus, the cell actually moves, but it is more convenient to consider thecell the location of one turn of the drum. Fixed to the cylinder are grids that,revolving with the drum, sweep the cossettes up until they slide off and fall intothe next cell. Thus, the cossettes and juice travel in opposite directions throughthe cylinder (35). The first extractors of this type were called Berge extractors.

The RT2 extractor was developed from the Berge extractor. It has adouble-helix, or double-threaded, screw, with each screw having twice the pitchof the single-helix screw (21). The diameter of the drum is enlarged at thecossette inlet side. The enlarged shell is perforated and rotates inside a fixedhead, which is not connected to the drum. The cossettes are flushed through thescalding pipe into the mobile head. Two grids are mounted inside the mobilehead and carry the cossettes into the drum with each turn. Scalding juice, to-gether with juice from the diffuser, flows through the perforated head into the

Sugars and Carbohydrates 261

circulating juice tank. Fresh water is introduced through the hollow shaft intothe second compartment and pulp press water is added to the fourth compart-ment, counting from the tail end. Diffusion juice, corresponding to the draft, ispumped from the circulating juice tank to the process. The remaining juice isheated and used for scalding and flashing the cossettes to the extractor (18).

Figures 13 to 15 show the general diagram of an RT2 extractor and itsassociated equipment. The largest RT2 extractors (7 m diameter and about 45m long) have a nominal capacity of 5000 t/d beet, which can be exceeded bymore than 50%. It is impossible to enlarge this model because of geometricaldifficulties with the slope of the cossettes passages and also because of theimportant empty volume inside the drum, which is expensive (21).

The RT4 extractor is directly derived from the RT2 model. In the RT4extractor the redesigned internal structure produces a more continuous move-ment of the cossettes during the rotation of the drum. The capacity of such anew drum is larger and the retention times of both juice and cossettes are

Figure 13 Constitutive elements of an RT2 extractor (40). 1 and 2, Helicoidal platesforming two separate juice channels; 3, transversal plate; 4, slopping passages for cosset-tes; 5, transversal screens.

262 Christodoulou

Figure 14 Juice end of an RT2 extractor (40). 1, Transversal screens; 2, front casing;3, juice and cossettes mixture inlet; 4, frontal and peripheral screens; 5, juice outlet tothe circulation tank.

Figure 15 RT2 extractor (40). 1, Slicer; 2, cossettes belt and belt weighing scale; 3,scalder; 4, circulation juice heater; 5, circulation juice tank; 6, fine pulp separator; 7,RT2 drum extractor; 8, press water tank; 9, fresh water tank; 10, raw juice to prelimer;11, exhausted cossettes.

Sugars and Carbohydrates 263

shorter. Even more simplified is the RT5 extractor, in which the idle times areshorter and the mixing phase is increased by about 7.5% (27).

The most common RT5 extractor has a diameter of 6.25 m and a lengthof 49 m. Its nominal capacity is 7500 t/d with sugar losses of 0.20% on beet ata draft of 115 kg/100 kg beet. It is capable of processing (with higher lossesand draft) up to 12,000 t/d. The largest model, of diameter 7.6 m and length 61m, has a nominal capacity of 12,000 t/d. It is important that the quality of thecossettes gives a good juice separation, which is obtained with a high Swedishnumber (�15) and a small number of fines (21).

B. Plate Conveyor De Smet Extractor

This Belgian equipment is the adaptation by the industry of an extractor ofvegetable oil from seeds using solvents (Fig. 16). It consists of a horizontalperforated plate belt conveyor about 30 m long, which may reach 7 m. On thisconveyor, a 1-m-thick layer of cossettes is placed. The conveyor moves forwardat low speed so that the cossettes are transported from the inlet to the outlet inabout 75 min. Above this layer of cossettes 18 juice distributors are installed.At the tail of the extractor, the last distributors are fed with press water andfresh water (21).

The juice distributors are fed from pumps that are arranged in groups ofthree in one housing on a common shaft and are driven by one motor. Eachpump receives the juice from the preceding slopped collecting hopper, which isinstalled between the upper and the lower belt. Floats are located in every hop-per to prevent the intrusion of air and the excess juice is allowed to overflowinto the next hopper. The cossettes are heated before they are deposited on thediffuser belt. They pass through an inclined trough and are preheated counter-currently with juice from the diffuser. The cooled juice is withdrawn from thistrough and sent to the process. Heated circulation flumes the cossettes througha specially designed pipe to the diffuser belt, where the thickness of the cossettelayer can be regulated by an adjustable dampening device (18).

An equipment is used that is similar to the cane extractor. If the pH valueand calcium concentration of the water are at the right levels, the temperatureinside the extractor can reach 75–77°C without juice percolation problems. Theretention time of the cossettes is about 75 min. The average juice retention timeis between 30 and 40 min. The main operating difficulty with this kind of extrac-tor comes from foam formation. Antifoaming agents (derivatives of fatty acidand products) are generally used at levels of 100–150 g/t of beet (21).

C. Chain-Type Extractors

The first continuous beet extractors, which were constructed in around 1930,were the chain-type Olier extractor in France and the Silver chain extractor in

264 Christodoulou

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Sugars and Carbohydrates 265

the United States (Fig. 17). They consisted of a series of U-shaped cells forminga serpentine tube. Cossettes were transported in the tube by transverse screensmoved by two strong chains. The necessary heat was supplied by steam jacketsfitted around the head end of the extractor. Some other systems of chain extrac-tors were also developed (Oppermann & Deichmann, J-diffusion). These extrac-tors are now generally out of use, chiefly because it is difficult to build for largethroughputs and also because of chain wear (21).

D. Tower Extractors

Two models of tower extractors have been developed simultaneously in Ger-many by BMA and Buckau-Wolf (Fig. 18). The models are comparable in theirprinciple of two main and distinct parts: the countercurrent mixer and the extrac-tion tower. The tower is a 14- to 20-m-high cylinder. Inside the tower, a tubularshaft rotates slowly at 0.2–0.8 min−1. Special steel pieces of helicoidal shape orflights are fitted on the shaft and give the cossettes their upward movement (Fig.19). The juice and the cossettes move countercurrently in the approximately 2-m-wide space between the outer casing and the inner shaft. Steel elements (sta-tionery flights) are fitted on the internal side of the tower wall to prevent thewhole of the cossette mass rotating with the shaft.

Figure 17 Silver chain extractor (40). 1, Cossettes inlet; 2, fresh water inlet; 3, ex-hausted cossettes outlet; 4, raw juice outlet; 5, juice transfer.

266 Christodoulou

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Sugars and Carbohydrates 267

Figure 19 BMA extraction equipment—central shaft and flights (40). 1, Flights; 2,wall arms.

The juice–cossettes mixture at a temperature of 70–73°C is pumped fromthe countercurrent mixer by the variable-speed pump to the tower wall. The circu-lation juice leaves the tower through the bottom screens occupying the wholebottom section area. The BMA tower has additional side screens in the lowertower wall. A revolving “scraper” device cleans the bottom screen from thecossettes. The juice then passes a cyclone sand separator and heat exchangerand enters the countercurrent mixer.

The press water and fresh water enter through two (radial header) pipeswith several inlets. The control of water feed consists of a level sensor in thetower that actuates the fresh water inlet valve. The cossettes are extracted byhorizontal screws placed at a higher level than the water inlet, so that the cos-settes are drained before being removed from the tower (21).

The packing of the juice–cossettes mixture depends on the rotationalspeed of the tower shaft. An increase in the speed reduces the packing, conse-quently decreasing the cossettes retention time and increasing the retention timedispersion. The consequence of these two phenomena is an increase in the sugarcontent of the exhausted cossettes. A lower rotating speed increases the packingin the tower and consequently the retention time, which gives better exhaustion,but it also reduces the permeability of the cossette mass and hence the through-put of the bottom screen. The necessary power for the drive of the tower shaftdepends on the packing of the cossettes in the tower. The power uptake is ameasure of that, as is the torque on the center shaft. The manufacturer recom-

268 Christodoulou

mends a packing of 650–700 kg of cossettes per cubic millimeter. Blocking ofthe bottom screens is controlled by pressure measurement above and below thebottom screens.

The Buckau Wolf tower extractors differ mainly in the following aspects:

- Position of extraction water inlets, which are placed in rotating armsattached to the tower shaft

- Configuration of the bottom screens without side screens- Shape of the bottom scraper inside which the cossettes–juice mixture

inlet is fitted- Slope (form) and size of the rotating transport elements (flights)- Absence of side screens- Heating arrangement; the necessary, flow of juice is picked up at the

outlet of the tower and passes through the heater before being intro-duced in the countercurrent mixer

Tower extractors work with cossettes of Silin numbers between 5 and 7m/100 g. They also need a low content of fine particles to guarantee good juicetransport. As in the case with DDS extractors, the rigidity of cossettes is im-proved by extraction water treatments such as acidification and calcium sulfateaddition. Thus, it is now possible to operate these extractors at temperatures ofup to 72–73°C.

The biggest extractors now constructed by BMA and Buckau have a nomi-nal throughput of 10,000 t/d. The BMA tower has a diameter of 11 m and aheight of 21.6 m. The maximal practicable throughput can exceed the nominalvalue by more than about 15%. The BMA tower extractor of Hohenau Suganasugar factory has a daily slice capacity of 10,000 t. At a draft value of 109%,sugar losses of 0.23% on beet are realized with beet containing an average of20% sugar (21).

E. Trough (DDS) Extractor

This extractor was invented by Bruniche–Olsen and developed by DDS (DeDanske Sukkerfabrikker) in the early 1950s (Fig. 20). In the United States, it isknown as the Silver DDS slope extractor. It consists essentially of a U-shapedsloping vessel in which two overlapping screws with opposite pitches rotate.Fresh cossettes fall from a conveyor belt to the lower end. The cossettes aretransported upward by the two screws to a paddle wheel, which lifts the ex-hausted cossettes out of the extractor. The raw juice leaves the extractor througha screen at the bottom end. The lower section of the extractor works in a similarway to the juice-cossettes heat exchangers: the juice is cooled and the cossettesare heated. The temperature difference between raw juice and fresh cossettes is

Sugars and Carbohydrates 269

Figure 20 DDS extractor—longitudinal and cross-sectional view (40). 1, Cossettesconveyor and scale; 2, screw elements; 3, press water inlet; 4, fresh water inlet; 5, ex-hausted cossettes; 6, heating jackets; 7, juice screen; 8, raw juice pump; 9, cossetteslevel; 10, juice level.

about 15°C. The necessary heat is supplied by 12 steam jackets having tempera-ture controls. There is no circulation juice.

At the nominal capacity of the extractor, the average retention time of thecossettes is between 125 and 140 min. The packing of the cossettes is high atabout 700 to 730 kg of cossettes per cubic millimeter. The mean juice retentiontime is about 55 min.

For a regular juice flow, it is very important that the permeability of cos-settes remain high. Figure 20 shows respectively the normal levels of juice andcossettes when the permeability of the cossettes is correct. In this case, theentrainment of the mass of cossettes by the screws gives a higher level of cosset-tes and juice in the middle of the extractor.

If the mush content is low, i.e., less than 5, it is possible to process cos-settes with a Silin number of 10 m/100 g. The DDS extractors are, however,normally fed with cossettes having a Silin number of 7–8. The maximal nominaldaily capacity of this type of extractor is 3600 t/d for equipment that is 28 m

270 Christodoulou

long and 8.5 m wide (21). This extractor can work with 40% overcapacity(28).

In Fig. 34 is given a general automation and control diagram of a beetextraction tower and in Fig. 35 the same diagram of a DDS beet extractiontrough. In Fig. 36 are explained the symbols of automation and control used inFigs. 34 and 35.

VIII. MASS AND HEAT BALANCE OF THE EXTRACTION

All of the industrial extractors follow the general procedure in Fig. 8. Exhaustedcossettes are pressed and the press water is reintroduced into the extractor. Thisflow of recycled water is supplemented by the addition of fresh water.

Figure 21 gives an example of a mass balance calculated according to thefollowing theoretical conditions:

Sugar content of cossettes 17%Marc content of fresh cossettes 4.5%Dry substance content of exhausted cossettes 10.55%Dry substance content of pressed pulp 25%Draft 120% on beetPress water purity 80%Raw juice purity 89.5%Known sugar losses 0.27% on beetUnknown sugar losses 0.10% on beet

The balance shows that the mass of exhausted cossettes extracted from the ex-tractor is significantly less than the mass of fresh cossettes. This is mainly aconsequence of the leaching and pressing effect that takes place in most extrac-tors (21).

A mass and heat balance of a DDS extractor are given in Fig. 22 (28).Initially the DDS extractor, which produced cold diffusion juice 15°C aboveambient temperature, needed less heat (approximately 1% beets standard steamwith 550 kcal/kg or 2300 kJ/kg heat of evaporation), whereas the other extrac-tors (RT, tower, etc.) needed more heat (approximately 5% beets standardsteam) because their diffusion juice was hot (50°C). This changed in the1980s. By modification inside the countercurrent mixer in tower extractors thediffusion juice produced was also cold; subsequently, the heat consumptionwas less (1% beets standard steam) (29). See also Fig. 23 where, according toBaloh, the correlation between the steam consumption in the extraction, thedraft, and the temperature of raw juice is given. The lowest steam consumptionis achieved by low draft 105% on beet and low temperature of raw juice 15°C(29).

Sugars and Carbohydrates 271

Figure 21 Example of an extraction mass balance (40).

IX. OPTIMIZING THE ECONOMIC PERFORMANCEOF BEET EXTRACTOR

Beet sugar manufacture yields three products: sugar, beet pulp, and molasses.The relative value of these products and the costs of processing materials andaids (i.e., mainly cost of fuel), as well as the processing of any additional juiceproduct, determines the optimum draft.

Van der Poel et al. demonstrated how the final fractions of sucrose ex-tracted can be divided between white sugar and molasses by establishing therelationship between the extraction of sucrose to that of potassium (30). Ander-son assumes that 60% of the decrease in sugar loss to the pulp will be recovered

272 Christodoulou

Figure 22 Mass and heat balance of a DDS extractor (28).

Sugars and Carbohydrates 273

Draft k

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as white sugar for use in the DDS Isocost chart. Thus, the ratio of pulp, sugar,and molasses can be estimated (31).

Production costs can then be assigned to the changing level of draft andimpurities, such as in purification, electrical costs for pumping, processing aids,and evaporation. The primary influence on cost is typically draft vs. evaporationcost. Combination of the values of the three products vs. the costs of processingallows the calculation of the overall economic impact of changing the extractionparameters, from which an optimum can be derived.

A typical example of an optimization calculation as used at CSM wasgiven by Van der Poel et al. (32). The figures show the optimum value at asugar loss in pressed pulp of 0.352% on beet and draft 120% on beet. Theabsolute levels of the yields are not right because many processing costs, suchas personnel and maintenance, are not included. However, the differences in

274 Christodoulou

relation to the sugar losses in pressed pulp as well as the position of the optimum(draft and sugar loss in pulp) are correct.

Adamopoulos et al. studied the optimization of the extraction process inDDS extractor of 3000 t/d on the basis of economical criteria (cost of sugarproduction), concluding that the optimal values for draft are 107–111% onbeets, the losses 0.11–0.24% on beets, the temperature 70–75°C (4).

Gudmundson described computer programs developed by the SwedishSugar Company, where an energy balance can be run to judge the impact acrossthe factory. A second program calculates the optimum between the draft andthe overall factory operation (33).

Christodoulou described the result of trial test runs of pulp presses in twodifferent factories of Hellenic Sugar Industry, Greece. Not only do the operatingparameters of extraction influence the good pressability of the pulp; the type ofextraction equipment does so as well (3).

Budicek and Hladikova presented an econometric model of a sugar factorythat is not limited to the extraction stage. Decision making is based on calcula-tion of the sugar factory’s annual gross profit. The model presents mass flowsas well as cost–benefit calculations. Mass balances consist of processed beet,accompanying impurities, and processing aids, thus providing an overall viewof the main products and byproducts, including their production costs (34).

X. COMPOSITION OF SUGAR CANE

Sugar cane is a type of giant grass belonging to the family, botanically knownas “Saccharum,” a generic name for sugar cane. One of the cultivated speciesof Saccharum is known as Saccharum officinarum. The wild form of the sugarcane belongs to the family, botanically known as “Saccharum spontaneum.”The cultivated wild varieties of sugar cane have played a very important role inthe evolution of new commercial varieties of sugar cane, now grown by hybrid-ization and selection techniques. Thus, all the sugar cane varieties grown indifferent countries have the parents belonging to the Saccharum officinarum andSaccharum spontaneum and, to some extent, other species (Fig. 24) (35). Thechemical composition of sugar cane varies widely depending on many factors.Therefore, only indicative average figures in percentage are given below (36):

75 water25 solids

13 fiber12 soluble solids

10.5 sugar9.8 sucrose

Sugars and Carbohydrates 275

0.7 invert sugar1.5 nonsugars

0.8 organic nonsugars0.2 nitrogenous substances

0.06 proteins0.14 amino acids

0.1 nitrogen-free substances0.03 carboxylic acids0.02 starch, wax, fats, phosphatides

0.5 unidentified substances0.7 mineral water

XI. EXTERNAL STRUCTURE OF THE SUGARCANE PLANT

A. Stalk

The sugar cane plant consists of a number of unbranched stalks that storethe sucrose. The stalks are tall and slender, roughly circular in cross-section,and bear two rows of leaves. The stalks are divided by the nodes, whichare distinctive areas where the leaves are attached (one leaf at each node)(Fig. 24).

Situated within the root band is the bud. There is normally one bud ateach node. The buds are situated on alternate sides of the stem. The bud is anembryonic shoot consisting of a small stem bearing miniature leaves, the outerones of which are scales. The size and shape of the buds as well as the form ofthe outer scales or flange vary considerably with variety (Fig. 25).

B. Leaf

The leaves are arranged alternately a single leaf arising from each node (Fig.26). They increase in size as the plant develops. Trash is formed when the leafages and dies.

C. Root System

There are three main types of sugar cane root: sett roots, shoot roots, and matureroots. The sett roots develop from the primordia in the root band of the cutting.The shoot roots develop from the root primordia on the lower nodes of youngshoots (Fig. 27). The mature roots arise from root bands of shoots after theinitial flash of shoot roots (37).

276 Christodoulou

Figure 24 Parts of sugar cane stem (40).

XII. TECHNICAL EXTRACTION OF SUGAR FROM CANE

A. Cane Unloading

Since cane unloading is a materials handling problem, the methods adopteddepend on the methods used for harvesting the cane and transporting from fieldto factory. Large factories use automated cane handling to a great extent. Wherecane payment or factory control systems demand, the cane unloading stationmust often include weighing the cane, recording its origins and sampling for the

Sugars and Carbohydrates 277

Figure 25 Buds are located in root bands on nodes of sugar cane stems (40).

Figure 26 Structure of a sugar cane leaf (40).

278 Christodoulou

Figure 27 Young cane plant showing two kinds of roots: sett roots and shoot roots(40).

determination of cane quality. In the end the cane handling system must supply thecane to the factory cane preparation equipment in as uniform a stream as possibleand at the appropriate rate. In some places—notably Hawaii—the cane is washed,but this operation can be avoided by good harvesting and handling methods.

B. Cane Preparation

For best efficiency of the extraction plant (crushing or diffusion) the cane mustbe as finely divided as is economically feasible. Modern factories use heavy-

Sugars and Carbohydrates 279

duty, swing hammer “shredders” to comminute the cane to a fibrous mass wherethe largest particles are pieces of rind of about 100 mm or shorter in length andof cross-section 4 × 2 mm or smaller. The rind is the toughest part of the stalkwhere the “fibrovascular bundles” are most tightly packed together. In the inte-rior of the stalk and particularly in the “internode“ sections, the fibrovascularbundles are less densely packed and there are more juice cells and juice.

C. Description of Heavy-Duty Shredders

Figure 28 is a diagrammatic representation of the cross-section of a swing ham-mer shredder. The swept diameter of the hammers ranges from 1500 to 1800mm. The rotor is typically 2100 mm long, rotates at about 1000 min−1, and isdriven via a reduction gear box by a steam turbine of up to 4500 kW. The billetsof chopped cane enter at the top [1] and slide down or are thrown against thefeed plate [2]. Below the feed plate is a series of heavy grid bars [3] that retardthe progress of the cane, retaining it in the range of the hammers to be brokeninto fine fibrous particles. The clearance between the extended hammer tips andthe grid bars is often 2 mm or less. Shredded cane is discharged at [4]. A set ofhammers numbers between 100 and 200, and each hammer weighs 15–20 kg.

Figure 28 Schematic cross-section drawing of heavy-duty cane shredder (40). 1, Canefeed; 2, feed plate; 3, grid; 4, prepared cane discharge; 5, hammer.

280 Christodoulou

Static and dynamic balancing of the rotor is essential. The hammers are hardfaced by welding or have hard replaceable inserts, and a refurbished set ofhammers must often be provided at weekly intervals. A high amount of soil inthe cane supply requires more frequent replacement if throughput and extractionefficiency are to be maintained at acceptable levels (38).

XIII. CANE MILLS

A. Crushing Trains

Extraction of sugar from cane by “crushing” or “grinding” is carried out in aseries of roller mills. The separation is carried out by volumetric reduction aidedby dilution of residual juice by counterflow washing. The normal “compoundimbibition” system is illustrated for a train of four mills in Fig. 29. For bestresults the imbibition liquid applied to the feed to the final mill is hot water—usually condensate at up to 85°C. The juice expressed from the first and secondmills is sent to process and is called “mixed juice.” The whole milling processis usually completed in about 20 min. Uniform application of imbibition liquidacross the bagasse “blanket” is important, although the whole blanket is satu-

Figure 29 Schematic arrangement of modern six-roll crushing mill showing diametersand depths (40). 1, Delivery roll; 2, top roll; 3, feed roll; 4, top pressure feeder roll; 5,bottom pressure feeder roll; 6, underfeed roll; 7, feed chute; 8, pressure feeder chute; 9,trash plate; 10, feed cane or bagasse; 11, delivery bagasse.

Sugars and Carbohydrates 281

rated with juice between the first pair of rolls at which juice is expressed andthere is no evidence that premixing of imbibition liquid and bagasse improvesextraction efficiency.

A modern crushing unit (mill) consists of up to six rolls arranged as shownin Fig. 30. The three main rolls (delivery–1, top–2, feed–3) are of nearly equaldiameter (except for differing wear). In the best modern practice the pressurefeed rolls 4 and 5 are of the same diameter as is the underfeed roll 6. Manyother combinations of auxiliary rolls and other “feeding devices” have beenused, but the six-roll mill arrangement illustrated gives high throughput andextraction efficiency (38).

XIV. CANE DIFFUSERS

There are a number of reports of cane diffusers (the common term diffuser isused here, although technically speaking this is an extractor) installed before1900, and at various other times and in different countries. However, it wasonly with the successful introduction of beet extractors that cane extraction (dif-fusion) became a practical scheme.

A batch cane diffusion system operated in Egypt for more than 50 years,but since the first successful continuous cane diffusers were installed in theearly 1960s, diffusers have been operated as continuous countercurrent solid-liquid systems. Early designs evolved from beet diffusers, and even the com-monly used name “diffuser” came from the beet industry, although the terminol-ogy is not really appropriate in the context of extraction of juice from cane.

Figure 30 Diagrammatic representation of a four-mill crushing train with ordinarycompound imbibition (40). 1, 2, 3, 4, Crushing units—mills 1 to 4; 5, shredder; 6, dragconveyor or rubber belt—prepared cane elevator; 7, 8, 9, drag conveyors—intermediatecarriers; 10, cane feed to shredder; 11, final bagasse to boilers, storage, or otherwise; 12,imbibition water; 13, imbibition recirculation system; 14, mixed juice to process.

282 Christodoulou

Differences between cane and beet extraction hinge around the consider-able differences in raw material to be extracted. Although beet is convenientlycut into cossettes, it was only when adequate preparation of cane for extractionwas achieved that cane diffuser installations became successful. A large num-ber of published reports on cane diffusion from 1965 to 1975 reflected theinterest generated at the adoption of cane diffusion. Since that time its adop-tion has been rapid in some countries, particularly in southern Africa, whereroughly 80% of all cane are processed in diffusers. However, milling is stillthe predominant extraction process in a large number of cane sugar–producingareas.

There are essentially two variants of the process, termed bagasse diffusionand cane diffusion. The former involves a single mill ahead of the diffuser, andin the later prepared cane is accepted directly into the diffuser. Early installa-tions favored bagasse diffusers because they represented a smaller step-changefrom milling, and are still required in countries where payment for cane is basedon an analysis of first expressed juice. However, cane diffusers have generallyshown themselves to be considerably more cost effective and are almost exclu-sively favored over bagasse diffusers in new installations. Therefore, this chap-ter covers only cane diffusers.

Diffusers must be provided with well-prepared cane. A dewatering stage,usually one or more mills, is required to dewater the very wet bagasse leavinga diffuser. In some installations a French screw press was used as a dewateringdevice but proved to be unreliable and subject to considerable wear. Conven-tional mills are now used universally for this duty. Because of the largeamount of liquid to be removed, dewatering is generally done in two stages iffour toll mills are used, or else in a single stage using pressure-fed mills (Fig.30). The types of cane diffuser that have been used can be categorized asfollows:

Uncontrolled transport of juice and cossettes (also called true countercur-rent diffusers), e.g., DDS, Saturne.

Controlled transport of juice and cossettes (also called moving-bed dif-fusers). There are either cross-flow moving belt extractors, such asB.M.A., De Smet (Fig. 31), Silver ring, Huletts-type extractors, or othertypes such as F&S/Van Hengel or Rotocel (39).

Moving bed diffusers are also countercurrent extraction devices but oper-ate on a staged basis. Juice is pumped onto a moving bed of prepared cane orbagasse, about 50–60 m long, in 10–18 stages. A schematic diagram of a canediffuser is shown in Fig. 31.

The De Smet cane diffuser (Sec. VII) is essentially the same as the DeSmet beet extractor. The cane or bagasse bed forms on a horizontal slow-mov-

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284 Christodoulou

ing screen. The Silver ring diffuser is essentially similar, but the screens movein a circle instead of along a straight line. The BMA and Hullets diffusers differfrom the De Smet in having a fixed screen, with a series of chains that transportthe cane bed across the screen. This generally results in a cheaper diffuser forthe same screen area (39). The dragging of cane by chains across the fixed-screen diffuser generally results in the formation of a more compact cane layerat the screen, which affects percolation.

The processing capacity of a cane diffuser–type BMA lies between 2000(the smallest unit) to 16,000 t/d (the biggest unit), while the same diffuser-typeBMA by bagasse processing has a capacity between 3000 (the smallest unit) to24,000 t/d (the biggest unit).

XV. FACTORS AFFECTING EXTRACTION EFFICIENCY

A. Cane Preparation

The process of breaking down the cane into small pieces is referred to as canepreparation. This is the most important variable affecting extraction in diffusers.If high extraction efficiencies are to be achieved it is essential that the cane beprepared in a heavy-duty shredder (Fig. 28) so that most of the sugar-containingcells of the cane stalk are ruptured. Laboratory and pilot plant work showedvery clearly that more intensive preparation of cane makes more of the sucrosecontaining juice readily accessible to the extracting liquid, minimizing theamount of sucrose that has to be extracted by a much slower diffusion mecha-nism. Unfortunately, measurement of the degree of cane preparation is difficultand existing measures are not always reliable.

The way in which the cane is prepared is also important. Ideally the typeof preparation should result in material where all cells are ruptured but wherelong fibers are still evident, resulting in a cane bed that is stable and openenough to allow high percolation rates to be achieved. In practice it has beenfound that this is best achieved in heavy-duty shredders with a minimum ofknifing, since intensive knifing reduces the average fiber length.

B. Imbibition Rate

As with any solid-liquid extraction process, the more extracting liquor that isadded the easier is the extraction. So it is with cane diffusion, where higherimbibition rates invariably result in higher extraction efficiencies. The amountof imbibition water added is generally related to the quantity of fiber beingprocessed, since it is the fiber that removes with it juices in final bagasse.

There is no maximal or minimal imbibition rate for diffusion. Since high

Sugars and Carbohydrates 285

imbibition rates enable a smaller diffuser to be utilized to achieve a given extrac-tion efficiency, the reduction in the cost of the diffuser would have to be bal-anced against the cost of additional evaporator capacity and/or the cost of steam.Therefore, the optimal imbibition rate for any factory is contingent on localfactors at that factory.

C. Effect of Number of Stages

The use of a number of stages rather than a single big mixed tank enables higherconcentration difference between sucrose in cane and true countercurrent flowis approached more closely. However, the benefit drops off as the number ofstages increases and marginal improvement becomes very small.

D. Effect of Percolation Rate

Although preparation is the most important variable affecting extraction effi-ciency in a cane diffuser, the percolation rate is probably the next most impor-tant variable. This is the rate at which liquid percolates down through the bedof prepared cane. Laboratory and pilot plant studies have shown that an increasein percolation rate promotes the rate of mass transfer and increases the propor-tion of the juice in open cells that is accessible to the extracting liquid.

E. Effect of Cane Retention Time

The longer the time the prepared cane spends in the diffuser the higher will bethe extraction efficiency. Provision of adequate retention time is probably oneof the most important design specifications. Since the raw juice offtake fromthe diffuser is roughly equal to the mass of cane entering the diffuser, juiceretention time in the diffuser is nearly twice the retention time of fiber, particu-larly if the juice trays below the diffuser are not kept empty.

F. Effect of Temperature

High temperatures are advantageous because they increase the rate of extrac-tion. However, this effect is not as important as the effect of preparation andliquid flow rate. Nonetheless it was estimated that an increase in temperaturefrom 75°C to 80°C would lead to an increase in extraction efficiency of about0.2% sugar on cane. The most important reason for keeping the temperatureabove 75°C is to control microbiological activity. Generally, diffusers are op-erated at about 85°C, allowing low-pressure steam to be used for heating pur-poses (39).

286 Christodoulou

XVI. STRUCTURE AND CHEMICAL COMPOSITIONOF CORN

The gross structural features, physical properties, structural details, and compo-sition of corn are described in the literature together with very nice photographsand microphotographs of corn particles (42). A proximate analysis of corn grainis given in Table 3 (42).

XVII. THE WET MILLING PROCESS OF CORNFOR EXTRACTION OF STARCH

Corn is abundant and relatively inexpensive; it has a high starch content andprotein of acceptable quantity and quality. Thus, its primary use is for animalfeed. It is also processed into valuable food and industrial products, such asethyl alcohol by fermentation, corn meal by dry milling, and highly refinedstarch by wet milling. The greatest volume is processed by wet milling to pro-duce starch products and sweetener products for foods. Nonfood products, suchas industrial starches, corn gluten feed, and corn gluten meal, are also manufac-tured.

The wet milling process involves an initial water soak under carefullycontrolled conditions to soften the kernels. The corn is then milled and its com-ponents separated by screening, centrifuging, and washing (Fig. 32), to producestarch, oil, feed byproducts, and sweeteners (by starch hydrolysis). Applicationsfor these products have shown steady growth, which has necessitated majorinvestment to expand production facilities in recent years (43).

Table 3 Proximate Composition of Corn Grain (42)

Component (%, wet basis) Range Average

Moisture 7–23 16.0Starch 61–78 71.7Protein 6–12 9.5Fat 3.1–5.7 4.3Ash (oxide) 1.1–3.9 1.4Pentosans (as xylose) 5.8–6.6 6.2Fiber (neutral detergent residue) 8.3–11.9 9.5Cellulose + lignin (acid detergent residue) 3.3–4.3 3.3Sugars, total (as glucose) 1.0–3.0 2.6

Sugars and Carbohydrates 287

Figure 32 Wet-milling process flow diagram, showing equipment arrangement for theseparation of the major components: steepwater, germ, fiber, gluten, and cornstarch (43).

288 Christodoulou

Figure 33 Direction of material transport in correlation to the basic process (sche-matic), W, water; Z, sugar (19).

A. Steeping

The first critical step in the wet milling of corn is steeping—the soaking ofcorn in water under controlled processing conditions of temperature, time, sulfurdioxide (SO2) concentration, lactic acid content, and so forth. These conditionshave been found necessary to promote diffusion of the water through the tip capof the kernel into the germ, endosperm, and their cellular components. Steepingsoftens the kernels, facilitating separation of components.

Corn is shipped in bulk to the wet milling plants by truck, hopper car, andbarge. It is then cleaned on vibrating screens to remove coarse material (retainedon 12.7-mm × 1/2-in. openings) and fine material (through 3.18-mm × 1/8-in.openings). These screenings are diverted to animal feed. If they are allowed toremain with the corn, they cause processing problems such as restricted waterflow through steeps and screens, increased steep liquor viscosity, and qualityproblems with the finished starch.

Steeping is accomplished by putting corn into tanks (steeps) that have acapacity of 50–330 t each. The corn is then covered with steepwater, heated to52°C, and held for 22–50 h. Steeps have cone bottoms with screens so that thewater can be separated from the corn and pumped elsewhere or recirculatedback to the top of the steep. To maintain steeping temperature, the recirculatedflow is heated directly by steam injection or indirectly by heat exchanger. The

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Figure 36 Symbols of automation and control.

water should not exceed 55°C to avoid destroying the bacteria needed to pro-duce lactic acid.

Steeping is a countercurrent system, utilizing a battery of 6–12 or moresteep tanks. Steeps are filled one at a time as they become empty. The corndoes not move—just the water, which is transferred from one steep to the next.However, steeping is accomplished in one plant by continuously adding drycorn at the top of the steep while continuously withdrawing steeped corn fromthe bottom.

Water for the steeps originates in the wet milling process, where it accu-mulates corn solubles. It is treated with SO2 to a concentration of 0.12–0.20%.The SO2 is purchased as a liquid or manufactured on site by burning of elemen-tal sulfur. The SO2 increases the rate of water diffusion into the kernel andassists in breaking down the protein-starch matrix, which is necessary for highstarch yield quality.

The SO2-treated water is added to the steep containing the oldest corn. Asthe water is advanced from steep to steep, the SO2 content decreases and bacte-rial action increases, resulting in the growth of lactic acid bacteria. The desiredlactic acid concentration is 16–20% (dry basis) after the water has advanced

292 Christodoulou

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Figure 38 Representation of the molecular electrostatic potential (MEP) profiles ofthe two relevant sucrose conformers. The MEPs are depicted on the corresponding con-tact surfaces in 16-color code ranging from violet (most electronegative potential) to red(most electropositive potential) (in original) in relative terms (46).

294 Christodoulou

Figure 39 Molecular lipophilicity profiles for the two sucrose conformers of Fig. 37,with blue (in original) corresponding to hydrophilic surface areas and yellow (in original)to most hydrophobic regions. For both sucrose conformers, the entire “backside” of thefructose moiety is decisively hydrophobic (46).

Sugars and Carbohydrates 295

Figure 40 Snapshot of an MD simulation of sucrose surrounded by 571 water mole-cules (47).

through the system and been withdrawn as light steepwater. Meanwhile, the SO2

content drops to 0.01% or less.The volume of water available for steeping is normally 1.2–1.4 m3/t of

corn. About 0.5 m3/t is absorbed by the corn to increase its moisture from 16%to 45% during the steeping. The remaining 0.7–0.8 m3/t is the quantity with-drawn from the steeping system. This water contains the solubles soaked out ofthe corn, which is 0.05–0.06 t solids/t corn processed. It is evaporated to 40–50% solids, mixed with corn fiber, dried, and sold as corn gluten feed (42).

B. Separations

The next step of the process is the separation of kernel components (germ, fiber,gluten, and corn starch) by means of degerminating rotating mills, hydrocycloneseparators, wedge-bar screens, disk-nozzle centrifuges, etc. (42). Solubles ex-tracted from the corn during steeping are routed to evaporators where 0.6–0.7m3 of water per ton of corn is removed to increase the solids from 5–10% to40–50%. The solids are then mixed with corn fiber and processed into corngluten feed. Falling film recirculating evaporator systems are used to increasethe light steepwater solids to 30%, using steam at a pressure of 1.2 kg/cm2 orless (42). Multiple effect design, in which 1 kg of steam can evaporate several

296 Christodoulou

Figure 41 Hydrophobic topographies for the amylose fraction of starch (46).

Sugars and Carbohydrates 297

kilograms of water, is necessary to minimize energy costs. Energy can be furtherreduced with mechanical recompression, which completely recycles the vapors,compresses them, and discharges them to the evaporator steam chest (42). Arelatively pure starch slurry from the wet milling operation contains 40% solids.

C. Trends, Automation

The wet milling process and associated equipment have matured sufficiently topermit consistent, reliable separation and product quality on a 24-h basis withminimal operating labor. The most notable achievements are attained in the newlarge plants where television-like displays and control systems distributed bycathode ray tubes are utilized to start process equipment sequentially on demandand then to monitor and control the system.

Furthermore, the technician can be alerted, by rate-of-change or trend-in-measurement functions, to variables that are about to get out of control. Thisallows time to make the necessary adjustments to avoid spills and off-quality.These computer systems are the latest development in the search for reducedcosts and better product quality. Better computer applications are probable asimproved on-line measuring devices for protein, starch, fiber, and soluble con-tent are perfected.

D. Utilities

Fresh water usage for a typical large sweetener plant is about 4.5–6.0 m3/t ofcorn processed, of which one-fourth is consumed in wet milling. Reduction inthe requirements for the wet milling portion is limited because the usage equalsthe need for properly steeping the corn. Greater water consumption for the fin-ishing processes is likely because the trend is toward more sophisticated prod-ucts that require more water.

Wet milling operations are high-energy users at 1.48 × 106 kcal/t of corn,even for the efficient large sweetener plants. About 20% of the total are forelectricity, which indicates that most of the energy goes into fuel to make steam.As products continue to become more sophisticated, the use of energy will in-crease unless new technology reverses the trend.

The use of mechanical vapor recompression evaporators in new facilitiesand in the replacement of old equipment is reducing energy usage because it isfive to six times more efficient than quadruple-effect steam evaporation. Moreimaginative use of heat exchangers can recover heat now being lost to the atmo-sphere via cooling towers. Steepwater evaporators now in service can utilizefeed dryer exhausts as their source of energy.

Superheated steam is showing promise for use in drying feed materials.Most (60–80%) of the energy for the primary heating of steam is recovered in

298 Christodoulou

low-pressure steam for use elsewhere in the process. Not only is 20–40% energysaving possible, but there is no discharge to the atmosphere, eliminating odorand particulate problems associated with feed drying. Reverse osmosis is im-proving due to breakthroughs in membrane technology. When operating with apressure differential of 35–70 kg/cm2 they may replace steepwater and sweet-ener evaporators, achieving a major reduction in energy requirements.

New steeping technology may be in the offing that improves the possibil-ity of success in concentrating the solids content of steepwater; it works byreducing the low molecular weight components such as alcohol that reverseosmosis, up to now, has had difficulty separating. Another benefit is reducedsteeping time. A technology being introduced to the industry to treat wastesrelies on anaerobic digestion, which has the advantage of using less energy-intensive processes and giving energy credits via recovery of methane gas,which is utilized as a fuel source. The cost of energy is being reduced by install-ing cogeneration. New coal-fired boilers operating at 40–80 atm drive electricgenerators. Steam for processing is then extracted in the 10 kg/cm2 range, andsteam for low-temperature heating (of water, for evaporation, etc.) is extractedat exactly 2 kg/cm2 absolute (42).

GLOSSARY (7, 40)

Bagasse Cane fiber leaving cane mill/diffuser after extractionof juice, e.g., first mill bagasse, etc., to final bagasse

Beet Sugar beet root, botanically the thick main root withhypocotyl in which sugar is stored

Beet brei Beet sample prepared for analysis is the form of fineparticles

Beet knife Rectangular piece of steel designed to slice beet intocossettes

Beet tail Elongated lower part of the beetBeet tops Beet leaves and petioles, which may or may not be

accompanied by crowns or pieces of crowns that areremoved in the field at the time of harvest

Beet washer Installation for cleaning beet (e.g., jet washer, re-volving-arm washer, drum washer, cyclone washer)

Cossettes Beet slices produced by a beet slicerCultivar A horticulturally or agriculturally derived variety of

a plant, as distinguished from a natural variety [culti-vated + variety)

Denaturation Deliberate alteration of beet cells, often by heat, inpreparation for extraction

Sugars and Carbohydrates 299

Draft The ratio percent weight of raw juice produced to theweight of cossettes introduced into the extractor

Deteriorated beet Beet of reduced suitability for processing due to ex-ternal causes, e.g., frost

Diffusion Gradual mixing of the molecules of two or more sub-stances, as a result of random thermal motion

Diffusivity Rate at which a substance diffuses between the oppo-site sides of a unit cube when there is unit concentra-tion difference between them. Also called “diffusioncoefficient.”

Dry substance In most cases a moisture-free substanceExhausted cossettes Cossettes leaving the extraction plantExtraction Process of obtaining juice from sugar beet or sugar

cane; the term “diffusion” should be used only forthe physicochemical process

Extraction fresh water Water introduced into the extraction in addition topress water

Extraction losses Quantity of sugar entered but not contained in theraw juice as a percentage of the beet or cane mass

Flume water Water used to transport beetImbibition Mixing of water or juice with bagasse during cane

millingInvert sugar Mixture of (close to) equal parts of glucose and fruc-

tose resulting from the hydrolysis of sucrose (inver-sion)

Juice purification Partial removal of nonsugar substances from the rawjuice while producing a thin juice

Knife block Box-like device used to hold a number of knives inthe disk or drum of a slicing machine

Molasses The sugar-bearing product of the sugar whose purityhas been reduced to the point that further crystalliza-tion of sugar is not economical feasible without spe-cial treatment of molasses

Mush content Portion of cossettes shorter than 1 cmNonsucrose content Difference between dry substance content and its su-

crose contentNonsugar Common overall term for substances contained in

the raw materials and products of the sugar industryexcept sucrose (sugar) and water

Osmosis Passage of a solvent through a semipermeable mem-brane into a more concentrated solution (Fig. 33)

300 Christodoulou

Polarization Term customarily used in sugar analysis for the opti-cal rotation of a sugar industry product measured un-der defined conditions (ICUMSA*), as a percentageof the rotation of pure sucrose, measured under thesame conditions

Press water Liquid effluent from the pulp pressesPress water pulp Particles of exhausted cossettes contained in the press

waterPressed pulp Pressed, exhausted cossettes, leaving the pulp pressesPurity Sugar content as percentage of dry substance contentRaw juice Juice obtained from beet or cane after extraction,

pressing, or milling, mixed juice (cane)Raw juice draft Mass of juice drawn from the extraction plant as per-

centage of mass of cossettes introducedSilin number Length in meters of 100 grams of cossettesSlicer (beet) Machine designed to hold knives in knife blocks to

produce cossettesSucrose Common term for the disaccharide α-D-glucopyran-

osyl-β-D-fructofuranosideSwedish number Ratio of the mass of cossettesWet pulp Commercial term for partially dewatered, exhausted

cossettes

REFERENCES

1. C Buttersack, D Schliephake. Extraction theory. In: RW Van der Poel, H Schiweck,T Schwartz, eds. Sugar Technology. Berlin, Germany: Verlag Bartens KG, 1998,pp. 310–327.

2. F Bonacker. Extraction. In: Die Zuckerherstellung, 3rd ed. Leipzig: Fachbuchver-lag, 1984, pp. 153–206.

3. P Christodoulou. Operating parameters of extraction systems for optimum pulppressing, Zuckerindustrie 119:1004–1007, 1994.

4. K Adamoupoulos, P Christodoulou, H Petropakis. Optimization of the process ofsugar extraction from beets. Hellenic Sugar Industry Quart Bull 50–51:57–79,1982.

5. K Bohn. Composition of sugarbeet and chemical behavior of constituents in pro-cessing. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology.Berlin: Verlag Bartens KG, 1998, pp. 115–151.

6. H Claassen. Die Zuckerfabrikation. In: Die Saftgewinnung. Magdeburg: Schal-lehn & Wollbruck, 1930, pp. 45–85.

*ICUMSA International Committee for Uniform Methods in Sugar Analysis.

Sugars and Carbohydrates 301

7. RA McGinnis, RN Sequeira. Chemistry of the beet and processing materials. Glos-sary of technical terms in common use in the beet sugar factory. In: RA McGinnis,ed. Beet Sugar Technology, 3rd ed. Fort Collins, CO: Beet Sugar DevelopmentFoundation, 1982, pp. 25–63, 759–782.

