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Principles of Food ChemistryThird Edition
John M. deMan, PhDProfessor Emeritus
Department of Food ScienceUniversity of Guelph
Guelph, Ontario
A Chapman & Hall Food Science Book
AN ASPEN PUBLICATION®Aspen Publishers, Inc.
Gaithersburg, Maryland1999
The author has made every effort to ensure the accuracy of the information herein. However, appropriateinformation sources should be consulted, especially for new or unfamiliar procedures. It is the responsi-bility of every practitioner to evaluate the appropriateness of a particular opinion in the context of actualclinical situations and with due considerations to new developments. The author, editors, and the pub-lisher cannot be held responsible for any typographical or other errors found in this book.
Aspen Publishers, Inc., is not affiliated with the American Society of Parenteral and Enteral Nutrition.
Library of Congress Cataloging-in-Publication Data
deMan, John M.Principles of food chemistry/
John M. deMan.—3rd ed.p. cm.
Includes bibliographical references and index.ISBN 0-8342-1234-X
1. Food—Composition. I. Title.TX531.D43 1999
664—dc2198-31467
CIP
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1 2 3 4 5
Preface
This book was designed to serve as a text forcourses in food chemistry in food science pro-grams following the Institute of Food Technolo-gists minimum standards. The original idea inthe preparation of this book was to present basicinformation on the composition of foods and thechemical and physical characteristics theyundergo during processing, storage, and han-dling. The basic principles of food chemistryremain the same, but much additional researchcarried out in recent years has extended anddeepened our knowledge. This required inclu-sion of new material in all chapters. The lastchapter in the second edition, Food Additives,has been replaced by the chapter Additives andContaminants, and an additional chapter, Regu-latory Control of Food Composition, Quality,and Safety, has been included. This last chapteris an attempt to give students some understand-ing of the scientific basis of the formulation oflaws and regulations on food and on the increas-ing trend toward international harmonization ofthese laws. A number of important food safetyissues have arisen recently, and these haveemphasized the need for comprehensive andeffective legal controls.
In the area of water as a food component, theissue of the glass transition has received muchattention. This demonstrates the important roleof water in food properties. Lipids have receivedmuch attention lately mainly because of public-ity related to nutritional problems. Sutructured
lipids, low caloric fats, and biotechnology havereceived a good deal of attention. Our under-standing of the functionality of proteins expandswith increasing knowledge about their composi-tion and structure. Carbohydrates serve manyfunctions in foods, and the noncaloric dietaryfiber has assumed an important role.
Color, flavor, and texture are importantattributes of food quality, and in these areas,especially those of flavor and texture, greatadvances have been made in recent years.Enzymes are playing an ever increasing part inthe production and transformation of foods.Modern methods of biotechnology have pro-duced a gamut of enzymes with new andimproved properties.
In the literature, information is found usingdifferent systems of units: metric, SI, and theEnglish system. Quotations from the literatureare presented in their original form. It would bedifficult to change all these units in the book toone system. To assist the reader in convertingthese units, an appendix is provided with conver-sion factors for all units found in the text.
It is hoped that this new edition will continue tofulfill the need for a concise and relevant text forthe teaching of food chemistry. I express gratitudeto those who have provided comments and sug-gestions for improvement, and especially to mywife, Leny, who has provided a great deal of sup-port and encouragement during the preparation ofthe third edition.
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Contents
Preface ....................................................................................................... vii
1. Water ................................................................................................... 1 Physical Properties of Water and Ice ................................................................. 1 Structure of the Water Molecule ........................................................................ 2 Sorption Phenomena ......................................................................................... 4 Types of Water ................................................................................................... 11 Freezing and Ice Structure ................................................................................. 14 Water Activity and Food Spoilage ...................................................................... 23 Water Activity and Packaging ............................................................................ 26 Water Binding of Meat ........................................................................................ 28 Water Activity and Food Processing .................................................................. 30
2. Lipids .................................................................................................. 33 Introduction ......................................................................................................... 33 Shorthand Description of Fatty Acids and Glycerides ....................................... 35 Component Fatty Acids ...................................................................................... 36 Component Glycerides ....................................................................................... 45 Phospholipids ..................................................................................................... 50 Unsaponifiables .................................................................................................. 51 Autoxidation ........................................................................................................ 54 Photooxidation .................................................................................................... 63 Heated Fats – Frying .......................................................................................... 65 Flavor Reversion ................................................................................................ 70 Hydrogenation .................................................................................................... 71 Interesterification ................................................................................................ 77 Physical Properties ............................................................................................. 81 Fractionation ....................................................................................................... 94
iv Contents
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Cocoa Butter and Confectionery Fats ................................................................ 95 Emulsions and Emulsifiers ................................................................................. 101 Novel Oils and Fats ............................................................................................ 106 Fat Replacers ..................................................................................................... 107
3. Proteins .............................................................................................. 111 Introduction ......................................................................................................... 111 Amino Acid Composition .................................................................................... 111 Protein Classification .......................................................................................... 113 Protein Structure ................................................................................................ 115 Denaturation ....................................................................................................... 118 Nonenzymic Browning ....................................................................................... 120 Chemical Changes ............................................................................................. 131 Functional Properties ......................................................................................... 134 Animal Proteins .................................................................................................. 138 Plant Proteins ..................................................................................................... 152
4. Carbohydrates ................................................................................... 163 Introduction ......................................................................................................... 163 Monosaccharides ............................................................................................... 163 Related Compounds .......................................................................................... 167 Oligosaccharides ................................................................................................ 169 Polysaccharides ................................................................................................. 183 Dietary Fiber ....................................................................................................... 203
5. Minerals .............................................................................................. 209 Introduction ......................................................................................................... 209 Major Minerals .................................................................................................... 209 Trace Elements .................................................................................................. 217 Metal Uptake in Canned Foods ......................................................................... 223
6. Color ................................................................................................... 229 Introduction ......................................................................................................... 229 CIE System ......................................................................................................... 229 Munsell System .................................................................................................. 236 Hunter System .................................................................................................... 237
Contents v
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Lovibond System ................................................................................................ 238 Gloss ................................................................................................................... 239 Food Colorants ................................................................................................... 239
7. Flavor .................................................................................................. 263 Introduction ......................................................................................................... 263 Taste ................................................................................................................... 263 Odor .................................................................................................................... 282 Description of Food Flavors ............................................................................... 291 Astringency ......................................................................................................... 294 Flavor and Off-Flavor ......................................................................................... 296 Flavor of Some Foods ........................................................................................ 297
8. Texture ................................................................................................ 311 Introduction ......................................................................................................... 311 Texture Profile .................................................................................................... 313 Objective Measurement of Texture .................................................................... 316 Different Types of Bodies ................................................................................... 320 Application to Foods ........................................................................................... 328 Textural Properties of Some Foods ................................................................... 334 Microstructure ..................................................................................................... 341 Water Activity and Texture ................................................................................. 347
9. Vitamins .............................................................................................. 355 Introduction ......................................................................................................... 355 Fat-Soluble Vitamins .......................................................................................... 355 Water-Soluble Vitamins ...................................................................................... 366 Vitamins as Food Ingredients ............................................................................ 385
10. Enzymes ............................................................................................. 389 Introduction ......................................................................................................... 389 Nature and Function ........................................................................................... 389 Hydrolases .......................................................................................................... 395 Oxidoreductases ................................................................................................ 413 Immobilized Enzymes ........................................................................................ 423
vi Contents
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11. Additives and Contaminants ........................................................... 429 Introduction ......................................................................................................... 429 Intentional Additives ........................................................................................... 431 Incidental Additives or Contaminants ................................................................ 449
12. Regulatory Control of Food Composition, Quality, and Safety .................................................................................................. 475 Historical Overview ............................................................................................. 475 Safety .................................................................................................................. 477 U.S. Food Laws .................................................................................................. 479 Canadian Food Laws ......................................................................................... 481 European Union (EU) Food Laws ...................................................................... 482 International Food Law: Codex Alimentarius ..................................................... 484 Harmonization .................................................................................................... 488
Appendices ............................................................................................... 491 Appendix A. Units and Conversion Factors ..................................................... 491 Appendix B. Greek Alphabet ........................................................................... 495
Index .......................................................................................................... 497
Water is an essential constituent of manyfoods. It may occur as an intracellular orextracellular component in vegetable andanimal products, as a dispersing medium orsolvent in a variety of products, as the dis-persed phase in some emulsified productssuch as butter and margarine, and as a minorconstituent in other foods. Table 1-1 indi-cates the wide range of water content infoods.
Because of the importance of water as afood constituent, an understanding of itsproperties and behavior is necessary. Thepresence of water influences the chemicaland microbiological deterioration of foods.Also, removal (drying) or freezing of wateris essential to some methods of food preser-vation. Fundamental changes in the productmay take place in both instances.
PHYSICAL PROPERTIES OF WATERAND ICE
Some of the physical properties of waterand ice are exceptional, and a list of these ispresented in Table 1-2. Much of this infor-mation was obtained from Perry (1963) andLandolt-Boernstein (1923). The exception-ally high values of the caloric properties ofwater are of importance for food processing
Table 1-1 Typical Water Contents of SomeSelected Foods
Product Water (%)
Tomato 95Lettuce 95Cabbage 92Beer 90Orange 87Apple juice 87Milk 87Potato 78Banana 75Chicken 70Salmon, canned 67Meat 65Cheese 37Bread, white 35Jam 28Honey 20Butter and margarine 16Wheat flour 12Rice 12Coffee beans, roasted 5Milk powder 4Shortening O
operations such as freezing and drying. Theconsiderable difference in density of water
Water
CHAPTER 1
and ice may result in structural damage tofoods when they are frozen. The density ofice changes with changes in temperature,resulting in stresses in frozen foods. Sincesolids are much less elastic than semisolids,structural damage may result from fluctuat-ing temperatures, even if the fluctuationsremain below the freezing point.
STRUCTURE OF THE WATERMOLECULE
The reason for the unusual behavior ofwater lies in the structure of the water mole-cule (Figure 1-1) and in the molecule's abil-ity to form hydrogen bonds. In the watermolecule the atoms are arranged at an angle
Table 1-2 Some Physical Properties of Water and Ice
Temperature (0C)
Water
Vapor pressure (mm Hg)Density (g/cm3 )Specific heat (cal/g°C)Heat of vaporization(cal/g)Thermal conductivity(kcal/m2h°C)Surface tension(dynes/cm)Viscosity (centipoises)Refractive indexDielectric constantCoefficient of thermal
expansion x 1 (T4
O
4.580.99981.0074597.2
0.486
75.62
1.7921 .333888.0
20
17.530.99820.9988586.0
0.515
72.75
1.0021 .333080.42.07
40
55.320.99220.9980574.7
0.540
69.55
0.6531.330673.33.87
60
149.40.98320.9994563.3
0.561
66.17
0.4661.327266.75.38
80
355.20.97181.0023551.3
0.576
62.60
0.3551 .323060.86.57
100
760.00.95831 .0070538.9
0.585
58.84
0.2821.318055.3
Temperature (0C)
Ice
Vapor pressure (mm Hg)Heat of fusion (cal/g)Heat of sublimation (cal/g)Density (g/cm3)Specific heat (cal/g 0C)Coefficient of thermal
expansion x 1 0~5
Heat capacity (joule/g)
O
4.5879.8677.80.91680.4873
9.2
2.06
-5
3.01
0.9171
7.1
-10
1.95
672.30.91750.4770
5.5
-15
1.24
0.9178
4.4
-20
0.77
666.70.91820.4647
3.9
1.94
-25
0.47
0.9185
3.6
-30
0.28
662.30.91880.4504
3.5
Figure 1-1 Structure of the Water Molecule
of 105 degrees, and the distance between thenuclei of hydrogen and oxygen is 0.0957 nm.The water molecule can be considered aspherical quadrupole with a diameter of0.276 nm, where the oxygen nucleus formsthe center of the quadrupole. The two nega-tive and two positive charges form the anglesof a regular tetrahedron. Because of the sepa-ration of charges in a water molecule, theattraction between neighboring molecules ishigher than is normal with van der Waals'forces.
that water has unusually high values for cer-tain physical constants, such as meltingpoint, boiling point, heat capacity, latent heatof fusion, latent heat of vaporization, surfacetension, and dielectric constant. Some ofthese values are listed in Table 1-3.
Water may influence the conformation ofmacromolecules if it has an effect on any ofthe noncovalent bonds that stabilize the con-formation of the large molecule (Klotz1965). These noncovalent bonds may be oneof three kinds: hydrogen bonds, ionic bonds,or apolar bonds. In proteins, competitionexists between interamide hydrogen bondsand water-amide hydrogen bonds. Accordingto Klotz (1965), the binding energy of suchbonds can be measured by changes in thenear-infrared spectra of solutions in TV-meth-ylacetamide. The greater the hydrogen bond-ing ability of the solvent, the weaker theC=O-H-N bond. In aqueous solvents theheat of formation or disruption of this bondis zero. This means that a C=O-H-N hydro-gen bond cannot provide stabilization inaqueous solutions. The competitive hydro-gen bonding by H2O lessens the thermody-namic tendency toward the formation ofinteramide hydrogen bonds.