8. RE Watkins, PV Schmidt. Changes in the chemical composition of beet. In: RWVan der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: VerlagBartens KG, 1998, pp. 276–277.

9. E Junghans. Morphology and physical properties of sugarbeet. In: RW Van derPoel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG,1998, p. 209.

10. C Buttersack. Ultrastructure of the native sugarbeet root. In: RW Van der Poel, HSchiweck, T Schwartz, eds. Sugar Technology. Berlin, Germany: Verlag BartensKG, 1998, pp. 211–212.

11. K Vukov. Physics and Chemistry of Sugar Beet Manufacture. Budapest: AkademiaiKiado, 1977, p. 595.

12. E Junghans. Physical properties of beet. In: RW Van der Poel, H Schiweck, TSchwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 212–214.

13. C Buttersack. Physical properties of beet after denaturation. In: RW Van der Poel,H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998,pp. 215–216.

14. C Buttersack. Separation by adsorption, chromatography or membranes. Zuckerin-dustrie 124:846–858, 1999.

15. P Christodoulou. Possibilities of application of the chaos and complexity theory insugar technology. Proceedings of 21st General Assembly of CITS, Antwerp, May24–28, 1999. Berlin: Verlag Bartens KG, 1999, pp. 331–349.

16. SI Nikolis. Chaos and Information Processing. Singapore: World Scientific, 1991.17. PM Silin. Technology of Beet Sugar Production and Refining. Jerusalem: Israel

Program for Scientific Translations, 1964, pp. 88–178.18. HO Ebell, M Storz. Diffusion. In: RA McGinnis, ed. Beet Sugar Technology, 3rd

ed. Fort Collins, CO: Beet Sugar Development Foundation, 1982, pp. 119–153.19. D Schliephake, A Wolf. Mass transport processes in the initial phase of aqueous

sugar extraction from cossettes. Zucker 21:484–493, 1968.20. H Van Malland. Technical extraction of beet. In: RW Van der Poel, H Schiweck, T

Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 328–334.21. J Vetter. Process parameters of extraction. Equipment for cell denaturation and the

cooling of raw juice. In: RW Van der Poel, H Schiweck, T Schwartz, eds. SugarTechnology. Berlin: Verlag Bartens KG, 1998, pp. 334–348.

22. F Schneider, E Reinefeld. Ruebenzerkleinerung. In: F Schneider ed. Technologiedes Zuckers, 2nd ed. Hannover: M&H Schaper, 1968, pp. 152–157.

23. K Bucholz, KM Bliesener, R Buczys, K Thieleckte, D Miehe. Bacterial control. In:RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: VerlagBartens KG, 1998, pp. 169–170.

24. J Dubourg. Sucrerie de Betteraves. Paris: Librairie JB Bailliere & Fils, 1952, pp.79–150.

25. H Bruniche-Olsen. Solid–Liquid Extraction. Copenhagen: Nyt Nordisk Forlag, Ar-nold Busck, 1962, p. 463.

302 Christodoulou

26. D Schliephake, B Mechias, E Reinefeld. Retention time distribution and movementmechanisms in extraction equipment without controlled transport of cossettes. Pro-ceedings of 15th General Assembly of CITS, Tienen, Belgium, Principles of Ex-traction, Vienna, Austria, 1975, pp. 31–123.

27. G Mantovani. Sugar technology–development since 50 years. In: Proceedings of21st General Assembly of CITS, Antwerp, May 24–28, 1999. Berlin: Verlag Bar-tens KG, 1999, pp. 47–51.

28. Anonymous. The Danish Sugar and Sweetener Engineering Beet Diffuser, 1999,Birkorood, Denmark.

29. A Baloh. Energiewirtshaft in der Zuckerindustrie. Berlin: Verlag Bartens KG, 1991,pp. 90–110.

30. JA Richmond, D Noble. Technical accounting and process control. In: RW Vander Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag BartensKG, 1998, pp. 1060–1061.

31. K Anderson. The DDS beet extraction. Zuckerindustrie 114:136–138, 1989.32. PW Van der Poel, NHM de Visser, CC Bleyenberg. Sugar Techn Rev 9:15–18,

1982.33. C Gudmundson. 18th Meeting of the CITS Subcommittee Measurement and Pro-

cess Control, Peterborough, 1993.34. L Budicek, L Hladikova. Econometric model of a sugar factory. Proceedings of

20th General Assembly of CITS, Munich. Berlin: Bartens KG, 1995, pp. 34–41.35. J Berge. Sucr Belge 72, 1930.36. PJM Rao, Industrial Utilization of Sugarcane and Its Co-products, New Delhi:

ISPCK, 1997, pp. 5, 12–13.37. GM Thomson. External structure of the sugar cane plant. In: RW Van der Poel, H

Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998,pp. 225–230.

38. CR Murry. Technical extraction of sugar from cane. In: RW Van der Poel, H Schi-weck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp.354–381.

39. PW Rein. Control of cane diffusers. In: RW Van der Poel, H Schiweck, TSchwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 381–383.

40. RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: VerlagBartens KG, 1998.

41. NC Carpita, DM Gibeaut. Structural models primary cell walls in flowering plants:consistency of molecular structure with the physical properties of the walls duringgrowth. Plant J 3:1–30, 1993.

42. SA Watson. Structure and composition. In: SA Watson, PE Ramstad, eds. Corn:Chemistry and Technology. St. Paul, MN: American Association of Cereal Chem-ists, 1994, pp. 53–82.

43. JB May. Wet milling process and products. In: SA Watson, PE Ramstad, eds. Corn:Chemistry and Technology. St. Paul, MN: American Association of Cereal Chem-ists, 1994, pp. 377–397.

44. W Hein, G Pollach. New findings with the use of hop products in the sugar indus-try. Zuckerindustrie 122:940–949, 1997.

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45. WE Boyce, RC Diprima. Elementary Differential Equations and Boundary ValueProblems, 6th ed. New York: John Wiley & Sons, 1997, pp. 129, 605.

46. WF Lichtenthaler. Computer simulation of chemical and biological properties ofsucrose, the cyclodextrine and amylose. Int Sugar J 97(1153):12–22, 1995.

47. WF Lichtenthaler, S Immel. The conformation of sucrose in water. A moleculardynamic approach. Liebigs Ann, 1995:1925–1937.

9Flavor and Aroma Substances

Youn-Woo Lee and Youn Yong LeeKorea Institute of Science and TechnologySeoul, Korea

Extraction of flavor and aroma substances from naturally occurring raw materi-als is important because the compositions of flavors are often too complex tobe synthesized economically. Raw materials are isolated from balsam, bark,berry, blossom, bud, fruit, grass, gum, heartwood, leaves, peel, root, seed, twig,wood, and resinous exudation. Extracts of flavor and aroma substances preparedfrom raw materials are termed pomade, concrete, absolute, resinoid, or tinctureaccording to their production technique (1). In processing essential oils andnatural extracts, steam distillation, solvent extraction, supercritical fluid extrac-tion, and expression are the major methods. Steam distillation provides the es-sential oil, whereas solvent extraction provides both the essential oil and theoleoresins. Oleoresins are concentrated natural liquid flavorings that containboth volatile and nonvolatile flavor components. Essential oils are obtainedfrom plant materials by steam distillation or water distillation. After condensa-tion of the vapor phase, the oil separates from the aqueous phase and is re-moved. Essential oils consist of volatile, lipophilic substances that are mainlyhydrocarbons or monofunctional compounds derived from the metabolism ofmono- and sesquiterpenes, phenylpropanoids, amino acids, and fatty acids. Po-mades consist of fats that contain fragrance substances and are produced by theenfleurage technique. Concretes are prepared by evaporation of residue sub-stances that are extracted from fresh plant material with nonpolar solvents suchas benzene, toluene, hexane, and petroleum ether, as shown in Fig. 1. Concretescontain viscous waxy compounds with volatile fragrance materials. Concretesand pomades are not completely soluble in ethanol. Absolutes are prepared bytaking up concretes in ethanol. Compounds that precipitate on cooling are thenremoved by filtration. After evaporation of the ethanol, a wax-free residue, the

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absolute, remains. Absolutes are completely soluble in ethanol. Resinoids areprepared by extracting plant exudates (balsams, oleo gum resins, natural oleores-ins, and resinous products) with solvents such as methanol, ethanol, or toluene.The products are usually highly viscous to improve their flow and processingproperties. Resinoids mainly consist of nonvolatile, resinous compounds and areprimarily used for their excellent fixative properties. Tinctures are alcoholicsolutions that are prepared by treating natural raw materials with ethanol orethanol-water solution. They can also be obtained by dissolving other extractsin these solvents.

Extraction is still one of the most important methods for producing flavorand aroma substances and is the selective extraction of a soluble constituentfrom solid (solid-liquid extraction) or liquid mixtures (liquid-liquid extraction)by liquid solvents (2). In the production of natural extracts, the most impressiveprogress is the introduction of supercritical fluid extraction technology (Chapter3). Characteristic applications are the extraction of caffeine from coffee or tea,or hops extraction.

I. SEPARATION PROCESSES (3)

A. Distillation

The most widely method used for producing flavor and aroma substances isdistillation. There are three different processes used: steam distillation, simpleboiling water distillation, and water and steam distillation, as shown in Fig. 2.In all cases, hot steam carries the most volatile compounds of the aromaticmaterial with it and is then condensed. The resulting distillate is composed ofthe essential oil and water. The mixture of condensed oil and water runs intothe layer separator where the lighter insoluble oil floats on the surface and accu-mulates slowly and from where it is drawn off periodically, as shown in Fig. 3.Some of soluble components may be lost in the water. Steam distillation uses anoutside source of steam and no water is allowed in the still, as shown in Fig. 2a.The steam passes through the aromatic material in the distillation unit and exitsinto the condenser. In simple boiling water distillation, the plant materials arefully submerged in water and water is heated to produce steam, which containsflavor and aroma substances, as shown in Fig. 2c. This is the oldest method ofdistillation and the most versatile. This process works best for powders and verytough materials like seeds or roots. Production of steam by direct heating of plantmaterial with water will result in hydrolysis and loss of fragrant esters and at leastsome pyrolysis of plant material. The result is low-grade oil. In water and steamdistillation, a basket (or a perforated grid) is used for supporting the plant materi-als; thus, only rising steam contacts with the plant materials, while water is boilingin the still, as shown in Fig. 2b. This is the best method for distilling leafy materi-als, but it doesn’t work well for woods, roots, seeds, and so forth.

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B. Solvent Extraction

Solvent extraction of the plant materials is a well-known method for the produc-tion of valuable oleoresins as well as essential oils. A solvent extracted oleoresinincludes essential oil, organic soluble resins, nonvolatile fatty acids, and otherplant materials, and can yield a more potent flavor profile. To capture delicatearomas like jasmine, linden blossom, and so forth, without thermal degradation,a process of solvent extraction is used.

Plant materials such as leaves, flowers, roots, or stems are dissolved intoa solvent such as hexane. They become wax-like concretes. Absolute is obtainedby alcohol extraction of concretes where most of the alcohol is later removed.Plant materials such as seeds are first crushed and dissolved into solvents. Hex-ane is the most commonly used solvent. The solvent is evaporated under vac-uum to give the oil. Resinoids are resins dissolved in a solvent such as benzeneor alcohol. Resins are naturally occurring solid or semisolid substances producedby plants or trees.

In general, the highest extraction yields are normally obtained comparedto the other extraction methods. The solid to be extracted is placed in a basketsuspended in the extraction vessel, as shown in Fig. 4. Solvent fed to the boiler,is heated and distilled off. It runs from the condenser, via the reflux head, intothe extraction vessel where it percolates through the solid to be extracted. Sol-vent loaded with extract returns, via a seal loop, to the boiler, where it evapo-rates again. This cycle continues until the extraction is complete.

C. Carbon Dioxide Extraction

Carbon dioxide extraction is another type of solvent extraction in which carbondioxide is used under high pressure to extract both essential oils and oleoresins.Supercritical and liquid CO2 can both be used as very inert, safe solvents. LiquidCO2 fits the requirements of solvent ability and low boiling point. It is alsononflammable, inert, inexpensive, and has low toxicity. As a solvent, liquidCO2 is highly selective, particularly toward those esters, aldehydes, ketones, andalcohols representative of aroma constituents (4). Low-temperature extractionusing liquid CO2 produces extracts that more closely resemble the aroma of theoriginal plant materials than any process involving heating of plant materials.Some examples of commercial extraction with liquid CO2 are as follows: anisestar, cardamon, celery seed, clove bud, coffee, coriander seed, ginger Jamaican,ginger Nigerian, ginger terpeneless, hop oil, hoparome absolute, juniper berry20%, mace, nutmeg, pepper oil black, pimento berry, and vanilla absolute.

In contrast to liquid CO2, the solvent power of the supercritical fluid ishighly dependent on its temperature and pressure. The advantage of the method,of course, is that no solvent residue remains, since at normal pressure and tempera-

Flavor and Aroma Substances 311

Figure 4 Single-stage extractor with solvent recycle.

ture the CO2 simply reverts to a gas and evaporates. However, when supercriticalCO2 is used to produce flavors and aromas, some unwanted waxes, resins, andpigments are usually present simultaneously. Some examples of commercial ex-traction with supercritical CO2 are as follows: celery, ginger oil, jasmine abso-lute, massoia oil, paprika flavor and color, pepper oil, rosemary, sage, and va-nilla absolute (5, 6).

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D. Cold Pressing

Cold pressing is also known as expression. In this method, the oil-containingouter layer of the fruit is pressed and filtered to yield pure essential oil. Aheating stage is used to help release the oil, usually not higher than 60–80°C.The pressed citrus oils are commercially produced by expression with eitherhydraulic press or screw-type press, as shown in Fig. 5. A drawback to coldpressing is that recovery of oil is lower than from solvent extraction. The yieldsto make an essential oil from raw materials are about 2 wt% in eucalyptus, 0.7wt% in lavender, 0.2 wt% in rosemary, 0.1 wt% in jasmine, and less than 0.05wt% in rose.

Figure 5 Plate press.

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E. Enfleurage

In this technique, the flavors released by flowers are absorbed with fats for along time. The petals of flower are spread over large sheets of glass that arecovered with lard. The sheets are then pressed together. The flowers are refilleduntil the lard becomes fully charged. The oil is extracted by an alcohol solvent.Carrot oil is produced by using a carrot root extract macerated in vegetable oil.Carrot oil is rich in β-carotene, vitamins B, C, D, and E. It is useful as a skinrejuvenator and is recommended for treatment of dry and aging skin.

F. Maceration (Hot Enfleurage)

Maceration is one of the oldest techniques for preserving plant flavor. In thismethod, flowers are directly dipped in hot liquid fats that absorb essential oils.This produces pomades which consist of fats with flavor.

II. EXTRACTION SOLVENTS

The choice of solvent is the primary consideration for an extraction process,and the selectivity of the solvent is of special importance. The required solventcan be selected based on the similar polarity with the solute.

According to the Part I of the Annex of the Council Directive on theapproximation of the laws of the member states on extraction solvents used inthe production of foodstuffs and food ingredients (7), propane, butane, butylacetate, ethyl acetate, ethanol, carbon dioxide, acetone, and nitrous oxide arethe solvents to be used in compliance with good manufacturing practice (GMP)for all use. An extraction solvent is considered as being used in compliance withgood manufacturing practice if its use results only in the presence of residues orderivatives in technically unavoidable quantities presenting no danger to humanhealth. In Parts II and III, extraction solvent for which conditions of use arespecified (Table 1).

III. APPLICATION OF SOLVENT EXTRACTIONOF FLAVORS AND AROMAS (1, 3–12)

A. Anise Star

Anise star (Illicium verum) is so named from the star shape of its fruit. In Asia,it is often chewed in small quantities after meals to promote digestion andfreshen the breath. The plant, which is cultivated in China, Indo-China, Japan,

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Table 1 Parts II and III of the Annex of the Council Directive

PART II Extraction solvents for which conditions of use are specified

Condition of use (summary Max. residue limits in extractedName description of extraction) foodstuff or food ingredient

Hexanea Production or fraction of fats 5 mg/kg in the fat or oil or co-and oils and production of co- coa buttercoa butter

Preparation of protein products 10 mg/kg in the food containingand defatted flours the protein products and defat-

ted floursPreparation of defatted cereal 5 mg/kg on the defatted cereal

germs germDefatted soya products 30 mg/kg in the soya product as

sold to the final consumerMethyl acetate Decaffeination of, or removal of 20 mg/kg in the coffee or tea

irritants and bittering fromcoffee and tea

Production of sugar from mo- 1 mg/kg in the sugarlasses

Ethylmethylketonea Fractionation of fats and oils 5 mg/kg in the far or oilDecaffeination of, or removal of 20 mg/kg in the coffee or tea

irritants and bittering fromcoffee and tea

Dichloromethane Decaffeination of, removal of ir- 2 mg/kg in the roasted coffeeritants and bittering from cof- and 5 mg/kg in the teafee and tea

Methanol For all uses 10 mg/kgPropan-2-ol For all uses 10 mg/kg

PART III Extraction solvents for which conditions of use are specified

Max. residue limits in foodstuff due to useof extraction solvents for preparation of flavorings

Name from natural flavoring materials (mg/kg)

Diethyl ether 2Hexane 1c

Methyl acetate 1Butan-1-ol 1Butan-2-ol 1Ethylmethylketone 1c

Dichloromethane 0.02

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Table 1 Continued

Max. residue limits in foodstuff due to use of extraction solventsName preparation of flavorings from natural flavoring materials (mg/kg)

Methlypropan-1-ol 1Propan-1-ol 1Cyclohexane 1

aHexane means a commercial product consisting essentially of acyclic saturated hydrocarbons con-taining six carbon atoms and distilling between 64°C and 70°C. The combined use of hexane andethylmethylketone is forbidden.bThe presence of n-Hexane in this solvent should not exceed 50 mg/kg. This solvent may not beused in combinatoin with hexane.cThe combined use of these two solvents is forbidden.Source: Ref. 7.

and Philippines, is an evergreen tree up to 12 m high with a tall, slender, whitetrunk. Its fruit consists of 5–13 seed-shaped follicles attached to a central axisin the shape of a star. The best time to collect the seeds when they begin to turnfrom green to grayish brown is in the morning while the dew is still on them.Store the seeds in a tightly sealed opaque container to preserve the volatile oil.The essential oil is traditionally obtained from seeds and dry flowering tops bysteam distillation, which is replaced by liquid CO2 extraction (8% yield) orsupercritical CO2 extraction (10% yield). The essential oil is a pale yellow liquidwith a warm, spicy, extremely sweet, licorice-like scent. Major components ofthe essential oil are trans-anethol, methyl chavicol, anisaldehyde, and cis-ocimene.It is mainly used in beverages and confectionery.

B. Beeswax

Beeswax absolute is a solid, waxy mass of pale yellow color and a very mild,sweet, oily odor, with a hay-like bodynote and a soft waxy backnote. Extractionis usually performed as a direct solvent washing of the beeswax. The mostcommon solvents used for commercial extraction are ethanol, ether, glycol, andwater. Selection of the solvent depends on the final use of the extract and ontechnical feasibility. The extraction yield is generally less than 1 wt%. Mostactive ingredients seem to be soluble in propylene glycol and ethanol. Feweringredients are soluble in water, but even water extracts show at least somebactericidal and fungicidal effects, as well as wound healing properties. Acetoneextracts have been used for production of shampoos and lotions.

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C. Calendula

Calendula (Calendula officianalis) oil is a herbaceous green sweet and darkbrown liquid. It is obtained by solvent extraction from the flowers and containscalendulin (a yellow resin), waxes, and a small amount of volatile oil. The CO2

extracted total of the calendula blossom is much thicker and richer than theselective extract because it contains the plant waxes and heavier phytochemicals.Calendula oil may be produced using cold pressed olive oil and infused withcalendula flowers that have been dried for 24–36 h to reduce the moisture con-tent.

D. Cardamon

Cardamon is cultivated in Guatemala, India, and Sri Lanka. It has a large, fleshyrhizome, and the alternate, lanceolate leaves are blades from 0.3 to 0.75 m long,smooth and dark green above, pale, glaucous green, and finely silky beneath.The oblong, gray cardamon (Elettaria cardamomum) fruit has many seeds andthey are gathered just before they are ripe. The dried ripe fruits (seeds) are handcrushed rather than machine treated so as to guarantee that the precious volatileoils are never subjected to heat. Then liquid CO2 extraction is used for avoidingheat. Traditionally it was extracted by steam distillation or solvent extractionwith ethanol or acetone. Cardamon seed essential oil is a colorless or very paleyellow liquid with a sweet-spicy, warm fragrance and a woody-balsamic note.Major components are 1,8-cineole and terpinyl acetate. It is used as a spice(especially in Arab countries) for baked gingerbread, fruits, and marinades andas an ingredient of curry.

E. Chamomile German

Chamomile German (Matricaria chamomilla), sometimes called wild chamo-mile, has flower heads about 1.9 cm broad, with about 15 white, strap-shaped,reflected ray florets and numerous tubular yellow, perfect florets. ChamomileGerman is now cultivated extensively in Hungary, Egypt, eastern Europe, andFrance. The simple daisy-like white flower head that is smaller than chamomileRoman is used to extract the essential oil. Chamomile German, produced bysupercritical carbon dioxide extraction, is very different from the traditionalsteam-distilled blue oil. The original color of the steam distilled essential oil isblue, but it becomes moderate green and finally dark brown when exposed tolight and air. Since CO2 extraction contains all the water-soluble parts of theplant, the oil is much thicker. The CO2 extracted oil has even stronger anti-inflammatory qualities than the steam-distilled essential oil, and has a morenatural, fruity odor.

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F. Coffee

In decaffeinating green coffee, a solvent must be selected that extracts caffeinewith high selectivity. Thus, the solvent does not extract other coffee ingredients.Moreover, the solvent employed must be completely removable from the treatedcoffee. Most known processes for decaffeinating green coffee employ benzene(C6H6), trichloroethylene (CCl2=CHCl), 1,2-dichloroethane (CH2Cl−CH2Cl),methylene chloride (CH2Cl2), or chloroform (CHCl3) on account of the selectivedissolving capability of these solvents for caffeine.

Successful extraction of caffeine from coffee bean was achieved by theGerman chemist Friedrich Ferdinand Runge in 1820 following the suggestionof his friend, the poet Johann W. von Goethe. Runge investigated the compo-nents of coffee that caused his insomnia and isolated caffeine for the first time.The first commercial decaffeination process was invented by Ludwig Roselius,a German coffee importer, who founded a company, Kaffee Hag in Bremen, in1906 with the brand Sanka. His success was based on the accidental steamtreatment of ruined coffee beans. Steaming elevates the moisture content andswells the bean to facilitate the extraction of caffeine. This discovery made itpossible to extract caffeine by benzene and produce decaffeinated coffee on alarge scale. In the first half of the 20th century, trichloroethylene was used untilit was shown to cause liver tumors in mice by the U.S. National Cancer Institute(NCI) in 1976. Since 1970s, most coffee makers have switched to other solvents,such as methylene chloride and ethyl acetate, or other types of processing todecaffeinate coffee.

The U.S. Food and Drug Administration (FDA) has authorized by regula-tion the use of both methylene chloride and ethyl acetate for coffee decaffeina-tion. According to an FDA report in the Federal Register, most decaffeinatedcoffee contains less than 0.1 ppm of residual methylene chloride, 100 times lessthan the maximal level of 10 ppm allowed by the FDA. Under European law,the level of methylene chloride residue in decaffeinated beans must be less than2 ppm. Since the use of the solvents has given rise to objections about health,most coffee producers no longer use methylene chloride.

In other processes, esters, ketones, light hydrocarbons, and ethers havebeen employed. However, esters are readily saponified, and it is difficult tosufficiently remove saponification products, i.e., acids and alcohols, from cof-fee. Due to their relatively high polarity ketones have insufficient selectivity forremoval of caffeine. Hydrocarbons and ethers dissolve caffeine only sparingly,so that relatively high temperatures and long treating periods must be employedwhen these solvents are used. One of the newest decaffeination methods usesan orange peel extract. The green beans are soaked in pure water and washedwith an orange peel extract to remove the caffeine.

There are four main methods to extract caffeine from coffee beans:

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1. In the decaffeination process with methylene chloride, the green beansare placed in a rotating drum and soaked by steam for approximately 30 min.This treatment swells the beans, increasing their surface area and making thecaffeine easier to remove. The next stage is extraction of the caffeine by methyl-ene chloride at a temperature close to the boiling point of the solvent. Methylenechloride (CH2Cl2) extracts the caffeine from the coffee beans by bonding to thecaffeine molecules (13). The caffeine-laden methylene chloride is drained away,and the beans are steamed for 9–12 h, evaporating the remaining solvent off thebeans. Since methylene chloride actually bonds to the caffeine instead of justdissolving it, the caffeine evaporates with the solvent and does not remain inthe coffee. At the final stage, air or vacuum drying removes excess moisturefrom the decaffeinated beans. In the indirect-contact method, sometimes calledas the “water process,” the green beans soak for several hours in a water-coffeesolution at a temperature near the boiling point. Gradually the solution drawsthe caffeine, as well as other flavors and aroma components, from the beans.The caffeine-water mixture solution is drained off and the caffeine is removedfrom water using methylene chloride, which absorbs the caffeine. The caffeine-free water, with all the remaining desirable flavor components, is returned tothe beans. After drying the beans, most of the coffee oils and flavor elementsare regained on to them. Methylene chloride itself never come in contact withthe beans. The roasting and grinding process further evaporates any minute resi-dues of methylene chloride. Although methylene chloride levels in the coffeebean are reduced to residual levels that are “legally safe," health-conscious con-sumers consider these residues unacceptable. Most decaffeinated coffee produc-ers no longer use methylene chloride because it is now strongly suspected tocause cancer in humans.

2. Ethyl acetate decaffeination is often referred to as “naturally decaf-feinated” because ethyl acetate is compound found in apples, peaches, pears,orange peel, and other fruits (14). Many processes employed natural ethyl ace-tate obtained from the fermentation of sugar cane. However, in decaffeination asynthetic chemical is used due to high cost of natural ethyl acetate. The processis the same as in conventional methylene chloride decaffeination (direct-contactmethod), except that ethyl acetate replaces methylene chloride as the solvent.There are still many commercial operations using ethyl acetate extraction inpractice in the world.

3. Water decaffeination is, for most people, synonymous with SwissWater Process, which is a trademarked name. The green coffee beans are firstsoaked thoroughly in pure water or steam under pressure, making the caffeinesoluble so that it can be drawn out easily (12). The aqueous solution is full ofmany desirable flavors and aroma components as well as caffeine. The solution isdrained off, and the coffee thrown away, because it is now flavorless. The caffeine-rich solution is then passed through an activated carbon bed, which selectivelyremoves the caffeine but not the flavor. This flavor-saturated solution is then

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poured onto a new batch of coffee. Because the liquid is already full of flavor,this flavor-charged water doesn’t extract any additional flavor from the coffeebeans. It does, however, extract the caffeine. Advantage of this method is that nochemicals are used, so that no residual chemicals are left in the beans. This methodsuccessfully removes approximately 95% of the caffeine while retaining more ofthe flavor compounds present in the essential oils than the chemical solvent extrac-tion. However, this method results in the loss of some other water-soluble compo-nents of coffee, such as carbohydrates. To overcome this problem, the activatedcarbon is pretreated with a carbohydrate, typically sucrose. This pretreatment pro-cess helps it to absorb caffeine without removing other compounds.

4. Carbon dioxide (CO2) decaffeination uses pressurized CO2 (a densefluid) as a solvent to dissolve and draw the caffeine from the coffee beans,leaving the larger-molecule flavor components behind (15). Green beans arefirst softened by steam for higher selectivity of caffeine before loading theminto an extraction vessel. Supercritical CO2 is introduced into the vessel to dis-solve the caffeine. The caffeine-laden CO2 is then transferred to a separatescrubbing vessel where the caffeine is absorbed by water. Carbon dioxide iscirculated until the caffeine content of the beans is reduced to 0.02% or less.The supercritical CO2 is then drawn off, leaving the beans free of caffeine. Inthis extraction process, no residual chemicals are left in the beans but it producesthe most flavorful decaffeinated coffee. The CO2 decaffeination process is supe-rior to other decaffeination methods because it removes caffeine while leavingthe coffee’s flavor compounds intact. CO2 is a gas at normal atmospheric pres-sure and temperature, so it is not usually thought of as a solvent. However,under pressure it becomes a dense, liquid-like fluid. While in this liquid state,CO2 has the ability to selectively bond with and dissolve other materials. It isthis selectivity that makes CO2 decaffeination unique. While CO2 removes99.9% of caffeine from coffee, it does not affect the carbohydrates (sugars,starch) and peptides (protein), which are ultimately responsible for the flavorand aroma of brewed coffee. Since carbohydrates and peptides are large polarmolecules, nonpolar CO2 extracts only the caffeine molecules, which are smallnonpolar molecules. CO2 decaffeination technology was discovered and devel-oped at the Max Planck Institute in Germany in the 1970s. Yet this process isnot widely used because of the high costs of opening a CO2 decaffeination plant.However, as competition for sales of decaffeinated coffee increases, consumerswill demand decaffeinated coffee that is processed without harmful chemicalsbut is full-flavored and aromatic.

G. Coriander

Coriander (Coriandrum sativum) is an annual herb that belongs to the carrotfamily (Umbelliferae). It is cultivated in East Asia, Hungary, the Mediterranean,Morocco, North America, Poland, and Russia. Coriander brown seeds, which

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are sweet and vaguely reminiscent of orange peel, are used to obtain a sweet-smelling, spicy essential oil. The essential oil obtained through steam distillationof the fruits is a colorless or pale yellow liquid, and the flavor is described asmild, sweet, and spicy-aromatic yet somewhat warm and slightly burning. Cori-ander oil would be prepared by steam distillation of dried fruits (16). To obtainthe maximal yield of essential oil and to reduce the processing time, it is neces-sary to crush the spice prior to distillation. The extraction yield of partiallydried ripe coriander seeds by steam distillation or hydrodistillation is up to 1.7%essential and up to 20% fatty acid. Liquid CO2 extract of seed has 3% essentialoil with 50% of low molecular weight saturated solid lipids. Supercritical CO2

oleoresin extract after separating the lipids has 1.3% volatile oil. Essential oilof coriander is used in perfumes, alcoholic beverages, baked goods, candies, icecreams, chewing gums, meats, sauces, curries, and tobacco. Demand is expectedto grow due to increased consumption of ethnic foods and growth in the meat-processing industry.

H. Elemi

Elemi (Canarium luzonicum ) is a tropical tree up to 30 m high grown inPhilippines and Indonesia. It yields a resinous pathological exudation witha flesh, citrus-like, peppery odor. Although it is called a gum, it is almostentirely made up of resin and essential oil. Steam distillation has beenused to prepare essential oils. Nowadays elemi essential oil is extractedby liquid CO2 from its resin and essential oil is a colorless to pale yellow li-quid with a light, fresh, balsamic-spicy, lemon-like odor. The major compo-nents of elemi oil are limonene, α-phellandrene, and sesquiterpene alcoholelemol.

I. Frankincense

Frankincense (Boswellia carterii), originated from Oman, Ethiopia, Somal-ia, South Arabia, and China, yields a natural oleo gum resin that is collectedby making incisions in the bark. At first, a milky white liquid appears whichthen solidifies into amber or orange-brown crystals of resin. It is preparedby CO2 extraction or steam distillation of selected oleo gum resin, and essen-tial oil is a warm, woody, sweet balsamic, spicy fragrance with a hint oflemon.

J. Galbanum

Galbanum (Ferula galbaniflus), originated from the Middle East, is a large pe-rennial herb with a smooth stem, shiny leaflets, and small flowers. It contains

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resin products that exude a milky juice, a natural oleoresin. The dried resinousexudate is collected by cutting at the base of the stem or upper part of theuncovered roots. Galbanum resinoid is produced by extraction of gum with anonpolar solvent. The essential oil is obtained from resinoid and the extractionmethod is turning from steam distillation to CO2 extraction. The essential oil isa yellow liquid with a green, slightly spicy odor. Both resinoid and essential oilare used for creating green top notes.

K. Ginger

Ginger (Zingiber officinale), originated from China, Fiji Islands, Indonesia, Ja-maica, Malaysia, Nigeria, Taiwan, and the West Indies, is a perennial herb upto 1 m high with thick, spreading, tuberous roots, which are very pungent. Gin-ger oil is produced by steam distillation of dried crushed rhizomes, whereasginger oleoresin is prepared by extracting ginger rhizomes with acetone or alco-hol. Ginger oil is a light yellow liquid and its major components are zingiberene(35–40%), Ar-curcumene (18%), and β-sesquiphellandrene. Ginger CO2 extractis much more aromatic than the steam-distilled version. Like other CO2-producedoils, ginger CO2 seems much closer to the scent and taste of a freshly gratedroot of fresh ginger, rather than the scent of the dried ginger found in the steam-distilled oil. Ginger concentrates are used in beverages like ginger ale, sweetbaked goods, confectionery, curry powder, meats, and cordials.

L. Heliocarrot

Heliocarrot (Radix daucus carota) essential oil prepared by CO2-extracted driedcarrot roots into pure jojoba oil is extremely high in β-carotene, as well asvitamins A and E. CO2 extract is much more concentrated than many of theinfused carrot root oils available on the commercial market.

M. Hop Oils

Hop oils consist principally of hydrocarbons, oxygenated compounds, and smallamounts of sulfur-containing compounds. The hydrocarbons typically make up80–90% of the total oil; the terpenes myrcene and β-pinene, and the sesquiter-penes β-caryophyllene and α-humulene are found in the largest quantities. Twoof these, β-caryophyllene and α-humulene, can be easily oxidized in air, thuscontributing to the oxygenated fraction of the oil as well. Other oxygenated com-pounds include alcohols such as linalool and geraniol and esters such as geranylisobutyrate and methyl dec-4-enoate. Although many brewers think that estersare all fermentation byproducts, hops can contribute a number of fruity aromas,e.g., grapefruit and pineapple.

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For years, oils and the hop aroma have been extracted with steam distilla-tion. However, the heat of steam distillation changes the hop aroma profile.Both oils and α acids have been extracted with solvents such as ethanol, ether,hexane, and methylene chloride. During the extraction with hexane, highly con-centrated soft resins in solution can result in a considerable uptake of hardresins. In ethanol extraction, a large amount of water-soluble components, suchas inorganic salts, nitrates, and tannins, will be extracted. Heating of the extractto remove solvent markedly modifies its aroma profile. There are also concernsthat some solvent remains. Recently, CO2 has been used to extract oils and αacids without these problems (17, 18). Liquid CO2 (typically at 6 MPa pressureand 5–10°C in extraction plants) is a relatively mild, nonpolar solvent that ishighly specific for hop soft resins and oils. The low temperature and selectivityassociated with liquid CO2 extraction allows recovery of the hop oils in undam-aged condition. Therefore, liquid CO2 becomes the most selective solvent usedcommercially for hops and hence produces the purest whole resin and oil ex-tract. It extracts none of the hard resins or tannins, much lower levels of plantwaxes, no plant pigments, and less water and water-soluble materials. However,the yield of α acids with liquid CO2 (89–93%) is lower than that of supercriticalCO2 (91–94%) or the organic solvents (93–96%). Above the critical point (typi-cally at 30 MPa pressure and at 60°C in extraction plants), CO2 has the proper-ties of both a gas and a liquid and is a much stronger solvent. Supercritical CO2

is more selective than the organic solvents and extracts less of the tannins andwaxes and less water and water-soluble components. It does extract some of theplant pigments like chlorophyll but rather less than the organic solvents do. Thismodification of the hop aroma profile also applies to some extent to supercriticalCO2 extracts, which are produced at about 60°C. Supercritical CO2 also extractsless oil than does liquid CO2 but more than the organic solvent extracts. Asmentioned, liquid CO2 extracts are the most pure whole-resin extracts; more-over, the low-temperature extraction (5–10°C) results in an aroma profile mostclosely resembling that of the leaf hops from which they were prepared. Theonly real disadvantage of liquid CO2 extract in comparison with the others is itshigher cost resulting from the lower extraction efficiency.

N. Jasmine

Jasmine extraction is one of the most important businesses nowadays. The ex-otic scent of jasmine comes from the flowers of Jasminum officinale, whichgrows in Algeria, Egypt, France, India, Italy, Morocco, South Africa, Spain, andTurkey. The star-shaped, very fragrant white flowers are used to obtain jasmineabsolute via the concrete. Hexane is normally used as solvent in solvent extrac-tion. Sometimes jasmine is extracted with benzene. Multistage extraction is car-ried out with hexane getting richer in perfumery content with each stage. Evapo-

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ration is then carried out in the falling-film evaporator until 90% of the hexaneof the mother liquor is evaporated. Concentrated liquor is distilled under vacuumto obtain the final jasmine concrete. The three-stage extraction of jasmine blos-soms with hexane gives a 0.3 wt% yield of concrete and the extraction of theconcrete with ethanol gives a 60 wt% yield of absolute. Jasmine is the mostexquisite of scents; the absolute is a deep, reddish brown liquid with a sweet,floral and exotic, slightly heady fragrance, whereas the concrete is a brownwaxy mass. Even though benzyl acetate is major volatile component of jasmineoil, indole, cis-jasmine, and methyl jasmonate contribute strongly to the typicaljasmine fragrance.

O. Linden

Linden is a deciduous tree growing to 30 m high with wide, heart-shaped leaveswith a whitish down on the underside, particularly on the veins. The small, ex-tremely fragrant white flowers grow in drooping clusters on long stalks. Linden(Tilia vulgaris) blossom absolute is extracted from concretes or pomades, whichare produced by steam. Other methods include solvent extraction, infusion, wa-ter distillation, and CO2 extraction. It is greenish, viscous, light, floral, and sweet.

P. Myrrh

Myrrh oil is obtained by either CO2 extraction or steam distillation of hand-picked and selected Burseraceae oleo gum resins produced by Balsamodendronmyrrha. CO2-extracted myrrh (Commiphora myrrha grows in Northeast Africaand Arabia), a rich golden resinous liquid, contains more components of theactual myrrh beads than does the steam distilled essential oil, which is a paleyellow to amber viscid clear oil with a characteristic warm, sweet balsamic,slightly spicy-medicinal odor. The most active ingredient of the myrrh flavor islindestrene. Myrrh resinoid is prepared by solvent extraction of the gum withhexane. The oil, resinoid, and tincture are used in pharmaceutical products, in-cluding mouthwashes, gargles, and toothpaste; they are also used in dentistry.The oil and resinoid are used as fixatives and fragrance components in soaps,detergents, cosmetics, and perfumes, especially oriental types and heavy floral.The essential oil is also used as a flavoring in most major food categories,alcoholic and soft drinks.

Q. Nutmeg

Nutmeg (Myristica fragrans), which is an apricot-like fruit of an evergreentree up to 20 m high with grayish brown smooth bark, dense foliage andsmall dull-yellow flowers, is cultivated in Madagascar, Indonesia, Sri Lanka,

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and the West Indies. Nutmeg essential oil is obtained from the dried nutmegseed by solvent (alcohol) extraction, steam distillation (6–16% yield), orliquid CO2 extraction (16% yield) (19). Most active ingredients of the nut-meg flavor are sabinene, α-pinene, β-pinene, and myristicin. It is a paleyellow liquid with a pleasant, warm, spicy, aromatic odor. Nutmeg oil is usedas seasonings for curries, vegetables, baked goods, and processed meats.

R. Rose

Rose concrete and rose absolute can be obtained by solvent (hexane) extraction,whereas rose essential oil is derived from steam distillation. The extraction ofrose flower with steam distillation gives about 0.025 wt% yield of rose oil. Thepremium rose perfume is the otto or attar, which is obtained by water and steamdistillation from the rose blossom (Rosa x damascena). A perforated grid is usedfor supporting the rose flowers; thus, only rising steam contacts with the plantmaterials while water boils in the still. The steam, which contains flavor andaroma, is condensed and collected. The waxy constituents of the rose oil thatfloat on the water are decanted off. The lower layer water is distilled again torecover any remaining oil. Both are combined to make rose oil. Since a lot ofphenyl ethyl alcohol is dissolved in the distillation water, the otto does notaccurately represent the rose flower flavor. The extraction yield is generally lessthan 0.02 wt%. Solvent extraction is now more frequently used to extract thearoma of rose blossoms. The product of solvent (hexane) extraction is a waxy,light brown, semisolid material known as a concrete. The concrete has phenyl-ethyl alcohol in the same ratio as the blossoms, so that it represents the roseflower flavor intimately. Rose absolute is obtained from the concrete by extrac-tion with a polar solvent like alcohol. The absolute is a reddish liquid with adeep, rich, sweet, rosy-spicy, honey-like fragrance. The phenylethyl alcoholcontent of its volatile portion is 60–70%. Typically, solvent extraction yieldsabout 10 times that obtained by steam distillation, in the order of 0.1–0.2% (1–2mL concrete/kg flowers). Rose oil is used primarily as a fragrance component inpharmaceutical preparations (e.g., ointments and lotions) and is extensively usedas a fragrance ingredient in perfumes, creams, and soaps. Rose oil and absoluteare also used extensively as flavor ingredients in fruit-type flavors. Food prod-ucts in which they are used include beverages, frozen dairy desserts, sweets, bakedgoods, gelatins, and puddings.

S. Rosehip

A soft golden red oil of rosehip (Rosa rubignosa Germany; originated southernAndes) is obtained from its red berry-like fruits or hips by CO2 extraction or

Flavor and Aroma Substances 325

cold pressing process. Rosehip is a rich source of vitamin C that is made into adrink for babies and young children. Those are contained in the rosehip shell.The oil extracted from the seeds contains essential polyunsaturated fatty acids,and trans-retinoic acid, which is now known to promote scar healing and moreyouthful-looking skin.

T. Tuberose

Tuberose (Polianthes tuberosa) is cultivated in Egypt, India, and Morocco.Large, very fragrant, white lily–like tuberous blossoms are used to extract abso-lute. The solvent extracted absolute is a dark orange or brown soft paste, witha heavy, sweet-floral, sometimes slightly spicy, tenacious fragrance.

U. Vanilla

Vanilla (Vanilla planifolia) has been one of the most important and most publi-cized flavors since its discovery in Mexico. The vanilla beans of commerce arethe cured, unripe fruit of Vanilla planifolia, Mexican or Bourbon vanilla, whichis native to Mexico, Central America, and northern South America; or Vanillatahitensis, Tahiti vanilla, which is native to Oceania. The principal sources ofvanilla are Madagascar, the Comoros, and Reunion, which together furnishabout 70–75% of the world’s supply, and Mexico, Uganda, and French Polyne-sia. Vanilla extracts are used extensively in chocolate, dairy products (milk, icecream, etc.), beverages (colas), baked goods, and confections. Vanilla is alsoused as a background note or flavor enhancer (or sweetness enhancer) to roundout the flavor profiles of many food products. The type of vanilla used dependson the product, the ingredients in the base formulation, and the desired flavorprofile. In solvent extraction, extracts are obtained by extracting crushed vanillabeans with a polar solvent (methanol, ethanol, acetone, or different concentra-tions of water/alcohol). Sometimes countercurrent extraction technique is usedto obtain vanilla essential oil. Concentrated vanilla extract is made by vacuumdistillation, which is often used to separate materials that might suffer thermaldegradation at the higher temperatures encountered in conventional distillation.However, distillation destroys some of the aromatic substances of vanilla flavor.Solvent is removed until the desired concentration is reached. Vanilla essentialoil is a viscous dark brown liquid with a rich, sweet, balsamic, vanilla-like odor.Liquid CO2 extracted absolutes (4.5% yield) have been tried with success inhigh-quality ice cream and desserts. Supercritical CO2 extraction with ethanolas a cosolvent gives essential oil (10% yield) rich in natural vanilla aroma andfine crystals of vanillin. The basic key ingredients of the vanilla extract arevanillin and phenol derivatives.

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V. Violet

Violet is extracted by solvent extraction. A small, tender, perennial plant with darkgreen, heart-shaped leaves and fragrant violet-blue flowers are the parts of theplant (Viola odorata) used for preparing absolutes. The leaf absolute is an intensedark green viscous liquid with a strong green-leaf odor and a delicate floral under-tone. The flower absolute is yellowish green viscous liquid with a sweet, rich,floral fragrance, characteristic of fresh flowers. Most active ingredient of the violetleaf absolute is 2-trans-6-cis-nonadienal, which is responsible for the flavor.