The water molecules around an apolar sol-ute become more ordered, leading to a lossin entropy. As a result, separated apolargroups in an aqueous environment tend to
Table 1-3 Physical Properties of SomeHydrides
In ice, every H2O molecule is bound by foursuch bridges to each neighbor. The bindingenergy of the hydrogen bond in ice amountsto 5 kcal per mole (Pauling 1960). Similarstrong interactions occur between OH andNH and between small, strongly electronega-tive atoms such as O and N. This is the rea-son for the strong association in alcohols,fatty acids, and amines and their great affin-ity to water. A comparison of the propertiesof water with those of the hydrides of ele-ments near oxygen in the Periodic Table(CH4, NH3, HF, DH3, H2S, HCl) indicates
Sub-stance
CH4
NH3
HFH2O
MeltingPoint(0C)
-184-78-92
O
BoilingPoint(0C)
-161-33+ 19+100
Molar Heat ofVaporization(cal/mole)
2,2005,5507,2209,750
associate with each other rather than with thewater molecules. This concept of a hydro-phobic bond has been schematically repre-sented by Klotz (1965), as shown in Figure1-2. Under appropriate conditions apolarmolecules can form crystalline hydrates, inwhich the compound is enclosed within thespace formed by a polyhedron made up ofwater molecules. Such polyhedrons can forma large lattice, as indicated in Figure 1-3.The polyhedrons may enclose apolar guestmolecules to form apolar hydrates (Speedy1984). These pentagonal polyhedra of watermolecules are unstable and normally changeto liquid water above O0C and to normal hex-agonal ice below O0C. In some cases, thehydrates melt well above 3O0C. There is aremarkable similarity between the smallapolar molecules that form these clathrate-like hydrates and the apolar side chains ofproteins. Some of these are shown in Figure1-4. Because small molecules such as theones shown in Figure 1-4 can form stablewater cages, it may be assumed that some of
the apolar amino acid side chains in apolypeptide can do the same. The concentra-tion of such side chains in proteins is high,and the combined effect of all these groupscan be expected to result in the formation ofa stabilized and ordered water region aroundthe protein molecule. Klotz (1965) has sug-gested the term hydrotactoids for these struc-tures (Figure 1-5).
SORPTION PHENOMENA
Water activity, which is a property of aque-ous solutions, is defined as the ratio of thevapor pressures of pure water and a solution:
whereaw = water activity
p = partial pressure of water in a food
po = vapor pressure of water at the sametemperature
According to Raoult's law, the lowering ofthe vapor pressure of a solution is propor-tional to the mole fraction of the solute: aw
can then be related to the molar concentra-tions of solute (n{) and solvent (n2):
= L = HI
"W Po ni+n2
The extent to which a solute reduces aw is afunction of the chemical nature of the solute.The equilibrium relative humidity (ERH) inpercentage is ERH/100. ERH is defined as:
equ
ERH = "—sat
P
where
Figure 1-2 Schematic Representation of theFormation of a Hydrophobia Bond by ApolarGroup in an Aqueous Environment. Open cir-cles represent water. Source: From LM. Klotz,Role of Water Structure in Macromolecules,Federation Proceedings, Vol. 24, Suppl. 15, pp.S24-S33, 1965.
Figure 1-4 Comparison of Hydrate-Forming Molecules and Amino Acid Apolar Side Chains. Source:From LM. Klotz, Role of Water Structure in Macromolecules, Federation Proceedings, Vol. 24, Suppl.15, pp. S24-S33, 1965.
Crystal Hydrate Formers Amlno Acid Side Chains
(Ala)
(VaI)
(Leu)
(Cys)
(Met)
(Phe)
Figure 1-3 Crytalline Apolar Polyhedrons Forming a Large Lattice. The space within the polyhedronsmay enclose apolar molecules. Source: From LM. Klotz, Role of Water Structure in Macromolecules,Federation Proceedings, Vol. 24, Suppl. 15, pp. S24-S33, 1965.
Figure 1-5 Hydrotactoid Formation AroundApolar Groups of a Protein. Source: From LM.Klotz, Role of Water Structure in Macromole-cules, Federation Proceedings, Vol. 24, Suppl.15, pp. S24-S33, 1965.
pequ- partial pressure of water vapor inequilibrium with the food at temper-ature T and 1 atmosphere total pres-sure
psat = the saturation partial pressure ofwater in air at the same temperatureand pressure
At high moisture contents, when theamount of moisture exceeds that of solids,the activity of water is close to or equal to1.0. When the moisture content is lower thanthat of solids, water activity is lower than1.0, as indicated in Figure 1-6. Below mois-ture content of about 50 percent the wateractivity decreases rapidly and the relation-ship between water content and relativehumidity is represented by the sorption iso-therms. The adsorption and desorption pro-cesses are not fully reversible; therefore, a
MOISTURE CONTENT g/g solids
Figure 1-6 Water Activity in Foods at DifferentMoisture Contents
distinction can be made between the adsorp-tion and desorption isotherms by determin-ing whether a dry product's moisture levelsare increasing, or whether the product'smoisture is gradually lowering to reach equi-librium with its surroundings, implying thatthe product is being dried (Figure 1-7). Gen-erally, the adsorption isotherms are requiredfor the observation of hygroscopic products,
REL. HUM. %Figure 1-7 Adsorption and Desorption Iso-therms
desorption
adsorptionMO
IST
UR
E
%RE
LATIV
E HU
MIDI
TY %
and the desorption isotherms are useful forinvestigation of the process of drying. Asteeply sloping curve indicates that the mate-rial is hygroscopic (curve A, Figure 1-8); aflat curve indicates a product that is not verysensitive to moisture (curve B, Figure 1-8).Many foods show the type of curves given inFigure 1-9, where the first part of the curveis quite flat, indicating a low hygroscopicity,and the end of the curve is quite steep, indi-cating highly hygroscopic conditions. Suchcurves are typical for foods with high sugaror salt contents and low capillary adsorption.Such foods are hygroscopic. The reverse ofthis type of curve is rarely encountered.These curves show that a hygroscopic prod-uct or hygroscopic conditions can be definedas the case where a small increase in relativehumidity causes a large increase in productmoisture content.
Sorption isotherms usually have a sigmoidshape and can be divided into three areas thatcorrespond to different conditions of thewater present in the food (Figure 1-7). The
REL HUM. %
Figure 1-9 Sorption Isotherms for Foods withHigh Sugar or Salt Content; Low CapillaryAdsorption
first part (A) of the isotherm, which is usu-ally steep, corresponds to the adsorption of amonomolecular layer of water; the second,flatter part (B) corresponds to adsorption ofadditional layers of water; and the third part(C) relates to condensation of water in capil-laries and pores of the material. There are nosharp divisions between these three regions,and no definite values of relative humidityexist to delineate these parts. Labuza (1968)has reviewed the various ways in which theisotherms can be explained. The kineticapproach is based on the Langmuir equation,which was initially developed for adsorptionof gases and solids. This can be expressed inthe following form:
a _ r K -| _a_? = TO + ^
wherea = water activityb = a constant
MO
ISTU
RE
%
REL HUM. %
Figure 1-8 Sorption Isotherms of HygroscopicProduct (A) and Nonhygroscopic Product (B)
MO
ISTU
RE
%
K = l/p0 and p0 = vapor pressure of waterat T0
V = volume adsorbedVm = monolayer value
When alV is plotted versus a, the result is astraight line with a slope equal to l/Vm andthe monolayer value can be calculated. Inthis form, the equation has not been satisfac-tory for foods, because the heat of adsorptionthat enters into the constant b is not constantover the whole surface, because of interac-tion between adsorbed molecules, andbecause maximum adsorption is greater thanonly a monolayer.
A form of isotherm widely used for foodsis the one described by Brunauer et al.(1938) and known as the BET isotherm orequation. A form of the BET equation givenby Labuza (1968) is
a = J_ , F a ( C - I ) I( l - a ) V V1nC
+ V VmC J
whereC = constant related to the heat of adsorp-
tion
A plot of a/(I - a) V versus a gives a straightline, as indicated in Figure 1-10. The mono-layer coverage value can be calculated fromthe slope and the intercept of the line. TheBET isotherm is only applicable for values ofa from 0.1 to 0.5. In addition to monolayercoverage, the water surface area can be calcu-lated by means of the following equation:
S° = Vm'M^>'N°'Att>°
= 3.5 XlO3V1n
whereS0 = surface area, m2/g solidM H Q = molecular weight of water, 18N0 = Avogadro's number, 6 x 1023
^H9O = area of water molecule, 10.6 x1020m2
The BET equation has been used in manycases to describe the sorption behavior offoods. For example, note the work of Sarava-cos (1967) on the sorption of dehydratedapple and potato. The form of BET equationused for calculation of the monolayer valuewas
p I C-I PO
W(P0^p) ~ W 1 C + W 1 C ' P
whereW = water content (in percent)p = vapor pressure of sampleP0 = vapor pressure of water at same tem-
peratureC = heat of adsorption constantW1 = moisture consent corresponding to
monolayer
The BET plots obtained by Saravacos fordehydrated potato are presented in Figure1-11.
Other approaches have been used to ana-lyze the sorption isotherms, and these aredescribed by Labuza (1968). However, theLangmuir isotherm as modified by Brunaueret al. (1938) has been most widely used withfood products. Another method to analyzethe sorption isotherms is the GAB sorptionmodel described by van den Berg and Bruin(1981) and used by Roos (1993) and Joup-pila and Roos (1994).
As is shown in Figure 1-7, the adsorptionand desorption curves are not identical. Thehysteresis effect is commonly observed; note,
for example, the sorption isotherms of wheatflour as determined by Bushuk and Winkler(1957) (Figure 1-12). The hysteresis effect isexplained by water condensing in the capil-
laries, and the effect occurs not only in regionC of Figure 1-7 but also in a large part ofregion B. The best explanation for this phe-nomenon appears to be the so-called ink bot-
Figure 1-11 BET Plots for Dehydrated Potato. Source: From G.D. Saravacos, Effect of the DryingMethod on the Water Sorption of Dehydrated Apple and Potato, / Food ScL, Vol. 32, pp. 81-84, 1967.
100-&- (%R.H.)Ko
FREEZE-DRIED
PUFF-DRIEDAIR-DRIED
10OpW(P0-P)
Figure 1-10 BET Monolayer Plot. Source'. From TP. Labuza, Sorption Phenomena in Foods, FoodTechnoL, Vol. 22, pp. 263-272, 1968.
Q( l - a ) V
intercept
slope
0.5
. _ !
" C V m
C - I" C V m
tie theory (Labuza 1968). It is assumed thatthe capillaries have narrow necks and largebodies, as represented schematically in Fig-ure 1-13. During adsorption the capillarydoes not fill completely until an activity isreached that corresponds to the large radiusR. During desorption, the unfitting is con-trolled by the smaller radius r, thus loweringthe water activity. Several other theories havebeen advanced to account for the hysteresisin sorption. These have been summarized byKapsalis (1987).
The position of the sorption isothermsdepends on temperature: the higher the tem-perature, the lower the position on the graph.
This decrease in the amount adsorbed athigher temperatures follows the ClausiusClapeyron relationship,
d(lna) _ _Qsd(l/T) ~~ ~~R
whereQ8 = heat of adsorption
P/Po
Figure 1-12 Sorption Isotherms of Wheat Flour, Starch, and Gluten. Source: From W. Bushuk andC.A. Winkler, Sorption of Water Vapor on Wheat Flour, Starch and Gluten, Cereal Chem., Vol. 34, pp.73-86, 1957.
FREEZE-DRIEDGLUTEN
SPRAY-DRIEDGLUTEN
STARCHFLOUR
X(MG
XG)
Figure 1-13 Ink Bottle Theory of Hysteresis inSorption. Source: From T.P. Labuza, SorptionPhenomena in Foods, Food TechnoL, Vol. 22,pp. 263-272, 1968.
R = gas constantT = absolute temperature
By plotting the natural logarithm of activityversus the reciprocal of absolute tempera-ture at constant moisture values, straightlines are obtained with a slope of -QJR(Figure 1-14). The values of <2S obtained inthis way for foods having less than fullmonolayer coverage are between about
2,000 and 10,000 cal per mole, demonstrat-ing the strong binding of this water.
According to the principle of BET iso-therm, the heat of sorption Qx should be con-stant up to monolayer coverage and thenshould suddenly decrease. Labuza (1968)has pointed out that the latent heat of vapor-ization Af/v, about 10.4 kcal per mole, shouldbe added to obtain the total heat value. Theplot representing BET conditions as well asactual findings are given in Figure 1-15. Theobserved heat of sorption at low moisturecontents is higher than theory indicates andfalls off gradually, indicating the gradualchange from Langmuir to capillary water.
TYPES OF WATER
The sorption isotherm indicates that differ-ent forms of water may be present in foods.It is convenient to divide the water into threetypes: Langmuir or monolayer water, capil-lary water, and loosely bound water. Thebound water can be attracted strongly andheld in a rigid and orderly state. In this form
In (
AC
TIV
ITY
)
VTFigure 1-14 Method for Determination of Heat of Adsorption. Moisture content increases from M1 toM5. Source: From T.P. Labuza, Sorption Phenomena in Foods, Food Technol, Vol. 22, pp. 263-272,1968.
the water is unavailable as a solvent and doesnot freeze. It is difficult to provide a rigiddefinition of bound water because muchdepends on the technique used for its mea-surement. Two commonly used definitionsare as follows:
1. Bound water is the water that remainsunfrozen at some prescribed tempera-ture below O0C, usually -2O0C.
2. Bound water is the amount of water in asystem that is unavailable as a solvent.
The amount of unfreezable water, based onprotein content, appears to vary only slightlyfrom one food to another. About 8 to 10 per-cent of the total water in animal tissue isunavailable for ice formation (Meryman1966). Egg white, egg yolk, meat, and fishall contain approximately 0.4 g of unfreez-able water per g of dry protein. This corre-
sponds to 11.4 percent of total water in leanmeat. Most fruits and vegetables contain lessthan 6 percent unfreezable water; wholegrain corn, 34 percent.
The free water is sometimes determined bypressing a food sample between filter paper,by diluting with an added colored substance,or by centrifugation. None of these methodspermits a distinct division between free andbound water, and results obtained are not nec-essarily identical between methods. This isnot surprising since the adsorption isothermindicates that the division between the differ-ent forms of water is gradual rather thansharp. A promising new method is the use ofnuclear magnetic resonance, which can beexpected to give results based on the freedomof movement of the hydrogen nuclei.
The main reason for the increased watercontent at high values of water activity mustbe capillary condensation. A liquid with sur-
Figure 1-15 Relationship of Heat of Sorption and Moisture Content as Actually Observed and Accord-ing to BET Theory. Source: From TR Labuza, Sorption Phenomena in Foods, Food TechnoL, Vol. 22,pp. 263-272,1968.
MOISTURE %
Vm
BET
observed
HE
AT
OF
SO
RP
TIO
N
face tension a in a capillary with radius r issubject to a pressure loss, the capillary pres-sure p0 = 2a/r, as evidenced by the rising ofthe liquid in the capillary. As a result, there isa reduction in vapor pressure in the capillary,which can be expressed by the Thomsonequation,
!„£.-_ 22. J lP0 ~ r RT
wherep = vapor pressure of liquidP0 = capillary vapor pressurea = surface tensionV = mole volume of liquidR = gas constantT = absolute temperature
This permits the calculation of water activityin capillaries of different radii, as indicatedin Table 1-4. In water-rich organic foods,such as meat and potatoes, the water ispresent in part in capillaries with a radius of1 (urn or more. The pressure necessary toremove this water is small. Calculated valuesof this pressure are given in Table 1-5 forwater contained in capillaries ranging from0.1 |im to 1 mm radius. It is evident thatwater from capillaries of 0.1 |0,m or largercan easily drip out. Structural damagecaused, for instance, by freezing can easilyresult in drip loss in these products. The factthat water serves as a solvent for many sol-utes such as salts and sugars is an additionalfactor in reducing the vapor pressure.
The caloric behavior of water has beenstudied by Riedel (1959), who found thatwater in bread did not freeze at all whenmoisture content was below 18 percent (Fig-ure 1-16). With this method it was possibleto determine the nonfreezable water. Forbread, the value was 0.30 g per g dry matter,
Table 1-4 Capillary Radius and Water Activity
Radius (nm) Activity (a)
O5 0.1161 0.3402 0.5835 0.806
10 0.89820 0.94850 0.979
100 0.9891000 0.999
and for fish and meat, 0.40 g per g protein.The nonfreezable and Langmuir water areprobably not exactly the same. Wierbicki andDeatherage (1958) used a pressure method todetermine free water in meat. The amount offree water in beef, pork, veal, and lamb var-ies from 30 to 50 percent of total moisture,depending on the kind of meat and the periodof aging, A sharp drop in bound water occursduring the first day after slaughter, and is fol-lowed by a gradual, slight increase. Hammand Deatherage (196Ob) determined thechanges in hydration during the heating ofmeat. At the normal pH of meat there is aconsiderable reduction of bound water.
Table 1-5 Pressure Required To Press Waterfrom Tissue at 2O0C
Radius Pressure (kg/cm2)
0.1 jim 14.841 ^m 1.48410 ̂ m 0.1480.1 mm 0.01481 mm 0.0015
FREEZING AND ICE STRUCTURE
A water molecule may bind four others ina tetrahedral arrangement. This results in ahexagonal crystal lattice in ice, as shown inFigure 1-17. The lattice is loosely built andhas relatively large hollow spaces; thisresults in a high specific volume. In thehydrogen bonds, the hydrogen atom is 0.1nm from one oxygen atom and 0.176 nm
from another hydrogen atom. When icemelts, some of the hydrogen bonds are bro-ken and the water molecules pack togethermore compactly in a liquid state (the averageligancy of a water molecule in water is about5 and in ice, 4). There is some structural dis-order in the ice crystal. For each hydrogenbond, there are two positions for the hydro-gen atom: O-H+O and O+H-O. Withoutrestrictions on the disorder, there would be4^ ways of arranging the hydrogen atoms inan ice crystal containing TV water molecules(2N hydrogen atoms). There is one restric-tion, though: there must be two hydrogenatoms near each oxygen atom. As a resultthere are only (3/2)^ ways of arranging thehydrogen atoms in the crystal.
The phase diagram (Figure 1-18) indicatesthe existence of three phases: solid, liquid,and gas. The conditions under which theyexist are separated by three equilibriumlines: the vapor pressure line TA, the meltingpressure line TC, and the sublimation pres-sure line BT. The three lines meet at point T,
Figure 1-17 Hexagonal Pattern of the LatticeStructure in Ice
TEMPERATURE 0C
Figure 1-16 Specific Heat of Bread of Different Water Contents (Indicated as %) as a Function ofTemperature. Source: From L. Riedel, Calorimetric Studies of the Freezing of White Bread and OtherFlour Products, Kdltetechn, Vol. 11, pp. 41-46, 1959.
SP
EC
. H
EA
T C
al/
g.°
C
TEMPERATURE 0C
Figure 1-18 Phase Diagram of Water
where all three phases are in equilibrium.Figure 1-18 shows that when ice is heated atpressures below 4.58 mm Hg, it changesdirectly into the vapor form. This is the basisof freeze drying.
It is possible to supercool water. When asmall ice crystal is introduced, the supercool-ing is immediately terminated and the tem-perature rises to O9C. Normally the presenceof a nucleus is required. Generally, nucleiform around foreign particles (heterogeneousnucleation). It is difficult to study homoge-neous nucleation. This has been studied inthe case of fat crystallization, by emulsifyingthe fat so that it is divided into a large num-ber of small volumes, with the chance of aglobule containing a heterogeneous nucleusbeing very small (Vanden-Tempel 1958). Ahomogeneous nucleus forms from the chanceagglomeration of water molecules in the iceconfiguration. Usually, such nuclei disinte-grate above a critical temperature. The prob-ability of such nuclei forming depends on thevolume of water; they are more likely to
form at higher temperature and in larger vol-umes. In ultrapure water, 1 mL can be super-cooled to -320C; droplets of 0.1 mm diame-ter to -350C; and droplets of 1 |im to -410Cbefore solidification occurs.
The speed of crystallization—that is, theprogress of the ice front in centimeters persecond—is determined by the removal of theheat of fusion from the area of crystalliza-tion. The speed of crystallization is low at ahigh degree of supercooling (Meryman1966). This is important because it affectsthe size of crystals in the ice. When largewater masses are cooled slowly, there is suf-ficient time for heterogeneous nucleation inthe area of the ice point. At that point thecrystallization speed is very large so that afew nuclei grow to a large size, resulting in acoarse crystalline structure. At greater cool-ing speed, high supercooling occurs; thisresults in high nuclei formation and smallergrowth rate and, therefore, a fine crystalstructure.
Upon freezing, HOH molecules associatein an orderly manner to form a rigid structurethat is more open (less dense) than the liquidform. There still remains considerable move-ment of individual atoms and molecules inice, particularly just below the freezingpoint. At 1O0C an HOH molecule vibrateswith an amplitude of approximately 0.044nm, nearly one-sixth the distance betweenadjacent HOH molecules. Hydrogen atomsmay wander from one oxygen atom toanother.
Each HOH molecule has four tetrahedrallyspaced attractive forces and is potentiallyable to associate by means of hydrogenbonding with four other HOH molecules. Inthis arrangement each oxygen atom isbonded covalently with two hydrogen atoms,each at a distance of 0.096 nm, and eachhydrogen atom is bonded with two other
vapor
solid
liquid
PR
ES
SU
RE
mm
Hg
hydrogen atoms, each at a distance of 0.18nm. This results in an open tetrahedral struc-ture with adjacent oxygen atoms spacedabout 0.276 nm apart and separated by singlehydrogen atoms. All bond angles are approx-imately 109 degrees (Figure 1-19).
Extension of the model in Figure 1-19leads to the hexagonal pattern of ice estab-lished when several tetrahedrons are assem-bled (Figure 1-17).
Upon change of state from ice to water,rigidity is lost, but water still retains a largenumber of ice-like clusters. The term ice-likecluster does not imply an arrangement iden-tical to that of crystallized ice. The HOHbond angle of water is several degrees lessthan that of ice, and the average distancebetween oxygen atoms is 0.31 nm in waterand 0.276 nm in ice. Research has not yetdetermined whether the ice-like clusters ofwater exist in a tetrahedral arrangement, asthey do in ice. Since the average intermolec-ular distance is greater than in ice, it follows
that the greater density of water must beachieved by each molecule having someneighbors. A cubic structure with each HOHmolecule surrounded by six others has beensuggested.
At OT!, water contains ice-like clustersaveraging 90 molecules per cluster. Withincreasing temperature, clusters becomesmaller and more numerous. At O0C, approx-imately half of the hydrogen bonds present at-1830C remain unbroken, and even at 10O0Capproximately one-third are still present. Allhydrogen bonds are broken when waterchanges into vapor at 10O0C. This explainsthe large heat of vaporization of water.
Crystal Growth and Nucleation
Crystal growth, in contrast to nucleation,occurs readily at temperatures close to thefreezing point. It is more difficult to initiatecrystallization than to continue it. The rate ofice crystal growth decreases with decreasingtemperature. A schematic graphical repre-sentation of nucleation and crystal growthrates is given in Figure 1-20. Solutes ofmany types and in quite small amounts willgreatly slow ice crystal growth. The mecha-nism of this action is not known. Membranesmay be impermeable to ice crystal growthand thus limit crystal size. The effect ofmembranes on ice crystal propagation wasstudied by Lusena and Cook (1953), whofound that membranes freely permeable toliquids may be either permeable, partly per-meable, or impermeable to growing ice crys-tals. In a given material, permeability to icecrystal growth increases with porosity, but isalso affected by rate of cooling, membranecomposition and properties, and concentra-tion of the solute(s) present in the aqueousphase. When ice crystal growth is retardedby solutes, the ice phase may become dis-
Figure 1-19 Hydrogen Bonded Arrangement ofWater Molecules in Ice
Oxygen
Hydrogen
Hydrogen bond
Chemicol bond
FP = temperature at which crystals start to form.
Figure 1-20 Schematic Representation of theRate of Nucleation and Crystal Growth
continuous either by the presence of a mem-brane or spontaneously.
Ice crystal size at the completion of freez-ing is related directly to the number of nuclei.The greater the number of nuclei, the smallerthe size of the crystals. In liquid systemsnuclei can be added. This process is knownas seeding. Practical applications of seedinginclude adding finely ground lactose to evap-orated milk in the evaporator, and recirculat-ing some portion of crystallized fat in a heatexchanger during manufacture of margarine.If the system is maintained at a temperatureclose to the freezing point (FP), where crys-tallization starts (Figure 1-20), only a fewnuclei form and each crystal grows exten-sively. The slow removal of heat energy pro-duces an analogous situation, since the heatof crystallization released by the few grow-ing crystals causes the temperature to remainnear the melting point, where nucleation isunlikely. In tissue or unagitated fluid sys-tems, slow removal of heat results in a con-tinuous ice phase that slowly moves inward,with little if any nucleation. The effect of
temperature on the linear crystallizationvelocity of water is given in Table 1-6.