W. Tolu Balsam

Tolu balsam is formed after injuries in the trunk of Myroxylon balsamum tree,native of the jungles of northern South America, particularly, in Colombia, Hon-duras, Peru, and Venezuela. The balsam is a brown, orange-brown, or darkyellowish brown mass. It is brittle when cold, and the fracture is glass-like orflint-like. Its odor is sweet balsamic, cinnamic in type, faintly floral, and with abacknote of vanillin. The fresh balsam is soft and sticky, but exposure to the airmakes it hard and brittle, more like resin, with a crystalline appearance. Theresinoid is prepared by extraction of the balsam, while essential oil is obtainedby distillation of the balsam. Tolu balsam absolute is used mainly as a fixativein citrus colognes, oriental perfumes, chypres, and floral bases.

REFERENCES

1. JA Rogers Jr, F Fischetti Jr. Flavors and spices. In: M Grrayson , ed. Kirk-OthmerEncyclopedia of Chemical Technology, 3rd ed., Vol. 10. New York: John Wiley &Sons, 1980, pp. 456–488.

2. W Hamm. Liquid-liquid extraction in the food industry. In: TC Lo, MHI Baird, CHanson, eds. Handbook of Solvent Extraction. New York: John Wiley & Sons,1983, pp. 593–603.

3. B Meyer-Warnod. Natural essential oils: Extraction processes and application tosome major oils. Perfumer and Flavorist 9:93–104, 1984.

4. WG Schultz, JM Randall. Liquid carbon dioxide for selective aroma extraction.Food Tech 24:1282–1286, 1970.

5. DA Moyler. Extraction of flavours and fragrances with compressed CO2. In: MBKing, TR Bott eds. Extraction of Natural Products Using Near-Critical Solvents.Glasgow: Chapman and Hall, 1993, pp. 140–183.

6. KU Sankar. Supercritical fluid carbon dioxide technology for extraction of spicesand other high value bio-active compounds. In: SSH Rizvi, ed. Supercritical FluidProcessing of Food and Biomaterials. London: Chapman and Hall, 1994, pp. 155–166.

7. Official Journal L 157, 24/06/1988 p. 0028–0033

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8. K Bauer, D Garbe, H Surburg. Flavors and fragrances. In: W Gerhartz, ed. Ul-mann’s Encyclopedia of Industrial Chemistry, 5th ed., Vol. A11. Weinheim, Ger-many: VCH, 1988, pp. 141–250.

9. C Karl. Herbs, spices and essential oils. In: E Ziegler, H Ziegler, eds. Flavourings.Weinheim: Wiley-VCH, 1998, pp. 184–218.

10. M Grieve, CF Leyel, eds. A Modern Herbal. New York: Dover Pubns, 197811. http://www.aromabar.com.au/database/essentialoils.htm12. http://www.darwinsgarden.com13. L Nutting, GS Chong. Continuous process for producing decaffeinated beverage

extract. US Patent 3,361,571 (1968).14. JM Patel, AB Wolfson, B Lawrence. Decaffeination process. US Patent No.

3,671,263 (1972).15. E Lack, H Seidlitz. Commercial scale decaffeination of coffee and tea using super-

critical CO2. In: MB King, TR Bott, eds. Extraction of Natural Products UsingNear-Critical Solvents. Glasgow: Chapman and Hall, 1993, pp. 101–139.

16. T Hirvi, I Salovaara, H Oksanen, E Honkanen. Progress in essential oil research.In: EJ Brunke, ed. Proceedings of International Symposium on Essential Oils. Ber-lin: Walter de Gruyter, 1986, pp. 111–116.

17. DS Gardner. Commercial scale extraction of alpha-acid and hop oils with com-pressed CO2. In: MB King, TR Bott, eds. Extraction of Natural Products UsingNear-Critical Solvents. Glasgow: Chapman and Hall, 1993, pp. 84–100.

18. DS Gardner. Industrial scale hop extraction with liquid CO2. Chem Ind 19:402–405, 1982.

19. BM Lawrence. Major tropical series on nutmeg and mace. Essential Oils 1976–1978:51–103, 1978.

10Extraction of Natural Antioxidants

Vassiliki OreopoulouNational Technical University of AthensAthens, Greece

I. INTRODUCTION

Natural antioxidants are compounds from plant or animal sources that retardoxidative rancidity of oils, fats, and fat-soluble components, thus protectingthem while delaying the development of unpleasant flavors and odors resultingfrom oxidation.

Antioxidants are present naturally in most raw food sources. Processingcan remove or degrade some of these antioxidants. Therefore, a supplementationwith suitable antioxidant compounds is needed to maintain acceptable qualityof the products. Especially oils, fats, and products with a high fat content aresusceptible to oxidation and require the addition of antioxidants. The mostwidely used antioxidants are synthetic ones, such as butylated hydroxyanisole(BHA), butylated hydroxytoluene (BHT), tertiary butylhydroquinone (TBHQ)and propyl gallate (PG). Doubts about the safety of synthetic antioxidants arosefirst in the 1960s and led to an increased interest and a broad research on naturalantioxidants (1, 2). Natural antioxidants are primarily phenolic compounds thatmay occur in all parts of a plant. They are multifunctional and can act as freeradical terminators, metal chelators, and singlet oxygen quenchers. The commonplant phenolic antioxidants are tocopherols, flavonoids, and related compoundslike coumarins, cinnamic acid derivatives and chalcones, phenolic diterpenes,and phenolic acids. Another widespread antioxidant in nature is ascorbic acid,but today its extraction from natural sources is not significant as all ascorbicacid used for food is chemically synthesized.

To be considered as an antioxidant for practical use, a compound extractedfrom a natural source must meet several criteria, including (a) absence of any

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toxic or physiological effect, (b) no impartation of any strong odor, flavor, orcolor to the product, and (c) considerable antioxidant activity at small concentra-tions in the product. In fact, antioxidants present in or added to foods are func-tional at very small concentrations, usually up to 0.02%.

Usually, in literature surveys on antioxidants, compounds that have noantioxidant activity themselves but enhance the activity of antioxidants, calledsynergists, are included. This chapter covers the main sources and extractionprocesses of natural antioxidants and synergists.

II. SOURCES

The main sources for the extraction of natural antioxidants are of plant origin.Some animal sources, such as shrimp, have also been reported (3). The mostcommon plant sources are (a) cereals, (b) citrus fruits, (c) cocoa and coffeebeans, (d) herbs and spices, (e) oilseeds, (f) olives, (g) onion and garlic, (h) tea,as well as some miscellaneous products.

A. Cereals

Several cereals have been reported to possess antioxidant activity. Oat has beenthe most widely investigated because it has greater activity than the others anda bland flavor. Oat flour has been incorporated in dairy products, potato crisps,sausages, mayonnaise, etc., and improved their stability. Malt and barley flouror meal acted as antioxidants on fats too (2).

Extracts of groats and hulls from oat were more active than syntheticphenolic antioxidants in the protection of oils during frying and of emulsions(4, 5). The antioxidant activity of oat hulls and groats is attributed to severalphenolic compounds, mainly ferulic, p-coumaric, caffeic, vanillic, p-hydroxy-benzoic acids and vanillin (6). The extract of defatted oat kernels also showedantioxidant activity similar to BHT and PG that was attributed to caffeic andferulic acid and their derivatives (7, 8). Oat hulls contain more phenolic acids thanoat flour; therefore, oat hulls are more efficacious for antioxidant extraction (9).

Byproducts of other cereals could be used as raw materials for antioxidantextraction, too. Buckwheat hulls extracts showed a good antioxidant activity,which was attributed to the presence of dihydroxy phenolic components, flavo-nols, and flavone glucosides (10, 11).

B. Citrus Fruits

Citrus fruits and especially oranges contain antioxidant components (12, 13).These antioxidants are mainly concentrated in the outer part of the peel, called

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flavedo. Extracts of orange and grapefruit flavedo had significant activityagainst d-limonene oxidation, but extracts of flavedo from lemon and lime hadvery little activity (13). On the contrary, extracts of lemon peel offer effectiveprotection from peroxidative damage in living systems (14).

Antioxidant properties of extracts from orange flavedo were attributed totocopherols (2, 13). Also citrus essential oils and terpenes showed antioxidantactivity in several food products (2). However, flavonoids isolated from variouscitrus extracts seem to be the most important antioxidant components present incitrus fruits. More than 60 individual flavonoids have been identified in citrusfruits (15). Among them, flavanones are the most abundant, and especially hes-peretin and naringenin are presented in the pulp of all citrus fruits (16). Highlymethoxylated flavones occur in much lower concentration but exhibit the great-est antioxidant activity. Two isoflavones are referred as the main antioxidantcomponents of osage orange peel (12), while a flavanone, eriocitrin and itsaglycon, eriodictyol, are referred as the main antioxidant components of lemonpeel (14). A review on citrus flavonoids and on the correlation of their structureto antioxidant activity has been presented by Benavente-Garcia et al. (17).

C. Cocoa and Coffee Bean

Cocoa bean husk powder and especially water and alcohol extracts were re-ported as potential antioxidants (18, 19). Fractions with antioxidant activity wereisolated from the extracts, but the active substances were not identified. Thebrown pigment was suggested as the antioxidant factor (2), though it was notactive to all substrates (20).

Roasted coffee powder showed antioxidant properties in oils. However,the antioxidant activity of the coffee powder was lower than that of its constit-uents: caffeic acid and quinic acid. Another potent antioxidant component incoffee is chlorogenic acid (2).

D. Herbs and Spices

Herbs and spices are the most broadly examined raw materials for the occur-rence and extraction of antioxidants. Since the pioneer work of Chipault et al.(21), many researchers have reported results on the antioxidant efficiency ofvarious herbs and spices and on the identification of active components. Thoughthe results are sometimes contradictory, the extracts have in most cases higheractivity than the relevant ground herbs or spices (2).

Rosemary has exhibited the greatest antioxidant activity of all spices andherbs. The extracts of rosemary were more effective than BHA and BHT in theprotection of fat and of products with a high fat content (22–24). Bleached,odorless, and tasteless antioxidants have been produced from rosemary and com-

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mercially exploited since the early 1980s (25, 26). The most active componentsof rosemary, identified in the earlier studies, were carnosol, rosemaridiphenol,and rosemariquinone (27–29). Nowadays several commercial rosemary extractsare available and their most active compounds are reported to be carnosol, car-nosic acid, and rosemarinic acid (30–32).

Sage belongs to the same family as rosemary—the Labiatae family—andpossesses almost equivalent antioxidant activity. Many investigations on rose-mary antioxidant efficiency also include data on sage efficiency (2), and thesame procedures for the extraction and purification of the antioxidant fractionfrom both spices have been suggested (25, 26). The main antioxidant constit-uents in sage were also found to be carnosol, carnosic acid, and rosemarinicacid, followed by other related phenolics (31, 33). In addition to the main sagespecies, i.e., Salvia officinalis, several other varieties have also shown remark-able antioxidant efficiency (34–36).

Other plants of the Labiatae family are potent sources of natural antioxi-dants (37). Oregano is one of the most promising. Extracts of oregano retardedlipid oxidation (37–40), while the essential oil of the spice also contained anti-oxidant components (41). Thyme seems to be another promising spice: extractsof the plant showed appreciable antioxidant activity (37, 42) and its essentialoil was effective (43). Dittany and marjoram extracts exhibited antioxidant ac-tivity too (2, 37).

Several other herbs, spices, and medicinal plants are rich in antioxidantcomponents (2, 42, 44–50).

E. Oilseeds

Oilseeds contain several antioxidant compounds that protect them against rancid-ity. Therefore, whole seeds or the byproducts obtained after oil extraction couldbe potential sources of antioxidants. Methanolic extracts of cottonseed exhibitedantioxidant activity attributed mainly to flavonoids (51). Flavonoids are alsoclaimed to be the major antioxidants of soybean methanolic extracts (52–54). Mostof these flavonoids remain in the waste during soybean curd processing and couldbe recovered (54). According to Rhee et al. (55), cottonseed meal had a higherphenolic content and antioxidant activity than soybean meal. Canola meal is an-other potent source of antioxidants with a total phenolic content remarkably higherthan both cottonseed and soybean meals (56). Other oilseeds studied with positiveresults include sunflower seed (57) and sesame seed (58).

F. Olives

Olives and olive oil are rich in antioxidant compounds, especially tocopherols.In addition to tocopherols, several other phenolic compounds, i.e., hydroxytyro-

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sol, caffeic, protocatechuic, ferulic, syringic, and vanillic acids, were identifiedin the extracts of virgin olive oil and were found effective in prolonging theshelf life of the refined olive oil (59, 60). Rape, a major byproduct of olive oilproduction, was used for the extraction of antioxidants. The extracts containedvarious phenolic acids, catechol, and tyrosol, and inhibited oxidative deteriora-tion of refined vegetable oils (61). Also, olive leaves contain phenolic antioxi-dant compounds, and some of them were identified (62–64).

G. Onion and Garlic

Onion contains appreciable amounts of the well-known flavonoid quercetin (16)and exhibits a remarkable antioxidant activity (65, 66). Onion skins are also richin quercetin and quercetin derivatives, and their methanolic extracts were effec-tive antioxidants (67, 68).

Garlic was found effective for some substrates, i.e., linoleic acid orminced pork (65), but ineffective in lard (44).

H. Tea

Tea leaves are a well-known source of natural antioxidants, especially catechins.Fresh or mildly processed commercial tea has been found to contain large quan-tities of these polyphenols, up to 30% of its dry mass (69). Comparable concen-tration has not been reported in any other foodstuff (70). Green tea extracts were21–24% more effective at radical quenching than black tea extracts in bothwater and lipid-soluble media (71). Aqueous extracts of green tea retarded theoxidation of oils and fats more than commercial rosemary extract (72), and werealso effective in emulsions (73) and fish meat (74). Commercial tea extracts areavailable with various catechin contents (74).

I. Miscellaneous Products

Various other fruits, vegetables, and grains contain considerable amounts ofantioxidants (16, 48, 75). Among them those that could serve as potential sourcesfor the extraction of antioxidants are the byproducts of agrofood industries or cropswith a limited utilization due to damage or to low commercial profit.

Grape skins and seeds are a byproduct of the wine making industry, witha high phenolic content and a remarkable free radical scavenging capacity andantioxidant activity in oils (76–80). The antioxidant activity of citrus peel andpulp was mentioned above, and similar byproducts from other fruit processingindustries can be used as well (81). Many other potential sources are referred,including bean hulls (82, 83), lupin derivatives (84), leaves of some agriculturalproducts (67, 85), potato peel waste (86), and so forth.

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III. EXTRACTION PROCESSES AND PARAMETERS

Although some of the raw materials mentioned above have been incorporatedas dry powders to certain foods or food systems in order to protect them fromoxidative deterioration, the use of extracts of the raw materials is suggested inmost cases. The aim of extraction is to concentrate the antioxidant componentsof the raw material, apart from inert substances, so that the product of the extrac-tion could be added to the food in smaller quantities.

The extraction process involves a more or less vigorous agitation of theground raw material with the extraction solvent at ambient or elevated tempera-ture and subsequent separation of the extract from the residue by filtration.Repeated extraction steps may be accomplished to increase the extract yield.Alternatively, a packed bed of the ground material can be used which is leachedby the extraction solvent under refluxing conditions.

A. Solvent Selection

The solvent used for the extraction is of major importance for the recovery ofthe antioxidant components, the coextraction of undesirable substances, and theprocess yield. For the extraction of antioxidants from the majority of the sourcesmentioned above organic solvents have been used. Water has been used to alesser extent, mainly for extracting antioxidants from tea and for the recoveryof essential oils through steam distillation. In addition, water is used in mixtureswith alcohols. Edible vegetable oils have been also reported and patented asextraction solvents.

Several studies focused on the efficiency of different organic solvents inextracting antioxidant components. The antioxidant components are of a pheno-lic nature; therefore, organic solvents of higher polarity are more effective inquantitative recovery of these substances than nonpolar solvents. For example,extracts obtained with methanol or ethanol from flour or hulls of cereals hadhigher antioxidant activity than extracts obtained with hexane, petroleum ether,ethyl ether or its mixtures with chlorophorm, acetone, and ethyl acetate (2, 4,10). Also, the extraction of various aromatic herbs by hexane, ethyl acetate, orethanol showed that the ethanol extracts were the most active in retarding theautoxidation process in lipid substrates (50). Similarly, in extraction of cocoabean husks with various organic solvents, the alcoholic extracts were the mosteffective (18). In fact, methanol or ethanol are widely used for the extraction ofantioxidants from rosemary, sage, and other herbs and spices, as well as fromoilseed and other agroindustrial byproducts. Aqueous solutions of methanol arealso used.

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Although methanol and ethanol are suitable for quantitative extraction oftotal phenolics, many undesirable substances are coextracted, and purification isnecessary to isolate the antioxidant fraction. Meanwhile, solvents of lower polar-ity may be used to recover and isolate special groups of phenolics. Thus ethylether yields a very low recovery of total phenols compared with 80% aqueousmethanol, but it can extract the low molecular weight phenolic acids (87) whichare good antioxidants. Both ethyl ether and hexane are suitable for the extractionof tocopherols and terpenoids with appreciable activity (40, 49). On the otherhand, pure methanol or aqueous solutions of methanol may cause some decom-position to certain phenolic glucosides, which has not been detected with ace-tone, aqueous acetone, or ethyl acetate (87).

Results obtained by successive extractions of various raw materials withdifferent solvents further indicated that the choice of the appropriate solventdepends on the raw material and the nature of the antioxidants it contains. Thus,in a sequential extraction of olive rape with hexane acetone and ethanol, thehexane extract, despite amounting to 9% of the dried rape, had very few poly-phenols and lower activity than either the acetone or the ethanol extract, whichhad similar behavior and amounted to 4% and 7% of the dried rape, respectively(61). A similar result was obtained when cocoa byproducts were extracted withpetroleum ether, or methanol, or successively extracted by petroleum ether fol-lowed by methanol. The methanol extract showed the highest activity whichwas further increased for the methanol extract obtained from the residue ofpetroleum ether extraction (88). Also, pretreatment of cocoa husks with petro-leum ether and chloroform before alcoholic extraction improved the antioxidantproperties of the alcohol extract (19).

In contrast in a sequential extraction of turmeric with hexane, benzene,and 80% aqueous methanol, all of the extracts had very good antioxidant proper-ties but the benzene extract showed the highest activity. The yield of the hexane,benzene, and 80% aqueous methanol extracts were 1.98%, 4.15%, and 5.24%on a dry basis, respectively (89). When another spice, oregano, was successivelyextracted with hexane, ethyl ether, ethyl acetate, and ethanol, the hexane andethyl ether extracts showed higher activity than the following extracts (40). Theyield of hexane extraction amounted to 8.8% on a dry basis and the main antiox-idant components were terpene derivatives. Yields of following extracts withethyl ether and ethyl acetate were rather low (1.5% and 2.8% d.b., respectively),while the ethanol extract yield was 12.3% d.b. The antioxidant components ofthe ethanol extracts were mainly flavonoids, but a lot of other substances werecoextracted; therefore, this extract had to be purified (90).

A comparative study of extraction yields and antioxidant activities of ex-tracts obtained by different solvents and sequential extraction steps, from severalaromatic herbs, was conducted by Dapkevicius et al. (46). Hydrodistillation was

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used to isolate the essential oils and the yields were greatly dependent on theplant, (as high as 6.9%). Except for thyme essential oil, the rest showed nosignificant antioxidant activity. Extracts obtained by acetone after hydrodistilla-tion or without previous hydrodistillation had good antioxidant activity. Theyield was higher when the acetone extraction was conducted without a previoushydrodistillation step and amounted to 11.3% and 6.6% for the rosemary andsage extract, respectively. Methanol-water (1:1) extraction of the residues ob-tained from the acetone extraction resulted in extracts with yields higher thanthe acetone extraction (except for rosemary), but with lower antioxidant activity.In the same study, the water extract of the herbs obtained after hydrodistillationwas tested and showed no antioxidant activity.

B. Effect of Extraction Parameters

The parameters that affect antioxidant recovery by aqueous ethanol extractionwere investigated by Wettasinghe and Shahidi (92) during the extraction of anoilseed meal. The concentration of ethanol in the extraction medium affectedmarkedly the recovery of antioxidants and best values lied in the range of 50–60%. The extraction temperature and time affected also the antioxidant activityof the extract. In particular, the activity increased with temperature and time upto 70°C and 60 min, respectively, and declined afterward. The optimal extrac-tion conditions were determined through response surface methodology as 52%ethanol in water, 74°C, and 62 min. In the same study, aqueous methanol oraqueous acetone were tested as extraction media with slightly lower antioxidantactivity than the ethanolic extracts.

Alcoholic extraction of other raw materials, like grape byproducts, is mosteffectively accomplished under acidic conditions. HCl acid is used and the pH isarranged around 3–3.5 (76). To avoid acid without decreasing phenolic recoveryseveral sequential extraction steps with alcohols and alcohol-water mixtureswere suggested (93). However, the hydrolysis of glucosidic bonds of polyphen-ols, induced by acid, is not necessarily undesirable as it might even increase theantioxidant activity of the relevant compounds.

Solvent extraction parameters for the isolation of phenolic antioxidantsfrom wood hydrolysates were studied by Cruz et al. (94). They used ethyl ace-tate or ethyl ether as extraction solvents and found that ethyl acetate was moreeffective in quantitative removal of phenolics. A single-stage extraction at asolvent/hydrolysate volume ratio of 3:1 or a two-stage extraction at a solvent/hydrolysate volume ratio of 1:1, and a contact time of 30 min were adequatefor the removal of 84% of the phenolics. Acidic pH conditions, i.e., pH 3,favored phenolic recovery, while temperature in the range of 10–40°C had noeffect. The antioxidant activity coefficient of the extract obtained under the bestconditions was equal to 64% of the relevant value of BHT.

Extract of Natural Antioxidants 337

Water extraction has been studied mainly for the extraction of variouscomponents from tea (95). The same kinetic model was fitted to the extractionof catechins, which are the main natural antioxidants present in tea, and ofcaffeine. The smaller catechin molecules (i.e., ungallated catechins) presented ahigher rate of extraction, supporting the hypothesis that the rate-determiningstep is a diffusion one through the leaf matrix to the surface (69). Also thepercent recovery after 20 min at 80°C of the gallated catechins decreased withincreasing concentration of the tea, while the recovery of the ungallated cate-chins was not affected (96). Another parameter that affected extraction effi-ciency was the pH of the water. Increase of the pH from 6 to 7.6 resulted indecrease of catechin recovery due to epimerization (96). Temperature depen-dence of the rate of extraction of individual catechins was different (69). There-fore, extraction parameters may be selected to allow preferential extraction ofsome catechins instead of others. Caffeine is coextracted with catechins by wa-ter, and its rate of extraction is higher than the corresponding catechin value forgreen tea and lower for black tea. Also, the effect of temperature on the rate isgreater for green than for black tea (69, 97).

Other parameters, which were proposed for the recovery of tea polyphen-ols, were extraction with water at 80°C, at a liquid-to-solid ratio of 14:1 (v/w),applying three successive extractions (98). For the recovery of flavonoids, etha-nol-water mixtures (7:3), at a liquid-to-solid ratio of 5:1 (v/w), at ambient tem-perature were used with success (99).

C. Extraction Procedures

Several extraction procedures have been proposed and some of them have beenpatented. In most cases purification of the extracts is included.

An extraction and purification process for rosemary and sage was patentedby Chang et al. (25). The solvent initially used was ethyl ether at a liquid-to-solid ratio of 2.4:1 and the extraction was conducted under reflux for 2 h. Theresidue obtained after filtration of the mixture was reextracted with ethyl etherunder the same conditions, and the filtrates were combined and freed of solventto yield up to 26% of crude antioxidant depending on the number of extractions.Methanol was also proposed as the extraction solvent in two reports (27, 91).To produce a bland, odorless, and tasteless antioxidant the crude extract wassubjected to molecular distillation or vacuum steam distillation to yield approxi-mately 10% of purified antioxidant. Kalsec Inc., which has the exclusive licenseto use this patent, modified the procedure to reduce color in its rosemary extractand manufactures two types of rosemary extracts with antioxidant inhibitors: anoil-soluble extract and a water-dispersible extract.

Sequential extraction of oregano with solvents of increasing polarity is apromising process. Hexane, ethyl acetate, and ethanol were used and the extrac-

338 Oreopoulou

tion was conducted under reflux through a fixed bed of the raw material. Theextracts obtained by hexane and ethyl acetate were light colored and could bebleached with active carbon, if desired. The methanol extract had to be furtherpurified in order to remove substances with no antioxidant activity. Final antiox-idant solutions in oil were prepared by thorough mixing with refined vegetableoils and removal of the solvent under vacuum. Hexane extraction could be re-placed by petroleum ether extraction with equal results. The process was alsoapplied successfully to a sage species (Salvia triloba L.).

Except of organic solvents, solutions of potassium or sodium bicarbonateor disodium phosphate were also used in an extraction process conducted at40–90°C under nitrogen. The extracts were subsequently decolorized, deodor-ized, and demineralized. The process was patented by Nestle SA (100) for theextraction of antioxidants from rosemary, sage, and parsley.

Oil extraction of ground spices with an edible animal or vegetable oil at120–125°C was patented by Campbell Soup Company (101). The extract wasseparated from the spice solids by centrifugation and filtration, and deodorizedby heating under vacuum while sparging with steam. Micronization of theground spice in an edible oil was later proposed by Bracco et al. (26). Theantioxidant components were separated from the lipid phase by molecular dis-tillation either on falling film or on a centrifugal system.

Several other processes have been patented for the extraction of naturalantioxidants (102, 103). These extracts are mainly characterized as flavoringsand are usually approved as such by the legal organizations. However, they alsohave the ability to manage oxidation. Rosemary extracts are by far the mostcommercially exploited products and have been reported to represent about 40–50% of the antioxidant market in Europe (102).

D. Extraction with Supercritical CO2

A relatively new process for the extraction of natural antioxidants involves su-percritical CO2 extraction. Extracts obtained by CO2 extraction of various herbsat 30 MPa and 40°C had similar or higher antioxidant activity than the relevantacetone extracts (46). The yields obtained by supercritical CO2 extraction werein most cases lower than the ones obtained by acetone extraction. Green tea wasextracted by CO2 at 31 MPa and 60°C, and the total yield as well as the yieldof individual catechins was considerably lower than that obtained by ethanol orwater extraction in a Soxhlet apparatus (104). The addition of aqueous ethanolsolutions as cosolvents in the supercritical extraction increased the yield of cate-chins and the ratio of catechins to caffeic and galic acid. The increase washigher as the concentration of ethanol in the solutions increased up to 95%.

The effect of extraction conditions on total yield of solutes and on theantioxidant activity of the extracts was investigated. Esquivel et al. (105) used

Extract of Natural Antioxidants 339

summer savory and found that an increase of pressure above 12 MPa did notincrease significantly the extraction yield that amounted approximately to 75%of that obtained by hexane. Extraction was conducted at 40°C and the otherconditions determined for maximal solute recovery was the solvent-to-solid ra-tio, equal to 120 kg CO2/h kg solid, and the duration of extraction, equal to 1h. The extracts obtained from summer savory under the studied conditions didnot show any antioxidant activity in oils at 120°C, as they consisted mainly ofessential oils, while the residue of extraction was effective. Extraction of rose-mary was accomplished on a pilot plant scale at 30–35 MPa and 40–60°C (106).The extracts were separated in two fractions with different antioxidant activityby arranging the conditions of pressure and temperature in two separator vessels.Methanol was also added as a modifier. The highest antioxidant activity wasdemonstrated by the extract obtained at 35 MPa and 50°C with no modifieradded and separated in the first separator by a pressure reduction to 20 MPa,resulting in a density reduction from 0.9 to 0.78 g/mL.

Supercritical CO2 extraction has also been proposed for the isolation oftocopherols from soy sludge (107). The most important raw material for theextraction of tocopherols is the deodorizer sludge obtained in the deodorizationof vegetable oils and fats. The processing parameters (temperature, vacuum,quantity of injected steam) are critical for the yield of tocopherols. Besides thevarious tocopherols these distillates also contain sterols, hydrocarbons, flavorcomponents, free fatty acids, and neutral triglycerides. The separation of tocoph-erols from other compounds is possible by esterification with a lower alcohol,followed by washing and vacuum distillation, by saponification, or by fractionalliquid-liquid extraction. Additional purification steps may include molecular dis-tillation, extraction, crystallization, or a combination of these processes (108).Due to the similar volatility of sterols, tocopherols, and fatty acids, it is quitedifficult to separate them by fractional distillation and steam stripping at highvacuum. Recently, enzymatic processes have been used for hydrolysis of theneutral triglycerides to fatty acids (109) and esterification of free fatty acids tomethyl esters (110) or butyl esters (109). The esterified products were thenfractionated by distillation at high vacuum to isolate tocopherols and sterolstogether.

E. Purification

Purification of the crude extracts is essential, especially for the alcoholic ex-tracts, in order to improve the antioxidant properties and to create a productwith a light odor, taste, and color, suitable as a food additive. Vacuum steamdistillation (25) and molecular distillation (26) are efficient methods, althoughthey may affect the heat-sensitive natural products. Removal of undesirablecomponents, such as lipids, carotenoids, and other fat-soluble materials, can be

340 Oreopoulou

done by washing the concentrated alcoholic extracts with an appropriate solvent,e.g., hexane (68). Isolation of an antioxidant fraction rich in flavonoids, fromalcoholic extracts, was achieved through solvent evaporation under vacuum, dis-solution of the solid residue in water, and liquid-liquid extraction with ethylether (90). Chloroform has been also suggested to remove caffeine or otherpigments from aqueous extracts of flavonoids, such as extracts of tea. Subse-quently an enriched flavonoid fraction was obtained by repeated extraction ofthe aqueous layer with equal volume of ethyl acetate (98, 99).

Fractionation with column liquid chromatography is suitable (27, 111) forthe purification of solvent extracts, but a limitation for large-scale productionarises from economic factors. A more convenient process proposed involveshigh-speed countercurrent chromatography with a multilayer coil separator-extrac-tor. It permits the direct efficient separation of undesirable constituents, such aschlorophylls and carotenoids, from the crude spice extracts, in a single step,without the need of a clean-up step (112).

Another approach is membrane separation of the polyphenolic fraction,which was applied to green tea extracts in aqueous ethanol (113). The membraneseparation focused on the removal of caffeine from the polyphenolic group ofcatechins, which are the main antioxidant components of green tea. Ethanolconcentration affected the performance of the nanofiltration, and higher concen-tration (e.g., 80%) gave higher retention of catechins and better separation fromcaffeine.

Supercritical CO2 fractionation of alcoholic extracts to isolate the antioxi-dant fraction has been also reported (114). The alcoholic extract was bleachedthrough an active carbon column, mixed with thermally treated aluminosilicate,and resulting suspension evaporated to dryness under reduced pressure. Thesolid residue was extracted with CO2 at 60°C at 10–40 MPa and at 100°C at 50MPa, for 6 h, at CO2 flow of 20 kg/h to give five fractions with higher antioxi-dant activity than the original extract. A total yield of 2.7% (calculated on thebasis of air-dried plant material) was obtained.

REFERENCES

1. AL Bran. Toxicology and biochemistry of BHA and BHT. J Am Oil Chem Soc52:372–375, 1975.

2. PA Dewdney, ML Meara. Natural fat-soluble antioxidants. Scientific and Techni-cal Surveys No 96. The British Food Manufacturing Industries Research Associa-tion, 1977.

3. LJ De Rosenzweig Pasquel, JK Babbitt. Isolation and partial characterization of anatural antioxidant from shrimp (Pandalus jordani). J Food Sci 56:143–145, 1991.

4. KJ Duve, PJ White. Extraction and identification of antioxidants in oats. J AmOil Chem Soc 68:365–370, 1991.

Extract of Natural Antioxidants 341

5. LL Tian, PJ White. Antioxidant activity of oat extract in soybean and cottonseedoils. J Am Oil Chem Soc 71:1079–1086, 1994.

6. Y Xing, PJ White. Identification and function of antioxidants from oat groats andhulls. J Am Oil Chem Soc 74:303–307, 1997.

7. DGH Daniels, HF Martin. Antioxidants in oats: monoesters of caffeic and ferulicacids. J Sci Food Agric 18:589–594, 1967.

8. DGH Daniels, HF Martin. Antioxidants in oats: glyceryl esters of caffeic andferulic acids. J Sci Food Agric 19:710–714, 1968.

9. F Sosulski, K Krygier, L Hogge. Free, esterified and insoluble-bound phenolicacids. 3. Composition of phenolic acids in cereals and potato flours. J Agric FoodChem 30:337–340, 1982.

10. M Watanabe, Y Ohshita, T Tsushida. Antioxidant compounds from buckwheat(Fagopyrum esculentum Moench) hulls. J Agric Food Chem 45:1039–1044, 1997.

11. BD Oomah, G Mazza. Flavonoids and antioxidant activities in buckwheat. J AgricFood Chem 44:1746–1750, 1996.

12. JR Clopton. Antioxidant material from osage orange (Bois D’Arc) fruit. J Am OilChem Soc 30:156–159, 1953.

13. SV Ting, WF Newhall. The occurrence of a natural antioxidant in citrus fruit. JFood Sci 30:57–63, 1965.

14. Y Miyake, K Yamamoto, Y Morimitsu, T Osawa. Isolation of C-glucosyflavonefrom lemon peel and antioxidated avtivity of flavonoid compounds in lemon fruit.J Agric Food Chem 45:4619–4623, 1997.

15. R Horowitz, B Gentili. Flavonoid constituents in citrus. In: S. Nagy, PE Shaw,MK Vedhuis, eds. Citrus Science and Technology. Westport, CT: AVI Publishing,1977, pp. 397–426.

16. U Justesen, P Knuthsen, T Leth. Quantitative analysis of flavonols, flavones, andflavonones in fruits, vegetables and beverages by high-performance liquid chro-matography with photo-diode array and mass spectrometric detection. J Chroma-togr 799:101–110, 1998.

17. O Benavente-Garcia, J Castillo, FR Marin, A Ortuno, JA Del Rio. Uses and prop-erties of citrus flavonoids. J Agric Food Chem 45:4505–4515, 1997.

18. WS Mueller. Antioxidative properties of cacao and their effect on butter oil. JDairy Sci 37:754–759, 1954.

19. AR Deschreider. Antioxidant properties of extracts of the shell of cacao beans,Inds Aliment Agric 75:759–766, 1958.

20. N Yamaguchi, S Naito, Y Yokoo. Studies on natural antioxidants. V. Antioxida-tive activity of pigment extracted from cacao bean husk. J Jap Soc Food Sci Techn29:534–537, 1982.

21. J Chipault, G Mizumo, W Lundberg. The antioxidant properties of spices in foods.Food Technol 10:209–211, 1956.

22. B Ostric-Matijasevic. Application des composants antioxygenes extrais du ro-marin a l’augmentation de la stabilite du sandoux et de la viande. Rev Fr CorpsGras 8–9:443–452, 1963.

23. S Barbut, DB Josephson, AJ Maurer. Antioxidant properties of rosemary oleoresinin Turkey sausage. J Food Sci 50:1356–1359, 1985.

342 Oreopoulou

24. JI Gray, RL Crackel, R Cook, A Gastel, R Evans, DJ Buckley. Observationson the antioxidant properties and thermal stability of an oleoresin rosemary. 3rdInternational Exhibition and Conference on Ingredients and Additives, November15–17, 1988.

25. SS Chang, B Ostric-Matijasevic, OAL Hsieh, C-L Huang. Natural antioxidantsfrom rosemary and sage. J Food Sci 42:1102–1106, 1977.

26. U Bracco, J Loliger, J-L Viret. Production and use of natural antioxidants. J AmOil Chem Soc 58:686–690, 1981.

27. JW Wu, M-H Lee, C-T Ho, SS Chang. Elucidation of the chemical structures ofnatural antioxidants isolated from rosemary. J Am Oil Chem Soc 59:339–345,1982.

28. CM Houlihan, C-T Ho, SS Chang. Elucidation of the chemical structures of anovel antioxidant, rosemaridiphenol, isolated from rosemary. J Am Oil Chem Soc61:1036–1039, 1984.

29. CM Houlihan, C-T Ho, SS Chang. The structure of rosemariquinine: a new anti-oxidant isolated from Rosemarinus officinalis L.. J Am Oil Chem Soc 62:96–98,1985.

30. SL Richheimer, MW Bernart, GA King, MC Kent, DT Bailey. Antioxidant activ-ity of lipid-soluble phenolic diterpenes from rosemary. J Am Oil Chem Soc 73:507–514, 1996.

31. ME Cuvelier, H Richard, C Berset. Antioxidant activity and phenolic compositionof pilot-plant and commercial extracts of sage and rosemary. J Am Oil Chem Soc73:645–652, 1996.

32. PJ Hidalco, JL Ubera, MT Tena, M Valcarcel. Determination of the carnosic acidcontent in wild and cultivated Rosemarinus officinalis. J Agric Food Chem 46:2624–2627, 1998.

33. ME Cuvelier, C Berset, H Richard. Antioxidant constituents in sage (Salvia offici-nalis). J Agric Food Chem 42:665–669, 1994.

34. J Protogeras, V Oreopoulou, C Tzia. Natural antioxidants from Salvia triloba forvegetable oils. Riv Ital Sostange Grasse 75:507–509, 1998.

35. J-P Yuan, H Chen, F Chen. Simultaneous determination of rosemarinic acid, lith-ospermic acid, and related phenolics in Salvia miltiorrhiza by HPLC. J AgricFood Chem 46:2651–2654, 1998.

36. XC Weng, W Wang. Antioxidant activity of compounds isolated from Salvia pleb-eia. Food Chem 71:489–493, 2000.

37. KD Economou, V Oreopoulou, CD Thomopoulos. Antioxidant activity of someplant extracts of the family Labiatae. J Am Oil Chem Soc 68:109–113, 1991.

38. N Nakatani, H Kikuzaki. A new antioxidative glucoside isolated from oregano(Origanum vulgare L.). Agric Biol Chem 51:2727–2732, 1987.

39. H Kikuzaki, N Nakatani. Structure of a new antioxidative phenolic acid fromoregano (Origanum vulgare L.). Agric Biol Chem 53:519–524, 1989.

40. SA Vekiari, C Tzia, V Oreopoulou, CD Thomopoulos. Isolation of natural antioxi-dants from oregano. Riv Ital Sostanze Grasse 70:25–28, 1993.

41. M Milos, J Mastelic, I Jerkovic. Chemical composition and antioxidant effect ofglycosidically bound volatile compounds from oregano (Origanum vulgare L. ssp.Hitrum). Food Chem 71:79–83, 2000.

Extract of Natural Antioxidants 343

42. K Schwarz, H Ernst. Evaluation of antioxidative constituents from thyme. J SciFood Agric 70:217–223, 1996.

43. RS Farag, AZMA Badel, FM Hewedk, GSA El-Baroty. Antioxidant activity ofsome spice essential oils on linoleic acid oxidation in aqueous media. J Am OilChem Soc 66:792–799, 1989.

44. JK Huang, CS Wang, WH Chang. Studies on the antioxidant activities of spicesgrown in Taiwan I. J Chin Agric Chem Soc 19:200–207, 1981.

45. N Kalaimani, V Muraleedharan, K George Joseph. Antioxidant effect of bettelleaf extract on dry-cured fish. Fish Technol 21:37–40, 1984.

46. A Dapkevicius, R Veskutonis, TA van Beek, JPH Linssen. Antioxidant activityof extracts obtained by different isolation procedures from some aromatic herbsgrown in Lithuania. J Sci Food Agric 77:140–146, 1998.

47. P Pietta, P Simonetti, P Mauri. Antioxidant activity of selected medicinal plants.J Agric Food Chem 46:4487–4490, 1998.

48. YS Velioglou, G Mazza, L Gao, BD Oomah. Antioxidant activity and total pheno-lics in selected fruits, vegetables and grain products. J Agric Food Chem 46:4113–4117, 1998.

49. A Demo, C Petrakis, P Kefalas, D Boskou. Nutrient antioxidants in some herbsand Mediterranean plant leaves. Food Res Int 31:351–354,1998.

50. NV Vanishlieva, EM Marinova. Activity and mechanism of action of natural anti-oxidants in lipids. Recent Res Dev Oil Chem 2:1–9, 1998.

51. CC Whittern, EE Miller, DE Pratt. Cottonseed flavonoids as lipid antioxidants. JAm Oil Chem Soc 61:1075–1078, 1984.

52. Q-H Meng, P Lewis, K Wahala, H Aldercreutz, MJ Tikkanen. Incorporation ofesterified soybean isoflavones with antioxidant activity into low density lipopro-tein. Biochem Biophys Acta 1438:369–376, 1999.

53. A Hirota, S Taki, S Kawaii, M Yano, N Abe. DPPH radical–scavenging compoundsfrom soybean miso and antiproliferative activity of isoflavones from soybean misotoward the cancer cell lines. Biosci Biotechnol Biochem 64:1038–1040, 2000.

54. E-A Bae, T-W Kwon, G-S Moon. Isoflavone content andantioxidative effects ofsoybeans, soybean curd and their by-products. J Korean Soc Food Sci Nutr 26:371–375, 1997.

55. KS Rhee, YA Ziprin, KC Rhee. Antioxidant activity of methanolic extracts ofvarious oilseed protein ingredients. J Food Sci 46:75–77, 1981.

56. F Shahidi. Food applications of natural antioxidants. In: SS Koseoglou, KC Rhee,RF Wilson, eds. Advances in Oils and Fats, Antioxidants, and Oilseed By-prod-ucts, Vol. 2. Champaign, IL: AOCS Press, 1996, pp. 241–246.

57. A Lazani, M Cardillo, E Fedeli. The antioxidant activity of chlorogenic acid iso-lated from sunflower seeds. Riv Ital Sostange Grasse 56:273–275, 1979.

58. Y Fukuda, T Osawa, M Namiki, T Ozaki. Studies on antioxidative substances insesame seed. Agric Biol Chem 49:301–306, 1985.

59. G Papadopoulos, D Boskou. Antioxidant effect of natural phenols on olive oil. JAm Oil Chem Soc 68:669–671, 1991.

60. MT Satue, S-W Huang, EN Frankel. Effect of natural antioxidants in virgin oliveoil on oxidative stability of refined, bleached and deodorized olive oil. J Am OilChem Soc 72:1131–1137, 1995.

344 Oreopoulou

61. FZ Sheabar, I Neeman. Separation and concentration of natural antioxidants fromthe rape of olives. J Am Oil Chem Soc 65:990–993, 1988.

62. E Notte, N Romito. Autoxidation of olive oil: influence of polyphenols 1. IndAgrarie 9:325–331, 1971.

63. FM Vela, AV Roncero. Antioxidantes naturales de las hojas de olivo II. GrasasAceites 13:124–127, 1962.

64. O Benavente Garcia, J Castillo, J Lorente, A Ortuno, JA Del Rio. Antioxidantactivity of phenolics extracted from Olea europaea L. leaves. Food Chem 68:457–462, 2000.

65. S Naito, N Yamaguchi, Y Yokoo. Studies on natural antioxidants. II Antioxidativeactivities of vegetables of Allium species. J Jap Soc Food Sci Technol 28:291–296, 1981.

66. C Karastogiannidou. Effect of onion quercetin on oxidative stability of cook-chillchicken in vacuum-sealed containers. J Food Sci 64:978–981, 1999.

67. Y-H Chu, C-L Chang, H-F Hsu. Flavonoid content of several vegetables and theirantioxidant activity. J Sci Food Agric 80:561–566, 2000.

68. HJ Suh, JM Lee, JS Cho, YS Kim, SH Chung. Radical scavenging compounds inonion skin. Food Res Int 32:659–664, 1999.

69. WE Price, JC Spitzer. The kinetics of extraction of individual flavonols and caf-feine from a Japanese green tea (Sen Cha Uji Tsuyu) as a function of temperature.Food Chem 50:19–23, 1994.

70. WE Bronner, GR Beecher. Method for determining the content of catechins in teainfusions by high-performance liquid chromatography. J Chromatogr 805:137–142, 1998.

71. PT Gardner, DB McPhail, GG Duthie. Electron spin resonance spectroscopic as-sessment of the antioxidant potential of teas in aqueous and organic media. J SciFood Agric 76:257–262, 1998.

72. Z-Y Chen, L-Y Wang, PT Chan, Z Zhang, HY Chung, C Liang. Antioxidativeactivity of green tea catechin extract compared with that of rosemary extract. JAm Oil Chem Soc 75:1141–1145, 1998

73. A Roedig-Penman, MH Gordon. Antioxidant properties of catechins and green teaextracts in model food emulsions. J Agric Food Chem 45:4267–4270, 1997.

74. Y He, F Shahidi. Antioxidant activity of green tea and its catechins in a fish meatmodel system. J Agric Food Chem 45:4262–4266, 1997.

75. CA Rice-Evans, NJ Miller, G Paganga. Antioxidant properties of phenolic com-pounds. Trends Plant Sci 2(4):152–159, 1997.

76. N Saint-Cricq de Gaulejac, Y Glories, N Vivas. Free radical scavenging effectsof anthocyanins in red wine. Food Res Int 32:27–333, 1999.

77. N Saint-Cricq de Gaulejac, Y Glories, N Vivas.Comparative study of polyphenolscavenging activities assessed by different methods. J Agric Food Chem 47:425–431, 1999.

78. V Katalinic. Grape catechins-natural antioxidants. J Wine Res 10:15–23,1999.79. N Gamez-Meza, JA Noriega-Rodriges, LA Medina-Juarez, J Ortega-Garcia, R

Cazarez-Casanova, O Angulo-Guerrero. Antioxidant activity in soybean oil of ex-tracts from Thomson grape bagasse. J Am Oil Chem Soc 76:1445–1447, 1999.

Extract of Natural Antioxidants 345

80. J Castillo, O Benavente-Garcia, J Lorente, M Alcaraz, A Redondo, A Ortuno,JA Del Rio. Antioxidant activity and radioprotective effects against chromosomaldamage induced in vivo by X-rays of flavan-3-ols (procyanidins)from grape seeds(Vitis vinifera): comparative study versus other phenolic organic compounds. JAgric Food Chem 48:1738–1745, 2000.

81. Z Ju, WJ Bramlage. Phenolics and lipid-soluble antioxidants in fruit cuticle ofapples and their antioxidant activities in model systems. Postharvest Biol Technol16:107–118, 1999.

82. P-D Dur, WY Yen, P-C Du, G-C Yen. Antioxidant activity of Mung bean hulls.J Am Oil Chem Soc 74:1059–1063, 1997.

83. SN Onyenebo, NS Hettriarachchy. Effect of Navy bean hull extract on the oxida-tive stability of soy and sunflower oils. J Agric Food Chem 39:1701–1704, 1991.

84. E Tsaliki, V Lagouri, G Doxastakis. Evaluation of the antioxidant activity of lupinseed flour and derivatives (Lupinus albus ssp. Graecus). Food Chem 65:71–75,1999.

85. BJF Hudson, SEO Mahgoub. Naturally-occuring antioxidants in leaf lipids. J SciFood Agric 31:646–650, 1980.

86. D Rodrigues de Sotillo, M Hadley, ET Holm. Potato peel waste: stability andantioxidant activity of a freeze-dried extract. J Food Sci 59:1031–1033, 1994.

87. R Julkunen-Tiitto. Phenolic constituents in leaves of Nothern willows: Methodsfor the analysis of certain phenolics. J Agric Food Chem 33:213–217, 1985.

88. AH Azizah, NM Nik Ruslawati, T Swee Tee. Extraction and characterization ofantioxidant from cocoa by-products. Food Chem 64:199–202, 1999.

89. S Chatterjee, SR Padwal Desai, P Thomas. Effect of γ-irradiation on the antioxi-dant activity of turmeric (Curcuma longa L.) extracts. Food Res Int 32:487–490,1999.

90. SA Vekiari, V Oreopoulou, C Tzia, CD Thomopoulos. Oregano flavonoids aslipid antioxidants. J Am Oil Chem Soc 70:483–487, 1993.

91. SS Chang, Y Bao. U.S. Patent US 5,043,100 (1991).92. M Wettasinghe, F Shahidi. Antioxidant and free radical-scavenging properties of

ethanolic extracts of defatted borage (Borago officinalis L.) seeds. Food Chem 67:399–414, 1999.

93. E. Revilla, J-M Ryan, G Martin-Ortega. Comparison of several procedures usedfor the extraction of anthocyanins from red grapes. J Agric Food Chem 46:4592–4597, 1998.

94. JM Cruz, JM Dominguez, H Dominguez, JC Parajo. Solvent extraction of hemi-cellulosic wood hydrolysates: a procedure useful for obtaining both detoxifiedfermentation media and polyphenols with antioxidant activity. Food Chem 67:147–153, 1999.

95. M Spiro, D Jago. Kinetics and equilibria of tea infusion. Part III: Rotating discexperiment interprated by a steady-state model. J Chem Soc Faraday Trans I 78:295–305, 1982.

96. Y Yoshida, M Kiso, T Goto. Efficiency of extraction of catechins from green tea.Food Chem 67:429–433, 1999.

97. M Spiro, D. Jaganyi, MC Broom. Kinetics and equilibria of tea infusion. Part IX:

346 Oreopoulou

The rates and temperature coefficients of caffeine extraction from green ChunMee and Black Bukial teas. Food Chem 45:333–335, 1992.

98. A Zhang, PT Chan, YS Luk, WKK Ho, ZY Chen. Inhibitory effect of jasminegreen tea epicatechin isomers on LDL-oxidation. J Nutr Biochem 8:334–340,1997.

99. M Milovanovic, K Picuric-Jovanovic, B Vucelic-Radovic, Z Vrbaski. Antioxi-dants effects of flavonoids of Anthriscus sylvestris in lard. J Am Oil Chem Soc73:773–776, 1996.

100. Nestle SA. Process for extracting antioxidants. British Patent 1,468,271 (1977).101. Campbell Soup Company. Spice antioxidant principle and process for the extrac-

tion of the antioxidant principle from spice. British Patent 1,331,050, U.S. Patent3,732,111 (1973).

102. BF Haumann. Antioxidants: Firms seeking products they can label as “natural”Inform 1(12):1002–1013, 1990.

103. J Giese. Antioxidants: Tools for preventing lipid oxidation. Food Technol 50(11):73–81, 1996.

104. CJ Chang, K-L Chiu, Y-L Chen, C-Y Chang. Separation of catechins from greentea using carbon dioxide extraction. Food Chem 68:109–113, 2000.

105. MM Esquivel, MA Ribeiro, MG Bernardo-Gil. Supercritical extraction of savoryoil: study of antioxidant activity and extract characterization. J Supercritical Fluids14:129–138, 1999.

106. FJ Senorans, E Ibanez, S Cavero, J Tabera, G Reglero. Liquid chromatographic-mass spectrometric analysis of supercritical-fluid extracts of rosemary plants. JChromatogr 870:491–499, 2000.

107. H Lee, BH Chung, YH Park. Concentration of tocopherols from soybean sludgeby supercritical carbon dioxide. J Am Oil Chem Soc 68:571–573, 1991.

108. P Schuler. Natural antioxidants exploited commercially. In: BJF Hudson, ed. FoodAntioxidants. London: Elsevier Applied Science, 1990, p 111.

109. S Ghosh, DK Bhattacharyya. Isolation of tocopherol and sterol concentrate fromsunflower oil deodorizer distillate. J Am Oil Chem Soc 73:1271–1274, 1996.

110. S Ramamurthi, AR McCurdy. Enzymatic pretreatment of deodorizer distillate forconcentration of sterols and tocopherols. J Am Oil Chem Soc 70:287–295, 1993.

111. S Nieto, A Garrido, J Sanhueza, LA Loyola, G Morales, F Leighton, A Valen-zuela. Flavonoids as stabilizers in fish oil: an alternative to synthetic antioxidants.J Am Oil Chem Soc 70:773–778, 1993.

112. N Fischer, B Weinreich, S Nitz, F Drawert. Application of high-speed countercur-rent chromatography for the separation and isolation of natural products. J Chro-matogr 538:193–202, 1991.

113. V Nwuha. Novel studies on membrane extraction of bioactive components ofgreen tea in organic solvents: Part I. J Food Eng 44:233–238, 2000.

114. Z Djarmati, RM Jankov, E Schwirtlich, B Djulinac, A Djordjevic. High antioxi-dant activity of extracts obtained from sage by supercritical CO2 extraction. J AmOil Chem Soc 68:731–734, 1991.

11Extraction of Alkaloids from NaturalPlants Using Supercritical Fluids

Marleny D. A. Saldana and Rahoma S. MohamedThe State University of Campinas–UnicampCampinas, Sao Paulo, Brazil

I. INTRODUCTION

Alkaloids are nitrogen-based organic compounds commonly found in leaves,seeds, and roots of plants, particularly the Papaveraceae (poppy and opium),Papilonaceae (lupins), Ranunculaceae (aconitum), and Solanaceae (tobacco)families (1). Some alkaloids have bitter taste and are highly toxic, but can havetherapeutic applications when used in moderate amounts. Almost all alkaloidshave pharmacological activity together with pronounced physiological actionsof interest to chemists, biochemists, and pharmacologists.

Alkaloids such as caffeine, morphine, cocaine, nicotine, quinine, codeine,emetine, pilocarpine, among others, form the active components in a variety ofstimulants and medication drugs. Among the most consumed alkaloid-contain-ing products are coffee, tea, and guarana, which contain caffeine, tea and cocoanuts, which contain theobromine; and tobacco, which contains nicotine. Thealkaloids found in these products have stimulant and/or sedative effects.

Until recently, most of research on alkaloids has focused on the chemicalaspects of their structure and isolation techniques (2). Increased attention is currentlydevoted to the identification of the pharmacological effects of these compounds onanimals and the extraction and characterization of new alkaloids. The extraction ofthe very small quantities of alkaloids in natural plants required the development ofnew and improved experimental equipment and instrumental techniques.

Alkaloids are normally removed from natural products using solvent ex-traction. For the purification of these alkaloids in the extracts, conventional

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separation processes such as fractional distillation, fractional crystallization, andfractional salt precipitation (3) as well as the new and promising technology ofextraction with supercritical carbon dioxide (4, 5) have been employed. Thepotential of each of these processes is determined by a compromise betweencost and selectivity. For large production volumes, the tendency is to choosethe conventional processes mentioned above, as they only require conventionalequipment. However, these processes bring with them the risk of contaminationof extracted products with residual chemical solvents used in extraction andpossible alteration in the quality of thermally labile products due to the hightemperatures used in distillation. Extraction with supercritical carbon dioxiderequires higher capital costs but is highly selective; therefore, it is recommendedfor the extraction and purification of valuable products such as alkaloids. Withthis technology, extraction and purification are carried out simultaneously sincesolvent power and selectivity of the supercritical fluid is easily modified withchanges in thermodynamic variables of temperature and pressure and the use ofa suitable cosolvent. In supercritical carbon dioxide extraction, there is no riskof thermal degradation as the extraction is carried out at temperatures close tothe critical temperature of carbon dioxide (31°C).

Few experimental data on the solubility and extractability of alkaloids suchas emetine, cephaeline, reserpine, vincristine (4), caffeine, theophylline, theobro-mine (6), codeine, thebaine, papaverine (7), quinine, morphine (8), pyrrolizidine(9), trigonelline (10), and ephedrine (11) in various supercritical solvents such ascarbon dioxide, ethylene, nitrous oxide, and fluoroform have been obtained overthe past few years. While there is a growing market for decaffeinated products(12), extraction of the many alkaloids encountered in natural plants with supercriti-cal carbon dioxide is yet to reach a desirable level of industrial application (5).

In this chapter, we present information on the potential application of thispromising technology: extraction using supercritical fluids in the isolation andrecovery of alkaloids from natural products. This presentation includes a briefsummary of the physiological effects and applications of alkaloids, a descriptionof commercial decaffeination applications of coffee and black tea, followed byexperimental laboratory data on the potential use of carbon dioxide for the isola-tion of a number of alkaloids from natural products, and future prospects in theisolation of important alkaloids.

II. PHYSIOLOGICAL EFFECTS AND APPLICATIONSOF PURINE ALKALOIDS

The principal purine alkaloids—caffeine, theobromine, and theophylline—areconsumed in large scale through the daily ingestion of coffee, tea, herbal mate,cola drinks, and chocolate as well as in analgesics, diuretics, and other pharma-

Extraction of Alkaloids 349

ceutical products. Physiological effects of methylxanthines on humans have re-ceived substantial attention, due largely to the high coffee consumption world-wide. All three purine alkaloids do, however, exhibit stimulant and psychomotoreffects, with caffeine and theophylline having highest activity. Theophylline isalso known to exhibit more diuretic effects as it inhibits the tubular reabsorptionof water and sodium in kidneys (13). It is also used as a vascular as well as abronchial dilator (14). Theobromine is found to have toxic effects on virtuallyall animals tested, including chickens (15), dogs (16), mice (17), rats (18), rab-bits (19), and humans (20). Recent interest in caffeine and theobromine is alsomotivated principally by their reproductive toxicities and their potential to in-duce fetal malformation through effects on embryo development. Furthermore,teratogen effects in animals are also known to occur as a result of biotransforma-tion when consuming products containing caffeine, theophylline, and theobro-mine (21). Finally, it is important to note that the presence of these alkaloids inproducts such as cocoa beans could prove to be a limiting and restrictive factorin its potential as a nourishing food in developing countries (22).

III. COMMERCIAL SCALE APPLICATIONS

Large commercial scale operations for the extraction and isolation of valuablealkaloids from natural plants for the pharmaceutical and cosmetic industrieshave been developed and operated for many years (23–25). Among the bestknown operations using supercritical carbon dioxide are the decaffeination pro-cesses of coffee and black tea and the removal of nicotine from tobacco. Withthe increase of consumption of decaffeinated coffee worldwide, caffeine is nowa major byproduct of coffee decaffeination with significant contribution in plantamortization (26–29). There are several patents and commercial processes forthe decaffeination of coffee beans (30–32) and black tea (33, 34). These pro-cesses employ a number of organic, nonorganic, and supercritical solvents. Ex-cept when supercritical carbon dioxide is used, the extraction process either isnonselective for caffeine or brings with it the potential risk of toxic residues inthe decaffeinated products that could prove to be harmful to the consumer.

Extraction with supercritical carbon dioxide as a substitute for chemicalsolvents eliminates the risks of potential toxic residues due to the inert natureof CO2 (35, 36) and thermal degradation of the decaffeinated product due to themild operating temperatures (near the critical temperature of CO2 of 31°C), be-sides being highly selective for caffeine. These characteristics result in the pres-ervation of most of the qualities of the original non-decaffeinated product.

In what follows, a more detailed description of decaffeination operationsof coffee beans and black tea using conventional and supercritical fluid solventsis presented.

350 Saldana and Mohamed

A. Decaffeination of Coffee Beans

Coffee, one of the most widely known natural products, contains 0.8–2.5%weight of caffeine, depending on the species (Coffea arabica, C. canephora, C.liberica, C. racemosa, etc.). It is used as a daily beverage in many parts of theworld. Green coffee beans are cultivated mainly in South America and Africa.

1. Decaffeination Using Conventional Solvents

Commercial coffee decaffeination is carried out on green coffee beans beforeroasting in order to minimize loss of flavor and aroma. These processes involvethe swelling of raw beans with water, caffeine removal with water-insolubleorganic solvents, steam stripping or distillation to remove residual solvent fromextracted beans, and drying of decaffeinated coffee beans to their initial hu-midity.

Organic solvents commonly used include toxic and flammable substancessuch as benzene, incombustible and highly volatile chlorinated methylene chlo-ride (37), ethyl acetate (38), methyl acetate, ethylmethylketone, and trichloroeth-ane (39, 40). Water (40–42) is an ideal solvent for decaffeination except for itshigh nonselectivity for caffeine, which results in the extraction of other water-soluble substances. Caffeine is subsequently recovered by washing the aqueousextracted solutions with organic immiscible solvents such as methyl chloride oradsorption of caffeine on activated carbon. As noted earlier, these conventionalprocesses suffer from two serious limitations: the risk of toxic residues in thedecaffeinated products when using organic solvents and/or the nonselective ex-traction when using water, which results in the removal of important constit-uents together with caffeine from the original products.

2. Decaffeination Using Supercritical Carbon Dioxide

Supercritical carbon dioxide (SC-CO2) is an excellent and selective solvent forcaffeine as it solubilizes caffeine and does not remove other important water-soluble components from coffee beans. The commercial decaffeination of coffeeis the first important industrial application of supercritical extraction in the foodindustry with plants constructed and operated in Germany and in the UnitedStates (Table 1). The plant owned and operated by General Foods has a yearlyproduction capacity of 50 × 106 kg of coffee beans (43). The German plantprocesses nearly 30 × 106 kg of green coffee beans per year (44). In this decaf-feination process, color and odor of the extracted beans are totally preserved,which is not always achieved when using conventional extraction processes(45). Extracted caffeine is reported to be of a very high purity and is easilyisolated and refined to provide a salable product (45).

Zosel’s patents on the decaffeination of green coffee beans using SC-CO2

(30, 31, 46, 47) served as the basis for the development of these commercial

Extraction of Alkaloids 351

Table 1 Commercial Plants Using SC- CO2 Extraction

Process/alkaloid extraction Manufacturer

Coffee decaffeination Kaffee HAG AG, Bremen, GermanyGeneral Foods, Texas, USAHermsen, Bremen, GermanySKW-Trostberg, Poszzillo, Italy

Tea decaffeination SKW-Trostberg, Munchmuenster, GermanyNicotine extraction Philip Morris, Virginia, USA

Source: Data from Ref. 23.

decaffeination plants. These patents propose, basically, three possible processes.In the first process, moistened green coffee beans placed in the extractor arecontacted with CO2 at 16–22 MPa and 70–90°C, where caffeine diffuses fromthe beans into the CO2 stream. This caffeine-saturated stream is passed into awater-washing tower operated at 70–90°C where caffeine is retained and thewater-saturated CO2 is recycled to the extractor. Caffeine retained in the washwater is separated by distillation. Decaffeinated coffee beans have a residualcaffeine content of only 0.02%.

In the second process, caffeine is extracted from coffee beans under thesame conditions as described in the first process while the caffeine-saturatedCO2 stream is passed through a bed of activated carbon where caffeine is re-tained. Adsorbed caffeine is subsequently recovered from activated carbon.

In a third alternative presented in these patents, a mixture of moistenedgreen coffee beans and activated carbon pellets is charged into the extractor (1kg of activated carbon/3 kg of coffee beans). CO2 is subsequently introducedinto the extractor and maintained at 22 MPa and 90oC. Caffeine diffuses fromcoffee beans into carbon dioxide until reaching the activated carbon. After theextraction, coffee beans are separated from the activated carbon pellets usingvibrating sieves.

Modifications of Zosel’s processes as well as new decaffeination pro-cesses were also suggested in U.S. and German patents as reported by Lackand Seidlitz (12) and McHugh and Krukonis (48). These new processes andmodifications involve (12, 48, 49) the following: (a) incorporation of twosmaller pressure vessels that are periodically charged and discharged with coffeebeans allowing a continuous extraction and using less quantity of CO2 than adiscontinuous process; (b) recovery of extracted caffeine by washing the CO2

stream with water and subsequent evaporation or crystallization; (c) use of othersolvents such as NO2, NH3, and CHF3 for caffeine extraction; (d) use of an ionexchange resin with temperature change to retain caffeine instead of activated car-bon; (e) use of liquid CO2 as a solvent; (f) addition of cosolvents such as acetone,methanol, and ethanol to increase solubility of caffeine in SC-CO2 and decrease

352 Saldana and Mohamed

the operating pressures. Other proposed modifications included the use of mixedsolvents such as carbon dioxide and propane, butane, or ethane (48), and the utiliza-tion of roasted coffee as described in the patents of Vitzthum and Hubert (32, 50)who employed a multistage process in which flavor and aroma were first extractedin the form of coffee oil. The oil-free coffee was subsequently moistened anddecaffeinated using a stream of water-saturated SC-CO2. The decaffeinated roastedcoffee is then spray dried and aromatized with the coffee oil extracted earlier.

Pilot plants in operation using the methods proposed by Zosel are alsodescribed in the literature (12, 45). These commercial and pilot plants consist,basically, of extractors, pumps, washing columns, and heat exchangers.

An interesting economic projection of a supercritical plant with daily feedrates of 32,000 and 64,000 kg of green coffee containing 12% of moisture oper-ated during 330 days was presented (51). Decaffeinated whole green coffeebeans with 3% of the original caffeine content and an aqueous caffeine solutionwere the main products. The extraction conditions considered were 14–35 MPaand 70–130°C and the separation conditions of 5–10 MPa and 15–50°C. Pro-cessing costs were estimated by considering (a) electric power, steam, coolingand process water, and carbon dioxide; (b) operation labor and supervision; and(c) maintenance, taxes, insurance, and plant overhead. Estimated costs werefound to be $0.83 and $0.68 per kg of coffee produced for plant capacities of32,000 and 64,000 kg, respectively.

3. Decaffeination with SC-CO2 and Conventional Solvents:Main Advantages and Disadvantages

Economical studies of Lack and Seidlitz (12) have shown that although theinitial investment cost of the SC-CO2 plant is higher, this process provides ahigher profit per ton of coffee processed than does the ethyl acetate process,due to the excellent quality of both decaffeinated coffee and caffeine producedand negligible losses in the SC-CO2 process. Carbon dioxide extracts caffeinewithout affecting the reduced sugar and amino acid contents. These compoundsneed to be preserved because they are converted to flavor and aroma during theroasting process (45). The flavor and appearance of decaffeinated coffee beansobtained with the carbon dioxide process are very close to those in the originalun-decaffeinated coffee and superior to the product obtained from other conven-tional decaffeination methods. This process is recognized by the consumer as a“natural” decaffeination technique for the high qualities of both products andbyproducts. The total quantity of caffeine recovered is much higher using com-pressed carbon dioxide than ethyl acetate. Supercritical carbon dioxide decaffei-nation is also considered a clean and environmentally acceptable technology.

Steam cost is another factor that contributes to the higher process costwhen using ethyl acetate. For the pretreatment of beans, both processes require

Extraction of Alkaloids 353

steam to swell the beans but only the ethyl acetate process requires almost fourtimes more steam to strip residual solvent from the final product.

The limiting factor for the use of the CO2 process is, however, the capitalcost due mainly to the highly specialized equipment needed for a safe plantoperation. Necessary equipment includes compressors for recirculation of CO2,separators and extractors with baskets that support high pressure and allow fastoperation, adequate piping, as well as safety and control systems to avoid inter-ruptions in production and precaution against explosion.

B. From Black Tea (Camellia sinensis)

Tea, a natural product, contains approximately 2.0–3.5% caffeine (52). Themost common types of tea are green and black. It is used as a daily beverageworldwide, mainly in China and India. Most decaffeinated tea is produced andconsumed in North America and Europe.

1. Tea Decaffeination Using Conventionaland SCCO2 Processes

These processes are similar to those used for coffee decaffeination and normallyemploy organic solvents, water, or supercritical fluids. Organic solvents such asmethylene chloride and ethyl acetate are employed in a similar manner to thatused in the decaffeination of coffee beans (45). As pointed out earlier, the opera-tions using organic solvents bring with them the risk of leaving toxic residuesin decaffeinated products, whereas the loss of valuable water-soluble tea constit-uents is markedly increased when using water as the solvent due to its nonselec-tivity for caffeine. Extraction with supercritical carbon dioxide is selective forcaffeine and eliminates the risk of possible toxic residues in the decaffeinatedproducts. Vitzthum and Hubert (34) describe a multistage procedure that avoidsthe loss of flavor and aroma when decaffeinating black tea. Aroma componentsare removed from tea by extraction with dry SC-CO2 at 25 MPa and 50°C. Thedearomatized leaves are subsequently moistened and decaffeinated with water-saturated CO2. The decaffeinated leaves are vacuum dried at 50°C and rearoma-tized by contact with the expanded CO2 solution containing the aroma compo-nents. This procedure is also used for the production of caffeine-free instant tea.In this case the decaffeinated tea is extracted with hot water and the extract isfreeze dried. The powder produced is impregnated with tea aromas using asolvent that is subsequently evaporated.

An SC-CO2 tea decaffeination plant is currently operated by SKW/Trost-berg in Meunschmeunster, Germany (53). Decaffeinated ice tea with tropicalflavors, where decaffeination is obtained using SC-CO2, was reported to havebeen introduced into the market by the Hansen Company (54).

354 Saldana and Mohamed

From a laboratory study, Klima et al. (33) reported the extraction of blacktea using wet carbon dioxide at 25.5–35 MPa and 50–80°C. The moistened teawith 15–50 wt% water content is packed into the extractor in alternate layerswith activated carbon. Before leaving the extractor, the CO2 stream passesthrough the bed of pure adsorbent placed inside of gas-permeable bags or tubeswhere extracted caffeine is adsorbed.

C. Potential Applications: Extraction of Methylxanthinesfrom Natural Products

The potential extraction of methylxanthines—caffeine, theophylline, and theo-bromine—a major group of purine alkaloids, from coffee beans (55–57), cocoanibs (58), and shells (59) and guarana seeds (60) has also been investigated overthe past few years. In what follows, we present a brief review of laboratoryexperiments and results obtained.

1. From Mate Tea (Ilex paraguariensis) Leaves

Mate tea is a beverage prepared by the infusion of dry mate leaves and istraditionally consumed in southern Brazil, Argentine, Paraguay, and Uruguay(61, 62). A careful inspection of the chemical constituents of Ilex species (Table2) reveals the reason for current and successful use of this natural product as astimulant, an antirheumatic, and a diuretic. Nevertheless, a high intake of herbalmate tea could provoke irritability and insomnia, and the possibility of develop-ing cerebral depression, nervous tremor, and numbness. Several laboratory stud-ies (63–66), on the other hand, have shown that polyphenols found in this teacould inhibit the formation and growth of tumors.

Similar to the decaffeination of coffee beans, removal of caffeine frommate tea leaves can be performed using organic solvents or water. While thereare problems associated with the use of chemical solvents (potential toxic resi-dues in extracted products) and water (nonselectivity, which results in loss ofvaluable flavor components) (68, 69), the use of carbon dioxide at supercriticalconditions proved to be potentially convenient in the extraction of methylxan-thines from natural products (44, 68).

While there are many patents for the use of SC-CO2 as a solvent to extractcaffeine from coffee beans and Camellia sinensis tea leaves (44, 56, 59), littleis known about the extraction of methylxanthines from mate tea.

During the past 2 years, our group has conducted an investigation on theextractability of methylxanthines from Ilex paraguariensis (70) where we useda semicontinuous-flow, high-pressure system purchased from Autoclave Engi-neers (Erie, PA). The major components of the apparatus included positive liq-uid displacement pumps for solvent delivery, high-pressure extraction vessels,

Extraction of Alkaloids 355

Table 2 Taxonomy, Chemical Compoisition, and Other Characteristicsof Ilex paraguariensis

Taxonomy Other characteristics

Division: Anthophyta Consumed part: leaves and some barksType/Subtype: Magnoliopsida/Rosidae Color: greenOrder: Celastrales Taste: astringent and sourFamily: AquifoliaceaeGendus: Ilex HumiditySpecie: paraguariensis, paraguayiensis - “in natura”: 50–60 wt%

- commercial: 8–10 wt%Common names: yerba mate, mate, ervamate, Paraguay Cayi, Paraguai Tea,South American Holly

Chemical composition/quantity (100 g) Alkaloids (mg/kg)Protein: 10.89 Theophylline: 142 ± 6,a 768 ± 3b

Carbohydrate: 12.04 Theobromine: 340 ± 7,a 209 ± 5b

Starch: 4.55 Caffeine: 5371 ± 161,a 8375 ± 251b

Glucose: 3.84Fiber: 16.96

aOld leavesbNew leaves from branches with fruitSource: Data from Refs. 61 and 67.

and three separator flasks in series (Fig. 1). Flow rates and accumulated gasvolumes passing through the apparatus were controlled with micrometeringvalves and measured with a flow computer-measuring device. Heating tapeswere used to maintain constant temperature in the extraction section and invalves to prevent freezing of solvents or solid solute precipitation followingdepressurization. Pressure in both extractors was monitored with a digital trans-ducer system. Mate leaves were extracted with liquid CO2 at 25.5 MPa and70°C; samples were collected and analyzed for purine alkaloids.

A supercritical extraction curve for mate tea revealed that high caffeine re-moval rates were obtained in the early stages of the extraction with extraction ratesdiminishing at later stages (Fig. 2). A similar qualitative behavior was observed fortheophylline and theobromine extraction (Fig. 3). These results reveal the higherselectivities of CO2 for caffeine followed by theobromine and theophylline. Thecumulative amounts of methylxanthines extracted from mate tea amounted to 4308,348, and 47 mg of caffeine, theobromine, and theophylline per kg of dry mate tea,respectively. Considering that different mate leaves were used, these values are ingood agreement with alkaloid contents in mate tea reported by Mazzafera (67).

After about 7 h of extraction, 94%, 68%, and 57% of extracted caffeine,

356 Saldana and Mohamed

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Extraction of Alkaloids 357

Figure 2 Caffeine extraction curve for the fractionation of mate tea at 25.5 MPa, 70oC,and a CO2 flow of 0.9–1.2 g/min.

theobromine, and theophylline in the plant matrix, respectively, had been recov-ered. By the last fractions, 99.9% of caffeine, 96% of theobromine, and 95% oftheophylline had been removed. Fractions obtained at the late stages becameeach time richer in theobromine and theophylline and could provide an interest-ing approach to the separation of extractable methylxanthines into fractions ofvarying concentrations.

2. From Cocoa (Theobroma cacao) Beans

Cocoa beans are the seeds of the tropical cocoa tree, Theobroma cacao. Thesebeans are used as raw material for producing chocolate and cocoa powder, whichare subsequently used in the manufacturing of other products such as cake fillings,pudding powders, ice cream, and cocoa beverages (chocolate), among others.

A theobromine content of about 1–2% and a caffeine content of 0.1% ofdry nut are reportedly found in the species Theobroma cacao (71, 72). Duringfermentation, theobromine and caffeine migrate to the shell. Theobromine con-tents of about 2% of the dry shell were reported (73). The shells represent about

358 Saldana and Mohamed

Figure 3 Dimethylxanthine extraction curves for fractionation of mate tea at 25.5MPa, 70oC, and a CO2 flow of 0.9–1.2 g/min.

10–12% of the nut’s weight. Once theobromine is extracted, shells are nowa-days used as animal ration. For example, considering the production of 150 ×106 kg of cocoa nuts [Brazilian production 1980–1981 according to CEPLAC(74)] results in 13 × 106 kg of shells and potentially 30,000 kg of theobromineavailable for use in food, pharmaceutical, and cosmetic industries (72).

Cocoa beans are also a potential source of high-quality cocoa butter (Table3) that is used in a variety of applications in the cosmetic and pharmaceuticalindustries. Furthermore, the residue material obtained in cocoa bean processingis fermented to produce organic fertilizers.

Cocoa butter with the characteristic and pleasant flavor and odor of cocoabeans contains many different types of triacylglycerols. Hexane is one of theconventional solvents used for the extraction of cocoa butter. Due to the toxicnature of hexane, a maximal residue of 5 mg/kg is allowed in the final cocoabutter.

Methylxanthines could be extracted from cocoa beans conventionally in afour-step process (75): extraction with warm water, purification using an adsor-bent, recovery of the xanthines from the adsorbent, and, finally, addition of

Extraction of Alkaloids 359

Table 3 Composition of Cocoa Beansand Cocoa Shells

Cocoa bean Cocoa shellComponent (wt%) (wt%)

Moisture 5 4.5Fat 54 1.5Theobromine 1.2 1.4Caffeine 0.2 —Polyhydroxyphenols 6 —Crude protein 11.5 10.9Mono- and oligosacharides 1 0.1Starch 6 —Pentosans 1.5 7Cellulose 9 26.5Carboxylic acids 1.5 —Other compounds 0.5 —Ash 2.6 8

Source: Data from Refs. 71 and 72.

residual concentrated aqueous extract to the cocoa beans to reestablish the solidcontent of the bean.

Extraction of theobromine and caffeine from cocoa seeds (58, 76) andshells (59) can be also done using supercritical carbon dioxide. Margolis et al.(77) described a process in which stimulants are removed from cocoa by SC-CO2 extraction of water-swollen nibs at 30 MPa and 90°C. It was earlier indi-cated that water acted as a chemical agent that freed caffeine linked to coffeesubstrate allowing the caffeine to be extracted by SC-CO2 (56). As caffeine issimilar to theobromine, one could assume that the same effect would be ob-served in the supercritical extraction of theobromine from cocoa beans.

A characterization of cocoa extracts obtained with SC-CO2 as a functionof temperature and pressure was presented by Rossi et al. (78). This workpointed to improved extraction yields at pressures exceeding 40 MPa but re-sulted in a nonselective process. For a better extraction of all cocoa fat, finemilling of the lipid bearing material was required. Experimental data on theextraction of cocoa butter from three substrates—nibs, shells, and liquor inwhich approximately 20%, 60%, and 100%, respectively, was obtained at 40MPa and 80oC in 5 h—were also presented (79).

Li and Hartland (58) used ethanol as a cosolvent for the SC-CO2 extractionof methylxanthines from cocoa nibs. The solubility of theobromine increasedwith ethanol concentration in the supercritical solvent. However, extraction with

360 Saldana and Mohamed

ethanol as a co-solvent proved to be more selective for cocoa butter than fortheobromine, contrary to what was observed when using an aqueous cosolvent(77). Brunner (59) investigated the influence of water content on the extractionof theobromine from cocoa seed shells using supercritical carbon dioxide at 30MPa and 80°C and reported that the solubility of theobromine in water-saturatedSCCO2 was found to be much less than that of caffeine. The extraction yieldwas also found to increase with pressure and moisture content. Furthermore,higher extraction temperatures favored the removal of xanthines and cocoa but-ter, whereas grinding and swelling of the starting material with water did notimprove the extraction (80).

Recently our group explored the extractability of theobromine from Bra-zilian cocoa beans (81). Results revealed the extraction of almost 89% of theoriginal theobromine at 20 MPa and 70°C using 10% ethanol as a cosolvent ofsupercritical carbon dioxide. When extraction was performed with dry or water-saturated carbon dioxide or at lower pressures or temperatures, additional timeand larger amounts of carbon dioxide were needed to achieve the same yield.Furthermore, the extraction was coupled to an adsorption step using activatedcarbon where extracted theobromine and cocoa butter were simultaneously ad-sorbed and separated from the CO2 solvent.

3. From Guarana (Paullinia cupuana) Seeds

Guarana is a bush plant native to Brazil, with the seeds being the only part usedfor human consumption. The seeds are commonly used in concentrated and softdrinks and as ingredients of a variety of pharmaceutical products. They are therichest source of caffeine, with 3–6% weight on a dry basis (68), and haveshown aphrodisiac and stimulant effects, acting on the cardiovascular and ner-vous systems and kidneys. These effects are attributed to the rich caffeine con-tent encountered in guarana seeds (82, 83).

South American Indians have traditionally toasted, ground, and mixed theseeds with water to form a paste that can be molded into different configurations(rods and sticks) to be used in special rituals. The toasted seeds can be groundto a powder and added to syrups as an essence or to water to form consumabledrinks.

Similar to coffee and tea decaffeination, caffeine removal of guarana seedscould be performed using organic solvents such as dimethyl chloride and water(84). The use of supercritical carbon dioxide eliminates the risks of toxic residuesin the extracted products and the long nonselective extraction presented with wateras a solvent. As observed with coffee beans, water can act as a valuable cosolventleading to a substantially improved extraction yield (56, 60, 68).

The literature contains the limited data of Mehr et al. (60) who reportedextractions for pressures of 13.74–27.47 MPa and temperatures of 35°C, 45°C,

Extraction of Alkaloids 361

and 55°C. Our recent findings (85) revealed some interesting information on theability of supercritical fluids in the decaffeination of widely consumed caffeine-rich natural guarana seeds with water-saturated supercritical carbon dioxide. Theextraction was performed using a semicontinuous-flow high-pressure microex-traction apparatus at 40°C and 70°C and pressures of 10, 20 and 40 MPa. Car-bon dioxide flow rates of 3 and 5 L/min were used.

Extraction curves showed the existence of a thermodynamic solubility–dependent, an intermediate, and a diffusion controlled regions. Extraction at 40MPa and 70oC using water-saturated supercritical carbon dioxide at a flow rateof 3 L/min allowed the removal of almost 99% of initial caffeine content in wetground guarana seeds in 240 min. When extractions were performed at lowerpressures or temperatures, additional time and larger amounts of carbon dioxidewere needed to achieve the same yield. Increasing the carbon dioxide flow ratedid not present any economic advantages unless the extraction was limited tothe thermodynamic solubility region. For total extraction of caffeine, the use oflow flow rates resulted in similar final product yield but at lower solvent con-sumption. A retrograde behavior for the extraction of caffeine from guaranaseeds was also observed at 10 MPa for the 40–70°C isotherms.

4. From Other Natural Matrices

Many alkaloids encountered in natural plant species are of interest as anticanceragents. Supercritical carbon dioxide extraction has provided a solution for theextraction of active components from some natural matrices without the risk ofdegradation or contamination with toxic solvents. Besides the applications al-ready cited, the extraction and isolation of chemotherapeutic pyrrolizidine alka-loids (monocrotaline) from the seeds of Crotalaria spectabilis using supercriti-cal carbon dioxide with ethanol and water as cosolvents has also been reported(9). In this operation, 94–100% of the monocrotaline was obtained using an ionexchange resin column, demonstrating the potential and effective extraction andisolation of other alkaloids of this class. Nicotine and nornicotines are alkaloidsencountered in Nicotiana (86, 87) and are used as a powerful insecticide as wellas in epidermal patches that help ease the difficulties associated with cigaretteaddiction. Nicotine has been extracted from tobacco using carbon dioxide–watermixtures (88, 89), and nowadays there is even an industrial plant for its extrac-tion operating in the United States.

Supercritical carbon dioxide has also been employed for the extraction ofalkaloids such as quinine and morphine (8). Morphine is one of the major alka-loids isolated from plants by conventional methods for industrial use. Annually,approximately 160,000 kg of morphine (24) is purified and 90–95% of that ismethylated to codeine (25), which is then either used directly or chemicallyconverted to a variety of derivatives that find use as analgesics. The illicit pro-

362 Saldana and Mohamed

duction of morphine for acetylation to heroin reaches almost 10 times that amount,totaling more than 1.2 × 106 kg/a (90).

Few alkaloids were also extracted with other supercritical fluids such asfluoroform (CHF3), which is an attractive solvent with a good polarity (1.6D)and accessible critical parameters. The solubilities of opium alkaloids—codeine,thebaine, papaverine, and noscapine—in supercritical CO2, N2O, and CHF3 havebeen studied by Stahl et al. (7, 91) at 15 MPa and 40°C. With the exception ofcodeine, all other alkaloids tested exhibited higher solubility in CHF3 than N2Oand CO2. Sanders (39) has also presented some production of alkaloid extractsfrom vegetable matter using supercritical carbon dioxide.

D. Future Prospects

During the past 20 years, the use of supercritical fluid solvent technology hasadvanced rapidly. Supercritical fluids have been applied to areas as diverse asfood industry, pharmaceutical, polymer, oils, petroleum, textile, biotechnology,among others.

Some alkaloids that offer future prospective for pharmaceutical applica-tions include emetine, an active ingredient of Ipeca (Cephaelis ipecacuanhaRubiaceae) used by South American Indians for the treatment of amoebic dysen-tery and other rubiaceous species. Quinine, medically used as in the treatmentof malaria, and quinidine, used as an antiarrhythmia drug (92), are also alkaloidsthat could potentially be extracted from natural plants like Cinchona shells.Physostigmine, also called eserine, is another alkaloid that could be extractedfrom the calabar bean of West Africa (Physostigma venenosum Balf) and usedclinically in the management of glaucoma by reduction of intraocular tension(93). Other potential alkaloids include colchicine, an ancient and well-knowndrug used for the management of gout (94), from the plant Colchicum autum-nale, the autumn crocus or meadow saffron, and the glory lily Gloriosa superba;bark extracts from the Yuzuriha tree (Daphniphyllum macropodum), which havebeen used for centuries as a folk remedy for asthma (95), and the tropane alka-loids in solanaceous plants that have been traditionally used for their medicinal,hallucinogenic, and poisonous properties (96, 97). The narcotic, anesthetic, andpsychostimulant cocaine (98) is a tropane alkaloid found outside of the Solana-ceae in Erythroxylum coca (Erythroxylaceae). The initial source of taxol wasthe bark of Taxus brevifolia, which proved to be efficient for refractory ovariancancer in 1992 and since 1994 being tested for breast cancer and other tumors(99).

Current efforts to apply metabolic engineering to increase the productionof economically important alkaloids continue and should expand to includemore plant species and compounds. The enormous worldwide effort to screenplants for new biologically active compounds is expected to bring new drugs,

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some of which will probably be alkaloids, to the market. In the coming years,considerable interest may thus be expected in the field of plant cell biotechnol-ogy and metabolic engineering of secondary metabolism by the pharmaceuticalindustry.

Recent data obtained by this research group at the State University ofCampinas (UNICAMP) in Brazil, in collaboration with Professor Brunner’sgroup at the Technical University of Hamburgh-Harburg (TUHH) in Germany,have revealed interesting and very useful information on the extraction of meth-ylxanthines from natural products such as guarana seeds, mate leaves, and cocoabeans using supercritical carbon dioxide and ethanol, and on the extraction ofcaffeine and theobromine along with cocoa butter from Brazilian cocoa beans(85, 100, 101). These findings point to new opportunities in the extraction ofother important active principles in natural plants of interest to pharmaceutical,food, and cosmetic industries.

For research on alkaloids, the future looks very exciting leading to a num-ber of interesting applications in the production of specialty chemicals and phar-maceuticals (92). Environmental and ecological factors can have a considerableimpact on future processes.

REFERENCES

1. KW Bentley. The Chemistry of Natural Products: The Alkaloids, 2nd ed., Vols. 1and 2. New York: Interscience, 1966, p. 260.

2. RHF Manske, HL Holmes. The Alkaloids: Chemistry and Physiology, Vol 1. NewYork, USA: Academic Press, 1950, pp. 1–90.

3. A McKillop. An Introduction to the Chemistry of the Alkaloids. London: Butter-worths, 1970, pp. 1–41.

4. E Stahl, W Schilz, E Schutz, E Willing. A quick method for the microanalitycalevaluation of the dissolving power of supercritical gases. Angew Chem Int EdEngl 17:731–738, 1978.

5. MDA Saldana, P Mazzafera, RS Mohamed. Uso de fluidos supercrıticos paraobtencao de alcaloides de plantas naturais. Boletim de Ciencia e Tecnologia deAlimentos 32 (1):80–88, 1998.

6. M Johannsen, G Brunner. Solubilities of the xanthines caffeine, theophylline andtheobromine in supercritical carbon dioxide. Fluid Phase Equilibria 95:215–226,1994.

7. E Stahl, E Willing. Extraction of natural materials with supercritical and con-densed gases-quantitative determination of the solubility of opium alkaloids. Mik-rochim Acta 2 (5–6):465–474, 1980.

8. AP Ndiomu, CF Simpson. Some applications of supercritical fluid extraction.Anal Chim Acta 213:237–243, 1988.

9. ST Schaeffer, LH Zalkow, AS Teja. Extraction and isolation of chemotherapeuticpyrrolizidine alkaloids from plant substrates. In: KP Johnston, JML Penninger,

364 Saldana and Mohamed

eds. Supercritical Fluid Science and Technology. Washington, DC: AmericanChemical Society, 1989, pp. 416–433.

10. MDA Saldana, P Mazzafera, RS Mohamed. Extracao dos alcaloides: cafeına etrigonelina dos graos de cafe com CO2 supercrıtico. Ciencia Tecnologia de Ali-mentos. 17 (4):371–376, 1997.

11. J Kim, YH Choi. SPE of ephedrine derivatives from Ephedra sinica. Proceedingsof 5th Int Symp on Supercritical Fluids, Atlanta, GA, 2000, pp. 1–12.

12. A Lack, H Seidlitz. Commercial scale decaffeination of coffee and tea using su-percritical CO2. In: MB King, TR Bott. Extraction of Natural Products UsingNear-Critical Solvents. Glasgow: Blackie Academic, 1993, pp. 101–139.

13. AG Gilman. As Bases Farmacologicas da Terapeutica, 8th ed. Rio Janeiro, Brasil:Guanabara Koogan SA, 1991, pp. 409–415.

14. R Viani. Physiologically active substances in coffee. In: RJ Clarke, R Macrae,eds. Coffee: Physiology, Vol. 3. New York: Elsevier Applied Science, 1988, pp.1–32.