If the temperature is lowered to below theFP (Figure 1-20), crystal growth is the pre-dominant factor at first but, at increasingrate of supercooling, nucleation takes over.Therefore, at low supercooling large crys-tals are formed; as supercooling increases,many small crystals are formed. Control ofcrystal size is much more difficult in tissuesthan in agitated liquids. Agitation may pro-mote nucleation and, therefore, reducedcrystal size. Lusena and Cook (1954) sug-gested that large ice crystals are formedwhen freezing takes place above the criticalnucleation temperature (close to FP in Fig-ure 1-20). When freezing occurs at the crit-ical nucleation temperature, small ice cry-stals form. The effect of solutes on nucle-ation and rate of ice crystal growth is amajor factor controlling the pattern of prop-agation of the ice front. Lusena and Cook(1955) also found that solutes depress thenucleation temperature to the same extentthat they depress the freezing point. Solutesretard ice growth at 1O0C supercooling, withorganic compounds having a greater effectthan inorganic ones. At low concentrations,
Table 1-6 Effect of Temperature on Linear Crys-tallization Velocity of Water
Temperature at Onset Linear Crystallizationof Crystallization (0C) Velocity (mm/min)^09 230-1.9 520-2.0 580-2.2 680-3.5 1,220-5.0 1,750-7.0 2,800
SUPERCOOLING
NUCLEATION
CRYSTALGROWTH
RATE
proteins are as effective as alcohols andsugars in retarding crystal growth.
Once formed, crystals do not remainunchanged during frozen storage; they havea tendency to enlarge. Recrystallization isparticularly evident when storage tempera-tures are allowed to fluctuate widely. Thereis a tendency for large crystals to grow at theexpense of small ones.
Slow freezing results in large ice crystalslocated exclusively in extracellular areas.Rapid freezing results in tiny ice crystalslocated both extra- and intracellularly. Nottoo much is known about the relationbetween ice crystal location and frozen foodquality. During the freezing of food, water istransformed to ice with a high degree ofpurity, and solute concentration in the unfro-zen liquid is gradually increased. This isaccompanied by changes in pH, ionicstrength, viscosity, osmotic pressure, vaporpressure, and other properties.
When water freezes, it expands nearly 9percent. The volume change of a food that isfrozen will be determined by its water con-tent and by solute concentration. Highly con-centrated sucrose solutions do not showexpansion (Table 1-7). Air spaces may par-tially accommodate expanding ice crystals.Volume changes in some fruit products uponfreezing are shown in Table 1-8. The effectof air space is obvious. The expansion ofwater on freezing results in local stresses thatundoubtedly produce mechanical damage incellular materials. Freezing may causechanges in frozen foods that make the prod-uct unacceptable. Such changes may includedestabilization of emulsions, flocculation ofproteins, increase in toughness of fish flesh,loss of textural integrity, and increase in driploss of meat. Ice formation can be influencedby the presence of carbohydrates. The ef-fect of sucrose on the ice formation process
Table 1-7 Volume Change of Water and SucroseSolutions on Freezing
Volume Increase DuringTemperature Change
Sucrose (%) from 70 0F to O 0F (%)
~ o a s10 8.720 8.230 6.240 5.150 3.960 None70 -1.0 (decrease)
has been described by Roos and Karel(1991a,b,c).
The Glass Transition
In aqueous systems containing polymericsubstances or some low molecular weightmaterials including sugars and other carbo-hydrates, lowering of the temperature mayresult in formation of a glass. A glass is anamorphous solid material rather than a crys-talline solid. A glass is an undercooled liquid
Table 1-8 Expansion of Fruit Products DuringFreezing
Volume IncreaseDuring Tempera-ture Change from
Product 70 0F to O 0F (%)
Apple juice 8.3Orange juice 8.0Whole raspberries 4.0Crushed raspberries 6.3Whole strawberries 3.0Crushed strawberries 8.2
of high viscosity that exists in a metastablesolid state (Levine and Slade 1992). A glassis formed when a liquid or an aqueous solu-tion is cooled to a temperature that is consid-erably lower than its melting temperature.This is usually achieved at high coolingrates. The normal process of crystallizationinvolves the conversion of a disordered liq-uid molecular structure to a highly orderedcrystal formation. In a crystal, atoms or ionsare arranged in a regular, three-dimensionalarray. In the formation of a glass, the disor-dered liquid state is immobilized into a disor-dered glassy solid, which has the rheologicalproperties of a solid but no ordered crystal-line structure.
The relationships among melting point(Tm), glass transition temperature (Tg), andcrystallization are schematically representedin Figure 1-21. At low degree of supercool-ing (just below Tm), nucleation is at a mini-mum and crystal growth predominates. Asthe degree of supercooling increases, nucle-ation becomes the dominating effect. Themaximum overall crystallization rate is at a
point about halfway between Tm and Tg. Athigh cooling rates and a degree of supercool-ing that moves the temperature to below Tg,no crystals are formed and a glassy solidresults. During the transition from the moltenstate to the glassy state, the moisture contentplays an important role. This is illustrated bythe phase diagram of Figure 1-22. When thetemperature is lowered at sufficiently highmoisture content, the system goes through arubbery state before becoming glassy (Chir-ife and Buera 1996). The glass transitiontemperature is characterized by very highapparent viscosities of more than 105 Ns/m2
(Aguilera and Stanley 1990). The rate of dif-fusion limited processes is more rapid in therubbery state than in the glassy state, and thismay be important in the storage stability ofcertain foods. The effect of water activity onthe glass transition temperature of a numberof plant products (carrots, strawberries, andpotatoes) as well as some biopolymers (gela-tin, wheat gluten, and wheat starch) is shownin Figure 1-23 (Chirife and Buera 1996). Inthe rubbery state the rates of chemical reac-
CrystalNucleation
CrystalGrowth
CrystallizationRate
glass
liquid
ice + liquidTEMP
ERAT
URE 0
C
Figure 1-21 Relationships Among CrystalGrowth, Nucleation, and Crystallization Ratebetween Melting Temperature (Tm) and GlassTemperature (Tg)
Figure 1-22 Phase Diagram Showing the Effectof Moisture Content on Melting Temperature(Tm) and Glass Transition Temperature (Tg)
CONCENTRATION %
tion appear to be higher than in the glassystate (Roos and Karel 199Ie).
When water-containing foods are cooledbelow the freezing point of water, ice may beformed and the remaining water is increas-ingly high in dissolved solids. When theglass transition temperature is reached, theremaining water is transformed into a glass.Ice formation during freezing may destabi-lize sensitive products by rupturing cell wallsand breaking emulsions. The presence ofglass-forming substances may help preventthis from occurring. Such stabilization offrozen products is known as cryoprotection,and the agents are known as cryoprotectants.
When water is rapidly removed from foodsduring processes such as extrusion, drying,or freezing, a glassy state may be produced(Roos 1995). The Tg values of high molecu-
lar weight food polymers, proteins, andpolysaccharides are high and cannot bedetermined experimentally, because of ther-mal decomposition. An example of measuredTg values for low molecular weight carbohy-drates is given in Figure 1-24. The value ofTg for starch is obtained by extrapolation.
The water present in foods may act as aplasticizer. Plasticizers increase plasticityand flexibility of food polymers as a result ofweakening of the intermolecular forces exist-ing between molecules. Increasing watercontent decreases Tg. Roos and Karel (199Ia)studied the plasticizing effect of water onthermal behavior and crystallization of amor-phous food models. They found that driedfoods containing sugars behave like amor-phous materials, and that small amounts ofwater decrease Tg to room temperature with
Wheat StarchGelatin
Wheat Gluten
Strawberry
Potatoes
Carrots
Gla
ss T
ran
sitio
n T
emp
., C
Water activity
Figure 1-23 Relationship Between Water Activity (aw) and Glass Transition Temperature (Tg) ofSome Plant Materials and Biopolymers. Source: Reprinted with permission from J. Cherife and M. delPinar Buera, Water Activity, Water Glass Dynamics and the Control of Microbiological Growth inFoods, Critical Review Food ScL Nutr., Vol. 36, No. 5, p. 490, © 1996. Copyright CRC Press, BocaRaton, Florida.
Figure 1-24 Glass Transition Temperature (Tg)for Maltose, Maltose Polymers, and Extrapo-lated Value for Starch. M indicates molecularweight. Source: Reprinted with permission fromY.H. Roos, Glass Transition-Related Physico-Chemical Changes in Foods, Food Technology,Vol. 49, No. 10, p. 98, © 1995, Institute of FoodTechnologists.
the result of structural collapse and formationof stickiness, Roos and Karel (199Ie) report alinearity between water activity (aw) and Tg inthe aw range of 0.1 to 0.8. This allows predic-tion of Tg at the aw range typical of dehy-drated and intermediate moisture foods.
Roos (1995) has used a combined sorptionisotherm and state diagram to obtain criticalwater activity and water content values thatresult in depressing Tg to below ambienttemperature (Figure 1-25). This type of plotcan be used to evaluate the stability of low-moisture foods under different storage condi-tions. When the T is decreased to belowambient temperature, molecules are mobi-lized because of plasticization and reactionrates increase because of increased diffusion,which in turn may lead to deterioration. Roosand Himberg (1994) and Roos et al. (1996)have described how glass transition tempera-tures influence nonenzymatic browning inmodel systems. This deteriorative reaction
WATER ACTIVITY
Figure 1-25 Modified State Diagram ShowingRelationship Between Glass Transition Temper-ature (Tg), Water Activity (GAB isotherm), andWater Content for an Extruded Snack FoodModel. Crispness is lost as water plasticizationdepresses Tg to below 240C. Plasticization isindicated with critical values for water activityand water content. Source: Reprinted with per-mission from Y.H. Roos, Glass Transition-Related Physico-Chemical Changes in Foods,Food Technology, Vol. 49, No. 10, p. 99, ©1995, Institute of Food Technologists.
showed an increased reaction rate as watercontent increased.
Water Activity and Reaction Rate
Water activity has a profound effect on therate of many chemical reactions in foods andon the rate of microbial growth (Labuza1980). This information is summarized inTable 1-9. Enzyme activity is virtually non-existent in the monolayer water (aw betweenO and 0.2). Not surprisingly, growth ofmicroorganisms at this level of aw is also vir-tually zero. Molds and yeasts start to grow ataw between 0.7 and 0.8, the upper limit ofcapillary water. Bacterial growth takes placewhen aw reaches 0.8, the limit of loosely
TEMP
ERAT
URE
(0C)
Starch
Maltohexaosa
Maltotriose
Maltose
Tg Curve GAB Isotherm
1/M
TEMP
ERAT
URE (
0C)
WATE
R CO
NTEN
T (g/
100 g
of S
olids
)
bound water. Enzyme activity increasesgradually between aw of 0.3 and 0.8, thenincreases rapidly in the loosely bound waterarea (aw 0.8 to 1.0). Hydrolytic reactions andnonenzymic browning do not proceed in themonolayer water range of aw (0.0 to 0.25).However, lipid oxidation rates are high inthis area, passing from a minimum at aw 0.3to 0.4, to a maximum at aw 0.8. The influ-
ence of aw on chemical reactivity has beenreviewed by Leung (1987). The relation-ship between water activity and rates of sev-eral reactions and enzyme activity is pre-sented graphically in Figure 1-26 (Bone1987).
Water activity has a major effect on thetexture of some foods, as Bourne (1986) hasshown in the case of apples.
Water Activity (% R.H.)
Figure 1-26 Relationship Between Water Activity and a Number of Reaction Rates. Source:Reprinted with permission from D.R Bone, Practical Applications of Water Activity and MoistureRelations in Foods, in Water Activity: Theory and Application to Food, L.B. Rockland and L.R.Beuchat, eds., p. 387, 1987, by courtesy of Marcel Dekker, Inc.
Free Fatty Acids
Mo
istu
re C
on
ten
tR
elat
ive
Act
ivit
y
Autox
ldatlo
n
Micro
organ
ismPr
olifer
ation
Free(Solute A Capillary)
CovalentIonic
StabilityIsotherm
BrowningReaction
AmlhocyanwnDegradation
Table 1-9 Reaction Rates in Foods as Determined by Water Activity
Reaction
Enzyme activityMold growthYeast growthBacterial growthHydrolysisNonenzymic browningLipid oxidation
MonolayerWater
ZeroZeroZeroZeroZeroZeroHigh
Capillary Water
LowLow*Low*Zero
Rapid increaseRapid increaseRapid increase
Loosely BoundWater
HighHighHighHighHighHighHigh
'Growth starts at aw of 0.7 to 0.8.