15. JG Black, NS Barron. The poisoning of life stock by cocoa products. Vet Res 55:166–167, 1943.

16. TR Strachan, A Bennet. Theobromine poisoning in dogs. Vet Res 134:284, 1994.17. K Owusu-Domfeh, DA Christensen, BP Owen. Nutritional value of some Ghana-

ian feed stuff. Can J Anim Sci 50:114–118, 1970.18. Y Wang, DP Waller. Theobromine toxicity on sertoli cells and comparison with

cocoa extracts in male rats. Toxicol Letter 70:155–164, 1994.19. MG Soffletti, C Nebbia, F Valenza, S Amadeo, G Re. Effects of theobromine in

mature and immature rabbits. J Comp Pathol 100:47–58, 1989.20. B Stavric. Methylxanthines-theobromine, paraxanthine and combined effect of

methylxanthine toxicity to humans. Food Chem Toxicol 26:725–733, 1986.21. MS Legator, S Zimmeving. Review of genetic effects of caffeine. J Environ Sci

Health 13:135–138, 1979.22. MU Eteng, EU Eyong, EO Akpanyung, MA Agiang, CY Aremu. Recent advances

in caffeine and theobromine toxicities: a review. Plant Foods Hum Nutr 51:231–243, 1997.

23. LT Taylor. Supercritical Fluid Extraction: Techniques in Analytical ChemistrySeries. New York: John Wiley, 1996, pp. 28–41, 136–147.

24. DD Songstad, WGW Kurz, CL Nessler. Tyramine accumulation in nicotiana-Tabacum transformed with a chimeric tryptophan decarboxylase gene. Phyto-chemistry 30 (10):3245–3246, 1991.

25. S Chavadej, N Brisson, JN McNeil, V de Luca. Redirection of tryptophan leadsto production of low indole glucosinolate canola. Proc Natl Acad Sci USA 91 (6):2166–2170, 1994.

26. SN Katz. Decaffeination of coffee. Colloque International Sur La Chimie DesCafes Verts, Torrefies Et Leurs Derives. London: Paris Association ScientifiqueInternationale du Cafe, 1980, pp. 295–302.

27. AJ King. The utility of chemical engineering principles in coffee processing tech-nology. Colloque International Sur La Chimie Des Cafes Verts, Torrefies Et LeursDerives, London, UK: Paris Association Scientifique Internationale du Cafe,1980, pp. 237–250.

Extraction of Alkaloids 365

28. M Sivetz. Coffee Processing Technology. Westport, CO: AVI, 1963.29. RJ Clarke. Packing of roast and instant coffee. In: RJ Clarke, R Macrae, eds.

Coffee: Technology, Vol. 2. London: Elsevier Applied Science, 1988, pp. 201–220.

30. K Zosel. Process for recovering caffeine. US Patent 3,806,619 (1974).31. K Zosel. Process for the decaffeination of coffee. US Patent 4,247,570 (1981).32. O Vitzthum, P Hubert. Extraction of caffeine from coffee. German Patent

2,212,281 (1973).33. Klima, Schutz, Vollbrech. European Patent Specification 0,200,150 (1990).34. O Vitzthum, P Hubert. Process for the manufacture of caffeine-free black tea.

USA Patent 4,167,589 (1979).35. RJ Clarke. International coffee standardization and legislation. Proceedings of the

VIII Colloque Association Scientifique International du Cafe, Abidjan, IvoryCoast. Paris, 1977, pp. 467–472.

36. RJ Clarke. International coffee standardization and legislation. Food Chem 4:81–96, 1979.

37. JM Patel, AB Wolfson. Semi-continuous countercurrent decaffeination processes,US Patent 3,671,263 (1972).

38. JM Patel, AB Wolfson, B Lawrense. Decaffeination process, US Patent 3,671,263(1972).

39. N Sanders. Food legislation and the scope for increased use of near-critical fluidextraction operations in the food, flavoring and pharmaceutical industries. In: MBKing, TR Bott, eds. Extraction of Natural Products Using Near-Critical Solvents.Glasgow: Blackie Academic, 1993, pp. 34–49.

40. SN Katz. Decaffeination of coffee. In: RJ Clarke, R Macrae eds. Coffee. Technol-ogy, Vol 2. London: Elvesier Applied Science, 1988, pp. 59–72.

41. NE Berry, RH Walters. Process of decaffeinating coffee, US Patent 2,309,902(1943).

42. L Nutting, GS Chong. Continuous process for producing decaffeinated beverageextract, US Patent 3,361,571 (1968).

43. LW Young. Supercritical fluid extraction. IOMA Broadcaster. March-April: 11–15, 1994.

44. MA McHugh, VJ Krukonis. Supercritical Fluid Extraction: Principles and Prac-tice. Boston: Butterworths, 1994.

45. M Sims. Decaffeinating with carbon dioxide. Tea Coffee Trade J, September 8–10, 1990.

46. K Zosel. Process for the decaffeination of coffee. USA Patent 4,260,639 (1981).47. K Zosel. Process for the direct decaffeination of aqueous coffee extract solutions.

US Patent 4,348,422 (1982).48. MA McHugh, VJ Krukonis. Supercritical Fluid Extraction: Principles and Prac-

tice. Boston: Butterworths, 1986, pp. 69–78.49. H Ebeling. Loslichkeit von Feststoffen in Hochcomprimierten uberkritischen Ga-

sen—untersucht am Beispiel des Coffeins. PhD dissertation, Universitat von Karl-sruhe, Germany, 1987.

50. O Vitzthum, P Hubert, W Sirtl. Decaffeination of crude coffee. German Patent2,357,590 (1975).

366 Saldana and Mohamed

51. RA Novak, DA Linnig, WE Leyers. Decaffeination with supercritical carbon diox-ide. Proceedings of ASIC 14o Colloque, San Francisco, 1991, pp. 357–364.

52. J Wurziger. Tee. In: Ullmann’s Encyclopedia for Technical Chemistry, Vol 22,4th ed. Weinhiem: VCH, 1982, pp. 405–410.

53. JF Brennecke. New applications of supercritical fluids. Chemistry and IndustryNovember: 831–834, 1996.

54. S Farr. 2001 a soft drinks odyssey. Food Manufacture, March 29–30, 1994.55. G Brunner. Mass transfer from solid material in gas extraction. Ber Bunsenges

Phys Chem 88:887–891, 1984.56. H Peker, MP Srinivasan, JM Smith, JB McCoy. Caffeine extraction rates from

coffee beans with supercritical carbon dioxide. AIChE J 38 (5):761–770, 1992.57. MDA Saldana, P Mazzafera, RS Mohamed. Extraction of caffeine, trigonelline

and chlorogenic acid from Brazilian coffee beans with supercritical fluids. Pro-ceedings of 4th International Symposium on Supercritical Fluids, Sendai-Japan,1997, pp. 219–222.

58. S Li, GS Varadarajan, S Hartland. Solubilities of theobromine and caffeine insupercritical carbon dioxide: correlation with density-based models. Fluid PhaseEquilibria 68:263–280, 1991.

59. G Brunner. Gas Extraction: An introduction to fundamentals of supercritical fluidsand the application to separation processes. In: H Baumgartel, EU Franck, WGrunbein, eds. Topics in Physical Chemistry, Vol. 4. Darmstadt: Springer-Verlag,1994, pp. 3–248.

60. CB Mehr, RN Biswal, JL Collins. Supercritical carbon dioxide extraction of caf-feine from guarana. J Super Fl 9 (3):185–191, 1996.

61. F Alikaridis. Natural constituents of Ilex species. J Ethnopharmacol 20:121–144,1987.

62. MJ Tormen. Economia ervateira brasileira. In: H Winge, AG Ferreira, JEA Mari-ath, LC Tarasconi, eds. Erva mate: Biologia e Cultura no Cone Sul. Porto Alegre,Brasil: Editora da Universidade Federal de Rio Grande do Sul, 1995, pp. 27–40.

63. ZY Wang, M Huang, Y Lou, J Xie, K Reuhl, H Newmark, C Ho, C Yang, AConney. Inhibitory effects of black tea, green tea, decaffeinated black tea, anddecaffeinated green tea on ultraviolet B light-induced skin carcinogenesis in 7,12-dimethylbenz[a]anthracene-initiated SKH-1 mice. Cancer Res 54:3428–3435,1994.

64. M Inagake, T Yamane, Y Kitao, K Oya, H Matsumoto, N Kikuoka, H Nakatani,T Takahashi, H Nishimura, A Iwashima. Inhibition of 1,2-dimethylhydrazine-induced oxidative DNA damage by green tea extract in rat. Jpn J Cancer Res 86:1106–1111, 1995.

65. R Hasegawa, T Chujo, K Sai-Kato, T Umemura, A Tanimura, Y Kurokawa. Pre-ventive effects of green tea against liver oxidative DNA damage and hepatotoxic-ity in rats treated with 2-nitropropane. Food Chem Toxicol 33 (11):961–970,1995.

66. SD Katiyar, CA Elmets, R Agarwal, H Mukhtar. Protection against ultra violet-Bradiation-induced local and systemic suppression of contact hypersensitivity andedema responses in C3H/HeN mice by green tea polyphenols. Photochem Photo-biol 62(5):855–861, 1995.

Extraction of Alkaloids 367

67. P Mazzafera. Caffeine, theobromine and theophylline distribution in Illex paragu-aryensis. Rev Bras Fisiol Veg 6 (2):149–151, 1994.

68. MDA Saldana. Extracao de cafeına, trigonelina e acido clorogenico de cafe comCO2 supercrıtico. MSc Thesis, Universidade Estadual de Campinas, Campinas,Brasil, 1997.

69. P Mazzafera, A Carvalho. A cafeına do cafe. Documentos IAC, Instituto Agronom-ico, Agosto 1991.

70. MDA Saldana, P Mazzafera, RS Mohamed. Extraction of purine alkaloids frommate (Ilex paraguariensis) using supercritical CO2. J Agric Food Chem 47:3804–3808, 1999.

71. F Marx, JGS Maia. Purine alkaloids in seeds of Theobroma species from theAmazon. Z Lebensm Unters Forsch 193:460–461, 1991.

72. A Sotelo, RG Alvarez. Chemical composition of wild Theobroma species andtheir comparison to the cacao bean. J Agric Food Chem 39:1940–1943, 1991.

73. VM Senanayake, ROB Wijesekera. Theobromine and caffeine content of the co-coa bean during its growth. J Sci Food Agric 22:262–263, 1971.

74. Centro de Pesquisas do Cacau (CEPLAC). Memorando 274/327 da Divisao deTecnologia e Engenharia Agrıcola. Itabuna, Bahia, Brasil, 1981.

75. D Magnolato, A Isely. European Patent Appl. 0.070.437 B1, 1985.76. S Li, S Hartland. Influence of co-solvents on solubility and selectivity in extrac-

tion of xanthines and cocoa butter from cocoa beans with supercritical carbonCO2. J Super Fl 5:7–12, 1992.

77. G Margolis, J Chiovani, FA Pagliaro. European Patent Appl 0,061,017 (1972).78. M Rossi, C Arnoldi, G Salvioni, A Schiraldi. Characterization of cocoa extracts

obtained with supercritical carbon dioxide. Ital J Food Sci 3:31–40, 1989.79. M Rossi. Supercritical fluid extraction of cocoa and cocoa products. In: JW King,

GR List, eds. Supercritical Fluid Technology in Oil and Lipid Chemistry. Cham-paign, IL: AOCS Press, 1996, pp. 220–229.

80. J Sebald, J Schulmeyr, M Gehrig. Removal of xanthines from cacao. In: R vonRohr, C Trepp, eds. High Pressure Chemical Engineering. London: Elsevier Sci-ence, 1996, pp. 333–338.

81. MDA Saldana, C Zetzl, RS Mohamed, G Brunner. Extraction of xanthines fromnatural products in a microextraction column using supercritical CO2 and ethanol.Proceedings of the 7th Meeting on Supercritical Fluids, Antibes-France, December6–8, 2000, pp. 787–792.

82. JE James. Caffeine and Health. San Diego: Academic Press, 1991.83. S Garatini. Some notes on coffee, caffeine and health. Proceedings XVI Colloque

Association Scientifique International du Cafe, Kyoto, Japan, 1995, pp. 73–78.84. GJ Hulbert, RN Biswal, TH Walker, CB Mehr, JL Collins. Solid/liquid extraction

of caffeine from guarana with methylene chloride. Food Sci Tech Int 4 (1):53–58,1998.

85. MDA Saldana, C Zetzl, RS Mohamed, G Brunner. Decaffeination of guaranaseeds in a microextraction column using water-saturated CO2. J Super Fl 22(2):119–127, 2002.

86. F Saitoh, M Noma, N Kawashima. The alkaloid contents of 60 Nicotiana species.Phytochemistry 24 (3):477–480, 1985.

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87. W Bauer, MH Zenk. 2 methylenedioxy bridge forming cytochrome P-450 depen-dent enzymes are involved in (s)-stylopine biosynthesis. Phytochemistry 30 (9):2953–2961, 1991.

88. O Vitzthum, P Hubert. Fluid extraction of hops, spices and tobacco with supercrit-ical gases. Angew Chem Int Ed Engl 17:710–715, 1978.

89. R Roselius, O Vitzhum, P Hubert. Nicotine recovery from tobacco. German Patent2,043,537 (1972).

90. M Amann, N Nagakura, MH Zenk. Purification and properties of (s)-tetrahy-droprotoberberine oxidase from suspension-cultured cells of berberis-Wilsoniae.Eur J Biochem 175 (1):17–25, 1988.

91. A Stahl, KW Quirin, D Gerard. Dense Gases for Extraction and Refining. NewYork: Springer-Verlag, 1988, p. 176.

92. R Verpoorte, R van de Heijden, J Memelink. Plant biotechnology and the produc-tion of alkaloid: prospects of metabolic engineering. In: GA Cordell, ed. The Al-kaloids: Chemistry and Biology, Vol 50. San Diego: Academic Press, 1998, pp.453–508.

93. B Witkop. From the “ordeal bean” (Physostigma venenosum) to the ordeal ofAlzheimer’s disease—some of the legacy of Percy Lavon Julian (1899–1975).Heterocycles 49:9–27, 1998.

94. M Levy, M Spino, SE Read. Colchicine—state of the art review. Pharmacothera-phy 11(3):196–211, 1991.

95. J Leonard. Alkaloids. In: RH Thomson, ed. The Chemistry of Natural Products.London: Chapman and Hall, 1993, pp. 218–258.

96. N Takao, M Kamigauchi, M Suguira, I Ninomija, O Miyata, T Natio. The struc-ture of the alkaloid macarpine. Heterocycles 16 (2):221–225, 1981.

97. T Tanahashi, MH Zenk. J New hydroxylated benzo[c]phenanthridine alkaloidsfrom eschscholtzia-Californica cell-suspension cultures. J Nat Prod 53 (3):579–586, 1990.

98. J Berlin, E Forche, V Wray, J Hammer, W Hosel. Formation of benzophenanthrid-ine alkaloids by suspension cultures of eschscholtzia-californica. Z Naturforsch C38(5–6):346–352, 1983.

99. ME Wall, MC Wani. History and future prospects of camptothecin and taxol. In:GA Cordell, ed. The Alkaloids: Chemistry and Biology, Vol 50. San Diego: Aca-demic Press. 1998, pp. 509–536.

100. MDA Saldana, C Zetzl, RS Mohamed, G Brunner. Extraction of methylxanthinesfrom guarana seeds, mate leaves and cocoa beans using supercritical carbon diox-ide and ethanol. J Agri Food Chem 50:4820–4826, 2002.

101. MDA Saldana, RS Mohamed, P Mazzafera. Extraction of cocoa butter from Bra-zilian cocoa beans using supercritical CO2 and ethane. Fluid Phase Equilibria 194–197, 885–894, 2002.

12Removal of Cholesterol from FoodProducts Using Supercritical Fluids

Rahoma S. Mohamed, Marleny D. A. Saldana,Alvaro B. A. de Azevedo, and Uiram KopcakThe State University of Campinas–UnicampCampinas, Sao Paulo, Brazil

I. INTRODUCTION

The necessity of some consumers to reduce or control weight due to healthproblems or for private reasons, as well as the general concerns of the popula-tion as a whole of the effects of diet on health are the major driving forces toproduce foods with low fat and cholesterol contents (1). When consumers in theUnited States were asked in a recent survey about their concerns of food prod-ucts, 61% were worried about fat and 35% about cholesterol (2, 3). The recom-mendation is that total fat calories in a daily diet should be limited to 30% withthe saturated fatty acids contribution not to exceed 10% (4). A high cholesteroland fat concentration in the blood can lead to their precipitation in the circula-tory system and cause arteriosclerosis.

Worries and concerns about cholesterol and fat on the part of the world-wide population can be appreciated when observing the decrease in the con-sumption of whole milk (5) and egg yolk–containing food products (6), and theresulting increase in the production of low-fat, skim milk, and cholesterol-freeegg yolk products to satisfy growing consumer demands for healthier products.Butter and butter oil have, traditionally, been the main derivatives of milk pro-cessing, with skim milk being a subproduct. The shift in consumer behavior insearch for light products has generated a large surplus of butter and butter oil.The surplus in butter oil reached 155 × 106 kg in 1990 and was expected toreach the 550 × 106 kg mark in 2000 (7).

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Cholesterol removal and fractionation of fat can be an attractive solutionto this surplus problem and a viable alternative in the production of new fatproducts with characteristics that meet new consumer concerns and demands.Cholesterol, a byproduct, can be used for a variety of applications, includingthe production of sterols, emollients, skin creams, and so forth (8).

Various technologies have been suggested for the removal of cholesterolfrom food products, including enzymatic conversion of cholesterol to a nonab-sorbable steroid (2), steam stripping (2, 9), distillation (10), complex formationswith or without adsorption (11), molecular distillation (12), melt crystallization(13, 14), complexation with β-cyclodextrins (15, 16), use of cholesterol-degrad-ing bacteria (17), and supercritical extraction using CO2 (18–21) and ethane assolvent (22). Rizvi and Bhaskar (19) made a comparison of the methods usedfor the extraction of cholesterol from milk fat and concluded that fat fractionsproduced with supercritical CO2 had favorably distinct and different physicaland chemical properties than those obtained by other methods. Fractionationtechniques have been reviewed by Hamm (23), who divided these processesinto dry crystallization, solvent fractionation, and detergent fractionation, to-gether with the new, attractive, and promising supercritical fluid fractionationtechnology as an alternative.

The technique to complex cholesterol with β-cyclodextrin is applied onan industrial scale to produce low-cholesterol milk with over 90% reduction incholesterol content (16). A low cholesterol liquid egg product (80% less incholesterol content) has also been introduced in North America but with littlesuccess due to the high cost of the β-cyclodextrin complexation process (24).

Other processes have also been suggested for the reduction of cholesterolcontent in food products, but they seem to be somewhat inconvenient becausethey introduce significant chemical changes in the protein and triglyceride con-tents in the raw material (24).

Consumer concern over chemical residues in foods and increasing demandfor high-quality healthy food products have rendered supercritical fluid extrac-tion processes with solvents that are nontoxic, inexpensive, and having a rela-tively low critical temperature, such as carbon dioxide, one of the most promis-ing alternatives to the use of chemical solvents in the removal of cholesterolfrom food products.

The solubility of cholesterol in supercritical fluids has been widely investi-gated (25–31). Furthermore, experimental data on the removal of cholesterolfrom dehydrated meat (32), chicken (33), pork (34), fish (35), eggs and deriva-tives (36–39), and butter oil (2, 8, 14, 40, 41) using supercritical carbon dioxidehave also been reported. Cully et al. (18) registered a patent on the process forthe removal of cholesterol or cholesterol esters from egg yolk powder and butterfat by extraction with compressed CO2. The possibility of producing milk fatwith 90% less cholesterol while maintaining the original color and flavor was

Removal of Cholesterol 371

described by Bradley (8). The removal of cholesterol and fractionation of butteroil with supercritical fluids have been reported by Shishikura et al. (42), Rizviet al. (19), and Mohamed et al. (22). The use of cosolvents to improve theremoval efficiency in the extraction of cholesterol with supercritical CO2 wasevaluated by Saldana et al. (21) and Singh et al. (30).

In any extraction process, it is often difficult to reconcile conditions thatlead to high recovery yields with those that result in high selectivity. A possibleapproach that could allow this reconciliation is through the coupling of the ex-traction process to a highly selective separation step. Shishikura (42) obtainedbutter oil fractions with only 5% of the cholesterol in the initial butter usingsupercritical extraction followed by adsorption on silica gel. However, in thisprocess it was only possible to obtain a maximal extracted oil yield of 50% ofthe original oil. Huber et al. (43) investigated selective removal of cholesterolfrom anhydrous milk fat using supercritical carbon dioxide in a multiseparatorprocess in which selective removal was only possible with the use of silica gelas an adsorbent. Mohamed et al. (22) recently presented data on the simultane-ous removal of cholesterol and fractionation of butter oil through a coupledextraction/adsorption process that uses carbon dioxide or ethane as the supercrit-ical solvent and alumina as the adsorbent.

The extraction of cholesterol from animal fats with supercritical CO2 usinga descending pressure profile with fractions collected in several separators con-nected in series was described by Chao et al. (44).

There seems to be a substantial lack of detailed study in the literature,however, on the changes in the sensory, nutritional, and physiological propertiesof foodstuffs with the loss of important lipophilic components other from cho-lesterol during the extraction.

In what follows, we present a brief summary of the physiological effectsof cholesterol on health; cholesterol analysis and solubilities in supercriticalfluids; a description of extraction and fractionation of cholesterol from differentfood products, followed by some experimental laboratory data on the potentialuse of supercritical fluids for the removal of cholesterol, and future prospects inthe reduction of cholesterol levels from important meals.

II. PHYSIOLOGICAL EFFECTS OF CHOLESTEROLAND IMPLICATIONS ON HUMAN HEALTH

Cholesterol (Fig. 1) is a sterol, differentiated from the main steroid ring structureby an aliphatic side chain, methyl groups, and a hydroxyl (OH) group. Thepresence of the OH group makes cholesterol a steroid alcohol or sterol (45).

Cholesterol is an extremely important biological molecule with a crucialrole in membrane structures as well as being a precursor for the synthesis of

372 Mohamed et al.

Figure 1 Cholesterol molecular structure.

steroid hormones, bile acids, and sexual hormones (46). The cholesterol mole-cule and the challenges faced by researchers to elucidate its role is illustrated inthe work of Vance and Van den Bosch (47). In a general manner, it is wellestablished that the human body not only synthesizes cholesterol but also ab-sorbs it through the intestine, as cholesterol is eventually transported to the liver(48, 49).

Some of the body cholesterol is produced by hepatocytes in the liver;therefore, every cell in the body is capable of producing cholesterol (50). In thecell, cholesterol is made available through synthesis in the endoplasm reticulumand by endocytosis of cholesterol-containing ligands (46). The complex mecha-nisms that govern the synthesis and the distribution of cholesterol in the cellsare well described in the work of Blanchette-Mackie (46). The accumulation ofcholesterol in the body depends on several factors such as nutrition, stress, andobesity (51).

It is well established that a high cholesterol concentration in the blood canlead to its precipitation in the circulatory system and arteriosclerosis (52). Thelow-density lipoprotein (LDL) cholesterol, called “bad cholesterol,” circulatingin the blood can slowly precipitate and accumulate in the walls of the arteriesthat feed the heart and brain. Together with other substances it can form plaque,a thick, hard deposit that clogs the arteries. This condition is known as arterio-sclerosis. On the other hand, one-third to one-fourth of total blood cholesterolis what is known as high-density lipoprotein (HDL), also called “good choles-terol.” A high level of HDL cholesterol is believed to prevent the precipitationof the LDL part and therefore arteriosclerosis that could lead to serious healthproblems such as heart attacks and strokes. The association of high blood cho-lesterol level with heart diseases or cancer is the motivating factor in recentresearch on the reduction of cholesterol levels in consumed meals (53, 54).

The American Heart Association recommends that there be no more than300 mg/d of cholesterol in daily diet. The cholesterol contents in some foods of

Removal of Cholesterol 373

animal origin are presented in Table 1. The removal of cholesterol from foodproducts without altering the composition or properties of the other constituentswould be necessary for these products to be accepted by consumers in the formof low-cholesterol butter, cheese, ice cream, and others (2).

III. DETERMINATION OF CHOLESTEROL CONTENTIN FOOD PRODUCTS AND EVALUATIONOF ITS SOLUBILITY IN SUPERCRITICAL FLUIDS

A. Cholesterol Compositional Analysis

Several different techniques are applied for the compositional analysis of choles-terol (56). Jimenez-Carmona and Luque de Castro (57) reported some choles-terol and cholesterol oxides analysis using gas chromatography (GC) and highperformance liquid chromatography (HPLC). When using GC, an appropriatecapillary column and a flame ionization detector are normally used, and beforethe analysis samples must be pretreated according to the standard AOAC 933.08method (58). Recent HPLC techniques use a nonaqueous reversed-phase systemwith saponified or esterified derivatives (59–62). The AOAC also reported dif-ferent methods for cholesterol analysis in food components such as eggs or oilsand fats. The method 941.09 (63) is a titrimetric method for determination ofcholesterol in eggs as a whole, while the method 43.290 (64) is recommendedfor determination in egg yolks. For oils and animal fats with low levels ofunsaponifiable matter the AOCS official method Ca 6a-40 (65) is recommended.

As reported by Pasin et al. (66), most of the methods used to determinethe cholesterol content in foods were developed with serum cholesterol proce-dures as the starting point. These methods can be divided into three groups:colorimetric, chromatographic, and enzymatic. The colorimetric methods werebased on the Liebermann-Burchard color reaction or the Zlatkis procedures (67,68). Some researchers have questioned the accuracy and the limitations of these

Table 1 Cholesterol Contents in Foods(mg/100 g)

Foods Raw Boiled

Pork 49–54 56–97Chicken 58–80 75–124Chicken with skin 104 139Beef 51–52 66–67Eggs 33–190 1000–1019

Source: Adapted from Ref. 55.

374 Mohamed et al.

methods (66). The chromatographic methods, which include gas chromatogra-phy (69), gas–liquid chromatography (70), HPLC (71), or capillary supercriticalfluid chromatography (37), require a pretreatment saponification step of samplesprior to analysis. These pretreatment steps are time consuming, cumbersome,and seldom environmentally friendly, all of which place strong limitations onsample throughput (66). Pasin et al. (66) also reported on an enzymatic methodfor determination of total cholesterol in fresh, frozen, and dried egg yolk usinga diagnostic cholesterol reagent. The results revealed that this enzymatic methodcould be used for the quantitative determination of cholesterol without the needfor saponification. Cholesterol determination is made by dye absorbance analy-sis. The intensity of the color produced is directly proportional to the total cho-lesterol in the sample (66).

B. Cholesterol Solubility in Supercritical Fluids

In order to explore the ability of supercritical fluid extraction in the reductionof cholesterol content in foods and products of animal origin (fats, meats, eggyolk, etc.), the solubility behavior of cholesterol in supercritical solvents mustbe determined. Some experimental data on the solubility of cholesterol in super-critical fluids, particularly CO2, have been presented in the literature. Some ofthese data, when compared with each other, show some disagreements and in-consistencies that are believed to be associated with the method used and thepurity of substances employed.

Yun et al. (29) used a continuous high-pressure flow apparatus to obtainsolubility data of cholesterol and triglycerides in supercritical CO2. A similarapparatus was used by Neves (20) for the same measurements, with the dataobtained found in good agreement with those of Yun et al. (29). When Yun etal. (29) compared their data with those of Chrastil (25) (who used a batch tech-nique) and also with those of Wong and Johnston (27) (who used a micro sam-pling apparatus), some discrepancies were observed; these were attributed todifferences in the experimental techniques used.

Chrastil (25) studied the cholesterol solubility in carbon dioxide at pres-sures ranging from 10 to 25 MPa and temperatures ranging from 40°C to 80°C,with 20° intervals. An increase in solubility with increase in pressure at constanttemperature was observed, as expected by the resultant increase in the fluiddensity and consequent increase in the solvent power of the supercritical fluid.A decrease in solubility with increase in temperature at constant pressure wasalso observed. This is the commonly encountered phenomenon known as retro-grade condensation, a characteristic of all supercritical extraction systems (72).A similar result was observed by Neves (20) at the same temperature and pres-sure ranges. This phenomenon is often attributed to the fact that the increase invapor pressure of the solute with increase in temperature cannot compensate theloss of solvent power caused by the resultant decrease in solvent density (72).

Removal of Cholesterol 375

Neves (20) reported that the change from retrograde to nonretrograde be-havior occurred at a pressure of 16 MPa. This pressure is known as the uppercrossover pressure below which the effect of vapor pressure prevails over theeffect of solvent density. This behavior can be explored among the many otherpossibilities in determining the optimal conditions for the extraction, separation,and fractionation of solutes present in a mixture.

To correlate the solubility data, Yun et al. (29) tested several models,including a model proposed by Chrastil based on association between solute andsolvent molecules to form a solvation complex. This model had successfullycorrelated the solubility of lipids (25). Another model proposed by Kumar andJohnston (73) provides a more realistic description of the solvent–solute molec-ular interactions.

The data available in the literature for the system of cholesterol-carbondioxide (25, 28, 29, 31) revealed low solubility values at relatively low pres-sures, with substantial increase in solubility at high pressures and relativelyhigh temperatures. Therefore, a viable extraction process of cholesterol withsupercritical CO2 implies high energy costs. An increase in solubility withoutthe use of substantially high pressure and temperatures could be brought aboutwith the use cosolvents or solvents with better affinities for cholesterol. Thecholesterol molecule has a large nonpolar tail (similar to a hydrocarbon). Forthis reason, the use of a hydrocarbon as a solvent or a cosolvent could be anattractive alternative. The use of hydrocarbon solvents such as supercritical eth-ane, which has a critical temperature very close to that of carbon dioxide, acritical pressure that is lower than CO2, and is an acceptable solvent in foodprocessing (30), has been explored. Singh et al. (30) presented solubility dataof cholesterol in supercritical ethane and in the binary solvent systems: ethane-propane (3.5 and 14 mol % propane) and ethane-carbon dioxide (3.5 and 14.5mol % ethane) at pressures ranging from 7 to 29 MPa and temperatures rangingfrom 308 to 338 K. Saldana et al. (21) presented solubility data of cholesterolin CO2/ethane mixed solvent systems at 55°C and pressures from 12 to 20 MPaand concentrations of ethane ranging from 8 to 96 mol %. These investigationsrevealed the same effects of pressure and temperature on cholesterol solubilityobtained earlier with CO2 but with cholesterol solubilities that are 3 to 15 timeshigher than in supercritical CO2 (25, 27, 28, 31). Results reported by Mohamedet al. (22) and Saldana et al. (74) are presented in Figs. 2 and 3, respectively.This higher solubility of cholesterol in ethane was attributed to the larger ethane-cholesterol dispersion forces in comparison with the weaker polar interactionforces between CO2 and cholesterol molecule. Mendes (75) and Mohamed et al.(22) reported similar observations for the β-carotene-ethane and cholesterol-ethane-butter oil systems, respectively.

As observed in Figs. 2 and 3, solubilities in mixed solvents are intermedi-ate to those obtained with pure solvents but not linear in behavior, as the in-crease in cholesterol solubility is not proportional to the amount of ethane added

376 Mohamed et al.

Figure 2 Cholesterol solubility in CO2 and ethane.

Figure 3 Solubility of cholesterol in mixtures of CO2 and ethane at 328.15 K.

Removal of Cholesterol 377

to CO2 (21). A similar increase in solubility was observed when using propaneas a cosolvent (30).

Moreover, Foster et al. (76) also observed the increase of cholesterol solu-bility with the increase in the concentration of acetone and hexane when addedas cosolvents to supercritical CO2 and supercritical ethane.

IV. POTENTIAL APPLICATIONS OF CHOLESTEROLREMOVAL FROM FOOD PRODUCTS

The removal of cholesterol from milk fat (77), eggs (38, 78, 79), chicken (33),pork (34), fish (35), beef (80), and others using supercritical carbon dioxide isamong the extraction and fractionation operations tested in laboratory with sev-eral patents reported in the literature (9, 12, 17). While the removal of choles-terol from these products can employ organic, nonorganic, or supercritical sol-vents, extraction with supercritical carbon dioxide eliminates the risks ofpotential toxic residues due to the inert nature of CO2 (19, 72), and thermaldegradation of extracted products due to mild operating temperatures (near theCO2 critical temperature of 31°C).

In what follows, a more detailed description of cholesterol extraction offood products using conventional and supercritical fluid solvents is presented.

A. From Milk Fat

Milk fat, a major product in the dairy industry, has been primarily used asbutter. It is a good source of essential fatty acids and contains a high proportionof short-chain fatty acids, which contributes to its easy digestibility. Milk fatcontains a mixture of triglycerides with a wide range of molecular weights. Themost desirable property of milk fat is its pleasant flavor not found in other fats.However, the relatively high saturated fatty acids content and the presence ofcholesterol in milk fat has raised nutritional and health concerns that have re-sulted in a substantial decrease in the direct consumption and utilization of milkfat as an ingredient in many industrial products. In order to reduce its cholesteroland saturated fatty acids contents, Arul et al. (14), Bradley (2, 8), and otherssuggested the fractionation of milk fat to obtain fractions with desirable physi-cal, functional, and nutritional properties.

Milk fat fractionation is a potential technology for the development ofnovel ingredients with varied functional properties for use in numerous foodformulations. Hamm (23) reports the possibility of three types of fractionation:(a) dry fractionation, also known as melt crystallization; (b) solvent fractionationusing such solvents as acetone or hexane; and (c) detergent fractionation,wherein a surfactant solution is used to transfer the crystallized material from

378 Mohamed et al.

the oil phase to the aqueous phase and facilitate its subsequent separation. Dryfractionation of milk fat is performed using the Tirtiaux process (81). Norris etal. (13) described the fractionation by crystallization at different temperatures(melt crystallization) with or without the use of solvents. Although the use ofsolvents or surfactants produces a good separation of triglycerides, these tech-niques are not environmentally friendly due to problems related to solvent re-moval and disposal. The commercial value of the secondary fraction producedin any fractionation process plays an important part in determining the viabilityof the overall process, and upgrading of this secondary fraction is an importantpart of the total production and marketing process (23).

Modification of milk fat can also be carried out using chemical methodssuch as interesterification (82) and hydrogenation (83), but these methods causelosses of many desirable characteristics and destroy the natural flavor of it (13).

The fact that none of the conventional methods could provide an adequateflavor concentrate has motivated the urgent search for another method for theremoval of cholesterol from milk fat. Several research groups have suggestedthe use of supercritical CO2 to fractionate and modify milk fat (2, 8, 14, 42,84–87) and to remove milk cholesterol (88).

Using supercritical carbon dioxide at 20 MPa and 80°C, Kaufmann et al.(40) fractionated butter oil into a liquid fraction and a solid fraction with differ-ent cholesterol contents. Arul et al. (14) also studied the distribution of choles-terol in milk fat fractions obtained with supercritical carbon dioxide at tempera-tures of 50°C and 70°C and pressures varying from 10 to 35 MPa, and comparedthe cholesterol contents in these fractions with those in fractions obtained bydistillation and crystallization. Cholesterol removal efficiency was highest withdistillation followed by supercritical fluid fractionation. Bradley (2) reportedthat 90% cholesterol removal efficiency from milk fat at 80°C and pressuresfrom 15.8 to 41.4 MPa is quite feasible for a broad range of dairy foods. Aredistribution of cholesterol in butter oil was also presented by Chen et al. (41)in which cholesterol concentrations were increased in fractions extracted at 40°Cand 10.3, 13.8, 24.1, and 27.6 MPa. A similar cholesterol redistribution in butteroil fractions extracted with supercritical CO2 was reported by Bradley (8). Ham-mam et al. (89) characterized the supercritical carbon dioxide fractionation prod-ucts, including the redistribution of cholesterol in such products according totheir physical properties. Bhaskar et al. (90) fractionated milk fat into fractionsof short chain (C4–C8), medium chain (C10–C12) and long chain (C14–C18) fattyacids and concluded that fractions collected at higher pressures were richer inthe higher molecular weight triglycerides and that carbon dioxide displays lowselectivity for cholesterol in relation to triglycerides. Shukla et al. (91) presentedphysicochemical and rheological properties of butter fractions obtained whenusing supercritical fluids for the fractionation of milk fat. The butter obtainedwith supercritical fluids showed a potential use at ambient and higher tempera-

Removal of Cholesterol 379

tures in contrast with the oiling off and leakage problems exhibited by normallyencountered market butter. Furthermore, the extracted butter was reported to bericher in unsaturated fatty acids and lower in cholesterol content (117.6 mg/100g) than those found in commercial butter (240.6 mg cholesterol/100 g). Rizviand Bhaskar (19) also separated milk fat into saturated and unsaturated fattyacids using supercritical CO2, identified the physical properties, and quantifiedthe cholesterol content. They concluded that the fractions obtained with super-critical CO2 were unique exhibiting different characteristic physicochemicalproperties. They also presented a summary of all recent research on the fraction-ation of butter oil with supercritical carbon dioxide and a basic study for scale-up of the process.

Solubility and fractionation data of cholesterol in supercritical carbon di-oxide and ethane were obtained by Mohamed et al. (22) using a high-pressureexperimental extraction apparatus (92). The amounts of cholesterol in butter oilextracted using supercritical ethane were found to be much higher than whenusing supercritical CO2 (Fig. 4).

Both supercritical CO2 and melt crystallization processes produce fractionswith varying physical properties (93). The advantage of using supercritical CO2

is that the flavor components are simultaneously concentrated during the frac-tionation. Furthermore, solid fractions obtained with supercritical CO2 have

Figure 4 Cholesterol contents in fractions obtained with the extraction of butter oilusing supercritical CO2 and ethane at different pressures and temperatures.

380 Mohamed et al.

lower cholesterol content than those obtained by melt crystallization. However,process cost favors melt crystallization (0.02–0.05 $/kg) over supercritical CO2

extraction (0.10–0.15 $/kg) as reported in a detailed study by Singh and Rizvi(7). The lower cholesterol content and consequently higher quality of milk fatfractions obtained with supercritical CO2 could still make this process a veryviable option.

Shishikura et al. (42) concluded that the preparation of a low-cholesterolbutter oil by simple extraction with supercritical CO2 is not practical due tothe observed relatively low butter oil capacity and cholesterol selectivity. Theyproposed the use of supercritical extraction in conjunction with an added adsor-bent. Cholesterol levels were substantially reduced when passing the extractthrough a silica or alumina adsorbent (22, 42). Adsorption and desorption ofcholesterol in continuous supercritical fluid processing of anhydrous milk fatwere also reported by Lim and Rizvi (87), with magnesium silicate as the adsor-bent.

The extraction/fractionation operation was also coupled with an adsorptionstep that uses alumina as the adsorbent (22). The combined extraction/adsorp-tion operation resulted in the removal of more than 97% of the cholesterol inthe original butter oil (Fig. 5). The increase in cholesterol content in fractionsobtained in late stages is attributed to the desorption of cholesterol from thealumina bed to the supercritical fluid stream. The operation has also resulted in

Figure 5 Cholesterol extraction curves of butter oil for the fractionation/adsorptionprocess with supercritical ethane.

Removal of Cholesterol 381

the generation of butter oil fractions with characteristic properties that are dis-tinctly different from those of the original oil.

The detailed economic analysis for the continuous supercritical CO2 pro-cessing of milk fat performed by Singh and Rizvi (7) and referred to earliershows this process to be economically viable. The final processing cost can beoffset by the value of the byproduct, cholesterol. Some economic analysis forcontinuous countercurrent processing of milk fat fractionation was also carriedout by Konig-Schreer (93). Assuming a plant capacity ranging from 800 to10,000 t/a for a continuous milk fat fractionation by supercritical CO2, a paybackperiod of about 5 years was estimated. These calculations confirm that an opti-mized milk fat fractionation process could be carried out on an industrial scale.The addition of an adsorption step for cholesterol would improve the economicsof this process. Semicontinuous systems are not as economically favorable ascontinuous large-scale systems.

B. From Eggs

Eggs are one of the most complete foods consumed by humans, being an impor-tant source of high-quality protein, vitamins, and minerals (38). However, eggyolk has a high concentration of cholesterol, and concerns about cholester-ol concentration in blood serums and potential health problems are the princi-pal causes of the worldwide decrease in egg consumption. Products containingwhite eggs, low or no egg yolk are already available in the market, resulting ina surplus of egg yolk (78). Removal of the cholesterol would be a promisingsolution for this surplus problem and the return to normal whole-egg consump-tion.

Extraction of lipids with organic solvents for the purpose of reducing thecholesterol content in egg yolks is no longer acceptable because organic solventsare nonselective, and such extractions result in the loss of other valuable compo-nents such as phospholipids which carry out important functional propertiesincluding emulsion stabilization (38). In these processes, proteins are denatured,which impedes the use of the resultant protein concentrate (38), and toxic resi-dues could potentially contaminate both final product and environment (79).Supercritical fluid extraction using carbon dioxide as a solvent has presentedgood results in cholesterol selective extraction, resulting in the extraction ofapproximately 70% of the cholesterol content of dried egg yolk (38) and thetotal removal of cholesterol from egg yolk powder (79). Therefore, the worksof Paraskevopoulou et al. (94) extracted about 45% of the lipids and 75% of thecholesterol content in dried egg yolk using a continuous supercritical fluid appa-ratus at 31.4 MPa and 35–45°C. Phospholipids were not coextracted with cho-lesterol and other lipids, which is highly desirable due to their important func-tional and organoleptic properties. Egg yolk, both dry and degreased, exhibited

382 Mohamed et al.

similar stability, and the concentrate had a satisfactory performance in the cakepreparation.

Wu and Hou (79) extracted egg yolk powder using supercritical carbondioxide as a solvent in a pilot plant at pressures and temperatures ranging from25 to 36 MPa and 35 to 70°C, respectively. A complete extraction of all eggyolk oil was obtained. The egg yolk powder particle size had no influence onthe extraction rate, and increasing the flow rate resulted in a faster extraction.A kinetic model developed to describe the extraction egg yolk presented resultsin good agreement with experimental pilot plant data.

C. From Beef, Chicken, Fish and Pork

Meats, important sources of protein in the human diet, generally have high con-tents of fat and cholesterol. Reduction of the fat and cholesterol contents in meatproducts is one of the challenges of the food industries to attend to consumerneeds for a healthy diet and important for those consumers who desire to loseweight or those with a heart problem.

Supercritical fluid extraction of cholesterol and fat from chicken (33),pork (34), fish (35), and beef (80), among others, has been studied. Results haverevealed the process to be efficient, in some cases resulting in the removalreducing of more than two-thirds of the original cholesterol and fat content.

Wehling (95) extracted fat and cholesterol in dehydrated beef using super-critical CO2 at pressures and temperatures ranging from 23.4 to 38.6 MPa and45 to 55°C, respectively. At a solvent density of 0.9 g/cm3, about 87% of thetotal cholesterol and fat content from dehydrated beef powder was removed,and high temperatures favored the extraction of lipids, which were in the solidstate at 45°C but were completely melted at 55°C. The authors also reported onthe effects of particle size on the extraction efficiency and on the color alterationof the dehydrated beef powder due to removal of pigments, which could be verydesirable as it allows the product to be a source of protein in various preparedfoods. Lin et al. (34) observed this same loss in pigments during the removal ofcholesterol from fried shredded pork using continuous supercritical carbon diox-ide extraction. Operating at pressures and temperatures ranging from 7.3 to 34.4MPa and 50 to 150°C, respectively, about 50–70% of the cholesterol in themeat sample was removed. The increase in temperature from 50°C to 150°Cresulted in higher pigment removal, which in this case is not desirable as white-ness could be perceived to indicate a poor-quality product. Sensory analysisindicated that products obtained with these extraction conditions could not bedifferentiated from those bought in a local supermarket, with 34 MPa and 150°Cbeing the optimal extraction conditions. Froning et al. (33) also demonstratedthe efficiency of the supercritical fluid process in the extraction of cholesterol

Removal of Cholesterol 383

and fat from dehydrated chicken meat powder and chunks, along with recoveryof important flavor components from the residues (extracted lipids).

A summary of the main products containing cholesterol and their extrac-tion with supercritical fluids is presented in Table 2. These results indicate thegreat potential of supercritical fluid extraction in the recovery of meat productswith acceptable cholesterol and fat contents.