WATER ACTIVITY AND FOODSPOILAGE
The influence of water activity on foodquality and spoilage is increasingly beingrecognized as an important factor (Rocklandand Nishi 1980). Moisture content and wateractivity affect the progress of chemical andmicrobiological spoilage reactions in foods.Dried or freeze-dried foods, which havegreat storage stability, usually have watercontents in the range of about 5 to 15 per-cent. The group of intermediate-moisturefoods, such as dates and cakes, may havemoisture contents in the range of about 20 to40 percent. The dried foods correspond tothe lower part of the sorption isotherms. Thisincludes water in the monolayer and multi-layer category. Intermediate-moisture foodshave water activities generally above 0.5,including the capillary water. Reduction of
water activity can be obtained by drying orby adding water-soluble substances, such assugar to jams or salt to pickled preserves.Bacterial growth is virtually impossiblebelow a water activity of 0.90. Molds andyeasts are usually inhibited between 0.88 and0.80, although some osmophile yeast strainsgrow at water activities down to 0.65.
Most enzymes are inactive when the wateractivity falls below 0.85. Such enzymesinclude amylases, phenoloxidases, and per-oxidases. However, lipases may remainactive at values as low as 0.3 or even 0.1(Loncin et al. 1968). Acker (1969) providedexamples of the effect of water activity onsome enzymic reactions. A mixture ofground barley and lecithin was stored at dif-ferent water activities, and the rates ofhydrolysis were greatly influenced by thevalue of a (Figure 1-27). When the lower avalues were changed to 0.70 after 48 days of
HYDR
OLYS
IS, %
STORAGE TIME, DAYS
Figure 1-27 Enzymic Splitting of Lecithin in a Mixture of Barley Malt and Lecithin Stored at 3O0Cand Different Water Activities. Lower aw values were changed to 0.70 after 48 days. Source: From L.Acker, Water Activity and Enzyme Activity, Food Technol, Vol. 23, pp. 1257-1270, 1969.
storage the rates rapidly went up. In theregion of monomolecular adsorption, enzy-mic reactions either did not proceed at all orproceeded at a greatly reduced rate, whereasin the region of capillary condensation thereaction rates increased greatly. Acker foundthat for reactions in which lipolytic enzymeactivity was measured, the manner in whichcomponents of the food system were put intocontact significantly influenced the enzymeactivity. Separation of substrate and enzymecould greatly retard the reaction. Also, thesubstrate has to be in liquid form; for exam-ple, liquid oil could be hydrolyzed at wateractivity as low as 0.15, but solid fat was onlyslightly hydrolyzed. Oxidizing enzymeswere affected by water activity in about thesame way as hydrolytic enzymes, as wasshown by the example of phenoloxidasefrom potato (Figure 1-28). When the lower avalues were increased to 0.70 after 9 days ofstorage, the final values were lower than with
the sample kept at 0.70 all through the exper-iment, because the enzyme was partiallyinactive during storage.
Nonenzymic browning or Maillard reac-tions are one of the most important factorscausing spoilage in foods. These reactionsare strongly dependent on water activity andreach a maximum rate at a values of 0.6 to0.7 (Loncin et al. 1968). This is illustrated bythe browning of milk powder kept at 4O0Cfor 10 days as a function of water activity(Figure 1-29). The loss in Iysine resultingfrom the browning reaction parallels thecolor change, as is shown in Figure 1-30.
Labuza et al. (1970) have shown that, evenat low water activities, sucrose may behydrolyzed to form reducing sugars that maytake part in browning reactions. Browningreactions are usually slow at low humiditiesand increase to a maximum in the range ofintermediate-moisture foods. Beyond thisrange the rate again decreases. This behavior
% DE
CREA
SE IN
TRAN
SMITT
ANCE
STORAGE TIME, DAYS
Figure 1-28 Enzymic Browning in the System Polyphenoloxidase-Cellulose-Catechol at 250C andDifferent Water Activities. Lower aw values were changed to 0.70 after 9 days. Source: From L. Acker,Water Activity and Enzyme Activity, Food Technol, Vol. 23, pp. 1257-1270, 1969.
CHANGE OFQW
can be explained by the fact that, in the inter-mediate range, the reactants are all dissolved,and that further increase in moisture contentleads to dilution of the reactants.
The effect of water activity on oxidation offats is complex. Storage of freeze-dried anddehydrated foods at moisture levels above
those giving monolayer coverage appears togive maximum protection against oxidation.This has been demonstrated by Martinez andLabuza (1968) with the oxidation of lipids infreeze-dried salmon (Figure 1-31). Oxida-tion of the lipids was reduced as water con-tent increased. Thus, conditions that are
WATER ACTIVITY
Figure 1-29 Color Change of Milk Powder Kept at 4O0C for 10 Days as a Function of Water Activity
YELL
OW
INDE
X
WATER ACTIVITY
Figure 1-30 Loss of Free Lysine in Milk Powder Kept at 4O0C for 10 Days as a Function of WaterActivity. Source: From M. Loncin, JJ. Bimbenet, and J. Lenges, Influence of the Activity of Water onthe Spoilage of Foodstuffs, J. Food Technol, Vol. 3, pp. 131-142, 1968.
LYSI
NE L
OSS
%
optimal for protection against oxidation maybe conducive to other spoilage reactions,such as browning.
Water activity may affect the properties ofpowdered dried product. Berlin et al. (1968)studied the effect of water vapor sorption onthe porosity of milk powders. When the pow-ders were equilibrated at 50 percent relativehumidity (RH), the microporous structurewas destroyed. The free fat content was con-siderably increased, which also indicatesstructural changes.
Other reactions that may be influenced bywater activity are hydrolysis of protopectin,splitting and demethylation of pectin, auto-catalytic hydrolysis of fats, and the transfor-mation of chlorophyll into pheophytin(Loncin et al. 1968).
Rockland (1969) has introduced the con-cept of local isotherm to provide a closerrelationship between sorption isotherms andstability than is possible with other methods.He suggested that the differential coefficient
of moisture with respect to relative humidity(AM/ARH), calculated from sorption iso-therms, is related to product stability.
The interaction between water and poly-mer molecules in gel formation has beenreviewed by Busk (1984).
WATER ACTIVITY AND PACKAGING
Because water activity is a major factorinfluencing the keeping quality of a numberof foods, it is obvious that packaging can domuch to maintain optimal conditions for longstorage life. Sorption isotherms play animportant role in the selection of packagingmaterials. Hygroscopic products always havea steep sorption isotherm and reach the criti-cal area of moisture content before reachingexternal climatic conditions. Such foodshave to be packaged in glass containers withmoistureproof seals or in watertight plastic(thick polyviny!chloride). For example, con-sider instant coffee, where the critical area is
Figure 1-31 Peroxide Production in Freeze-Dried Salmon Stored at Different Relative Humidities.Source: From F. Martinez and T.P. Labuza, Effect of Moisture Content on Rate of Deterioration ofFreeze-Dried Salmon, J. Food ScL, Vol. 33, pp. 241-247, 1968.
TIME - HOURS
0IdH
Ox
«3d
N30A
XO •
'">
MiCR
OLlT
ERS
OXYG
EN P
£R G
RAM
LlPiO
at about 50 percent RH. Under these condi-tions the product cakes and loses itsflowability. Other products might not behygroscopic and no unfavorable reactionsoccur at normal conditions of storage. Suchproducts can be packaged in polyethylenecontainers.
There are some foods where the equilib-rium relative humidity is above that of theexternal climatic conditions. The packagingmaterial then serves the purpose of protect-ing the product from moisture loss. This isthe case with processed cheese and bakedgoods.
Different problems may arise in compositefoods, such as soup mixes, where several dis-tinct ingredients are packaged together. InFigure 1-32, for example, substance B withthe steep isotherm is more sensitive to mois-ture, and is mixed in equal quantities withsubstance A in an impermeable package.*
*The initial relative humidity of A is 65 percentand of B, 15 percent.
The initial moisture content of B is X1, andafter equilibration with A, the moisture con-tent is X2. The substances A and B will reacha mean relative humidity of about 40 percent,but not a mean moisture content. If this werea dry soup mix and the sensitive componentwas a freeze-dried vegetable with a moisturecontent of 2 percent and the other compo-nent, a starch or flour with a moisture con-tent of 13 percent, the vegetable would bemoistened to up to 9 percent. This wouldresult in rapid quality deterioration due tononenzymic browning reactions. In this case,the starch would have to be postdried.
Salwin and Slawson (1959) found that sta-bility in dehydrated foods was impaired ifseveral products were packaged together. Atransfer of water could take place from itemsof higher moisture-vapor pressure to thoseof lower moisture-vapor pressure. Theseauthors determined packaging compatibilityby examining the respective sorption iso-therms. They suggested a formula for calcu-lation of the final equilibrium moisturecontent of each component from the iso-
MO
IST
UR
E %
REL HUM. %
Figure 1-32 Sorption Isotherms of Materials A and B
therms of the mixed food and its equilibriumrelative humidity:
_ (W1- V g w l O + (W2-52-flw2,)(W1-S1)H-(W2-S2)
whereW1 = gram solids of ingredient 1S1 = linear slope of ingredient 1aw[' = initial aw of ingredient 1
WATER BINDING OF MEAT
According to Hamm (1962), the water-binding capacity of meat is caused by themuscle proteins. Some 34 percent of theseproteins are water-soluble. The main portionof meat proteins is structural material. Onlyabout 3 percent of the total water-bindingcapacity of muscle can be attributed towater-soluble (plasma) proteins. The mainwater-binding capacity of muscle can beattributed to actomyosin, the main compo-nent of the myofibrils. The adsorption iso-therm of freeze-dried meat has the shapeshown in Figure 1-33. The curve is similarto the sorption isotherms of other foods andconsists of three parts. The first part corre-sponds to the tightly bound water, about 4percent, which is given off at very low vaporpressures. This quantity is only about one-fifth the total quantity required to cover thewhole protein with a monomolecular layer.This water is bound under simultaneous lib-eration of a considerable amount of energy, 3to 6 kcal per mole of water. The binding ofthis water results in a volume contraction of0.05 mL per g of protein. The binding islocalized at hydrophilic groups on proteinssuch as polar side chains having carboxyl,amino, hydroxyl, and sulphydryl groups and
also on the nondissociable carboxyl andimino groups of the peptide bonds. The bind-ing of water is strongly influenced by the pHof meat. The effect of pH on the swelling orunswelling (that is, water-binding capacity ofproteins) is schematically represented in Fig-ure 1-34 (Honkel 1989). The second portionof the curve corresponds to multilayer ad-sorption, which amounts to another 4 to 6percent of water. Hamm (1962) consideredthese two quantities of water to represent thereal water of hydration and found them toamount to between 50 and 60 g per 100 g ofprotein. Muscle binds much more than thisamount of water. Meat with a protein contentof 20 to 22 percent contains 74 to 76 percentwater, so that 100 g of protein binds about350 to 360 g of water. This ratio is evenhigher in fish muscle. Most of this water ismerely immobilized—retained by the net-
WAT
ER C
ON
TEN
T %
REL HUM. %
Figure 1-33 Adsorption Isotherm of Freeze-Dried Meat
pH 5.5 pH 7.0
Figure 1-34 Water Binding in Meat as Influ-enced by pH
work of membranes and filaments of thestructural proteins as well as by cross-link-ages and electrostatic attractions betweenpeptide chains. It is assumed that changes inwater-binding capacity of meat during aging,storage, and processing relate to the freewater and not the real water of hydration.The free water is held by a three-dimensionalstructure of the tissue, and shrinkage in thisnetwork leads to a decrease in immobilizedwater; this water is lost even by applicationof slight pressure. The reverse is also possi-ble. Cut-up muscle can take up as much as700 to 800 g of water per 100 g of protein atcertain pH values and in the presence of cer-tain ions. Immediately after slaughter there isa drop in hydration and an increase in rigid-ity of muscle with time. The decrease inhydration was attributed at about two-thirdsto decomposition of ATP and at about one-third to lowering of the pH.
Hamm (1959a, 1959b) has proposed thatduring the first hour after slaughter, bivalentmetal ions of muscle are incorporated intothe muscle proteins at pH 6, causing a con-traction of the fiber network and a dehydra-tion of the tissue. Further changes in hy-dration during aging for up to seven days canbe explained by an increase in the number ofavailable carboxyl and basic groups. Theseresult from proteolysis. Hamm and Deather-age (196Oa) found that freeze-drying of beefresults in a decrease in water-binding capac-ity in the isoelectric pH range of the muscle.The proteins form a tighter network, which isstabilized by the formation of new salt and/orhydrogen bonds. Heating beef at tempera-tures over 4O0C leads to strong denaturationand changes in hydration (Hamm andDeatherage 196Ob). Quick freezing of beefresults in a significant but small increase inthe water-holding capacity, whereas slowfreezing results in a significant but smalldecrease in water binding. These effectswere thought to result from the mechanicalaction of ice crystals (Deatherage and Hamm1960). The influence of heating on waterbinding of pork was studied by Sherman(196Ib), who also investigated the effect ofthe addition of salts on water binding (Sher-man 196Ia). Water binding can be greatlyaffected by addition of certain salts, espe-cially phosphates (Hellendoorn 1962). Suchsalt additions are used to diminish cookinglosses by expulsion of water in canning hamsand to obtain a better structure and consis-tency in manufacturing sausages. Recently,the subject of water binding has been greatlyextended in scope (Katz 1997). Water bind-ing is related to the use of water as a plasti-cizer and the interaction of water with thecomponents of mixed food systems. Retain-ing water in mixed food systems throughouttheir shelf life is becoming an important
requirement in foods of low fat content. Suchfoods often have fat replacer ingredientsbased on proteins or carbohydrates, and theirinteraction with water is of great importance.
WATER ACTIVITY AND FOODPROCESSING
Water activity is one of the criteria forestablishing good manufacturing practice(GMP) regulations governing processingrequirements and classification of foods(Johnston and Lin 1987). As indicated inFigure 1-35, the process requirements for
foods are governed by aw and pH; aw con-trolled foods are those with pH greater than4.6 and aw less than 0.85. At pH less than 4.6and aw greater than 0.85, foods fall into thecategory of low-acid foods; when packagedin hermetically sealed containers, thesefoods must be processed to achieve commer-cially sterile conditions.
Intermediate moisture foods are in the aw
range of 0.90 to 0.60. They can achieve sta-bility by a combination of aw with other fac-tors, such as pH, heat, preservatives, and Eh
(equilibrium relative humidity).
Federal Regulations
21 CFR 113 & 108.35
Thermally ProcessedLow-Acid Foods Packaged
in HermeticallySealed Containers
pH > 4.6
a w > 0.85
"AcidifiedFoods
&Acid
Foods
p H < 4 . 6
aw > 0.85
Acid & aw
ControlledFoods
pH < 4.6
aw < 0.85
aw Controlled Foods
pH > 4 . 6
a w < 0.85
•Acidified Foods - 21 CFR 114 & 106.25
Figure 1-35 The Importance of pH and aw on Processing Requirements for Foods. Source: Reprintedwith permission from M.R. Johnston and R.C. Lin, FDA Views on the Importance of aw in Good Man-ufacturing Practice, Water Activity: Theory and Application to Food, L.B. Rockland and L.R. Beuchat,eds., p. 288, 1987, by courtesy of Marcel Dekker, Inc.
PH
a w
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Berlin, E., B.A. Anderson, and MJ. Pallansch. 1968.Effect of water vapor sorption on porosity of dehy-drated dairy products. J. Dairy ScL 51: 668-672.
Bone, D.P. 1987. Practical applications of water activ-ity and moisture relations in foods. In Water activity:Theory and application to food, ed. L.B. Rocklandand L.R. Beuchat. New York: Marcel Dekker, Inc.
Bourne, M.C. 1986. Effect of water activity on textureprofile parameters of apple flesh. J. Texture Studies17:331-340.
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Deatherage, EE., and R. Hamm. 1960. Influence offreezing and thawing on hydration and charges ofthe muscle proteins. Food Res. 25: 623-629.
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Hamm, R., and EE. Deatherage. 196Oa. Changes inhydration and charges of muscle proteins duringheating of meat. Food Res. 25: 573-586.
Hamm, R., and EE. Deatherage. 196Ob. Changes inhydration, solubility and charges of muscle proteinsduring heating of meat. Food Res. 25: 587-610.
Hellendoorn, E.W. 1962. Water binding capacity ofmeat as affected by phosphates. Food Technol. 16:119-124.
Honkel, K.G. 1989. The meat aspects of water andfood quality. In Water and food quality, ed. TM.Hardman. New York: Elsevier Applied Science.
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Jouppila, K., and YH. Roos. 1994. The physical stateof amorphous corn starch and its impact on crystalli-zation. Carbohydrate Polymers. 32: 95-104.
Kapsalis, J.G. 1987. Influences of hysteresis and tem-perature on moisture sorption isotherms. In Wateractivity: Theory and application to food, ed. L.B.Rockland and L.R. Beuchat. New York: MarcelDekker, Inc.
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INTRODUCTION
It has been difficult to provide a definitionfor the class of substances called lipids. Earlydefinitions were mainly based on whether thesubstance is soluble in organic solvents likeether, benzene, or chloroform and is not solu-ble in water. In addition, definitions usuallyemphasize the central character of the fattyacids—that is, whether lipids are actual orpotential derivatives of fatty acids. Every def-inition proposed so far has some limitations.For example, monoglycerides of the short-chain fatty acids are undoubtedly lipids, butthey would not fit the definition on the basisof solubility because they are more soluble inwater than in organic solvents. Instead of try-ing to find a definition that would include alllipids, it is better to provide a schemedescribing the lipids and their components, asFigure 2-1 shows. The basic components oflipids (also called derived lipids) are listed inthe central column with the fatty acids occu-pying the prominent position. The left col-umn lists the lipids known as phospholipids.The right column of the diagram includes thecompounds most important from a quantita-tive standpoint in foods. These are mostlyesters of fatty acids and glycerol. Up to 99percent of the lipids in plant and animal
material consist of such esters, known as fatsand oils. Fats are solid at room temperature,and oils are liquid.
The fat content of foods can range fromvery low to very high in both vegetable andanimal products, as indicated in Table 2-1. Innonmodified foods, such as meat, milk, cere-als, and fish, the lipids are mixtures of manyof the compounds listed in Figure 2-1, withtriglycerides making up the major portion.The fats and oils used for making fabricatedfoods, such as margarine and shortening, arealmost pure triglyceride mixtures. Fats aresometimes divided into visible and invisiblefats. In the United States, about 60 percent oftotal fat and oil consumed consists of invisi-ble fats—that is, those contained in dairyproducts (excluding butter), eggs, meat, poul-try, fish, fruits, vegetables, and grain prod-ucts. The visible fats, including lard, butter,margarine, shortening, and cooking oils,account for 40 percent of total fat intake. Theinterrelationship of most of the lipids is repre-sented in Figure 2-1. A number of minorcomponents, such as hydrocarbons, fat-solu-ble vitamins, and pigments are not includedin this scheme.
Fats and oils may differ considerably incomposition, depending on their origin. Bothfatty acid and glyceride composition may
Lipids
CHAPTER 2
result in different properties. Fats and oilscan be classified broadly as of animal or veg-etable origin. Animal fats can be further sub-divided into mammal depot fat (lard andtallow) and milk fat (mostly ruminant) andmarine oils (fish and whale oil). Vegetableoils and fats can be divided into seed oils(such as soybean, canola), fruit coat fats(palm and olive oils), and kernel oils (coco-nut and palm kernel).
The scientific name for esters of glyceroland fatty acids is acylglycerols. Triacylglyc-erols, diacylglycerols, and monoacylglycer-ols have three, two, or one fatty acid esterlinkages. The common names for these com-pounds are glycerides, triglycerides, diglyc-erides, and monoglycerides. The scientificand common names are used interchange-ably in the literature, and this practice is fol-lowed in this book.
Figure 2-1 Interrelationship of the Lipids
FATTYALDEHYDES
PHOSPHORIC ACIDAMINO ALCOHOLS
FATTYALCOHOLS
Plasmalogens
Phosphatidylesters
Waxes
FATTYACIDS
GLYCEROL
Etheresters
Glycer ylether
Mono, Di, Tr iglycerides
Sterol estersSTEROLS
Sphingomyelin
Cercbrosides
SPHINGOSINE
HEXOSES
Table 2-1 Fat Contents of Some Foods
Product Fat (%)
Asparagus 0.25Oats 4.4Barley 1.9Rice 1.4Walnut 58Coconut 34Peanut 49Soybean 17Sunflower 28Milk 3.5Butter 80Cheese 34Hamburger 30Beef cuts 10-30Chicken 7Ham 31Cod 0.4Haddock 0.1Herring 12.5
SHORTHAND DESCRIPTION OFFATTY ACIDS AND GLYCERIDES
To describe the composition of fatty acidsit is sometimes useful to use a shorthanddesignation. In this convention the composi-tion of a fatty acid can be described by twonumbers separated by a colon. The firstnumber indicates the number of carbonatoms in the fatty acid chain, the secondnumber indicates the number of doublebonds. Thus, 4:0 is short for butyric acid,16:0 for palmitic acid, 18:1 for oleic acid,etc. The two numbers provide a completedescription of a saturated fatty acid. Forunsaturated fatty acids, information aboutthe location of double bonds and their stereoisomers can be given as follows: oleic acid(the cis isomer) is 18:lc9; elaidic acid (the
trans isomer) is I8:lt9. The numbering ofcarbon atoms in fatty acids starts normallywith the carboxyl carbon as number one. Insome cases polyunsaturated fatty acids arenumbered starting at the methyl end; forinstance, linoleic acid is represented as18:2n-6 and linolenic acid 18:3n-3. Thesesymbols indicate straight-chain, 18-carbonfatty acids with two and three methyleneinterrupted cis double bonds that start at thesixth and third carbon from the methyl end,respectively. These have also been describedas 006 and co3. The reason for this type ofdescription is that the members of eachgroup n-6 or n-3 are related biosyntheticallythrough processes involving desaturation,chain elongation, and chain shortening(Gunstone 1986) (Figure 2-2).
Triglycerides can be abbreviated by usingthe first letters of the common names of thecomponent fatty acids. SSS indicates tri-stearin, PPP tripalmitin, and SOS a triglycer-ide with two palmitic acid residues in the 1and 3 positions and oleic acid in the 2 posi-tion. In some cases, glyceride compositionsare discussed in terms of saturated and unsat-urated component fatty acids. In this case, Sand U are used and glycerides would be indi-cated as SSS for trisaturated glyceride andSUS for a glyceride with an unsaturated fattyacid in the 2 position. In other cases, the totalnumber of carbon atoms in a glyceride isimportant, and this can be shortened to glyc-erides with carbon numbers 54, 52, and soon. A glyceride with carbon number 54could be made up of three fatty acids with 18carbons, most likely to happen if the glycer-ide originated from one of the seed oils. Aglyceride with carbon number 52 could havetwo component fatty acids with 18 carbonsand one with 16 carbons. The carbon numberdoes not give any information about satura-tion and unsaturation.
COMPONENT FATTY ACIDS
Even-numbered, straight-chain saturatedand unsaturated fatty acids make up thegreatest proportion of the fatty acids of natu-ral fats. However, it is now known that manyother fatty acids may be present in smallamounts. Some of these include odd carbonnumber acids, branched-chain acids, andhydroxy acids. These may occur in naturalfats (products that occur in nature), as well asin processed fats. The latter category may, inaddition, contain a variety of isomeric fattyacids not normally found in natural fats. It iscustomary to divide the fatty acids into dif-ferent groups, for example, into saturated andunsaturated ones. This particular division isuseful in food technology because saturatedfatty acids have a much higher melting pointthan unsaturated ones, so the ratio of satu-rated fatty acids to unsaturated ones signifi-cantly affects the physical properties of a fator oil. Another common division is intoshort-chain, medium-chain, and long-chainfatty acids. Unfortunately, there is no gener-ally accepted division of these groups. Gen-
erally, short-chain fatty acids have from 4 to10 carbon atoms; medium-chain fatty acids,12 or 14 carbon atoms; and long-chain fattyacids, 16 or more carbon atoms. However,some authors use the terms long- and short-chain fatty acid in a strictly relative sense. Ina fat containing fatty acids with 16 and 18carbon atoms, the 16 carbon acid could becalled the short-chain fatty acid. Yet anotherdivision differentiates between essential andnonessential fatty acids.
Some of the more important saturated fattyacids are listed with their systematic andcommon names in Table 2-2, and some ofthe unsaturated fatty acids are listed in Table2-3. The naturally occurring unsaturatedfatty acids in fats are almost exclusively inthe c«-form (Figure 2-3), although trans-acids are present in ruminant milk fats and incatalytically hydrogenated fats. In general,the following outline of fatty acid composi-tion can be given:
• Depot fats of higher land animals consistmainly of palmitic, oleic, and stearicacid and are high in saturated fatty acids.