V. FUTURE PROSPECTS

During the past 20 years, the use of supercritical fluid technology has advancedquite rapidly. Supercritical fluids have been applied in diverse areas, includingthe food and pharmaceutical industries, biotechnology, new materials technolo-gies, petroleum, among others. Removal of cholesterol contents from differentfood products, including fish oil capsules, foodstuffs sausages, mayonnaise,noodles, and cheese (71, 96), have been recently reported, and the determinationof cholesterol contents in food products and blood serum using capillary super-critical fluid chromatography is believed to be a potential and fast analysis tech-nique for programmed control of the optimal consumption diets.

The analytical determination of cholesterol in food, as described by Jime-nez-Carmona (57) using a reverse micelle formation to accelerate the extractionof cholesterol and in which a surfactant is added to the sample from whichcholesterol will be extracted, is also a potential new application.

Table 2 Supercritical Fluid Extraction of Cholesterol with CO2 from Productsof Animal Origin

Cholesterol(mg/g)

Product Ref. P (Mpa) T (°C) Before After Yield (%)

Dried egg yolk 39 16.5–37.8 40–55 18.52 6.34 65.8Dried egg yolk 68 24.1–37.8 45–55 18.94 0.38 98.0Dehydrated beef 86 23.4–38.6 45–55 1.56 0.19 87.8Beef patties (cooked) 70 17.2–55.1 40–50 1.94 0.12 93.8Pork (cooked) 34 7.3–34.4 50–150 0.80 0.22 70.1Dried chicken meat 33 30.6–37.6 45–55 4.96 0.54 90.0Milk fat 20 10.1–36.4 40–70 2.50 0.21 91.5Milk fata 22 8.0–24.0 40–70 2.50 0.20 93.4

aUsing supercritical ethane as solvent.

384 Mohamed et al.

In general, alternative processes using supercritical fluids have beenclaimed to be highly versatile techniques, which could be applicable in a numberof situations where conventional processes could not be applied or have seriouslimitations. Some new applications of supercritical fluid processes for the reduc-tion of cholesterol content include the enzymatic conversion of cholesterol tosterols that are unabsorbed by humans. In this application, a supercritical fluidas a reaction medium in enzymatic catalytic processes is considered as a meansto increase the solubility of hydrophobic components and revealed to be highlysuccessful as shown in the recent literature (97, 98). King et al. (99) reportedthe use of supercritical carbon dioxide as a reaction medium with lipase as acatalyst in a natural process, applicable to produce additives that can be incorpo-rated directly in food formulations.

Some sterol esters can be added to foods acting as cholesterol-loweringagents, with important implications for the food and nutraceutical industries.Enzyme-catalyzed esterification of cholesterol using vinyl acetate as a cosolventin supercritical ethane and pressurized hexane was studied by Sarkari et al. (97),who concluded that the enhancement in the esterification rate is a strong func-tion of the pressure and cosolvent concentration in the system when using super-critical ethane as a reaction medium,. When using hexane, the effect of pressurewas less pronounced. These facts are much important as they point to the possi-bility of controlling the reaction rate by choosing the appropriate solvent andpressure system.

REFERENCES

1. CG Greenwald. Overview of fat and cholesterol reduction technologies. Chap. 3.In: C Haberstroh, CE Morris, eds. Fat and Cholesterol Reduced Foods: Technolo-gies and Strategies. Advances in Applied Biotechnology Series, Vol. 12. Wood-lands, TX: Gulf Pub, 1991, pp. 21–32.

2. RL Bradley Jr. Removal of cholesterol from milk fat using supercritical carbondioxide. J Dairy Sci 72:2834–2840, 1989.

3. S Yokohama. Tecnologia para a reducao de colesterol em alimentos. In: SeminarioColesterol: Analise, ocorrencia, reducao em alimentos e implicacoes na saude.Campinas. Anais. ITAL, Campinas, SP, Brasil, 1996, pp. 15–18.

4. GA Bray. A health and nutritional perspective on fat, cholesterol, and risks tohealth. Chap. 2. In: C Haberstroh, CE Morris, eds. Fat and Cholesterol ReducedFoods: Technologies and Strategies. Advances in Applied Biotechnology Series,Vol. 12. Woodlands, TX: Gulf Pub, 1991, pp. 13–19.

5. MA Kantor. Light dairy products: The needs and consequences. Food Technol 44:81, 1990.

6. W Wu, Y Hou. Mathematical modeling of extraction of egg yolk oil with supercrit-ical CO2. J Super Fl, 19:149–159, 2001.

7. B Singh, SSH Rizvi. Our industry today: design and economic analysis for continu-

Removal of Cholesterol 385

ous countercurrent processing of milk fat with supercritical carbon dioxide. J DairySci 77:1731–1745, 1994.

8. RL Bradley Jr. Removing cholesterol from milk fat using supercritical carbon diox-ide. Chap. 17. In: C Haberstroh, CE Morris, eds. Fat and Cholesterol ReducedFoods: Technologies and Strategies. Advances in Applied Biotechnology Series,Vol 12. Woodlands, TX: Gulf Pub, 1991, pp. 221–232.

9. SS Marschner, JB Fine. Physical process for simultaneous deodorization and cho-lesterol reduction of fats and oils. U.S. Patent 4,804,555 (1989).

10. CO Pfaudler. Wiped film evaporator. Bulletin No. SB 39–100-1, Rochester, NY,1982.

11. E Sundfeld, S Yun, JM Krochta, T Richardson. Separation of cholesterol frombutter oil using Quilliajas saponinis. I. Effects of pH, contact time and adsorbent.J Process Eng 16(3):191–205, 1993.

12. U Bracco. Butter-like food product. British Patent 1,559,064 (1980).13. R Norris, IK Gray, AKR McDowell. Chemical composition and physical properties

of fractions of milk fat obtained by a commercial fractionation process. J DairyRes 38:179–191, 1971.

14. J Arul, A Boudreau, J Makhlouf. Distribution of cholesterol in milk fat fractions. JDairy Res 55:361–371, 1988.

15. J Courrelonge, JP Maffrand. Procede d’elimination du cholesterol contenu dansuna M.G. d’ origine Animal et M.G. appauvrie en cholesterol obtenue. EuropeanPatent Application EP 0256911 A1 (1988).

16. H Roderbourg, D Dalemans, R Bouhon. Fundamentals of processing with supercrit-ical fluids. Chap. 1. In: SSH Rizvi, ed. Supercritical Fluid Processing of Food andBiomaterials. New York: Blackie Academic, 1994, pp. 1–26.

17. P Chosson, C Deshayes, J Frankinet. Degradation of sterols. European Patent Ap-plication EP 0278794 A1 (1988).

18. J Cully, JR Vollbrecht, E Schutz. Verfahren zur entfernung von cholesterin bzwcholesterinestern aus lebensmitteln. German Patent Application DE 3929555 A1(1989).

19. SSH Rizvi, AR Bhaskar. Supercritical fluid processing of milk fat: Fractionation,scale-up and economics. Food Tech 49(2):90–100, 1995.

20. GBM Neves. Solubilidade do colesterol e do oleo de manteiga em dioxido de car-bono supercrıtico. MSc thesis, Universidade Estadual de Campinas, Campinas-Brasil, 1996.

21. MDA Saldana, EM Homen, RS Mohamed. Extracao do colesterol com misturas dedioxido de carbono e etano supercrıtico. Ciencia e Tecnol. Aliment. 17(4):389–392,1997.

22. RS Mohamed, MDA Saldana, FH Socantaype, TG Kieckbusch. Reduction in thecholesterol content of butter oil using supercritical ethane, extraction and adsorptionon alumina. J Super Fl 16:225–233, 2000.

23. W Hamm. Review: Trends in edible oil fractionation. Trends Food Sci Technol6(April): 121–126, 1995.

24. D Oakenfull, GS Sidhu. Low-cholesterol egg and dairy products. Outlook on Agri-culture 21(3):203–208, 1992.

386 Mohamed et al.

25. J Chrastil. Solubility of solids and liquids in supercritical gases. J Phys Chem 86:3016–3021, 1982.

26. MDA Saldana, EM Homen, RS Mohamed. Uso de etano como co-solvente na ex-tracao do colesterol com CO2 supercrıtico. In: Anales del II Congresso Iberoameri-cano de Ingenierıa de Alimentos. Universidad Nacional del Sur, Bahia Blanca-Argentina, 1998, pp. 1–6.

27. JM Wong, KP Johnston. Solubilization of biomolecules in carbon dioxide basedsupercritical fluids. Biotechnol Prog 2:29–39, 1986.

28. RS Mohamed, GM Neves, TG Kieckbusch. Reduction in cholesterol and fraction-ation butter oil using supercritical carbon dioxide with adsorption on alumina. IntJ Food Sci Technol 33(5):245–253, 1998.

29. SLJ Yun, KK Liong, GS Guardial, NR Foster. Solubility of cholesterol in supercrit-ical carbon dioxide. Ind Eng Chem Res 30:2476–2482, 1991.

30. H Singh, SLJ Yun, SJ Macnaoughton, DL Tomasko, NR Foster. Solubility of cho-lesterol in supercritical ethane and binary gas mixtures containing ethane. Ind EngChem Res 22:2841–2848, 1993.

31. E Kosal, CH Lee, GD Holder. Solubility of progesterone, testerone, cholesterol insupercritical fluids. J Super Fl 5:169–179, 1992.

32. GW Froning, SL Cuppett, L Niemann. Extraction of cholesterol and other lipidsfrom dehydrated beef using supercritical carbon dioxide. J Agric Food Chem 40:1204–1207, 1992.

33. GW Froning, F Fieman, RL Wehiling, S Cuppett, L Nielmann. Supercritical carbondioxide extraction of lipids and cholesterol from dehydrated chicken meat. PoultrySci 73:571–575, 1994.

34. TY Lin, YJ Wang, PY Lai, FJ Lee, JTS Cheng. Cholesterol content of fried-shred-ded pork extracted by supercritical carbon dioxide. Food Chem 67:89–92, 1999.

35. I Hardardottir, JE Kinsella. Extraction of lipid and cholesterol from fish musclewith supercritical fluids. J Food Sci 53:1656–1658,1661, 1988.

36. GW Froning. Supercritical fluid extraction of cholesterol from dried eggs. Chap.22. In: C Haberstroh, CE Morris, eds. Fat and Cholesterol Reduced Foods: Tech-nologies and Strategies. Advances in Applied Biotechnology Series, Vol. 12.Woodlands, TX: Gulf Pub, 1991, pp. 277–288.

37. RA Novak, WJ Reightler, G Pasin, AJ King, G Zeidler. Supercritical fluid extrac-tion of cholesterol from liquid egg. Chap. 23. In: C Haberstroh, CE Morris, eds. Fatand Cholesterol Reduced Foods: Technologies and Strategies. Advances in AppliedBiotechnology Series, Vol 12. Woodlands, TX: Gulf Pub, 1991, pp. 289–298.

38. GW Froning, RL Wehiling, S Cuppett, L Nielmann. Moisture content and particlesize of dehydrated egg yolk affect lipid and cholesterol extraction using supercriti-cal carbon dioxide. Poultry Sci 77:1718–1722, 1998.

39. GW Froning, RL Wehiling, S Cuppett, MM Pierce, L Nielmann, DK Siekan. Ex-traction of cholesterol and other lipids from dried egg yolk using supercritical car-bon dioxide. J Food Sci 55:95–98, 1990.

40. VW Kaufmann, G Biernoth, E Frede, W Merk, D Precht, H Timmen. Fraktionierigvon buttergett durch extraktion mit uberkritischim CO2. Milchwisserscharft 37(2):92–96, 1982.

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41. H Chen, SJ Schwartz, GA Spanos. Fractionation of butter oil by supercritical car-bon dioxide. J Dairy Sci 75:2659–2669, 1992.

42. A Shishikura, K Fujimoto, T Kaneda. Modification of butter oil by extraction withsupercritical carbon dioxide. Agric Biol Chem 50(5):1209–1215, 1986.

43. W Hubert, A Molero, C Pereyra, EM de la Ossa. Dynamic supercritical CO2 extrac-tion for removal of cholesterol from anhydrous milk fat. Int J Food Sci Technol31:141–151, 1996.

44. RR Chao, SJ Mulvaney, ME Bailey, LN Fernando. Supercritical CO2 conditionsaffecting extraction of lipid and cholesterol from ground beef. J Food Sci 56:183–187, 1991.

45. K Bloch. The structure of cholesterol and bile acids. Trends Biochem Sci 7:334–336, 1982.

46. E J Blanchette-Mackie. Intracellular cholesterol trafficking: role of the NPC1 pro-tein. Biochim Biophys Acta 1486:171–183, 2000.

47. DE Vance, H Van den Bosch. Cholesterol in the year 2000. Biochim Biophys Acta1529:1–8, 2000.

48. K Lu, MH Lee, SB Patel. Dietary cholesterol absorption; more than just bile.Trends Endocrinol Metab 12(7):314–320, 2001.

49. M Hernandez, J Montenegro, M Steiner, D Kim, C Sparrow, PA Detmers. Intestinalabsorption of cholesterol is mediated by a saturable, inhibitable transporter. Bio-chim Biophys Acta 1486:232–242, 2000.

50. S Yokoyama. Release of cellular cholesterol: molecular mechanism for cholesterolhomeostasis in cells and in the body. Biochim Biophys Acta 1529:231–244, 2000.

51. TA Miettinem. Cholesterol absorption efficiency and sterol metabolism in obesity.Atherosclerosis 153:241–248, 2000.

52. N Kumar, OP Singhal. Cholesterol oxides and atherosclerosis: a review. J Sci FoodAgric 55:497–510, 1991.

53. RM Feeley, PE Criner, BK Watt. Cholesterol content of foods. J Am Diet Assoc61:134–139, 1972.

54. DE LaCroix, WA Mattingl, NP Wong, JA Alford. Cholesterol, fat and protein indairy products. J Am Diet Assoc 62:275–279, 1973.

55. N Bragagnolo. Determinacao dos nıveis de colesterol em carnes, ovos e macarraocom ovos. FEA, UNICAMP, 1993. In: Revista da Carne 222: 82, 1995.

56. JP Sweeney, JL Weihrauch. Summary of available data for cholesterol in foods andmethods for its determination. CRC Crit Rev Food Sci Nutr 8:131–159, 1976.

57. MM Jimenez-Carmona, MD Luque de Castro. Reverse miscelle formation for ac-celeration of the supercritical fluid extraction of cholesterol from food samples.Anal Chem 70:2100–2103,1998.

58. AOAC. Residue (unsaponifiable) of oils and fats-ether extraction method. In: Offi-cial Methods of Analysis. Arlington, VA: AOAC, 1990, p. 971.

59. WJ Hurst, MD Aleo, RA Martin. High-performance liquid-chromatographic analy-sis of cholesterol. J Dairy Sci 66:2192–2194, 1983.

60. DR Newkirk, AJ Sheppard. High-pressure liquid-chromatographic determination ofcholesterol in foods. JAOAC 64:54–57, 1981.

61. K Sugino, J Terao, H Murakami, S Matsushita. High-performance liquid-chromato-

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graphic method for the quantification of cholesterol epoxides in spray-dried egg. JAgric Food Chem 34:36–39, 1986.

62. HG Brown, Adaptation of an HPLC system to quantify cholesterol. JAOCS 64(1):106–108, 1987.

63. AOAC. Method 941.09. In: Official Methods of Analysis, 15th ed. Arlington, VA:AOAC, 1990.

64. AOAC. Method 43.290. In: Official Methods of Analysis, 15th ed. Arlington, VA:AOAC, 1990.

65. AOCS Official Method Ca 6a-40. In: Official Methods and Recommended Prac-tices, 4th ed. Champaign, IL: AOCS, 1993.

66. G Pasin, GM Smith, M O’Mahony. Rapid determination of total cholesterol in eggyolk using commercial diagnostic cholesterol reagent. Food Chem 61(1/2):255–259, 1998.

67. B Zak. Cholesterol methodologies—review. Clinical Chem 23(7):1201–1214,1977.

68. CF Bohac. Assessment of methodologies for colorimetric cholesterol assay meats.J Food Sci 53:1642–1644, 1998

69. MW Marshall, BA Clevidence, RH Thompson Jr, JT Judd. Problems in estimatingamounts of food cholesterol: three methods for mixed diets. J Food Comp Anal 2:2–12, 1989.

70. M Fenton, JS Sim. Determination of egg yolk cholesterol content by on-columncapillary gas chromatography. J Chrom 540:323–329, 1991.

71. F Ulberth, H Reich. Gas-chromatographic determination of cholesterol in processedfoods. Food Chem 43(5):387–391, 1992.

72. MA McHugh, VJ Krukonis. Supercritical Fluid Extraction Principles and Practice.Boston: Butterworths, 1986.

73. SK Kumar, KP Jhonston. Modeling the solubility of solids in supercritical fluidswith density as the independent variable. J Super Fl 1:15–22, 1988.

74. MDA Saldana, EM Homen y RS Mohamed, Extraccion del colesterol com mezclasde dioxido de carbono y etano supercrıtico. Informacion Tecnologica 10(1):95–100, 1999.

75. RL Mendes, BP Nobre, JP Coelho, AP Palavra. Solubility of β-carotene in super-critical carbon dioxide and ethane. J Super Fl 16:99–106, 1999.

76. NR Foster, H Singh, SLJ Yun, DL Tomasko, SJ Macnaughton. Polar and non-polarco-solvent effects on the solubility of cholesterol in supercritical fluids. Ind EngChem Res 32:2849–2853, 1993.

77. W Huber, A Molero, C Pereyra, EM de la Ossa. Determination of cholesterol inmilk fat by supercritical fluid chromatography. J Chromatogr A 715(2):333–336,1995.

78. MW Warren Jr, HR Ball, GW Froning, DR Davis. Lipid composition of hexaneand carbon dioxide reduced cholesterol dried egg yolk. Poultry Sci 70:1991–1997,1991.

79. W Wu, Y Hou. Mathematical modeling of extraction of egg yolk oil with supercrit-ical CO2. J Super Fl 19:149–159, 2001.

80. JW King, JH Johnson, WL Orton, FK McKeith, PL O’Connor, J Novakofski, TR

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Carr. Fat and cholesterol content of beef patties as affected by supercritical CO2

extraction. J Food Sci 58:950–958, 1993.81. E Deffense. Multi-step butter oil fractionation and spreadable butter. Fett Wiss

Technol 89(13):502–507, 1987.82. V Antila. The fractionation of milk fat. Milk Ind 81:17–20, 1979.83. RA Yoncoski, VH Holsinge, LP Posati, MJ Pallansc. Changes in thermal and opti-

cal properties of butter oil on hydrogenation. JAOCS 46(9):489–492, 1969.84. SSH Rizvi, Z Yu, A. Bhaskar, L Rosenberry. Phase equilibria and distribution

coefficients of δ-lactones in supercritical carbon dioxide. J Food Sci 58:996–1000,1993.

85. S Lim, GB Lim, SSH Rizvi. Continuous supercritical CO2 processing of milk fat.In: MA McHugh, ed. Proceedings of 2nd International Symposium on SupercriticalFluids. Baltimore, May 20–22, Johns Hopkins Univ. Press, 1991, pp. 292–296.

86. S Lim. Performance characteristics of a continuous supercritical carbon dioxide sys-tem coupled with adsorption. PhD dissertation, Cornell University, Ithaca, NY, 1992.

87. S Lim, SSH Rizvi. Continuous supercritical fluid processing of anhydrous milk fatin a packed column. J Food Sci 60:889–893, 1995.

88. AB de Haan, J de Graauw, JE Schaap, HT Badings. Extraction of flavors frommilk fat with supercritical carbon dioxide. J Super Fl 3:15–19, 1990.

89. I Hammam, I Soderberg, B Sivik. Physical properties of butter oil fractions ob-tained by supercritical carbon dioxide extraction. Fat Sci Technol 93(10):374–378,1991.

90. AR Bhaskar, SSH Rizvi, JW Sherbon. Anhydrous milk-fat fractionation with con-tinuous countercurrent and supercritical carbon dioxide. J Food Sci 58:748–752,1993.

91. A Shukla, AR Bhaskar, SSH Rizvi, SJ Mulvaney. Physicochemical and rheologicalproperties of butter made from supercritical fractionated milk fat. J Dairy Sci 77:45–54, 1994.

92. MDA Saldana, P Mazzafera, RS Mohamed. Extraction of purine alkaloids frommate (Ilex paraguariensis) using supercritical CO2. J Agric Food Chem 47:3804–3808, 1999.

93. M Konig-Schreer. Anwendung der Milchfett Fraktionierung Mittels Uberkritish-cehm Kohlendioxid zur Gewinnung Spezifischer Fettqualitaten. Dissertation derUniversitat Bonn, Germany, 1994.

94. A Paraskevopoulou, K Panayiotou, V Kiosseoglou. Functional properties of eggyolk concentrate with a reduced cholesterol content prepared by treatment withsupercritical CO2. Food Hydrocolloids 11(4):385–391, 1997.

95. RL Wehling. Supercritical fluid extraction of cholesterol from meat products. Chap.10. In: C Haberstroh, CE Morris, eds. Fat and Cholesterol Reduced Foods: Tech-nologies and Strategies. Advances in Applied Biotechnology Series, Vol. 12.Woodlands, TX: Gulf Pub, 1991, pp. 133–139.

96. JW King. Applications of capillary supercritical fluid chromatography and super-critical fluid extraction to natural-products. J Chromatogr Sci 28:9–14, 1990.

97. M Sarkari, BL Knutson, CS Chen. Enzymatic catalysis in co-solvent modified pres-surized organic solvents. Biotechn Bioeng 65(3):258–264, 1999.

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98. MA Kane, GA Baker, S Pandey, FV Brigh. Performance of cholesterol oxidasesequestered within reverse miscelles formed in supercritical carbon dioxide. Lang-muir 16(11):4901–4905, 2000.

99. JW King, JM Snyder, H Frykman, A Neese. Sterol ester production using lipaseca-talyzed reactions in supercritical carbon dioxide. Eur Food Res Technol 212(5):566–569, 2001.

13Solvent Extraction: Safety, Health,and Environmental Issues

Phillip J. WakelynNational Cotton CouncilWashington, D.C., U.S.A.

Peter J. WanU.S. Department of AgricultureNew Orleans, Louisiana, U.S.A.

I. INTRODUCTION

Safety, health, and environmental issues for edible oil extraction facilities vary,depending on the extraction method used. For example, aqueous extraction, su-percritical fluid extraction, and mechanical extraction facilities have differentrequirements than organic solvent extraction facilities. In the United States, edi-ble oil extraction using organic solvents, is a mature industry that has beenunder increasing regulatory pressure in recent years. In this chapter, regulatoryconcerns and toxicity of extraction solvents, commercial hexane, and other po-tential alternative solvents are discussed along with the current methods of edi-ble oil extraction (i.e., mechanical, prepress solvent, and solvent extraction).

Fats, which are solid at ambient temperature, and liquid oils are recoveredfrom diverse biological sources by mechanical separation, solvent extraction, ora combination of the two methods (1, 2). These materials include animal tissues(e.g., beef, chicken, and pork); crops specifically produced for oil or protein(e.g., soy, sunflower, safflower, rape/canola, palm, and olive); byproducts ofcrops grown for fiber (e.g., cottonseed and flax); crops for food and their copro-ducts (e.g., corn germ, wheat germ, rice bran, coconut, peanuts, sesame, wal-nuts, and almonds); nonedible oils and fats (castor, tung, jojoba); and other oilsources (oils and fats from microbial products, algae, and seaweed). There are

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many physical and chemical differences among these diverse biological materi-als. However, the similarities are that oils (edible and industrial) and other usefulmaterials (e.g., vitamins, nutraceuticals, fatty acids, phytosterols) can be ex-tracted from these materials by mechanical pressing, solvent extracting, or acombination of pressing and solvent extraction. The preparation of the variousmaterials to be extracted varies. Some need extensive cleaning, drying (op-tional), fiber removal (cottonseed), dehulling, flaking, extruding, and so forth,all of which affect the solvent–substrate interaction and therefore the yield,composition, and quality of the oils and other materials obtained.

Historically, the advancement of processing technology for recovering oilsand other useful materials has been primarily driven by economics. Each extrac-tion process was optimized through trial and error with the available technologyto produce maximal yield of high-quality products at the lowest cost. For thou-sands of years, stone mills, and for several centuries, simple hydraulic or leverpresses were used as batch systems. The continuous mechanical presses onlybecame a reality during the early 1900s. It was not until the 1930s that extrac-tion solvents were used more widely, which greatly enhanced the recovery ofoil from oilseeds or other oil-bearing materials. In recent years, safety, health,and environmental regulations for the solvents used for extracting oil from oil-seeds have prompted research efforts to find solvents to replace commercialhexane. These solvents, including ethanol, isopropanol, water, supercritical car-bon dioxide, and others, are technically feasible as oil extraction solvents but atpresent are economically unacceptable (1). Recent research to use commercialisohexane in two separate cottonseed oil mills (3, 4) has demonstrated energysavings and throughput increases. In the near term, it appears that commercialisohexane can be used as an alternative to commercial hexane.

II. OIL EXTRACTION PROCESS

Four types of processing systems are used to extract oil from oil-bearing materi-als: hydraulic press, expeller or screw press, prepress solvent extraction, anddirect solvent extraction. Oil-bearing materials have to be prepared for extrac-tion to separate the crude oil from the meal (1, 2) (Fig. 1). Careful control ofmoisture and temperature during processing must be exercised to maintain thequality of the protein in the meal, to minimize the damage to the oil, and tomaximize oil extraction. Crude oils are refined by conditioning with phosphoricacid to promote the removal of phospholipids and treating with sodium hydrox-ide (alkali refining) (Fig. 2). Refined oil is bleached with activated clay to re-move color pigments and residual soap. Bleached oils are then deodorized bysteam distillation. When the soybean oil is of good quality and the free fattyacid (FFA) content is low, the oil can be physically refined. Physical refininginvolves water washing to recover lecithin followed by phosphoric acid pretreat-

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Figure 1 Preparation for extraction to separate the crude oil from the meal in oil-bearing materials.

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Figure 2 Refining of crude oil.

ment and bleaching to remove the nonhydratable gum. After bleaching, the oilis steam deodorized to remove the FFAs. The refined, bleached, and deodorizedoil (RBD oil) is used to produce finished products, e.g., salad and cooking oils,shortenings, and margarine. Some of the finished products also require the oilto be hydrogenated, which changes the consistency and solid content of the oil,and increases stability to oxidation, which extends the shelf life of the finishedproducts. Also, some of the oils (e.g., cottonseed and sunflower) are winterizedto remove the higher melting constituents or wax. The solid fraction removedfrom winterization can be used in confectionery products; the winterized oil isless likely to become cloudy in refrigerated storage.

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A. Preparation for Extraction

1. Storage

For optimal extraction and quality of oil, the oil-bearing material should bestored dry and at a relatively low temperature. If the material is wet, it shouldbe processed as soon as possible after harvest. Oils in the presence of water candeteriorate rapidly, forming FFAs and causing greater refining loss.

2. Seed Cleaning

The first step in the commercial processing of oilseeds is “cleaning” to removeforeign materials, such as sticks, stems, leaves, other seeds, sand, and dirt usingdry screeners and a combination of screens and aspiration. Permanent electro-magnets are also used for the removal of trash iron objects. Final cleaning ofthe seed usually is done at the extraction plant just prior to processing.

3. Dehulling

The dehulling process may include the removal of excess moisture by drying,cracking the seed, and removing the outer seed coat (hull) of the seed. The hullcontains little or no oil, so that its inclusion makes the extraction less efficientand dilutes the protein content of the meal. Also, the hull will reduce the totalyield of oil by absorbing and retaining oil in the press cake. An acceptable levelof hull removal must be determined, depending on the desired protein level ofthe final meal. Hulls are removed by aspiration, and un-dehulled seeds are re-moved from the kernels by screening and returned to the huller. Some meatsstill adhere to the hulls, which are beaten, then screened again to obtain themeat. In the case of high oil content seed for direct solvent extraction, such ascottonseed, sunflower seed, etc., certain quantity of hull material is added to thekernels to provide the structural strength and matrix needed for the solvent ex-traction process.

4. Flaking

After dehulling, the meats are reduced in size and “flaked” to increase surfacearea and to facilitate oil removal. Proper moisture content of the seeds is essen-tial for flaking, and if the moisture level is too low, the seeds are “conditioned,”with water or steam, to raise the moisture to about 11%. In the case of soybean,heat will be applied to soften the meats prior to flaking. For solvent extraction,flakes are commonly not less than 0.203–0.254 mm (0.008–0.010 in), whichcan be solvent extracted efficiently with less than 1% residual oil. Thinner flakestend to disintegrate during the solvent extraction process and reduce the miscellapercolation rate.

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5. Cooking

Prior to extraction, the flakes are heated. The purpose of cooking the flakes isto (a) break down cell walls to allow the oil to escape; (b) reduce oil viscosity;(c) control moisture content; (d) coagulate protein; (e) inactivate enzymes andkill microorganisms; and (f) fix certain phosphatides in the cake, which helpsto minimize subsequent refining losses. Flakes are cooked in stack cookers tomore than 87.8°C (190°F) in the upper kettle. Flakes with high phosphatidecontent may benefit from being cooked at slightly lower temperatures to avoidelevating refining losses. The temperature of the flakes is raised to 110–132.2°C(230–270°F) in the lower kettles. The seeds are cooked for up to 120 min.Overcooking lowers the nutritional quality of the meal and can darken both theoil and the meal. Poor-quality seeds with high levels of FFAs cannot be cookedfor as long as high-quality seeds because of darkening. Darker oil requires addi-tional refining to achieve a certain bleachable color. For soybeans, this heattreatment is often done prior to flaking.

6. Preparation of Collets with Expanders

Sometimes low-shear extruders, called expanders, are used. This equipment hasthe capability to process both low and high oil content materials. The meats arefed to an extruder after dehulling, flaking, and cooking and are heated as theyare conveyed by a screw press through the extruder barrel. The meats are underconsiderable shear, pressure, and temperature when they reach the exit of theextruder. The change in pressure as the material leaves the extruder causes it toexpand whereupon most of the oil cells are ruptured, releasing the oil, which israpidly reabsorbed to the porous “collets” or pellets. The expanded collets pro-duced are then cooled and extracted with solvent.

B. Oil Extraction

1. Mechanical Extraction

Olive oil is still routinely obtained from olives by using a low-temperature hydrau-lic press process, referred to as “cold pressing.” This is done to minimize the heat-related degradation of olive oil. Palm fruit and some cottonseed are extracted withan expeller or screw press, which is a continuous process. To achieve a higheryield of oil, often a higher heat treatment of the cottonseed flakes is carried outprior to expelling. This process can extract up to 90% of the available oil fromthe cottonseed kernels and leaves about 3–5% residual oil in the pressed cake.

2. Prepress Solvent Extraction

In prepress solvent extraction, the oil-bearing material is first mildly pressedmechanically by means of a continuous screw press operation to reduce the oil

Solvent Extraction 397

by one-half to two-thirds of its original level, followed by solvent extraction toremove the remaining oil in the prepressed cake. Pressing followed by solventextraction is more commonly used when high oil content materials (e.g., canola/rapeseed, flaxseed, corn germ, and cottonseed) are processed. This process re-duces the amount of oil to be extracted by solvent and, therefore, requires asmaller extractor and less solvent than a direct solvent extraction facility of thesame throughput.

3. Direct Solvent Extraction

Direct solvent extraction involves the use of a nonpolar solvent, usually com-mercial hexane, to dissolve the oil from oilseed flakes or collets without remov-ing proteins and other non-oil-soluble compounds. Solvent extraction yieldsabout 11.5% more oil than does the screw press method, and less oil remains inthe meal. The cooked flakes or collets are mixed with solvent in a batch orcontinuous countercurrent extraction operation. The vapor pressure of hexanelimits the practical operating temperature of the extraction and its contents toabout 50–55°C. The resulting miscella (or full miscella) (oil-solvent mixture),usually about 18–24% of oil by weight, and the marc (solvent-laden colletsor flakes) are heated to evaporate the solvent, which is collected and reused.Occasionally, overheating of the oil-solvent miscella can cause irreversible colorchanges in the oil.

The oil is freed from the full miscella, by using a series of stills (solventevaporators), stripping columns, and associated condensers. The hexane-free oil(i.e., crude oil) is filtered and cooled before leaving the solvent extraction plantfor storage or further treatment. This is the crude oil normally traded in thecommodity market. To minimize the settling problem during storage and ship-ping, crude soybean oil is often degummed before it is traded.

Due to economic factors and product quality concerns, most of the cotton-seed mills in the United States further integrate oil refining as part of the routineoperation. The majority of cottonseed mills conduct miscella refining with so-dium hydroxide to produce a once refined or prime bleachable summer yellow(PBSY) cottonseed oil. The primary benefit of this additional refining operationis to achieve a more consistent oil quality in terms of its color and reducedrefining loss. Several U.S. cottonseed oil mills further process the cottonseedoil to finished RBD cottonseed oil (Fig. 2). In some cases, the oil laiden bleach-ing clay (which is a source of calcium to livestock) is added back to the mealduring the deodorization/toasting step. Some operation, besides refining (R),bleaching (B) and deodorization (D), may also do winterization (W) to reducethe amount of the saturated portion of the vegetable oil to produce a finishedoil referred to as RBWD or RBDW oil. For these facilities, additional regulatoryrequirements may be necessary.

The extraction solvent used in the extractor is normally recovered from the

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miscella and solvent-laden flakes or collets and reused. The small amount of sol-vent loss that occurs through vents, crude oil, and desolventized flakes is unavoid-able (i.e., fugitive loss). Estimated solvent loss from each of these emission pointsis given in Fig. 3. Management and control of emission loss during solvent extrac-tion is an important issue for worker safety and protection of our environment.

III. TOXICITY OF EXTRACTION SOLVENTS

Many halogenated and aromatic solvents have been examined in the past andare effective in extracting edible oils (1). However, these solvents have variousdegrees of toxicity and therefore are not likely to be used as alternates/replace-ments for commercial hexane for edible oil extraction. Several hydrophilic sol-vents also have been studied as oil extraction solvents. They are not toxic buttheir overall performance as an oil extraction solvent is not acceptable. This iswhy hydrocarbon-based solvents will likely be used for oil extraction for theforeseeable future.

A. Commercial Hexane

Commercial hexane has been used for decades as the solvent to extract oilsfrom biological sources. It is a mixture of six carbon saturated compounds, withn-hexane as the predominant component. Pure n-hexane causes peripheral nervedamage in rats and humans when inhalation exposures are maintained for sev-eral months at 500 ppm in rats or 125 ppm in humans (5, 6). However, commer-cial hexane, which contains 52% n-hexane and a mixture of hexane isomers (seecomposition below), does not cause peripheral nerve damage in animals.

The composition of commercial hexane tested (7) is as follows:

52% n-hexane16% methylcyclopentane16% 3-methylpentane13% 2-methylpentane3% cyclohexane

This was shown by extensive animal inhalation studies, which were mandatedby the U.S. Environmental Protection Agency (EPA) under Section 4 of theToxic Substances Control Act (TSCA) (8). The test results, summarized byGalvin (7), showed that this commercial hexane blend was not a neurotoxin. Inaddition, the following tests also were negative: acute toxicology, subchronicneurotoxicity, mutagenicity (both in vitro and in vivo studies), oncogenicity,and development and reproductive studies (with the above-described commer-

Solvent Extraction 399

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cial hexane) at a vapor concentration as high as 9000 ppm for 6 h/d, 5 d/weekup to 13 weeks.

B. Commercial Isohexane and Hexane Isomers

An alternative to commercial hexane, which would require minimal retrofit ofexisting extraction facilities, is commercial isohexane. This solvent, a blend ofhexane isomers [2-methylpentane or isohexane, 3-methylpentane, 2,3-dimethyl-butane, 2,2-dimethylbutane and n-hexane (<1%)], has not been tested as exten-sively as commercial hexane. However, the individual components have beentested in various toxicological assays. Based on the available information, com-mercial isohexane is not a neurotoxicant (7).

C. Other Solvents

Many other solvents have examined by various research teams as potential alter-native solvents for commercial hexane for the extraction of edible oils. Sometoxicity information of the most common ones is summarized in Table 1.

More discussion on these solvents can be found in the summary by Wa-kelyn and Adair (9). Most of the solvents mentioned in this section, with theexception of acetone (9a), can undergo photochemical oxidation in the atmo-sphere in the presence of sunlight and nitrogen oxides (NOx) to form ozone ina greater rate than ethane (C2H6) and, therefore, are classified as volatile organiccompounds (VOCs). The main component of the hydrocarbon solvents used foredible oil extraction, n-hexane, is a neurotoxin and considered a hazardous airpollutant (HAP) by EPA. Thus, n-hexane containing solvents are more strin-gently regulated than isohexane (see IV. Regulatory Issues).

IV. REGULATORY ISSUES

Many workplace/occupational health and safety, environmental, food safety, andother regulations (see Tables 2 and 3 for summary information on U.S. lawsand regulations; a list of terms/abbreviations is given in the Glossary) apply tooilseed processors (10). Most of the environmental and workplace legislationcame about in the 1970s in response to increased general public awareness,concern, and desire for a cleaner environment, safer workplace, and safer foodsupply. This has led to an increasing number of regulations, and it is expectedthat there will continue to be more and stricter regulation in the future.

How are regulations established? In the United States, first Congresspasses a law, e.g., the 1990 amended Clean Air Act, giving the legislative au-thority to the regulatory agency. Next the federal regulatory agency that has the

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Table 1 Some Toxicity Information for Potential Alternate Solvents

Solvent (CAS no.) LD50 (g/kg) Other toxic concerns

Acetone (67-64-1) 5.8 in rats A central nervous system depressant20 in rabbits in animals and humans

2-Butanone (methylethylketone) 2.74 in rats Eye and skin irritation and cause nar-(78-93-3) 13 in rabbits cosis

Cyclohexane (110-82-7) 12.7 in rats Moderate irritation to eyes and mu-cous membrane

Cyclopentane (287-92-3) NarcoticEthyl acetate (141-78-6) 5.6 in rats Irritation to the eyes, mucous mem-

3.0 in cats branes, respiratory tractEthyl alcohol (ethanol) (64-17-5) 7.06 in rats Irritation to the eyes, mucous mem-

branesHeptane (142-82-5) 2.22 in mice Central nervous system depressantIsopropyl acetate (108-27-4) 3.0 in rats Irritation to the eyes, mucous mem-

branesIsopropyl alcohol (isopropanol) 5.05 in rats Irritation to the eyes, nose, throat

(67-63-0) 12.8 in rabbitsMethyl alcohol (methanol) (67- 5.628 in rats Headaches and visual impairment

56-1) 15.8 in rabbitsMethylcyclohexane (108-87-2) — Similar to that of heptaneMethylene chloride (dichloro- — Decreased visual and auditory func-

methane) (75-09-2) tion; headaches, dizziness, nausea,and memory loss; a B2 probablehuman carcinogen

n-Propyl acetate (109-60-4) 9.37 in rats Irritation to the eyes, respiratorysystem

legislative authority establishes standards/regulations through notice and com-ment rule making. As long as there is legislative authority, regulations can beestablished or revised when necessary. A proposed standard is published in theFederal Register announcing that the agency is undertaking a rule making (no-tice) and asking for comments; a hearing can be held also. After comments arereceived and the comment period closes, the agency prepares and publishes afinal standard in the Federal Register. The effective dates of various require-ments of the new standard are usually phased in under a predetermined timetable(e.g., see the regulation for vegetable oil production under Hazardous Air Pollut-ants). Each facility is expected to establish programs to ensure proper compli-ance with the various regulations and that proper corrective actions are takenwhere necessary. Programs can include written programs, auditing, record keep-ing, reporting, warning labels, worker training programs, and so forth, according

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Table 2 U.S. Environmental Laws and Regulations

Law or regulation Purpose

Environmental Protection Agency (EPA) To protect human health and welfare and(established 1970) the environment.

Clean Air Act (CAA) (42 U.S. Code To protect the public health and welfare.7401 et seq.) Provides EPA with the authority to set

NAAQS, to control emission from newstationary sources, and to control haz-ardous air pollutants.

Federal Water Pollution Control Act The major law protecting the “chemical,(known as the Clean Water Act) physical and biological integrity of the(CWA) (33 U.S. Code 1251 et seq.) nation’s waters.” Allows the EPA to es-

tablish federal limits on the amounts ofspecific pollutants that can be releasedby municipal and industrial facilities.

Toxic Substances Control Act (TSCA) Provides a system for identifying and eval-(15 U.S. Code 2601 et seq.) uating the environmental and health ef-

fects of new chemicals and chemicalsalready in comerce.

Resource Conservation and Recovery Act A system for handling and disposal of(RCRA) (42 U.S. Code 6901 et seq.) nonhazardous and hazardous waste.

Comprehensive Environmental Response, Known as “Superfund,” gives the EPACompensation and Liability Act power to recover costs for containment,(CERCLA) (42 U.S. Code 9601 et seq.) other response actions, and cleanup of

hazardous waste disposal sites andother hazardous substance releases.

Emergency Planning and Community (Part of Superfund) Provides authority forRight-to-Know Act (EPCRA; also communities to devise plans for pre-“SARA Title III”) (42 U.S. Code 1101 venting and responding to chemicalet seq.) spills and release into the environment;

requires public notification of the typesof hazardous substances handled or re-lease by facilities; requires state and lo-cal emergency plans.

to the particular regulations that pertain to that industry. Regulations are en-forced by the appropriate federal or state agency.

The regulations discussed here are required in the United States. Manyother countries have similar requirements, but if they do not, it would be prudentfor oilseed solvent extraction operations to consider meeting these regulationsand for these industries to have environmental, health and safety, and qualitymanagement programs (11, 12).

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Table 3 U.S. Worker Health and Safety Laws and Regulations

Laws:Occupational Safety and Health Act of 1970 (OSH Act) (PL 91-596 as amended

by PL 101 552; 29 U.S. Code 651 et. seq.)OSHA Health Standards:

Air Contaminants Rule, 29 CFR 1910.1000Hazard Communication Standard, 29 CFR 1910.1200Occupatioanl Exposure to Hazardous Chemicals in Laboratories, 29 CFR

1910.1450Bloodborne Pathogens, 29 CFR 1910.1030

OSHA Safety Standards:Process Safety Management, 29 CFR 1910.119Emergency Action Plan, 29 CFR 1910.38(a)(1)Fire Prevention Plan, 29 CFR 1910.38(b)(1)Fire Brigades, 29 CFR 1910.156Permit-Required Confined Space, 29 CFR 1910.146Lockout-Tagout, 29 CFR 1910.147Powered Industrial Truck Operator Training, 29 CFR 1910.178Occupational Noise Exposure, 29 CFR 1910.95 and Hearing Conservation Pro-

gram, 29 CFR 1910.95(c)Personal Protection Equipment:

General Requirements, 29 CFR 1910.132Eye and Face Protection, 29 CFR 1910.133Respiratory Protection, 29 CFR 1910.134Head Protection, 29 CFR 1910.135Foot Protection, 29, CFR 1910.136

A. Environmental Protection

The purpose of the EPA, which administers all regulations affecting the environ-ment and chemicals in commerce, is to protect human health and welfare andthe environment. The individual states and state environmental regulatory con-trol boards implement and enforce most of the regulations. The legislation thatserves as the basis for the regulations can be divided into:

1. Statutes that are media specific [Clean Air Act (CAA) and Clean Wa-ter Act (CWA)];

2. Statutes that manage solid and hazardous waste [Resources Conserva-tion and Recovery Act (RCRA) and Comprehensive EnvironmentalResponse, Compensation and Liability Act (CERCLA)]; and,

3. Statutes that directly limit the production rather than the release of

404 Wakelyn and Wan

chemical substance [Toxic Substances Control Act (TSCA) and Fed-eral Insecticide, Fungicide and Rodenicide Act (FIFRA)].

The reader is referred to Table 2 for a summary of the information on environ-mental laws and regulations, Table 4 for an overview of environmental require-ments for air and water, Table 5 for a summary of air threshold emissions, andTable 6 for an overview of environmental requirements for waste.

1. Clean Air Act (CAA; 42 U.S. Code 7401 et seq.):

The purpose of the CAA is to protect the public health and welfare. To satisfythe CAA requirements, states and state air control boards are required to imple-ment regulations and develop state implementation plans (SIPs) (13, 14). Crite-ria pollutants [e.g., ozone (O3), particulate matter (PM), nitrogen oxides (NOx),sulfur oxides (SOx), carbon monoxide (CO), and lead (Pb)] are regulated with

Table 4 U.S. Environmental Regulations, Air and Water

Chemical name (CAS no) VOC HAP CWAa

n-Hexane (110-54-3) Yes Yes YesCommercial hexane (none) Yesn-Heptane (148-82-5) Yes No YesCyclohexane (110-82-7) Yes No YesCyclopentane (287-92-3) Yes No YesHexane isomers (none) Yes No YesCommercial isohexane (none) (Same as No Yes

hexane isomers)2-Methyl pentane (isohexane) (107-83-5) (a hexane Yes No Yes

isomer)3-Methyl pentane (96-14-0) (a hexane isomer) Yes No YesMethyl cyclopentane (96-37-7) (a hexane isomer) Yes No Yes2,2-Dimethyl butane (neohexane) (75-83-2) (a Yes No Yes

hexane isomer)2,3-dimethyl butane (79-29-8) (a hexane isomer) Yes No YesMethyl cyclohexane (108-87-2) Yes No YesIsopropyl alcohol (2-propanol) (67-17-5) Yes No YesEthyl alcohol (ethanol) (64-17-5) No YesAcetone misc. (67-64-1) Nob No Yes

aUnder the Clean Water Act there could be storm water and NPDES permit requirements; none ofthe solvents are listed as priority toxic pollutants in 40 CFR 401.15.bAcetone is considered by the U.S. EPA not to be a VOC (60 FR 31643; June 16, 1995).CAS no., Chemical Abstracts Service Registry number; VOC, volatile organic chemical; HAP,hazardous air pollutant; CWA, Clean Water Act.

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Table 5 Summary of Threshold Emission Levels

Regulation Threshold emission level Requirementa

Major source (40 CFR 70) 100 t criteria pollutant (or Title V permit (federal oper-lessb) or 10/25 t HAP (per ating permit)year)

Hazardous Air Pollutants 10 t of one HAP/25 t total NESHAP/MACT standard(HAP) HAP (per year)

NAAQS: 03 (VOC)c 10–100 t (depending on de- RACT/BACT standardgree of severity of nonat-tainment)

PM 70 t nonattainment; 100 t RACM/BACM standardattainment

NSR: PSDd 250 t Preconstruction/building per-mit, BACT; much paper-work

Nonattainment NSR 100 t/a Preconstruction permit,LAER, emission offsets

Toxic Release Inventory 10,000–25,000 lb/a Annual reporting (Form R)(TRI)

112(r)e Hexane/hexane isomers notcovered

PSMf >10,000 lb in one tank (all Written plan, controls, train-flammable solvents includ- ing, etc.ing hexane/hexane isomersare covered)

aFor NESHAP, MACT, RACM, BACM, RACT, BACT, and LAER, see Glossary.bFor example, the threshold can range from 10 to 100 t depending on degree of severity of ozone(O3) nonattainment.cGround level ozone (O3) is not emitted directly into the air but is formed when sunlight acts onemissions of nitrogen oxides (NOx) and volatile organic compounds (VOC) like hexane.dPSD = prevention of significant deterioration, a requirement of New Source Review (NSR) inattainment areas.esection 112(r) of the Clean Air Act (40 CFR 68) is for prevention of chemical accidents.fPSM = process safety management (29 CFR 1910.119); OSHA standard to prevent or minimizethe consequences of catastrophic releases.

National Ambient Air Quality Standards (NAAQS) and hazardous air pollutants(HAPs), such as n-hexane, with National Emissions Standards for HazardousAir Pollutants (NESHAP).

The 1990 CAA expanded the list of HAPs to 188, including n-hexane,and more strictly regulates nonattainment areas for criteria pollutants such asO3, PM, CO and NOx.

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Table 6 U.S. Environmental Regulations, Solid Waste

(EPCRA/SARATitle III)a

(RCRA)Sec. 304

Chemical name RCRA CERCLA Sec.313(CAS no.) Codeb (RQ) Sec.311/312 (TRI)

n-hexane (110-54-3) 5000c Yes YesCommercial hexane (none) Yesd

n-heptane (142-82-5) Yes NoCyclohexane (110-87-7) U056 1000 Yes YesCyclopentane (287-92-3) Yes NoHexane isomers (none) Yes Noe

Commerical isohexane (none) Noe

2-Methyl pentane (isohexane) (107-83-5) Yes Noe

3-Methyl pentane (96-14-0) Yes Noe

Methyl cyclopentane (96-37-7) Yes Noe

2,2-Dimethyl butane (neohexane) (75-83-2) Yes Noe

2,3-Dimethyl butane (79-29-8) Yes Noe

Methyl cyclohexane (108-87-2) Yes NoIsopropyl alcohol (2-propanol) (67-63-0) Yes Nof

Ethyl alcohol (64-17-5) Yes NoAcetone (67-64-1) U002 5000 Yes No

aFrom Title III Lists of Lists, U.S. EPA, EPA 740-R-95-001 (April 1995); 40 CFR 52-99; (59 FR4478; January 31, 1994) hexane added to TRI list; (60 FR 31633; June 16, 1995) acetone removedfrom TRI list.b40 CFR 261.33, listed hazardous waste—EPA RCRA Hazardous Waste Number. All the solventsthat are on the RCRA list are listed because of Section 3001 of RCRA (part for identification andlisting of hazardous waste) except hexane which is on because of CAA Section 112 (HAP).cRQ for hexane finalized June 12, 1995 (60 FR 30939)dOnly the amount of commercial hexane that is n-hexane has to be reported (e.g., if the commercialhexane is 62% n-hexane, only 62% of the emissions have to be reported for TRI).eThe EPA clarified that the listing for hexane was only for n-hexane, other isomers of hexane arenot included. (59 FR 61457; Nov. 30, 1994).fThe EPA has indicated (62 FR 22318; April 25, 1997) that IPA itself does not meet the criteriafor listing on the TRI list. The EPA will remove IPA from the TRI list.RQ, reportable quantity in pounds.

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Hazardous Air Pollutants (HAPs) or Air Toxics (40 CFR 61). If a facil-ity is a major emitter (i.e. a major source or significant area source) of any ofthe chemicals on the CAA list of HAPs (presently 188), EPA requires sourcesto meet national emissions standards (13, 15, 16). n-Hexane is on the HAP listbut isohexane, acetone, and other solvents listed in Table 4 are not.

The air toxic requirements of the CAA for establishing control measuresfor source categories are technology-based emission standards (not health based)established for major sources (10 t/a of one HAP or 25 t/a of total HAPs perfacility) that require the maximal degree of reduction emissions, taking costs,other health and environmental impacts, and energy requirements into account.Standards are set based on known or anticipated effects of pollutants on thepublic health and the environment, the quantity emitted, and the location ofemissions. Compliance with a NESHAP involves the installation of maximumachievable control technology (MACT)—MACT essentially is maximal achiev-able emission reduction. For new sources, MACT standards must be no lessstringent than the emission control achieved in practice by the best-controlledsimilar source.

On April 12, 2001 (66 FR 19006) EPA published the NESHAP for solventextraction (40 CFR 63) for vegetable oil production. On April 5, 2002 (66 FR16317) U.S. EPA amended this NESHAP. The amendments clarify startup, shut-down, and malfunction (SSM) requirements for owners or operators subect tothe NESHAP rules applicable to vegetable oil production facilities and alsoclarifies the applicability of NESHAP General Provisions (40 CFR63). EPAconsiders solvent extraction for vegetable oil production processes as majorsources of the HAP n-hexane. Figure 3 shows a general flow diagram of atypical vegetable oil production facility, identifying the most common emissionsources of hexane (17). Hexane emissions occur from ten general sources: (a) themain vent; (b) meal dryer vent; (c) meal cooler vent; (d) crude meal; (e) crudeoil; (f) equipment leaks; (g) solvent storage tanks; (h) process wastewater collec-tion; (i) facility startup/shutdowns; and (j) operational upsets. Facilities coveredare those that produce crude vegetable oil and meal products by removing crudeoil from listed oilseeds (corn germ, cottonseed, flax, peanuts, rapeseed, saf-flower, soybeans, and sunflower) through direct contact with solvent. The rulerequires all existing and new solvent extraction processes that are major sources(potential to emit 10 t/a or more of n-hexane) to meet these HAP emissionstandards as a 12-month rolling average based on a 64% n-hexane content. HAPemission standards (solvent lose factors) vary for each oilseed (Table 7) andreflect the application of MACT. Industry will have 3 years to achieve compli-ance (i.e., Apr 12, 2004). Since the emission loss factor values are 12-monthrolling averages, the first compliance report would be due 48 months after thestandard is promulgated (i.e., Apr 12, 2005). The requirements cover normaloperations and SSM, which was further clarified by EPA (4/5/2002; 67 FR

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Table 7 U.S. Oilseed Solvent Loss Factors for Allowable HAP Loss (12-Mo.rolling ave.)

Oilseed solventloss factor (gal/t)

Type of oilseed Existing Newprocess A source that . . . sources sources

Corn germ, wet processes corn germ that has been separated 0.4 0.3milling from other corn components using a wet

process of centrifuging a slurry steeped ina dilute sulfurous acid solution.

Corn germ, dry processes corn germ that has been separated 0.7 0.7milling from other corn components using a dry

process of mechanical chafing and airsifting.

Cottonseed, large processes 120,000 t or more of a combina- 0.5 0.4tion of cottonseed and other listed oilseedsduring all normal operating periods in a12-month operating period.

Cottonseed, small processes less than 120,000 t of a combina- 0.7 0.4tion of cottonseed and other listed oilseedsduring all normal operating periods in a12-month operating period.

Flax processes flax. 0.6 0.6Peanuts processes peanuts. 1.2 0.7Rapeseed processes rapeseed (e.g., canola). 0.7 0.3Safflower processes safflower. 0.7 0.7Soybean, conven- uses a conventional style desolventizer to pro- 0.2 0.2

tional duce crude soybean oil products and soy-bean animal feed products.

Soybean, specialty uses a special style desolventizer to produce 1.7 1.5soybean meal products for human and ani-mal consumption.

Soybean, small processes soybeans in both specialty and con- 0.25 0.25combination ventional desolventizers and the quantityplant of soybeans processed in specialty desol-

ventizers during normal operating periodsis less than 3.3% of total soybeans pro-cessed during all normal operating periodsin a 12-month operating period. The corre-sponding solvent loss factor is an overallvalue and applies to the total quantity ofsoybeans processed.

Sunflower processes sunflower. 0.4 0.3

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16317). A plant processing an oil that is not a listed oil, i.e., not a regulatedentity (40 CFR 63.2872; e.g., rice bran oil plant or meadfoam oil processingplant, etc.), and that is a major source, is not covered by the MACT standardbut the state is likely to require it to meet a particular level as part of its TitleV permit.

There are also variable emission requirements depending on the oilseedfor allowable emissions for vegetable oil processing in Europe (Table 8) (18).The EEC directive for Europe is to be fully implemented by all member statesby Oct. 30, 2004 for new plants and Oct. 30, 2007 for old plants. However,there are some intermediate targets before full compliance—the target solventconsumption is 1.5 times the final level starting Oct. 30, 2001 for new plantsand starting Oct. 30, 2005 for old plants. The EEC counts the total input ofsolvent into a plant per calendar year or any 12-month period and also requiresa solvent management plan, which contains the following and is to be updatedyearly: (a) verification of compliance to the EEC regulation through a massbalance; (b) identification of future reduction options; and (c) development of awaste minimization plan.

In the United States a health-based standard would be for a boundary linelevel of a solvent (e.g., n-hexane) based on the inhalation reference concentra-tion (RfC) (19). The current RfC for n-hexane is 200 µg/m3. Recent researchsuggests that the RfC for n-hexane should be at least 10 times higher (>2000µg/m3).

Table 8 European Maximal Solvent Loss Factors(EEC Directive for Vegetable Oil Extraction) (18)

Max VOCusage/t of seed/yr

Oilseed kg/t gal/t

Olives 2.5 1.0Castor 3.0 1.2Rapeseed 1.0 0.4Sunflower 1.0 0.4Soybeans 0.8 0.3Soy flash 1.2 0.5Other seeds 3.0 1.2Oil refining and fractionation:

Fractionation w/o degumming 1.5 0.6Degumming plant 4.0 1.6

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NAAQS (40 CFR 50). The NAAQS are set at levels sufficient to protectpublic health, including the health of sensitive populations (primary air qualitystandards) and public welfare (secondary air quality standards; “welfare effects”include protection against decreased visibility, damage to wildlife, crops, vege-tation, and buildings, and effects on personal comfort and well-being) from anyknown or anticipated adverse effect of the pollutant with an adequate (appro-priate) margin of safety.

VOCs are essentially considered the same as the criteria pollutant ozone(14–16). Ground level ozone is not emitted directly into the air but is formedwhen sunlight acts on emissions of NOx and VOCs. VOCs are very broadly de-fined by the EPA (40 CFR 51.100): any compound of carbon, excluding carbonmonoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, andammonium carbonate, that participates in atmospheric photochemical reactions.This includes any organic compound other than those specifically listed as hav-ing been determined to have negligible photochemical reactivity. ReactiveVOCs are essentially all those judged to be clearly more reactive than ethane—the most reactive member of the “negligibly reactive” class. C4—C6 paraffinsare of relatively low kinetic reactivity but produce NO2 and potentially ozone(20). n-Hexane, hexane isomers, and the other solvents discussed, except ace-tone, would be considered VOCs (Table 4) that can undergo photochemicaloxidation in the atmosphere to form ozone. In the United States, acetone wasadded to the list of compounds excluded from the definition of VOCs in 1995because it was determined to have negligible photochemical reactivity (11).

Most U.S. vegetable oil extracting facilities would be major sources ofVOCs and would be covered by the requirements for ozone emissions and at-tainment, unless they used a solvent that was not classified as a VOC. The defini-tion of “major source” changes as the severity of the ozone nonattainment areaincreases. Plants in marginal and moderate areas are major if they emit 100 tVOC/a; in serious areas, 50 t/a; in severe areas, 25 t/a; and in extreme areas, 10t/a. All facilities in ozone nonattainment areas could be required to reduce emis-sions through implementing reasonable available control technology (RACT) orbest available control technology (BACT) standards.

Particulate matter (PM) is the solid or liquid matter suspended in the atmo-sphere. Most vegetable oil production facilities are major sources of PM. De-pending on the oilseed processed, PM emissions can be 0.045–0.136 kg of totalsuspended particulate (TSP) per ton of seed processed. This PM is about 50%PM10 (particulate 10 µm and less) and less than 3% PM2.5 (particulate 2.5 µmor less), assuming that PM emissions from oilseed handling operations are simi-lar in particle size distribution to other agricultural operations, such as cottongins (21). PM controls (Table 5) would also have to be part of a facility’s federaland state permits. Vegetable oil production facilities probably also have to in-clude NOx, SOx, and CO emissions in their federal and state permits.

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New Source Review. Any new or significantly modified facility wouldhave to comply with the new source review (NSR) requirements. NSR is a precon-struction-permitting program. If new construction or making a major modifica-tion will increase emissions by an amount large enough to trigger NSR require-ments (Table 5), then the source must obtain a permit before it can beginconstruction. Permits for sources in attainment areas are prevention of signifi-cant deterioration (PSD) permits and those in nonattainment areas are nonattain-ment NSR permits. For a PSD permit a source must apply BACT and for nonat-tainment NSR lowest achievable emission rate (LAER) is required.

Odor. There are no specific federal regulations for odor. However, statescan regulate odor if they choose to. For example, Colorado requires hog lagoonsto be covered because of a state referendum vote. Also, odor can generate com-plaints that cause states to require more stringent emission controls.

Federal Permits (40 CFR 70). All major sources of regulated solventsare required to have federally enforceable operating permits (FOPs) (13, 14)(also referred to as Title V permits).

State Permits. Most states require state permits for facilities that emitlisted air pollutants (13, 14). In some states federal permits and state permitsare combined, whereas in other states facilities are required to have both a stateand a county (air district) permit and a federal permit. As part of annual emis-sion inventory reporting requirements, many states already require reporting ofHAPs and VOCs because of their state implementation plan (SIP).

2. Clean Water Act (CWA; 33 U.S. Code 1251 et seq.)

The CWA is the major law protecting the “chemical, physical and biologicalintegrity of the nation’s waters.” Under it, the EPA establishes water qualitycriteria used to develop water quality standards, technology-based effluent limita-tion guidelines, and pretreatment standards and has established a national permitprogram [National Pollution Discharge Elimination System (NPDES) permits; 40CFR 122] to regulate the discharge of pollutants. The states have responsibilityto develop water quality management programs. Oilseed processing and oil re-fining are covered by: (a) basic discharge effluent limitations (40 CFR 122); (b)storm water regulations (40 CFR 122 and 123); and (c) oil spill prevention andresponse plans (40 CFR 112) (14).

Basic Discharge and Stormwater. Vegetable oil extracting facilities andoil refining are covered by basic discharge effluent limitations [direct dischargesto receiving waters or indirect discharges to publicly owned treatment works(POTWs)], and stormwater regulations (14). The amount of solvent in effluentdischarges and in stormwater (for those covered) needs to be determined and

412 Wakelyn and Wan

possibly monitored as part of an NPDES permit and as part of the visual exami-nation or testing of stormwater quality. None of the solvents normally used inoilseed extraction and refining are listed as priority toxic pollutants (40 CFR401.15).

Oil and Hazardous Substances Spills and Response Plans. Under OilPollution Prevention and Response (40 CFR 112) there are requirements foroilseed extraction and oil refining for storage and transportation of vegetableoil. In 2002 EPA amended this rule (67 FR 47042; July 17, 2002). This ruleincludes requirements for Spill Prevention Control and Countermeasure (SPCC)Plans (i.e., prevention plans) and for Facility Response Plans (FRPs). SPCCplans are required for on site storage and are intended to prevent spills of oil(of any kind; this includes animal fats and vegetable oils) by non-transportation-related on-shore and off-shore facilities into the waters of the United States oradjoining shorelines. The requirements for preparation and implementation ofSPCC plans state that if a facility discharges or could reasonably be expectedto discharge oil in harmful quantities [a discharge of oil that can cause a sheen(indecent appearance) on the surface of water; 40 CFR 110.3] into navigablewaters of the United States or adjoining shoreline, an SPCC plan is required. Inthe 2002 amended rule facilities with total above-ground storage capacitygreater than 5000 L (1320 gal) are covered but underground storage tanks (UST)are exempt if regulated under the federal UST rule (40 CFR 280). Also, underthe final rule as published, SPCC Plans must be in compliance by Feb. 17, 2003and implementation of the amended SPCC Plan must be completed by Aug. 18,2003. Some of the amendments are controversial and EPA is planning to sus-pend this deadline at least by one year. Spill reporting is required for any spill≥3788 L (1000 gal) and for any two spills in any consecutive 12-month periodof 159 L (42 gal) or greater.

Pursuant to the Oil Pollution Act of 1990 (OPA-90), EPA amended the oilpollution prevention regulations by adding response plan requirements for non-transportation-related on-shore facilities that handle, store, or transport oil. Facili-ties that could cause substantial harm to the environment are required to prepareand submit response plans to EPA. The “Flowchart of Criteria for SubstantialHarm” (14) was published in the Federal Register on July 1, 1994 (59 FR 34104);if the facility transfers oil over water to or from vessels and has a total oil storagecapacity greater than or equal to 159,000 L (42,000 gal) or the facility has a totalstorage capacity greater than or equal to 3.8 × 106 L (1 million gal), a responseplan is required. EPA further amended this rule (65 FR 40776; June 30, 2000) toprovide guidance for handling, storing, or transporting of vegetable oils and animalfats.

Under OPA-90, comprehensive oil spill response plans are also requiredfor transportation of nonpetroleum oils (i.e., vegetable oil) by rail and road in

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amounts of 159,000 L (42,000 gal) or more [regulated by Research and SpecialPrograms Administration (RSPA), Department of Transportation (DOT); FinalRule, 61 FR 30533; June 17, 1996] and for marine transportation–related facili-ties [regulated by the DOT, Coast Guard; Final Rule, 65 FR 40820; June 30,2000].

3. Resource Conservation and Recovery Act (RCRA;42 U.S. Code 6901 et seq.)

RCRA gives EPA authority to regulate the handling and disposal of hazardousand nonhazardous waste. RCRA subtitle C (40 CFR 261) is a federal “cradle-to-grave” system to manage hazardous waste (including provisions for cleaningup releases and setting statutory and regulatory requirements). Subtitle D coversnonhazardous wastes. Materials or items are hazardous wastes if and when theyare discarded or intended to be discarded. The act requires generators, transport-ers, and disposers to maintain written records of waste transfers, and requiresEPA to establish standards, procedures, and permit requirements for disposal.The act also requires states to have solid waste management plans, prohibitsopen dumping, and requires EPA to establish criteria for sanitary landfills. EPAunder RCRA also regulates underground storage tanks that store or have storedpetroleum or hazardous substances.

Hazardous wastes are either listed wastes (40 CFR 261.30-33) or charac-teristic wastes (40 CFR 261.21-24). The EPA defines four characteristics forhazardous waste: ignitability (40 CFR 260.21); corrosivity (40 CFR 260.22);reactivity (40 CFR 260.23); and toxicity (40 CFR 260.24). Any waste that ex-hibits one or more of these characteristics is classified as hazardous underRCRA. The ignitability definition includes a liquid that has a flash point of lessthan 60°C (140°F); the EPA included ignitability to identify wastes that couldcause fires during transport, storage, or disposal (e.g., used solvents). Since allof the solvents in Table 6 have flash points less than 60°C, all could be anRCRA ignitability waste.

Spent bleaching clay is not an RCRA hazardous waste (40 CFR 302). It isusually disposed of by taking it to a regular landfill. Sometimes a spontaneouscombustion (oxidation of unsaturated fatty acids in the retained oil causing self-heating leading to combustion) may occur when it is taken to the landfill. Thepotential for spontaneous combustion in spent bleaching earth depends on the typeand amount of oil retained and rises with increasing unsaturation of the fatty acidsin the retained oil. U.S. DOT classifies materials liable to spontaneous combus-tion as Class 4.2 hazardous materials [49 CFR 173.124(b) and Appendix E3].Spent bleaching clay can be finely ground and put in small quantities into theanimal meal in operations that do oil extraction. Zschau (22) describes otherways to utilize spent bleaching clay, including environmentally friendly ways.

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4. Comprehensive Environmental Response, Compensationand Liability Act (CERCLA, “Superfund”; 42 U.S. Code 9601et seq.)

CERCLA “Superfund” gives EPA the authority to force those responsible forhazardous waste sites or other releases of hazardous substances, pollutants, andcontaminants to conduct cleanup or other effective response actions.

Section 103 of CERCLA requires the person in charge of a facility toimmediately report any release of a hazardous substance in an amount equal toor greater than its reportable quantities (RQ) to the National Response Center(NRC) (see also discussion below on EPCRA Section 304). n-Hexane, acetone,and other chemicals in Table 6 as well as NOx (RQ = 4.5 kg/24 h [10 lb/24 h]),hydrogen sulfide, and ammonia (both 45 kg/24 h [100 lb/24 h]) are CERCLAhazardous substances and have a CERCLA RQ for releases. However, CERCLAand EPCRA do not require notification to the NRC, the State Emergency Re-sponse Commissions (SERCs), and the Local Emergency Planning Committees(LEPCs) of “federally permitted releases” of hazardous air releases/emissionsas defined in CERCLA Section 101(10)(H): any emission into the air subject toa permit or control regulation under Section 111, Section 112, Title I Part C,Title I Part D, or state implementation plans in accordance with Section 110 ofthe CAA. In 2002 EPA published final guidance clarifying the CERCLA Sec-tion 101(10)(H) federally permitted release definition for “Certain Air Emis-sions” (67 FR 18899; April 17, 2002) and for “Clean Air Act ‘Grandfathered’Sources” (67 FR 19750; April 23, 2002). Essentially if the CAA requirementsare being met, even without a permit, it is a federally permitted release exemptfrom reporting.

5. Emergency Planning and Community Right-to-Know Act(EPCRA; 42 U.S. Code 11001 et seq.)

Enacted as Title III of the 1986 Superfund Amendments and ReauthorizationAct (“SARA”), the Act mandates EPA to monitor and protect communities re-garding release of chemicals into the environment. It requires states to establishemergency planning districts with local committees to devise plans for prevent-ing and responding to chemical spills and releases.

Section 304 (40 CFR 355.40). Section 304 of EPCRA requires the owneror operator of a facility to immediately notify the State SERCs and LEPCs ofany accidental releases (that are not “federal permitted”), in quantities equal toor greater than their RQ, of an EPCRA designated extremely hazardous sub-stance (EHS) or a CERCLA hazardous substance (40 CFR 302, Table 302.4)and provide written follow-up notice as soon as practicable thereafter (also seediscussion on CERCLA Section 103). n-Hexane, cyclohexane, acetone, and

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some of the other solvents discussed are CERCLA hazardous substances andhave CERCLA RQ for air releases (Table 5).

Section 311, 312 (40 CFR 370.20-.21). Businesses must make MSDSs,for chemicals that are required to have an MSDS, available to state and localofficials. Since all of the solvents discussed require MSDSs under the OSHAHCS, all are covered by these requirements.

Section 313 (40 CFR 372), Toxic Release Inventory (TRI). Businessesare required to file annual reports with federal and state authorities of releasesto air, water, and land above a certain threshold for chemicals on the TRI/Section 313 list (40 CFR 372.65) by July 1 each year for the previous year’sreleases (23). TRI requirements are triggered if a facility is involved in manufac-turing with 10 or more full-time employees, manufactures, processes, or other-wise uses one or more listed substance(s) in a quantity above the statutory re-porting threshold of 11 t/a (25,000 lb/yr) (manufactured or processed) or 4.5 t/a(10,000 lb/yr) (otherwise used). Beginning with the 1991 reporting year, suchfacilities also must report pollution prevention and recycling data for such chem-icals pursuant to Section 6607 of the Pollution Prevention Act (42 U.S. Code13106).

n-Hexane was added to the TRI list in 1994 with reporting for 1995 emis-sions (11, 23). The other solvents discussed are not on the TRI list. The EPAcan add new chemicals to or delete chemicals from the TRI list as is deemednecessary and any person may petition the EPA to add chemicals or deletechemicals from the list. Acetone (11) in 1995 (60 FR 31643; June 16, 1995)and phosphoric acid in 2000 (65 FR 39552; June 27, 2000) were deleted andno longer require reporting under TRI.

6. Toxic Substances Control Act (TSCA; 15 U.S. Code 2600et seq.)

If a chemical’s manufacture, processing, distribution, use, or disposal wouldcreate unreasonable risks, the EPA, under the TSCA (40 CFR section 700, etseq.), can regulate it, ban it, or require additional testing.

Section 4(a). Under Section 4(a) of TSCA, the EPA can require testing(referred to as a “Section 4 test rule”) of a chemical substance or mixture todevelop data relevant for assessing the risks to health and the environment.

Section 5(a)(1). Section 5(a)(1) of TSCA (40 CFR 720) mandates theEPA to monitor and control the use of toxic substances by requiring the Agencyto review the health and environmental effects of new chemicals (referred to as“Premanufacturing Notice” or “PMN”) and chemicals already in commerce. TheEPA also has Significant New Use Rules (SNURs) under Section 5(a)(2) of

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TSCA (40 CFR 721), which provides a way for EPA to restrict use of a chemi-cal substance already in commerce that is proposed for new uses. n-Hexane,hexane isomers, acetone, and the other solvents discussed are already commer-cially available, so a PMN would not apply. However, some solvents other thann-hexane could be subject to an SNUR (40 CFR 721, subpart A), since they arenot presently being used as extraction solvents in large quantities.

Section 8. Reporting and Retention of Information: Section 8(d) ofTSCA (Health and Safety Data Reporting; 40 CFR 716) requires that lists ofhealth and safety studies conducted or initiated with respect to a substance ormixture be submitted to the EPA. Section 8(e) of TSCA (no 40 CFR Ref.)requires that all new toxicological data of the effects of a chemical not pre-viously mentioned must be reported immediately, if the data reasonably supportthe conclusion that such substance or mixture presents a substantial risk of in-jury to health or the environment. Testing (Section 4 test rule) was required forseveral of the solvents earlier [e.g., commercial hexane for which new toxico-logical information was reported to the EPA since 1992 (24)], and any newtoxicological information will have to be reported to the EPA under Section8(e) and 8(d).

Inventory Update Rule (IUR) (40 CFR 710). The IUR was established in1986 to require manufacturers and importers of chemicals listed on the masterTCSA inventory to report current data every four years on the production volumeof chemicals imported or produced. Food and feed products produced from naturalagricultural product, such as oilseeds, are not required to be reported, but all oiland meal products obtained by solvent extraction that is sold for other than foodor feed use (e.g., oils as chemical raw materials and meal as fertilizer) are. Thislist from 1990 was used to determine the high production volume (HPV) chemicals(greater than 453 t/a [1 million lb/yr]) that are part of the HPV testing program.Vegetable oils are listed as a category 1 that does not require toxicity testing atthis time.

B. Workplace/Occupational Safety and Health

In the United States, workplace regulations (Table 3) are promulgated and en-forced by the Occupational Safety and Health Administration (OSHA), whichis part of the U.S. Department of Labor. The purpose of OSHA is to ensure thatthe employers maintain a safe and healthful workplace. OSHA general industrystandards (29 CFR 1910) apply to oilseed extraction and oil refining, and severalof these workplace standards that particularly apply to oilseed extraction facilitiesare discussed. Many other health and safety standards (e.g., blood-borne patho-gens, noise, operation of fork-lift trucks, and lockout/tagout) apply that cover allindustries (25). Also even if there is not a specific standard, OSHA can cite a

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facility under the “general duty clause” [Sec. 5(a)(1) of the OSH Act], since theOSH Act requires the employer to maintain a safe and healthful workplace.

1. Air Contaminants Standard (29 CFR 1910.1000)

Air contaminant standards are intended to reduce risk of occupational illness forworkers by reducing permissible exposure limits (PELs) for chemicals. Table 9lists the PELs for n-hexane, hexane isomers, and some other solvents and chemi-cals. PELs are 8-h time-weighted average exposures. To achieve compliancewith PEL, administrative or engineering controls must first be determined andimplemented, whenever feasible. When such controls are not feasible to achievefull compliance, personal protective equipment, work practices, or any otherprotective measures are to be used to keep employee exposure below the PEL.

In the case of a mixture of contaminants, an employer has to compute theequivalent exposure when the components in the mixture pose a toxic effect onthe same target organ to a worker’s health (26, 27). The mixture calculation isexpressed as:

Em = (C1/L 1) + (C2/L 2) + . . . + (Cn/L–n) (1)where:

Em is equivalent exposure for the mixture (Em should be ≤1 for compli-ance), C is concentration of a particular substance, and L is PEL (the exposurelimit for that substance specified in 29 CFR 1910).

2. Hazard Communication Standard (HCS)(29 CFR 1910.1200)

The HCS requires all employers to provide information to their employees onthe hazardous chemicals to which they are exposed through a written hazardcommunication program, labels and other forms of warning, material safety datasheet (MSDS), training programs, and record keeping.

A substance is a “hazardous chemical” if it is a “physical hazard” or a“health hazard” [29 CFR 1910.1200(c)]. A flammable or explosive liquid is a“physical hazard.” A flammable liquid means “any liquid having a flashpointbelow 37.8°C (110°F), except any mixture having components with flashpointsof 37.8°C (110°F) or higher, the total of which make up 99% or more of thetotal volume of the mixture.” “Health hazard” means “a chemical for whichthere is statistically significant evidence based on at least one valid study thatacute or chronic health effects may occur in exposed employees.” Hexane re-quires an MSDS, since all flammable liquids (physical hazard), as defined byOSHA and/or possible health hazards that have an U.S. OSHA PEL, require anMSDS. n-Hexane isomers (e.g., isohexane) do not have an OSHA PEL but dohave an American Conference of Governmental Industrial Hygienists (ACGIH)

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Table 9 U.S. Workplace Regulations,a Air Contaminants

Chemical name Permissible exposure limit (PEL)(CAS no.) [Health risk: basis for the PEL]

n-Hexane (110-54-3) 500 ppm-1800 mg/m3; new PEL was 50 ppm–180 mg/m3

same as ACGIH (TLV); [neuropathy]Commercial hexanea (none) (Same as n-hexane)nHeptane (148-82-5) 500 ppm–2050 mg/m3; new PEL was 400 ppm–1640

mg/m3, (500 ppm STEL) same as ACGIH (TLV); [nar-cosis]

Cyclohexane (110-82-7) 300 ppm–1050 mg/m3; ACGH (TLV) 300 ppm–1030mg/m3; [sensory irritation]

Cyclopentane (287-92-3) None; new PEL was 600 ppm, same as ACGIH (TLV);[narcosis]

Hexane isomers None; new PEL was 500 ppm–1760 mg/m3 (1000 ppmSTEL) same as ACGIH (TLV); [narcosis]

Commercial isohexanea (none) (Same as hexane isomer)2-Methyl pentane (2-MP) (isohexane) (Same as hexane isomer)

(107-83-5)(Same as hexane isomer)3-Methyl pentane (3-MP) (96-14-0)

Methyl cyclopentane (MCP) (96-37-7) (Same as hexane isomer)2,2 Dimethyl butane (2,2-DMB) (neo- (Same as hexane isomer)

hexane) (75-83-2)2,3 Dimethyl butane (2,3-DMB) (79- (Same as hexane isomer)

29-8)Methyl cyclohexane (107-87-2) 500 ppm; new PEL was 400 ppm/1610 mg/m3, same as

ACGIH (TLV); [narcosis]Isopropyl alcohol (IPA) (2-propanol) 400 ppm–980 mg/m3; ACGIH (TLV) same plus 500

(67-17-5) ppm–1230 mg/m3 STEL; [sensory irritation]Ethyl alcohol (ethanol) (64-17-5) 1000 ppm–1880 mg/m3; ACGIH (TLV) same; [narcosis,

irritation]Acetone (67-64-1) 1000 ppm/2400 mg/m3; ACGIH (TLV) 750 ppm (1000

ppm STEL); [sensory irritation]Particulate not otherwise regulated

(PNOR):Total dust 15 mg/m3

Respirable dust 5 mg/m3; [physical irritation]Phosphoric acid (7664-36-2) 1 mg/m3; [sensory irritation]Sodium hydroxide (1310-73-2) 2 mg/m3; [sensory irritation]Sulfuric acid (7664-93-9) 1 mg/m3; [sensory irritation]

aCAS No. is the Chemical Abstracts Service Registry Number; PEL is from 29 CFR 1910.1000,Table Z-1; American Conference of Governmental Industrial Hygienists (ACGIH), threshold limitvalue (TLV); under the HCS, a MSDS is required for all of the compounds (physical and/or chem-cial hazard); all of the solvents are flammable liquids or gasses, under the OSHA definition, andare regulated under the PSM standard.bCommercial hexane as used in the U.S. is usually about 64% n-hexane, and the rest is hexaneisomers [e.g., methyl cyclopentane (MCP), 2-methyl pentane (2-MP), and 3-methyl pentane (3-MP)], and it contains less than 10 ppm benzene.cMixture of 2-MP (45–50%), 3-MP, 2,2-DMB, and 2,3-DMB (3).

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threshold limit value (TLV) (28) of 500 ppm (Table 9), which many states andcountries enforce as a mandatory standard.

Chemical manufacturers and importers are required to review the availablescientific evidence concerning the hazards of chemicals they produce or import,and to report the information to manufacturing employers who use their prod-ucts [29 CFR 1910.1200(b)]. If a chemical mixture has not been tested as awhole to determine whether it is a hazardous chemical, the mixture is assumedto present the same hazards as the components that comprise 1% or more of themixture or a carcinogenic hazard if it contains a component in concentration of0.1% or more that is a carcinogen [29 CFR 1910.1200(a)(5)]. Commercial hex-ane, containing 52% n-hexane, has been tested and found not to be neurotoxic,unlike pure n-hexane (7, 29, 30). So mixtures with less than 52% n-hexaneshould not be considered neurotoxic, although n-hexane would have to be listedon the MSDS if present in greater quantity than 1% of the mixture.

3. Process Safety Management Standard (29 CFR 1910.119)

Process safety management (PSM) is for the prevention or minimization of theconsequences of catastrophic releases of toxic, reactive, flammable, or explosivechemicals. This regulation applies to all processes that involve one or more of137 listed chemicals (29 CFR 1910.119, Appendix A) above their thresholdquantities or have 4.5 t (10,000 lb) or more of a flammable liquid or gas, asdefined by the OSHA HCS [29 CFR 1910.1200(c)]. This includes n-hexane,hexane isomers, and the other solvents listed in Table 9. The requirements formeeting this regulation are described in more detail by Lajeunesse (31) andStrube (32).

In addition to the PSM standard, OSHA has been enforcing two otherregulations for operations/processes with flammable liquids. First, under Per-sonal Protective Equipment—General Requirements (29 CFR 1910.132), OSHAhas cited or obtained voluntary agreement from organizations relative to flame-resistant (FR) clothing. Operators and other employees working in the area of aflammable process are being required to wear flame-resistant work clothing. Forfacilities that use commercial hexane and other flammable solvents, it would beprudent to require FR clothing for all personnel working in areas where there isan exposure to a flammable liquid. Second, OSHA has cited organizations forfailure to meet related safety regulations under Fire Brigades (29 CFR1910.156), specifically for standards such as training, both initial and annualrefresher training; protective equipment availability and testing; and fitness forduty including periodic physicals. If an on-site fire brigade is part of the site’semergency response plan (29 CFR 1910.38), then these requirements must alsobe met. In addition, the requirement of the PSM standard for an emergencyresponse plan triggers the requirements of emergency action plan [29 CFR1910.38(a)].

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C. Food Safety

Oilseed extraction solvents and food processing substances, to be legally usedin the United States, must have been subject to an approval by the U.S. Foodand Drug Administration (FDA), which regulates all aspects of food, includingfood ingredients and labeling, or the U.S. Department of Agriculture (USDA)during 1938–1958 for this use (“prior sanction”); be generally recognized assafe (GRAS) for this use; or be used in accordance with food additive regula-tions promulgated by the FDA.

Many prior sanctions and GRAS determinations are not codified in theFDA regulations. However, extracting solvents used in food manufacturing,such as n-hexane, have been labeled as food additives, solvents, defoamingagents, component of a secondary food and color additives, minor constituent,or incidental additives (i.e., “additives that are present in a food at significantlylow levels and do not have any technical or functional effect in that food”)depending on the application. Incidental additives can be “processing aids,” (i.e.,“substances that are added to a food during processing but removed from thefood before it is packaged”). Most food processing substances, including sol-vents, can be regarded as “incidental additives” and thus are exempt from labeldeclaration in the finished food product. Even if exempt from label declaration,all extraction solvents must be used in accordance with the FDA good manufac-turing practice (GMP; 21 CFR 100).

Commercial hexane, containing about 50–85% n-hexane, has been in ma-jor use since the 1940s as an oilseed extraction solvent on the determinationthat it is GRAS and it may also be subject to a prior sanction. Like many otherfood processing substances, there is no FDA regulation specifically listing n-hexane as GRAS or prior sanctioned. However, under FDA regulations hexanehas been cleared as a solvent in the manufacture of food additives and has beencleared as a minor constituent (not more than 5 ppm) of a cocoa butter substitutethat is a direct food additive that has been affirmed by FDA as GRAS for fooduse (52 FR 47918, December 17, 1987; 21 CFR 184.1259). In Europe the maxi-mum residue limit (MRL) in vegetable oils has been established as 5 ppm n-hexane [European Union (EU) Communitee Directive 88/344/EEC of 13 June1988; Off. J. Eur. Commun. L 157, 24 June 1988, pp. 0028–0033].

The Flavor and Extract Manufacturers Association (FEMA) has conducteda program since 1958 using a panel of expert pharmacologists and toxicologiststo determine substances that are GRAS. This safety assessment program (“FEMAGRAS”) is widely accepted and considered an industry/government partnershipwith the FDA (33). A number of papers published since 1961 (34–36) list thesubstances that the panel has determined to be GRAS and the average maximallevels in parts per million (ppm) at which each has been reported to be GRASfor different categories of food. The FDA has not incorporated these substances

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in their regulations but recognizes the findings of the Expert Panel of FEMA asGRAS substances.

Since vegetable oil and other human food grade oils undergo deodoriza-tion (steam distillation) and other purification processes (i.e., refining andbleaching) as part of the manufacturing process prior to being used as a foodproduct, they should not contain any of the extraction solvent, if proper manu-facturing practices are followed. Refining removes nonglyceride materials (e.g.,phospholipids, color, and trace metals) and free fatty acids; bleaching with acid-activated bleaching earth or clay (e.g., bentonite), removes color-producing sub-stances; and deodorization, the last major processing step in edible oils refining,removes volatile compounds (undesirable ingredients occurring in natural oilsand those that may be imparted by prior unit processes or even storage, manyof which are associated with undesirable flavors and odors) (37, 38). Most com-mercial deodorizers operate at a temperature of 245–275°C (475–525°F) undera negative pressure of 2–10 mm Hg (37, 38). It has been reported that no n-hexane residue remains in the finished oil after processing due to its high volatil-ity (39). In addition, animal feeding studies with expeller and solvent-extractedmeals have not indicated any adverse health effects related to the extractionsolvent (40).

In summary, GRAS status may be determined by a company (“GRASself-determination”), an industry, an independent scientific organization (e.g.,FEMA GRAS), or the FDA. The Federal Food, Drug and Cosmetic Act (FFDCA;21 U.S. Code 321 et seq.) does not provide for the FDA to approve all ingredientsused in food, and the FDA explicitly recognizes that its published GRAS list isnot meant to be a complete listing of all substances that are in fact GRAS foodsubstances. Although there is no requirement to inform the FDA of a GRAS self-determination or to request FDA review or approval on the matter, the FDA hasestablished a voluntary GRAS affirmation program under which such advicewill be provided by the agency. If a facility is considering changing its extract-ing solvent for the extraction of the various edible biological materials, solventsthat do not have prior sanction, a GRAS determination, or a tolerance set proba-bly should be evaluated for compliance under food safety requirements.

V. SUMMARY

In summary, there most likely will be new demands for highly specialized ex-traction solvents as newly domesticated species that make useful novel oils (41)and other products and new or altered biological products with enhanced nutri-tional and industrial properties will be developed through conventional breedingand genetic engineering for use as “functional foods” (42) (e.g., phytosterols toachieve cholesterol lowering); as oils with altered lipid profiles (43) (e.g., for

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lower saturated fat) or with more vitamin E; new drugs/nutraceuticals, industrialchemicals (e.g., fatty acids for lubricants, as cosmetics, coatings, detergents,surfactants, flavors, polymers, etc.); as sources for specialty chemicals; as value-added products; and so forth (42–49). There will be demands for solvent sys-tems for simultaneous removal of undesirable meal components (e.g., mycotox-ins, gossypol, flavors, and odors) that offer the potential for upgrading meal foruse as higher value animal feeds and human foods. Solvents that offer energysavings and that pose lower health, environmental, and fire hazards will also besought. However, it will be more necessary than ever to be aware of regulatoryrequirements for and the toxicity of the solvents used.

GLOSSARY

ACGIH American Conference of Governmental IndustrialHygienists, an independent standards setting organi-zation.

BACM best available control measures.BACT best available control technology.CAA Clean Air Act, 42 U.S. Code 1251 et seq.CERCLA (Superfund) Comprehensive Environmental Response, Compen-

sation, and Liability Act, 42 U.S. Code 9601 et seq.CFR Code of Federal Regulations. This is where the U.S.

federal regulations after promulgation are codified.The preceding number is the Title, the succeedingnumber (after CFR) is the Part of Section (e.g., 29CFR 1910 is Title 29 Code of Federal Regulationsat Part 1910).

CWA Clean Water Act (Federal Water Pollution ControlAct), 33 U.S. Code 1251 et seq.

EPA Environmental Protection Agency, 42 U.S. Code4321 et seq.

EPCRA Emergency Planning and Community Right-to-Know Act, part of CERLA/Superfund, Title III ofSARA, the 1986 amended Superfund.

FR Federal Register. This is where regulatory announce-ments and new rules and their justification are pub-lished. The preceding number is the volume; the suc-ceeding number (after FR) is the page, usuallyfollowed by the date when it appeared (e.g., 51 FR27956 is Volume 51 Federal Register, page 27956).

GRAS generally recognized as safe.