Figure 2-2 The n-3 Family Polyunsaturated Fatty Acids Based on Linolenic Acid. The heavy arrowsshow the relationship between the most important n-3 acids through desaturation (vertical arrows) andchain elongation (horizontal arrows)
16 : 3 <- 18 : 3 -* 20 : 3 -+ 22 : 3 -> 24 : 3
16 : 4 «- 18 : 4 -»-20 : 4 -* [22 : 4] -» 24 : 4
18 : 5 «- 20 : 5 -»-22 : 5 -> 24 : 5 -> 26 : 5 -* [28 : 5] -> 30 : 5
22 : 6 -> 24 : 6 -> 26 : 6
The total content of acids with 18 carbonatoms is about 70 percent.
• Ruminant milk fats are characterized bya much greater variety of componentfatty acids. Lower saturated acids with 4to 10 carbon atoms are present in rela-tively large amounts. The major fattyacids are palmitic, oleic, and stearic.
• Marine oils also contain a wide varietyof fatty acids. They are high in unsatur-ated fatty acids, especially those unsatur-ated acids with long chains containing20 or 22 carbons or more. Several ofthese fatty acids, including eicosapen-taenoic acid (EPA) and docosahex-aenoic acid (DHA), have recently re-
ceived a good deal of attention becauseof biomedical interest (Ackman 1988b).
• Fruit coat fats contain mainly palmitic,oleic, and sometimes linoleic acids.
• Seedfats are characterized by low con-tents of saturated fatty acids. They con-tain palmitic, oleic, linoleic, and linolenicacids. Sometimes unusual fatty acidsmay be present, such as erucic acid inrapeseed oil. Recent developments inplant breeding have made it possible tochange the fatty acid composition of seedoils dramatically. Rapeseed oil in whichthe erucic acid has been replaced by oleicacid is known as canola oil. Low lino-lenic acid soybean oil can be obtained, as
Table 2-2 Saturated Even- and Odd-Carbon Numbered Fatty Acids
Systematic Name
n-Butanoicn-Hexanoicn-Octanoicn-Decanoicn-Dodecanoicn-Tetradecanoicn-Hexadecanoicn-Octadecanoicn-Eicosanoic/7-Docosanoicn-Pentanoicn-Heptanoic/i-Nonanoicn-Undecanoicn-Tridecanoicn-Pentadecanoicn-Heptadecanoic
Common Name
ButyricCaproicCaprylicCapricLaurie
MyristicPalmiticStearic
ArachidicBehenicValeric
EnanthicPelargonic
Margaric
Formula
CH3-(CH2J2-COOHCH3-(CH2J4-COOHCH3-(CH2J6-COOHCH3-(CH2J8-COOHCH3-(CH2J10-COOHCH3-(CH2J12-COOHCH3-(CH2J14-COOHCH3-(CH2J16-COOHCH3-(CH2J18-COOHCH3-(CH2J20-COOHCH3-(CH2J3-COOHCH3-(CH2J5-COOHCH3-(CH2J7-COOHCH3-(CH2J9-COOHCH3-(CH2J11-COOHCH3-(CH2J13-COOHCH3-(CH2J15-COOH
ShorthandDescription
4:06:08:010:012:014:016:018:020:022:05:07:09:011:013:015:017:0
can sunflower and linseed oils with moredesirable fatty acid composition.
The depot fats of higher land animals,especially mammals, have relatively simplefatty acid composition. The fats of birds aresomewhat more complex. The fatty acidcompositions of the major food fats of this
group are listed in Table 2-4. The kind offeed consumed by the animals may greatlyinfluence the composition of the depot fats.Animal depot fats are characterized by thepresence of 20 to 30 percent palmitic acid, aproperty shared by human depot fat. Many ofthe seed oils, in contrast, are very low inpalmitic acid. The influence of food con-
Figure 2-3 Structures of Octadec-cw-9-Enoic Acid (Oleic Acid) and Octadec-Jraws-9-Enoic Acid(Elaidic Acid)
Table 2-3 Unsaturated Fatty Acids
Systematic Name
Dec-9-enoicDodec-9-enolcTetradec-9-enoicHexadec-9-enoicOctadec-6-enoicOctadec-9-enoicOctadec-11-enoicOctadeca-9: 1 2-dienoicOctadeca-9: 1 2: 1 5-trienoicOctadeca-6:9: 1 2-trienoicOctadeca-9: 11:1 3-trienoicEicos-9-enoicEicosa-5:8:1 1 :14-tetraenoicEicosa-5:8:1 1:1 4:17-
pentaenoic acidDocos-13-enoicDocosa-4:7:10:13:16:19-
hexaenoic acid
CommonName
MyristoleicPalmitoleicPetroselinicOleicVaccenicLinoleicLinolenicy-LinolenicElaeostearicGadoleicArachidonicEPA
ErucicDHA
Formula
CH2=CH.(CH2)rCOOHCH3.CH2.CH=CH.(CH2)rCOOHCH3.(CH2)3.CH=CH.(CH2)rCOOHCH3.(CH2)5.CH=CH.(CH2)rCOOHCH3.(CH2)10.CH=CH.(CH2)4.COOHCH3.(CH2)7.CH=CH.(CH2)7-COOHCH3.(CH2)5.CH=CH.(CH2)9.COOHCH3.(CH2)4.(CH=CH.CH2)2.(CH2)6.COOHCH3.CH2.(CH=CH.CH2)3.(CH2)6.COOHCH3.(CH2)4.(CH=CH.CH2)3.(CH2)3.COOHCH3.(CH2)3.(CH=CH)3.(CH2)rCOOHCH3.(CH2)9.CH=CH-(CH2)7.COOHCH3.(CH2)4.(CH=CH.CH2)4.(CH2)2.COOHCH3.CH2.(CH=CH.CH2)5.(CH2)2.COOH
CH3.(CH2)7-CH=CH-(CH2)1 1 -COOHCH3.CH2(CH=CH.CH2)6.(CH2).COOH
ShorthandDescription
10:112:114:116:118:118:118:118:2co618:3co318:3(0620:320:120:40)620:5(03
22:122:6(03
sumption applies equally for the depot fat ofchicken and turkey (Marion et al. 1970; Jenet al. 1971). The animal depot fats are gener-ally low in polyunsaturated fatty acids. Theiodine value of beef fat is about 50 and oflard about 60. Iodine value is generally usedin the food industry as a measure of totalunsaturation in a fat.
Ruminant milk fat is extremely complex infatty acid composition. By using gas chro-matography in combination with fractionaldistillation of the methyl esters and adsorp-tion chromatography, Magidman et al.(1962) and Herb et al. (1962) identified atleast 60 fatty acids in cow's milk fat. Severaladditional minor fatty acid components havebeen found in other recent studies. About 12fatty acids occur in amounts greater than 1percent (Jensen and Newburg 1995). Amongthese, the short-chain fatty acids frombutyric to capric are characteristic of rumi-nant milk fat. Data provided by Hilditch andWilliams (1964) on the component fattyacids of some milk fats are listed in Table 2-5.Fatty acid compositions are usually reportedin percentage by weight, but in the case offats containing short-chain fatty acids (orvery long-chain fatty acids) this method maynot give a good impression of the molecularproportions of fatty acids present. Therefore,in many instances, the fatty acid composi-
tion is reported in mole percent, as is thecase with the data in Table 2-5. Accordingto Jensen (1973) the following fatty acids arepresent in cow's milk fat: even and odd satu-rated acids from 2:0 to 28:0; even and oddmonoenoic acids from 10:1 to 26:1, with theexception of 11:1, and including positionaland geometric isomers; even unsaturatedfatty acids from 14:2 to 26:2 with some con-jugated geometric isomers; polyenoic evenacids from 18:3 to 22:6 including some con-jugated trans isomers; monobranched fattyacids 9:0 and 11:0 to 25:0—some iso andsome ante-iso (iso acids have a methylbranch on the penultimate carbon, ante-isoon the next to penultimate carbon [Figure2-4]); multibranched acids from 16:0 to28:0, both odd and even with three to fivemethyl branches; and a number of keto,hydroxy, and cyclic acids.
It is impossible to determine all of the con-stituents of milk fatty acids by a normalchromatographic technique, because many ofthe minor component fatty acids are eithernot resolved or are covered by peaks of othermajor fatty acids. A milk fat chromatogramof fatty acid composition is shown in Figure2-5. Such fatty acid compositions as re-ported are therefore only to be considered asapproximations of the major component fattyacids; these are listed in Table 2-6. This
Table 2-4 Component Fatty Acids of Animal Depot Fats
Fatty Acids Wt %
Animal
PigBeefSheepChickenTurkey
14:0
14311
16:0
2425212420
76:7
35266
18:0
13192566
18:1
4136344038
18:2
10451724
78:3
1Trace
312
table reports the most recent results of themajor component fatty acids in bovine milkfat as well as their distribution among the sn-1, sn-2, and sn-3 positions in the triacylglyc-erols (Jensen and Newburg 1995).
In most natural fats the double bonds ofunsaturated fatty acids occur in the cis con-figuration. In milk fat a considerable propor-tion is in the trans configuration. Thesetrans bonds result from microbial action inthe rumen where polyunsaturated fatty acidsof the feed are partially hydrogenated. Cata-lytic hydrogenation of oils in the fat industry
also results in trans isomer formation. Thelevel of trans isomers in milk fat has beenreported as 2 to 4 percent (deMan anddeMan 1983). Since the total content ofunsaturated fatty acids in milk fat is about34 percent, trans isomers may constituteabout 10 percent of total unsaturation. Thecomplexity of the mixture of different iso-mers is demonstrated by the distribution ofpositional and geometric isomers in themonoenoic fatty acids of milk fat (Table 2-7)and in the unconjugated 18:2 fatty acids(Table 2-8). The iodine value of milk fat is
Figure 2-4 Examples of Iso- and Ante-Iso-Branched-Chain Fatty Acids
Table 2-5 The Component Fatty Acids of Some Milk Fats in Mole %
Fatty Acid
4:06:08:010:0Total short chain12:014:016:018:020:010-12 unsaturated16:118:118:220-22 unsaturated
Cow
9.54.10.83.2
17.62.9
11.526.77.61.81.14.3
22.43.11.0
Goat
7.54.74.3
12.829.36.6
11.824.14.70.41.42.2
16.52.80.2
Sheep
7.55.33.56.4
22.74.59.9
21.610.30.81.02.0
21.64.31.3
Source: From TP. Hilditch and P.M. Williams, The Chemical Constitution of Natural Fats, 4th ed., 1964, JohnWiley & Sons.
in the range of 30 to 35, much lower thanthat of lard, shortening, or margarine, whichhave similar consistencies.
Marine oils have also been found to con-tain a large number of component fatty acids.Ackman (1972) has reported as many as 50or 60 components. Only about 14 of theseare of importance in terms of weight percentof the total. These consist of relatively fewsaturated fatty acids (14:0, 16:0, and 18:0)and a larger number of unsaturated fattyacids with 16 to 22 carbon atoms and up to 6double bonds. This provides the possibilityfor many positional isomers.
The complexity of the fatty acid composi-tion of marine oils is evident from the chro-matogram shown in Figure 2-6 (Ackman1994). The end structure of the polyunsatu-
rated fatty acids is of nutritional impor-tance, especially eicosapentaenoic acid(EPA), 20:5co3 or 20:5 n-3, and docosa-hexaenoic acid (DHA), 22:6co3 or 22:6 n-3.The double bonds in marine oils occurexclusively in the cis configuration. EPAand DHA can be produced slowly fromlinolenic acid by herbivore animals, but notby humans. EPA and DHA occur in majoramounts in fish from cold, deep waters,such as cod, mackerel, tuna, swordfish, sar-dines, and herring (Ackman 1988a; Simo-poulos 1988). Arachidonic acid is theprecursor in the human system of pros-tanoids and leukotrienes.
Ackman (1988b) has drawn attention tothe view that the fatty acid compositions ofmarine oils are all much the same and vary
Figure 2-5 Chromatogram of Milk Fat Fatty Acid Composition Analyzed as Butyl Esters on a 30-mCapillary Column. Source: Reprinted from R.G. Ackman, Animal and Marine Lipids, in Improved andTechnological Advances in Alternative Sources of Lipids, B. Kamel and Y. Kakuda, eds., p. 298, ©1994, Aspen Publishers, Inc.
HEXA
NE
BUTA
NOL
only in the proportions of fatty acids. Thepreviously held view was that marine oilswere species-specific. The major fatty acidsof marine oils from high-, medium-, andlow-fat fish are listed in Table 2-9 (Ackman1994).
The fatty acid composition of egg yolk isgiven in Table 2-10. The main fatty acidsare palmitic, oleic, and linoleic. The yolkconstitutes about one-third of the weight ofthe edible egg portion. The relative amountsof egg yolk and white vary with the size ofthe egg. Small eggs have relatively higheramounts of yolk. The egg white is virtuallydevoid of fat.