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HAP hazardous air pollutant, 40 CFR 61.HCS Hazard Communication Standard, 29 CFR

1910.1200.IUR Inventory Update Rule, 40 CFR 710.LAER lowest achievable emission rate.MACT maximum achievable control technology.MSDS material safety data sheet, required under OSHA

HCS.NAAQS National Ambient Air Quality Standard, 40 CFR 50.NESHAP National Emission Standard for Hazardous Air Pol-

lutants under the CAA.Nonattainment Areas that do not meet NAAQS, 40 CFR 51.100 et

seq.NPDES National Pollution Discharge Elimination System.

The national permit program under the CWA, 40CFR 122.

NSR new source review.OPA-90 Oil Pollution Act of 1990.OSHA Occupational Safety and Health Administration (part

of the U.S. Dept. of Labor), 29 U.S. Code 651 etseq.

Ozone (O3) One of the compounds on the NAAQS list that isformed through chemical reaction in the atmosphereinvolving VOC, NOx, and sunlight; also a primaryconstituent of smog.

NOx nitrogen oxides.PEL permissible exposure limit for an air contaminant un-

der OSHA standards.PM particulate matter. One of the NAAQS; denotes the

amount of solid or liquid matter suspended in theatmosphere. The EPA regulates PM as PM10

(“coarse” particulate 10 µm and less) and PM2.5

(“fine” particulate 2.5 µm or less).POTW publicly owned treatment works, for indirect waste-

water discharge.PSD prevention of significant deterioration, a requirement

of NSR.PSM process safety management standard.RACM reasonably achievable control measures.RACT reasonably available control technology.RCRA Resource Conservation and Recovery Act, 42 U.S.

Code 6901 et seq.

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RCRA-Characteristic hazardous wastes that are ignitable, corrosive, reac-Wastes tive, or toxic, 40 CFR 260.64.

RCRA-Listed Wastes Specially listed hazardous wastes in 40 CFR 261.30-33.

SARA Superfund Amendments and Reauthorization Act.TCLP toxic characteristic leaching potential under RCRA,

40 CFR 261.24.Title V The part of the Clean Air Act that deals with federal

permits, 40 CFR 70.TLV threshold limit value for an air contaminant under

ACGIH regulations.TRI toxic release inventory, under section 313 of

EPCRA.TSCA Toxic Substances Control Act.TWA time weighted average.U.S. Code The United States Code where legislation, including

health, safety, and environmental legislation, is codi-fied once passed by Congress (e.g., 42 U.S. Code7401 is Title 42 U.S. Code at paragraph 7401).

UST Underground storage tank (any tank completely cov-ered with earth); Technical Standards and CorrectiveAction Requirements for Owners and Operators ofUST (40 CFR 280).

VOC volatile organic compounds. A group of chemicalsthat react in the atmosphere with nitrogen oxides(NOx) in the presence of heat and sunlight to formozone; does not include compounds determined byEPA to have negligible photochemical reactivity.

REFERENCES

1. PJ Wan, PJ Wakelyn, eds. Technology and Solvents for Extracting Oilseeds andNon-petroleum Oils. Champagne, IL: AOCS Press, 1997, pp. 1–353.

2. MA Williams, RJ Hron. Obtaining oils and fats from source materials. In: YH Hui,ed. Bailey’s Industrial Oils and Fat Products, 5th ed., Vol. 4, Edible Oil and FatProducts: Processing Technology. New York: John Wiley & Sons, 1996, pp. 61–154.

3. PJ Wan, RJ Hron, MK Dowd, MS Kuk, EJ Conkerton. Alternate hydrocarbon sol-vents for cottonseed extraction: plant trials. J Am Oil Chem Soc 72:661–664, 1995.

4. M Horsman. Using isohexane to extract cottonseed. Oil Mill Gaz 105(8):20–23,2000.

5. American Petroleum Institute. Neurotoxicity: n-Hexane and Hexane Isomers. Medi-cal Research Pub. 30-32846 and 30-32858. Washington, DC: API, 1983.

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6. Berichte Deutsche Gesellschaft fur Mineralolwissenschaft und Kohlechemie. Effectsof n-Hexane in Man and Animals. Research Report 174-2. Hamburg: BDGNK e.V.,1982. [Also see U.S. EPA Integrated Risk Information System (IRIS), n-hexanehttp://www.epa.gov.iris.subst/0486.htm]

7. JB Galvin. Toxicity data for commercial hexane and hexane isomers. In: PJ Wan,PJ Wakelyn, eds. Technology and Solvents for Extracting Oilseeds and Nonpetro-leum Oils. Champaign, IL: AOCS Press, 1997, pp. 75–85.

8. U.S. Environmental Protection Agency. Commercial Hexane Final Test Rule. (53Federal Register 3382; February 5, 1988.)

9. PJ Wakelyn, PK Adair. Toxicity data for extraction solvents other than hexane andisohexane/hexane isomers. In: PJ Wan, PJ Wakelyn, eds. Technology and Solventsfor Extracting Oilseeds and Nonpetroleum Oils. Champaign, IL: AOCS Press,1997, pp. 86–100.

9a. PJ Wakelyn, RJ Hron, FJ Flider, PJ Wan. Acetoneian environmental preferablechoice? INFORM 12(9):887–893, 2001.

10. PJ Wakelyn. Regulatory considerations for extraction solvents for oilseeds andother nonpetroleum oils. In: PJ Wan, PJ Wakelyn, eds. Technology and Solventsfor Extracting Oilseeds and Nonpetroleum Oils. Champaign, IL: AOCS Press,1997, pp. 48–74.

11. PJ Wakelyn, PK Adair. Assessment of risk and environmental management. In: SSKoseoglu, KC Rhee, RF Wilson, eds. Emerging Technologies, Current Practices,Quality Control, Technology Transfer, and Environmental Issues, Vol. 1. Proceed-ings of the World Conference on Oilseed and Edible Oil Processing. Champaign,IL: AOCS Press, 1998, pp. 305–312.

12. PJ Wakelyn, PK Adair, SR Gregory. Management of quality and quality control incottonseed extraction mills. Oil Mill Gaz 103(6):23–30, 1997.

13. PJ Wakelyn. Overview of the Clean Air Act of 1990: Cottonseed oil mills. CottonGin and Oil Mills Press 92(17):12–15, 18, 1991.

14. PJ Wakelyn, LA Forster. Regulatory issues regarding air, water, solid waste andsalmonella. Oil Mill Gaz 99(12):21–30, 1994.

15. PJ Wan, PJ Wakelyn. Regulatory considerations of VOC, HAP. Inform 9:1155–1160, 1998.

16. PJ Wan, PJ Wakelyn. Regulatory considerations of VOC and HAP from oilseedextraction plants. Oil Mill Gaz 104(6):15–24, 1998.

17. C Zukor, BL Riddle. Vegetable oil process description. Memorandum to EPA Veg-etable Oil Project File, May 29, 1998.

18. L Venne. Volatile organic compounds. Paper presented at the 1997 AOCS AnnualMeeting, Seattle, WA, 1997.

19. U.S. Environmental Protection Agency. n-Hexane. U.S. EPA Integrated Risk Infor-mation System (IRIS) Substance File, 1999 (www.epa.gov/ngispgm3/IRIS/subst/0486.htm).

20. U.S. Environmental Protection Agency. Study of Volatile Organic Compound Emis-sions from Consumer and Commercial Products. U.S. Environmental ProtectionAgency, Office of Quality Planning and Standards, 1995 (EPA–453/R-94-066-A).

21. SE Hughs, PJ Wakelyn. Physical characteristics of cyclone particulate emissions.Appl Eng Agric 13(4):531–535, 1997.

426 Wakelyn and Wan

22. W Zschau. Spent bleaching earth—practical solutions. Inform 5:1375–1377, 1994.23. U.S. Environmental Protection Agency. Toxic Chemical Release Inventory Report-

ing Form R and Instructions (Revised 1995 Version). U.S. EPA, Office of PollutionPrevention and Toxics, 1996, pp. II-1. (EPA 745-K-96-001).

24. JK Dunnick. Toxicity Studies of n-Hexane in F344/N Rats and B6C3F1 Mice.National Toxicology Programs, U.S. Dept. of Health and Human Services, NTPTOX 2, NIH Publication No. 91-3121, 1991.

25. DW Smith, PJ Wakelyn, PK Adair, B Nevius, LA Forster. Cottonseed Oil MillOSHA Compliance Guide. Memphis, TN: National Cottonseed Products Associa-tion, 1996.

26. Occupational Health and Safety Administration. Field Operations Manual, ChapterIV: Violations, C. Health Standards Violations, (OSHA Instruction 2.45B CH-4,Dec. 13, 1993), Washington, DC: Bureau of National Affairs, 1994, pp. 77:2513–2518.

27. Occupational Health and Safety Administration. Technical Manual, Section I—Sampling, Measurement Methods, and Instruments, Chapter 1—Personal Samplingfor Air Contaminants, Appendix I:1-6. Sampling and Analytical Errors (SAEs)(OSHA Instruction TED 1.15, September 22, 1995; amended by OSHA InstructionTED 1.15 CH-1, May 24, 1996).

28. American Conference of Governmental Industrial Hygienists. 1997 TLVs andBEIs, Threshold Limit Values for Chemical Substances and Physical Agents andBiological Exposure Indices. Cincinnati, OH: The American Conference of Gov-ernmental Industrial Hygienists, 1997, pp. 12–40.

29. JB Galvin, CJ Kirwin, DW Kelly. Risk assessment of hydrocarbon solvents. Inform6:951–952, 1995.

30. HH Schaumberg, PS Spencer. Degeneration in central and peripheral nervous sys-tems produced by pure n-hexane: An experimental study. Brain 99:183–192, 1976.

31. DR Lejeunesse. Process safety management. In: PJ Wan, PJ Wakelyn, eds. Tech-nology and Solvents for Extracting Oilseeds and Nonpetroleum Oils. Champaign,IL: AOCS Press, 1997, pp. 153–169.

32. M Strube. OSHA’s Rules and Solvent Plant Automation. Inform 8:125–126, 1977.33. AH Allen. GRAS Self-Determination: Staying Out of the Regulatory Soup. Food

Product Design (April 1996 supplement to Food Product Design), 1996.34. P Newberne, RL Smith, J Doull, VJ Feron, JI Goodman, IC Munro, PS Portoghese,

WJ Waddell, BM Wagner, CS Weil, TB Adams, JB Hallagan. GRAS flavoringsubstances 19. Food Technol 54(7):66–84, 2000.

35. BL Oser, RA Ford. Recent progress in the consideration of flavoring ingredientsunder the food additive amendments 6. GRAS substances. Food Technol 27(1):64,1973.

36. RL Hall, BL Oser. Recent progress in the consideration of flavoring ingredientsunder the food additive amendments 6. GRAS substances. Food Technol 19:151,1965.

37. AM Galvin. In: PJ Wan, ed. Introduction to Fats and Oils Technology. Champaign,IL: AOCS Press, 1991, pp. 137–164.

38. LA Jones, CC King. In: YH. Hui, ed. Bailey’s Industrial Oil and Fats Products, 5th

Solvent Extraction 427

ed., Vol. 2, Edible Oil and Fat Products: Oils and Oil Seeds. New York: JohnWiley & Sons, 1996, pp. 177–181.

39. HW Lawson. Standards for Fat and Oils. Westport, CT: AVI, 1985, p. 34.40. SW Kuhlmann, MC Calhoun, JE Huston, BC Baldwin, Jr. Total (+)- and (-)- gossy-

pol in plasma and liver of lambs fed cottonseed meal processed by three methods.J Anim Sci 72 (suppl. 1):145, 1994.

41. DJ Murphy. Development of new oil crops in the 21st century. Inform 11:112–117,2000.

42. MA Ryan. Functional foods hit supermarket shelves. Today’s Chemist at Work8(9):59–63, 1999.

43. BF Haumann. Structured lipids allow fat tailoring. Inform 8:1004–1011, 1997.44. B Flickinger, E Hines. A look into the future of food. Food Quality 6(7):18–24,

1999.45. CT Hou. Value added products from oils and fats through bio-processes. In: S.

Shizhong, L. Ruizheng, eds. Proceedings of International Symposium on New Ap-proaches to Functional Cereals and Oils. Beijing, P. R. China: Chinese Cereals andOil Association, 1997, p. 669.

46. DJ Kyle. New specialty oils: development of a DHA-rich nutraceutical product. In:S. Shizhong, L. Ruizheng, eds. Proceedings of International Symposium on NewApproaches to Functional Cereals and Oils. Beijing, P. R. China: Chinese Cerealsand Oil Association, 1997, p. 681.

47. JK Daun. Modified fatty acid profiles in Canadian oilseeds. In: S. Shizhong, L.Ruizheng, eds. Proceedings of International Symposium on New Approaches toFunctional Cereals and Oils. Beijing, P. R. China: Chinese Cereals and Oil Associa-tion, 1997, pp. 659–668.

48. CM Henry. Nutraceuticals: fad or trend? Chem Eng News 77(48):42–47, 1999.49. R Ohlson. Bonn symposia explore industrial uses. Inform 10:722–724, 1999.

Index

Absolute (extract), 211, 305–307, 310, [Alkaloids extraction]cosolvent use, 348, 351, 356, 359–361311, 315, 322–326

Acentric factor, 71 equipment for, 353from natural plants, 347–368Acetone, 37, 56, 78, 117, 228, 315, 316,

321, 328 solvent extraction of, 347supercritical carbon dioxide, 349,Activity:

absolute, 12, 18, 22 350, 352–354, 359–363yield, 359–361coefficient, 18, 19, 29

relative, 18, 19, 22, 26 Ambrose group contribution method, 40Anise star, 313, 315Affinity of reaction, 32, 33

Alkaloids (purine), 230, 347–349, 351, Antioxidants:activity, 330–336, 338–340354, 355, 361–363

pharmaceutical and cosmetic (applica- natural, 59, 329–346synergists, 330tions, industries), 349, 358, 362,

363 synthetic, 329Architecture, of matrix, solid, structure,pharmaceutical products, 360

pharmacological activity (effect), 347 45, 48Avogadro’s number, 30physiological (action) effect, 347,

348–349 Azeotrope, 21, 24toxic effect, 349

Alkaloids extraction: Beeswax, 315Beet, 35, 43commercial scale applications,

349–362 Bubble point, 22, 24

429

430 Index

Caffeine, 43, 77, 79, 80, 122, 125, 134, [Cholesterol extraction]solubility in supercritical fluids,307, 317–319, 347–355, 357,

359–361, 363 374–377solvents in, 370, 375, 377, 381, 382Cake (from pressing), 113, 115, 200,

202, 206, 397 supercritical fluid extraction withCO2, 370, 371, 375, 377–384Calendula, 315

Cane, 35, 43 yield, 371, 383Cholesterol removal:Carbohydrates, 235, 237

Carbon dioxide (CO2), 65–68, 73 applications, 377–383from eggs, 381–382liquid CO2, 65, 121, 310, 315, 316,

320, 322, 324 from food products, 369–390from meat, 382–383supercritical (SC-CO2) or liquefied,

58, 59, 79 from milk fat, 370, 371, 377–381technologies for, 370Cardamom, 315

Cell, architectural arrangement of, 47– Citrus fruit (antioxidant source), 330,33149, 51

Cereal (antioxidant source), 330 Cocoa bean, antioxidant source, 331Cocoa bean extraction:Chamomile, German, 315

Change of transition of state, 25 cosolvent in, 359, 360of methylxanthines from, 357–360Chemical effect, 72

Chemical reaction, 2, 11, 30 solvents in, 358, 359, 360Coefficient:equilibrium, 30, 32

Chemical potential, 12, 17, 21, 27, 28, liquid-phase mass transfer, 40mass transfer, 5, 40, 51235

Chemicals: of thermal expansion, 10Coffee, 43, 58, 77, 79, 80, 99, 117, 181,explosive, 419

flammable, 419 307, 317–319, 347–349extractor system for, 99hazardous, 417

health hazard, 417 SFE industrial plant, 134Coffee bean:permissible exposure limits (PELs),

417 antioxidant source, 331decaffeination of, 350–353reactive, 419

toxic, 419 Cold pressing, 312Collets, 396–398Cholesterol, 74, 75, 421

determination in food products, Compressibility, 10, 25critical, 25373–374

fractionation, 370, 371, 379–381 factor, 25, 131Concrete, 305, 310, 322, 323free from, 369

low content, 369, 370, 373, 380 Conditions state (steady or unsteady), 39Constraints, 137, 140–142, 144–149,physiological effect on health (health

problems), 369, 371–373, 381 151, 152, 154, 156–160, 162,163, 165–169, 172reduction of, 370–372, 374, 382, 384

Cholesterol extraction: boundary, 142equality, 141, 146, 149, 151, 154,of animal fats, 371

cosolvent use, 371, 375, 377, 384 156, 157, 159

Index 431

[Constraints] [Decaffeination of coffee beans]toxic residues in decaffeinated prod-functional, 144, 147

inequality, 141, 146, 149, 151, 154, ucts, 349, 350, 353, 354, 360water process, 318156, 160

regional, 144, 147, 148, 172 Zosel’s process, 350, 351, 352Decaffeination of tea (black), 348, 349,Conveyer elements (or systems) in ex-

tractors, 101–104, 107, 110, 117 351, 353–354solvents for, 353Cooking of proteins, 115

Coriander, 319 supercritical carbon dioxide and sol-vent, 354Corn, 286, 288, 291, 295, 297

extraction of starch from, 286–298 Decision variables, 137, 139, 141, 142,145, 148, 152, 153, 161–163 (seeseparations, 295

steeping, 288 also Variables or Parameters orControl factors)wet milling, 286

Cossettes (see Sugar beet) Degrees of freedom, 20Density, 3, 4Cost, 79, 82, 85, 103, 104, 118, 134,

136–142, 144, 146, 164–173, of CO2, 65of SCFs182, 187, 197, 227, 254, 271,

273, 274, 282, 285, 297, 298, in supercritical region, 61, 62, 71in near-critical region, 63318, 319, 322, 348, 352, 353,

370, 375, 380, 381, 390, 392 Design problem, 140, 237Desolventizing, 44, 97, 117Criterion or function, 138–140, 142,

144–146, 148, 165, 172, (see also Dew point line, 22, 24Dielectric constant, 64Performance function or Objective

function or Response or Perfor- Diffusion, 35, 38–45, 50, 52, 110, 136,182–186, 235, 237, 240, 245,mance measure)

Critical constants, 60, 70 246, 254, 255, 259, 261, 265,270, 281, 282, 284, 288, 291,Critical fluids, 58

Critical point, 21, 24, 60 337, 361coefficient (apparent, effective), 39–

48, 50, 52, 63, 64, 67, 68, 75, 77,Decaffeination of coffee beans, 58, 319,350–353 80, 81

Diffusivity, 39, 41–43, 45, 48, 51, 184by adsorption of caffeine, 125allowed by FDA, 317 of CO2, 67

in SCF, 64, 65alternative methods with CO2, 122commercial, 317, 318, 348, 350–353 Dilution, 14, 15, 27, 28cosolvent use in extraction, 351, 356high-pressure liquid-fluid extraction, Egg (see Cholesterol removal)

Electrolyte, 28–30125solvent, 350 Elemi, 320

Energy, 2, 4, 6–8, 11solvents for, 350–352supercritical carbon dioxide, 319, balance, cost, requirements, saving,

103, 124, 140, 146, 167, 170,350–352supercritical carbon dioxide and sol- 173, 183, 192, 199, 200, 202,

240, 297, 298, 375, 392vent, 352

432 Index

Enfleurage, 313 Explosion of solvent, 110, 117Expression, 111, 312Enhancement factor (in SFE), 74, 75

Enthalpy, 8, 25, 26, 31 Extract, 35, 36, 38, 40, 41, 85, 134separation, 36Entrainer in SFE, 78

Entropy, 7, 8, 25, 26 recovery, 85Extractability, 51, 103Environmental issues or demands or pro-

tection, 96, 117, 134 (see also Extracting time, 97Extraction, 35, 37, 40 (see also Oils,Health and Safety issues)

Equation: Proteins, Sugars, Starch, Flavorand Aroma, Antioxidants, Alka-Benedict-Webb-Rubin, 68

Gibbs, 12 loids, Cholesterol, Solvent, Liq-uid-liquid, Supercritical fluidGibbs-Duhem, 12, 16

of state, 4, 69 (SFE) and Supercritical CO2 ex-traction)of state of ideal gas, 16

Stokes-Einstein, 41 carbon dioxide, 79, 118, 121, 310,311Van der Waals, 16, 69, 70

Equilibrium, 5, 8, 10, 11, 17, 19–22, from cellular material, 48commercial, 310, 311, 315, 349–36225, 26, 30, 32, 33, 40, 41, 51 (see

also Phase equilibrium) curve, 76efficiency, 95, 99, 102, 110, 182constant, 32

Equipment for extraction (see Extraction fractional, 83liquid CO2, 310, 315, 316, 320, 322,systems or Supercritical fluid ex-

traction systems) 324mathematical model for, 47, 245–Essential amino acids, 218, 219, 224

Essential oils, 182, 305, 307, 310, 312, 247near-critical, 60313, 315, 316, 319–321, 323–326

compound classes, 207, 209 preparation for, 38, 111, 392,395–396distillation equipment (retort, con-

denser, oil separator), 210 rate, 37, 44, 45, 48, 51, 52solid-liquid, 35, 235, 307hydrodistillation methods (by water,

water and steam, direct steam), solid-fluid, 82solvent in, 35–36, 305, 307, 314–209, 210

refining, 211 315, 350–354, 358–360, 362supercritical fluid (SFE), 57–93, 305,production of, 209

Essential oils extraction: 307, 315theories of, 182by cold fat or volatile solvents, 211

by supercritical carbon dioxide or pro- time, 38yield, 80, 99, 102, 103, 111pane, 212

Ethanol, 36, 37, 57, 59, 64, 78, 79, 118, Extraction methods (conventional):immersion, 96–99, 102, 192192, 218, 220, 305, 307, 313–

316, 322, 323, 325, 334–338, percolation, 96–99, 101, 102, 192Extraction operation (conventional):340, 351, 359–361, 363, 392

Evaporation, 22 batch, 96charging, 96, 100, 107, 122, 129Expander, 115, 396

Expeller, 113–115, 197, 199, 392 continuous, 97, 134

Index 433

[Extraction operation (conventional)] [Extractor(s)]sliding cell, 105, 107counter current (flow), 97, 98, 101,

102, 104, 105, 107, 109, 110, solid-conveying system, 102, 103solvent pump, 102, 107122, 125

cross current (flow), 96, 98, 102 stationary, 98, 99, 104, 105for sugar, 99, 259–270discharging, 96–98, 100, 103, 104,

107, 122, 129 system for coffee (FIC), 99Extruder, 112, 115, 122, 207, 396holdup, 99

loading facilities, 99moving-bed, 99, 102, 103 Factor(s), 139–141, 165, 167, 172 (see

also Parameters or Decision vari-multistage, 97quasi-continuous, 96, 97, 134 ables)

Fats and oils, 329, 391 (see also Oils)recirculation, 101–104, 107residence time, 99, 102 Fines, 107, 110, 111

Flakes, 42, 43, 45, 51, 103, 110, 185,semicontinuous, 134single-stage, 97 193, 207, 219–222, 228–230,

395–398Extraction systems (see also Extractors):conventional, 95–118 Flavonoids, 329, 331, 332, 335, 337,

340with supercritical fluids (SFE), 69,79, 81, 85, 120–135 Flavor, 58, 78, 79, 96, 99

Flavor and aroma extraction, 307Extractor(s), 38, 81–83 (see also Extrac-tion systems) cosolvent use, 325

with liquid CO2, 310, 315, 316, 320,basket elevator (Bollman), 107, 109belt, 104 324

by solvent, 305, 310, 313–326Carrousel, 103, 104crown loop, 105 solvents in, 305, 307, 314–315

solvents conditions of use, 314–315conveyer elements, 101, 102, 104,107, 110, 117 supercritical with CO2, 311, 315, 316,

319, 322, 325cost, 103, 104, 118efficiency, 96 yield, 310, 312, 315, 320, 322–325

Flavor and aroma substances, 305–327extruder, 112, 115foodtech system of FMC, 118 concentrates, 212

distillation (steam, water), 305, 307–horizontal/vertical (working), 104industrial (conventional extraction), 309, 315, 316, 320–326

essential oils of spices, 209102–110multiplate tower (Bonotto), 110 extracts, 305

plant (material), 305, 307, 310, 313,for oilseeds, 192–197percolator, 96, 99, 100, 115, 192, 193 316, 324, 326

raw materials, 305for pretreatment, 95rotating parts, 103, 104 Flavorings, 305, 314, 315, 323

Fractionation, 57–59, 74–76, 79, 83,rotating, 104Rotocel (deep-bed), 103, 118 377

butter, 371screw conveyor, 98, 102, 109, 110single-stage, 311 butter oil, 371, 379, 380

cholesterol from food products, 371sliding bed, 107

434 Index

[Fractionation] Hop, 58, 96, 118, 307industrial SFE of, 134column, 83–85

fat, 370 oils of, 321Hydraulic press, 312milk fat, 377, 378, 381

supercritical CO2, 340 Hydrocolloids, 35supercritical fluid (SFF), 83, 84

Frankincense, 320 Interaction between solute-solvent, 36Interfacial tension (IFT), 75, 76Fruit juices, 118

Fugacity, 16, 17, 22, 26, 27, 28 Ionic activity coefficient, 29Ionic strength, 30coefficient, 17

Fusion, 24 Isohexane (commercial), 392, 400Isopropanol, 62, 78, 192, 392

Galbanum, 320Garlic (antioxidant source), 333 Jasmine, 322Gas mixture, 15–16Gibbs energy, 9 Law(s) (see also Regulations):

of corresponding states, 70Ginger, 321Gossypol, 227, 228 Debye-Huckel Limiting, 29

Fick’s, 39–42, 45, 47, 245Green processing, 59Grit, 42, 51 Fourier, 245

Henry’s, 27Guarana seeds:extraction of methylxanthines from, ideal gas, 60

Raoult’s, 26, 27360–361solvent in extraction, 360 Leaching, 35, 38, 52, 96, 97, 102, 103,

172, 181immersion, 96Hazardous air pollulants (HAPs), 407

Hazards (for health, environment, fire), Legislation (see Regulations)Linden, 323422

Health issues (see Safety issues) Linoleic acid, 43Lipids, 35, 79Heat, 2, 7, 8, 31

Heat capacity, 7, 10, 68 Liquid-liquid extraction, 307, 339, 340Heat transfer, 68Heliocarrot, 321 Maceration (hot enfleurage), 313

Mass, 4Helmholtz free energy, 8Henry’s constant, 27 Mass transfer (or transport), 35–38, 40,

51, 52, 75Herbs (antioxidant source), 331–332,338 mechanism, 35, 36

Mathematical model, 138–142, 145–Hexane, 36, 37, 42–44, 51, 95, 192,229, 230, 305, 310, 314, 315, 147, 165, 167, 169, 173, 174

for extraction from a two-dimensional322–324, 334, 335, 337–340,358, 377, 384, 392, 397, 409, matrix, 47

evaluation, 147410, 414–417, 419, 420commercial, 398 mass transfer in SFE, 76–78

phase equilibria, 70, 71emission sources of, 407GRAS for food use, 420 shrinking core leaching, 77

validation, 147toxicity, 398

Index 435

Mathematic programming, 138, 149, 164 Natural products, 58, 59, 347, 348, 350,353–362Meal, 35, 96, 103, 104, 117, 205, 207,

218, 220, 226, 227, 229, 230, Near-critical:extraction, 60392, 396

Meats, 206, 396 (see also Cholesterol re- fluids, 73region, 63, 68, 71moval)

Mechanical extraction: Nicotine, 347, 349, 351, 361Nonelectrolyte, 28, 29deoiling (expression), 111, 113, 115

extraction of fruit juices, 118 Number:Biot for mass transfer, 48extraction of oils, 396

pressing, 111–115, 117, 118, 181 Fick’s, 40Sherwood, 52Methanol, 64, 78, 79, 307, 314, 325,

332–339, 351 Schmidt, 52Nutmeg, 323Methylxanthines, 349, 354, 355, 357,

359, 363Microstructure (food, solid matrix), 37, Objective function, 140–142, 145, 146,

149, 151–153, 156, 157, 162,38, 42, 44, 45, 50, 52Milk fat (see Cholesterol removal) 163, 165 (see also Performance

function or Response or CriterionMills (for oilseed preparation prior to ex-traction), 111, 112, 392 or Performance measure)

Oil extraction, 42, 43, 51, 77, 81, 104,Miscella, 96, 101–104, 107, 109, 110,115, 118, 184, 193, 397 181–215, 338, 396–398

continuous multistage countercurrentMixing functions, 14Mixture, 13–26 extraction, 191

efficiency, 182Modeling, extraction from cellular mate-rial, 48 expelling (expeller pressing), 197,

207Molar quantities, 5, 6Myrrh, 323 extraction battery, 188

extractors, 192–197hydraulic pressing, 197Natural antioxidants, 329–346

criteria for use, 329, 330 mechanical, 392mechanical pressing or deoiling (ex-phenolic, 329

plant sources, 330–333 pression), 111, 173, 174, 199, 200miscella, 184Natural antioxidants extraction:

cosolvent use, 338 operation mode, 183prepress-solvent extraction, 199, 206,parameters, 336–337

procedures, 337–338 207rate of, 183, 184, 185process, 334

liquid–liquid, 339, 340 recovery, 197, 392(direct) solvent, 197, 199, 202, 206,purification, 337, 339–340

recovery (of antioxidants, solute), 207, 392solvents for, 191–192334–337, 339

solvents, 334–336 solvent choice, 182steady-state of operation (single- orwith supercritical CO2, 338–339

yield (of extract, of process, of sol- multistage), 186, 190supercritical, 126vent), 334–340

436 Index

[Oil extraction] Olives (antioxidant source), 332, 333Onion (antioxidant source), 333theories, 183–191

yield, 182, 197, 206, 392, 397 Optimal:design, 137Oil gland, 209

Oils(s), 76, 77, 83 (see also Fats) extraction, 395pressing result, 112content of materials, 104, 111, 112,

115, 117 solution, 137, 148, 151Optimization, 137, 237, 274cost (production), 118, 392

crude, 392 and control, 169problem, 139quality, 197, 202, 392, 395, 397

rancidity, 329 theory, 139Optimization applications in food extrac-refined, 392

refining, 392, 394, 397, 421 tion processing, 164–175Optimization in food processing:sources, 391

stability to oxidation, 392 chemical engineering, 164dehydration, 165Oilseeds (or oil-bearing materials), 35,

80, 81, 83, 197 equipment, 165formulation, 166antioxidant source, 332

canola (rapeseed), 199, 200, 202, 204, industrial operations, 169management, 169205

cleaning, 395 thermal processing, 165thermal systems, 164, 165conditioning prior extraction, 95, 110,

111, 113, 115, 202 Optimization of extraction process,170–172cooking, 200, 396

corn, 207 proteins, 172oil (by expression or SFE), 173,cottonseed, 199

dehulling, 205, 206, 395 174in analytical procedures, 175drying, 395

flaking, 395 Optimization of food product:acceptability (sensory), 166, 167mills for preparation, 111, 112

post-treatment after extraction, 204, nutritional quality, 166quality, 167–169205

preparation (cleaning) or pretreat- Optimization methods, 138, 152–164ment, 182, 183, 200

preparation with expander (extruder), categorization, 152–153constrained (equality or inequality396

preparation for extraction, 111, 392, constraints), 156–161direct or indirect, 153395–396

raffinate, 184 dynamic, 161–162experimental-response surface (RSM),soybean, 206, 207

storage, 395 168graphical, 153sunflower seed, 199, 205, 206

Oleoresins, 305, 307, 310, 320, gradient-based, 153numerical, 153321

Olive oil, 173, 396 programming, 162–163, 168

Index 437

[Optimization methods] Plant (industrial), 134, 192, 381of decaffeination, 319, 350–353unconstrained, 154–156

theoretical background, 149–152 extraction, 118, 322Plant materials, 181, 197, 209, 210, 217,Optimization performing, 148

Optimization problem, 138–140, 142– 218, 305, 307, 310proteins from, 217–233149, 151, 152, 154, 159, 161,

163, 165, 167 fats and oils from, 181–215Plants:formulation, 142, 145

solution, 148, 152 for alkaloids, 347, 349, 360–363for antioxidants, 329, 330, 332, 336Optimization procedure, 142, 143, 149

Optimized extraction parameters, 218 Plant tissue (cell), 38Points of water (boiling, freezing, tri-Optimum conditions, 137

Oxidation, 329 ple), 3Polysaccharides, 99Pomade, 305, 313, 323Parameters, 137, 139, 141–148, 163,

165, 168, 170–175 (see also Deci- Porosity, 44, 45Potential energy of interaction, 62sion variables or Control factors)

Particle size (solid), 38, 40, 42, 43, 52 Presses, 112–115, 118, 312, 392, 396Pressing, 111–115, 118 (see also Me-Peanut, 42

Penetration distance, 40 chanical pressing)cold, 396Performance function, 140, 141, 145,

148 (see also Objective function final, 117, 118of oilseeds, 111, 113or Response or Criterion or Perfor-

mance measure) prepressing, 112, 113, 118Pressure, 3Performance measure, 140, 144, 145,

148 (see also Performance func- critical, 21, 25, 60, 62Pretreatment for solvent extraction,tion or Objective function or Re-

sponse or Criterion or Criterion 115grinding (or milling), 111function)

Permeability (of cell wall, membranes, mechanical, 110–115thermal, 115solute), 38

Permittivity, 62, 64 Principle, of corresponding states, 25Process, 5Phase, 4, 5, 21

supercritical, 60 efficiency, 99at equilibrium, 10Phase equilibrium (a), 2, 17, 19–26,

69–71 industrial scale, 81, 85Product:model, modeling, 70, 71

Phenolic compounds (phenolics), 329, quality, 95, 96, 99, 113, 134, 218,220, 227330, 332, 334–336, 340

Phospholipids, 79 yield, 99, 113Programming optimization (see Optimi-Physical properties, of gas, liquid and

SCF, 63 zation methods)Protein (vegetable), 35Phytochemicals, 35

Pigments, 311, 322 canola, 225–227cereal, 217, 218, 224, 226, 229Pilot plant, 339, 352, 382

438 Index

[Protein (vegetable)] [Regulations for environment, health,safety]commercial products, 218, 219, 222,

223 air toxics requirements, 407Clean Air Act (CAA), 404concentrates, 219–221, 229

corn, 44 Clean Water Act (CWA), 411Comprehensive Environmental Re-cottonseed, 227–229

flour, 219–221, 224, 225, 229, 230 sponse, Compensation and Liabil-ity Act (CERCLA), 414functional properties, 217, 218, 22,

223, 224, 227, 229 Emergency Planning and CommunityRight-to-know Act, 414, 415gluten, 224–225

isolates, 217–222, 227, 229, 230 Federal permits, 411hazards (for health, environment,legumes, 217, 218, 230

lupin, 230 fire), 422New source review (NSR) require-meal (oilseed), 218, 220, 226, 227,

229, 230 ments, 411in oil production, 401, 402, 407, 409,nutritional value (or studies), 217–

219, 226 410–413, 416Resource Conservation and Recoveryoilseed, 217, 218, 226

peanut, 230 Act (RCRA), 413state permits, 411phenolic compounds, 220, 223, 226,

227, 229 Toxic Substances Control Act(TSCA), 415, 416precipitation, 226, 227, 229, 230

quality, 218, 220, 224, 226, 227 for Volatile Organic Compounds(VOPs), 410rapeseed, 225–227

sesame seed, 229 for workplace, 416–419Resinoid, 305, 307, 310, 321, 323, 326soluble in, 217, 218, 220, 224, 227,

230 Resins, 307, 310, 311, 316, 320, 322,326soy, 218–223

sunflower seed, 229 Response (variable), 139, 140, 167, 169,172, 173 (see also Performancewheat, 218, 224, 225

Protein extraction, 50, 172, 217–220, function or Objective functionor Criterion or Performance225–230

aqueous (or water), 218 measure)Rose, 324commercial processes (production),

218, 219, 221, 222, 225, 227 Rosehip, 324cooking, 115optimization, 172 Safe plant operation, 353

Safe process, 81yield, 218, 224, 227Purification, 38, 50, 337, 339–340, 348 Safety (environment, health):

assessment program (FEMA GRAS),420Quantities, 2, 4

emission requirements in oil produc-tion, 409Reaction quotient, 33

Regulations for environment, health, of extraction solvents, 192, 421flammable limit of solvent, 110safety, 400, 419

Index 439

[Safety (environment, health)] [Solvent(s)]emission standards, 407food, 420

GRAS food substances, 420, 421 escape, 97extraction solvents (for flavor), 313of oil extraction facilities, 391

of oil production, 407 flammable limit, 110for flavor extraction, 305, 307,of oilseed solvent extraction, 420

particulate matter (PM) emission, 410 314–315GRAS, 78of plants, 192

programs for, 402 hazardous air pollutant, 405loss, 398regulations, 400

of solvent extraction plant, 110 loss factor of oilseeds, 407, 408for natural antioxidants, 334–336by solvent loss, 398

of synthetic antioxidants, 329 for edible oil extraction, 191–192,398, 400Screw press, 112, 113, 197, 200, 312,

392 power, 72recovery, 85, 95, 97, 115, 117,Separation, 35, 36, 38, 50

SI system and units, 2–4, 7, 12, 15 124–126regulations, 391, 392Simulation (computer), 138, 141, 142,

145, 147, 168, 174, 175 safe (or safety of), 182, 310, 318selection, 37, 334–336Solid-liquid extraction, 35–55, 235, 307

Solid matrix, 35, 36, 37, 44 selectivity, 74, 95, 182toxicity, 391, 398, 400, 401, 422diffusion, 38

structural features, 41, 42 Solvent extraction, 36, 38, 52, 97, 112,181, 182, 199, 202, 206, 207,Solubility, 37

of lipid components (lipids), 73, 74 310, 313–326, 347, 391, 392prepress, 199, 396in SC-CO2, 73, 134

of solutes in SFC, 71, 73 direct, 397Soybean, 42, 43of sugars in CO2, 74

Solubility parameter, 72, 73 Spice (antioxidant source), 331–332, 338dittany, 332Solubilization, 36

Solute, 26, 35, 36, 97 oregano, 332, 337rosemary, 331, 332, 337–339load of solvent, 97

recovery, 85 sage, 332, 337thyme, 332Solution, 26–30

Solvent(s), 26, 35, 36, 97, 392, 397, 400 Spices, 58, 77, 209Standards for safety, health, environ-accepted for use, 37

for alkaloids extraction, 350–354, ment:air contaminants, 417358–360, 362

allowed by FDA for decaffeination, hazard communication, 417process safety management (PSM),317

approval for oilseed extraction, 420 419State effect, 72capacity, 96

for cholesterol, 370, 375, 377, 381, Standard state, 18, 27Starch, 235, 237, 240, 276, 286, 288,382

criteria selection for SFE, 79 291, 295, 297

440 Index

[Starch] [Sugar beet extraction]osmosis, 246extraction, 235, 237

Sublimation, 24 overdiffusivity, 245percolation rate, 254Sucrose, 35, 42, 235, 240

of sucrose, 43, 50 pressing, 257, 258pulp, 237, 238, 248, 254–259, 261,Sugar (beet, cane), 42, 181, 235

Sugar beet: 270, 271, 273, 274purity of juice, 237bacterial activity and control, 256,

258 slicing (control and machines), 248,249, 251cellular structure, 235

cell types, 239, 240 slicing knives and cleaning, 251sucrose concentration, 235, 237, 240,cellulose fibrils, 242

chemical composition, 238 245–247(mathematical) theory of, 237,morphology of roots, 238, 239

physical properties, 240 245–247yield (or sugar manufacture), 237,Sugar beet extraction, 240, 246, 247

cossettes, 235, 237, 240, 242, 245– 271, 273Sugar cane:248, 251, 252, 254, 255, 258–

261, 263, 265, 267–270, 282 bagasse, 237, 280–282, 284composition, 274–276cossette quality, 252, 254

cost (Silin number, Swedish number, structure of plant, 276, 277Sugar cane extraction:mush content, draft), (of sugar pro-

duction), 273, 274 diffusion and diffusers, 281, 282, 284efficiency, 280, 281, 284, 285denaturation, 240, 242, 246, 255

deteriorated, 258, 259 factors affecting, 284–285extractors in, 99diffusion (rate), 235, 237, 240, 245,

246, 255 juice, 280, 281, 284, 285mills, 280, 281diffusion coefficients, 245, 246

diffusion juice, 254, 259, 261, 265, preparation and equipment, 237, 278,279, 284270

mechanism, 245 unloading, 277, 278Supercritical carbon dioxide (SC-CO2)parameters (pH, temperature, time,

water, aeration, antifoaming extraction, 57–59, 62, 64, 65, 68,69, 73–77, 79–81, 121, 129, 134,agents), 254–259

rate of, 255 311, 315, 319, 320, 322, 324,325, 338–340, 349, 350, 352–extractors (RT, De Smet, Chain-type,

DDS, Tower) in, 99, 258, 354, 359–363, 370, 371, 375,377–384, 392259–270

fractal structure of tissues, 242, 245 cosolvent use, 325, 338, 348, 351,356, 359–361, 371, 375, 377, 384juice, 237, 238, 246, 254–256, 258

juice purification, 238, 255, 258, 259 Supercritical fluid extraction (SFE), 57–93, 122, 130, 192, 307marc content, 238

mass and heat balance, 270 adsorption (on activated carbon), 122,124–126optimal temperature, 255

optimization, 274 applications, 58, 120, 125, 126

Index 441

[Supercritical fluid extraction (SFE)] [System]one-phase open, 11–13charging or discharging, 122, 129

curve, 75 vapor-liquid, 22–24System properties, 4dehulling, 126

depressurizing or pressurizing, 122,124, 125 Tea:

alkaloid containing, 347, 348entrainer effect, 77, 78industrial application, 134 antioxidant source, 333

black tea, 353, 354liquid carrier use, 122liquid CO2, 121 decaffeination, 348, 349, 351,

353–354matrix effect, 80–81pretreatment, 126 Tea extraction:

of methylxanthines from Mate, 354–process(es), 59, 64, 69, 75, 76, 77,79–82, 85, 86 355, 357

solvents in, 353processing schemes, 81–85processing (batch or continuous), 122 Temperature, 3–5

critical, 21, 25, 60, 62Supercritical fluid extraction (SFE)system: Temperature scales, 3

Terpenes, 329, 331, 335butterfly valve, 120, 121compressor, 120, 129, 130 Thobromine, 347–349, 354, 355, 357–

360, 363equipment, 69, 79, 81, 85, 120extraction (pressure) vessels construc- Thermal conductivity, 68, 69

Thermochemistry, 30tion and operation, 120, 122, 124,127, 129 Thermodynamic functions, 8, 11, 14, 15

Thermodynamic laws, 2, 7, 8gas hydrate formation, 134heat exchanger, 120, 121, 127, 129 Thermodynamic quantity, 14

Thermodynamics, 1, 2plants, 121, 122pressurizing device, 120 Tincture, 305, 307, 323

Tobacco, 347, 349, 361pumps characteristics and efficiency,121, 129–134 Tocopherols, 329, 331, 332, 335, 339

Tolu balsam, 326separator, 120–122solvent cycle, 120, 121 Tortuosity, 44, 45

Transport phenomena, 37solvent recovery, 120, 121, 124, 125,127 Triple point, 20

Tuberose, 325Supercritical fluids (SCFs), 57, 60density of, 61–63, 71diffusion coefficient in, 67 Vanilla, 325

Variables, 137, 140–142, 144–151, 153,enhancement factor, 74, 75properties, 59 155–160, 162–164, 167, 171,

174, 175 (see also Decision vari-selectivity, 74, 134thermal conductivity, 68, 69 ables or Parameters or Control fac-

tors)Supercritical phase, 60System, 4, 5 Violet, 326

Virial coefficient and equation, 15in equilibrium, 10, 11one-phase closed, 8–11 Viscosity, 37, 41, 52, 63, 64, 66

442 Index

Volume, 3, 4 [Water (in extraction)]99, 218, 220, 237, 246, 248, 315,critical molar, 60317, 318, 331, 333, 334, 337,338, 392Washing, 181

Water (in extraction), 35, 36, 41–43, 57, Waxes, 311, 316, 322Work, 2, 6, 859, 62, 64, 66, 69, 74, 78, 79,