The vegetable oils and fats can be dividedinto three groups on the basis of fatty acidcomposition. The first group comprises oilscontaining mainly fatty acids with 16 or 18
carbon atoms and includes most of the seedoils; in this group are cottonseed oil, peanutoil, sunflower oil, corn oil, sesame oil, oliveoil, palm oil, soybean oil, and safflower oil.The second group comprises seed oils con-taining erucic (docos-13-enoic) acid. Theseinclude rapeseed and mustard seed oil. Thethird group is the vegetable fats, comprisingcoconut oil and palm kernel oil, which arehighly saturated (iodine value about 15),and cocoa butter, the fat obtained fromcocoa beans, which is hard and brittle atroom temperature (iodine value 38). Thecomponent fatty acids of some of the mostcommon vegetable oils are listed in Table2-11. Palmitic is the most common satu-rated fatty acid in vegetable oils, and onlyvery small amounts of stearic acid arepresent. Oils containing linolenic acid, such
Table 2-6 Major Fatty Acids of Bovine Milk Fat and Their Distribution in the Triacylglycerols
Fatty Acids(mol%)
4:06:08:010:012:014:015:016:016:117:018:018:118:218:3
Bovine Milk Fat
TG
11.84.61.9
373.9
11.22.1
23.92.60.87.0
24.02.5
Trace
sn-1
1.41.94.99.72.0
34.02.81.3
10.330.0
1.7
sn-2
0.90.73.06.2
17.52.9
32.33.61.09.5
18.93.5
sn-3
35.412.93.66.20.66.41.45.41.40.11.2
23.12.3
Source: Reprinted with permission from R.G. Jensen and D. S. Newburg, Milk Lipids, in Handbook of Milk Compo-sition, R.G. Jensen, ed., p. 546, © 1995, Academic Press.
as soybean oil, are unstable. Such oils canbe slightly hydrogenated to reduce the lino-lenic acid content before use in foods.Another fatty acid that has received atten-tion for its possible beneficial effect onhealth is the n-6 essential fatty acid,
gamma-linolenic acid (18:3 n-6), whichoccurs at a level of 8 to 10 percent inevening primrose oil (Carter 1988).
The Crucifera seed oils, including rape-seed and mustard oil, are characterized bythe presence of large amounts of erucic acid
Table 2-8 Location of Double Bonds in Unconjugated 18:2 lsomers of Milk Lipids
CIS, CIS
11,1510,159, 158, 15 and/or 8, 127, 15 and/or 7, 126, 15 and/or 6, 12
cis, trans or trans, c/s
11, 16 and/or 11, 1510, 16 and/or 10, 159, 15 and/or 9, 168, 1 6 and/or 8, 1 5
and/or 8, 12
trans, trans
12, 1611, 16and/or11, 1510, 16 and/or 10, 159, 16 and/or 9, 15
and/or 9, 13
Source: From R.G. Jensen, Composition of Bovine Milk Lipids, J. Am. Oil Chem. Soc., Vol. 50, pp. 186-192, 1973.
Source: From R.G. Jensen, Composition of Bovine Milk Lipids, J. Am. Oil Chem. Soc., Vol. 50, pp. 186-192,1973.
Table 2-7 Positional and Geometric lsomers of Bovine Milk Lipid Monoenoic Fatty Acids (Wt%)
Position ofDoubleBond
5678910111213141516
c/s lsomers
14:1
1.00.80.90.696.6
16:1
Tr1.35.6Tr
88.7Tr2.6Tr
17:1
3.42.120.171.3Tr2.9Tr
trans lsomers
18:1
1.795.8Tr2.5
16:1
2.27.86.75.032.81.710.612.910.6
18:1
1.00.83.210.210.535.74.110.59.06.87.5
(docos-13-enoic) and smaller amounts ofeicos-11-enoic acid. Rapeseed oil of the vari-ety Brassica napus may have over 40 percentof erucic acid (Table 2-12), whereas Bras-sica campestris oil usually has a much lowererucic acid content, about 22 percent.Because of possible health problems result-ing from ingestion of erucic acid, new vari-eties of rapeseed have been introduced inrecent years; these are the so-called low-eru-cic acid rapeseed (LEAR) varieties, whichproduce LEAR oil. When the seed is alsolow in glucosinolates, the oil is known ascanola oil. Plant breeders have succeeded inreducing the erucic acid level to less than 1percent and as a result canola oil has a very
high level of oleic acid (Table 2-12). Thebreeding of these varieties has in effectresulted in the creation of a completely newoil. Removal of the erucic and eicosenoicacids results in a proportional increase in theoleic acid content. The low erucic acid oil isa linolenic acid-containing oil and is there-fore similar in this respect to soybean oil.The fatty acid composition of mustard oil isgiven in Table 2-12. It is similar to that of B.campestris oil.
Vegetable fats, in contrast to the oils, arehighly saturated, have low iodine values, andhave high melting points. Coconut oil andpalm kernel oil belong to the lauric acid fats.They contain large amounts of medium- and
Figure 2-6 Chromatogram of the Fatty Acid Composition of Fish Oil (Menhaden). Analysis ofmethyl esters on a 30-m capillary column. Source: Reprinted from R.G. Ackman, Animal and MarineLipids, in Improved and Technological Advances in Alternative Sources of Lipids, B. Kamel and Y.Kakuda, eds., p. 308, 1994, Aspen Publishers, Inc.
short-chain fatty acids, especially lauric acid(Table 2-13). Cocoa butter is unusual in thatit contains only three major fatty acids—palmitic, stearic, and oleic—in approxi-mately equal proportions.
Table 2-10 Fatty Acid Composition of Egg Yolk
Fatty Acid %
Total saturated 36.214:0 0.316:0 26.618:0 9.3
Total monounsaturated 48.216:1 4.018:1 44.1
Total polyunsaturated 14.718:2 13.418:3 0.320:4 1.0
COMPONENT GLYCERIDES
Natural fats can be defined as mixtures ofmixed triglycerides. Simple triglycerides arevirtually absent in natural fats, and the distri-bution of fatty acids both between and withinglycerides is selective rather than random.When asymmetric substitution in a glycerolmolecule occurs, enantiomorphic forms areproduced (Kuksis 1972; Villeneuve andFoglia 1997). This is illustrated in Figure 2 -7. Glycerol has a plane of symmetry or mir-ror plane, because two of the four substitu-ents on the central carbon atom are identical.When one of the carbon atoms is esterifiedwith a fatty acid, a monoglyceride resultsand two nonsuperimposable structures exist.These are called enantiomers and are alsoreferred to as chiral. A racemic mixture is amixture of equal amounts of enantiomers.Asymmetric or chiral compounds are formedin 1-monoglycerides; all 1, 2-diglycerides; 1,
Table 2-9 Total Fat Content and Major Fatty Acids in High-, Medium-, and Low-Fat Fish
Total fatFatty acid14:016:016:118:120:122:120:5n-322:6A?-3Total
High Fat
Capelin
14.1
7.19.9
11.013.416.312.68.66.7
85.6
Sprat
12.9
5.517.55.8
18.07.4
12.87.4
11.786.1
Medium Fat
BlueWhiting
7.4
3.911.56.1
14.810.712.410.412.682.4
Capelin
4.0
7.39.78.3
14.513.610.49.2
11.084.0
Loiv Fat
Dogfish
1.7
1.615.34.9
20.811.27.96.0
15.584.8
Saith,Gutted
0.4
1.712.42.7
13.15.93.5
12.730.682.6
Source: Reprinted from R.G. Ackman, Animal and Marine Lipids, in Improved and Technological Advances in Alter-native Sources of Lipids, B. Kamel and Y. Kakuda, eds., p. 302, 1994, Aspen Publishers, Inc.
3-diglycerides containing unlike substitu-ents; and all triglycerides in which the 1- and3- positions carry different acyl groups.
The glyceride molecule can be representedin the wedge and slash form (Figure 2-8). Inthis spatial representation, the wedge indi-cates a substituent coming out of the planetoward the observer, and the slash indicates asubstituent going away from the observer.The three carbon atoms of the glycerol are
then described by the stereospecific number-ing (STZ) with the three carbon atoms desig-nated sn-l from the top to sn-3 at the bottom.
When a fat or oil is characterized by deter-mination of its component fatty acids, therestill remains the question as to how theseacids are distributed among and within theglycerides. Originally theories of glyceridedistribution were attempts by means of math-ematical schemes to explain the occurrence
Table 2-1 1 Component Fatty Acids of Some Vegetable Oils
Fatty Acid Wt%
Oil
CanolaCottonseedPeanut*OliveRice branSoybeanSunflowerSunflower high oleicPalmCocoa butter
16:0
42713101611544426
18:0
22322455434
18:1
56183878422220813935
18:2
26514173753698113
18:3
10TraceTrace
18
Total C18
96738390848995965474
'Peanut oil also contains about 3% of 22:0 and 1% of 22:1 .
Table 2-12 Component Fatty Acids of Some Crucifera Seed Oils (Wt%)
Fatty Acid
Seed Oil
Rapeseed (B. campestris)Rapeseed (B. napus)Canola (LEAR)Mustard (B. juncea)
16:0
4344
18:0
212
18:1
33175522
18:2
18142624
18:3
991014
20:1
1211212
22:1
2245<120
TotalC18
62419660
Source: Data from B. M. Craig et al., Influence of Genetics, Environment, and Admixtures on Low Erucic AcidRapeseed in Canada, J. Am. Oil Chem. Soc., Vol. 50, pp. 395-399, 1973; and M. Vaisey-Genser and N.A.M. Eskin,Canola Oil: Properties and Performance, 1987, Canola Council.
Enantiomers
Figure 2-7 Plane of Symmetry of a GlycerolMolecule (Top) and Mirror Image of Two Enan-tiomers of a Mono-Acylglycerol (bottom).Source: Reprinted with permission from P. Ville-neuve and TA. Foglia, Lipase Specificities:Potential Application in Bioconversions, Inform,8, pp. 640-650, © 1997, AOCS Press.
thesis. Hilditch proposed the concept of evendistribution (Gunstone 1967). In the rule ofeven (or widest) distribution, each fatty acidin a fat is distributed as widely as possibleamong glyceride molecules. This means thatwhen a given fatty acid A constitutes about35 mole percent or more of the total fattyacids (A + X), it will occur at least once in alltriglyceride molecules, as represented byGAX2. If A occurs at levels of 35 to 70 molepercent, it will occur twice in an increasingnumber of triglycerides GA2X. At levelsover 70 percent, simple triglycerides GA3 areformed. In strictly random distribution theamount of GA3 in a fat would be propor-tional to the cube of the percentage of Apresent. For example, at 30 percent A therewould be 2.7 percent of GA3, which underrules of even distribution would occur onlyat levels of A over 70 percent (Figure 2-9).
of particular kinds and amounts of glyceridesin natural fats. Subsequent theories havebeen refinements attempting to relate to thebiochemical mechanisms of glyceride syn-
Table 2-13 Component Fatty Acids of Some Vegetable Fats (Wt %)
Fatty Acid
Vegetable Fat
CoconutPalm kernelCocoa butter
6:0
0.5
8:0
9.02.7
10:0
6.87.0
12:0
46.446.9
14:0
18.014.1
16:0
9.08.8
26.2
18:0
1.01.3
34.4
18:1
7.618.537.3
18:2
1.60.72.1
Figure 2-8 Stereospecific Numbering of theCarbons in a Triacylglycerol
Mirror
Symmetric
Mirrorplane
The theory of restricted random distribu-tion was proposed by Kartha (1953). In thistheory the fatty acids are distributed at ran-dom, but the content of fully saturated glycer-ides is limited to the amount that can remainfluid in vivo. This theory is followed by the1,3 random, 2 random distribution hypothesisof Vander WaI (1964). According to this the-ory, all acyl groups at the 2-positions of theglycerol moieties of a fat are distributedtherein at random. Equally, all acyl groups atthe 1- and 3-positions are distributed at ran-dom and these positions are identical. Appli-cation of this theory to the results obtainedwith a number of fats gave good agreement(Vander WaI 1964), as Table 2-14 shows.
In vegetable fats and oils, the saturatedfatty acyl groups have a tendency to occupythe 1- and 3- positions in the glycerides andthe unsaturated acyl groups occupy the 2-
position (Figure 2-10). Since these fats con-tain a limited number of fatty acids, it is cus-tomary to show the glyceride composition interms of saturated (S) and unsaturated (U)acids. The predominant glyceride types inthese fats and oils are S-U-S and S-U-U.Lard is an exception—saturated acyl groupspredominate in the 2-position. The glyceridedistribution of cocoa butter results in a fatwith a sharp melting point of about 30 to340C. It is hard and brittle below the meltingpoint, which makes the fat useful for choco-late and confectionery manufacture. Otherfats with similar fatty acid composition, suchas sheep depot fat (see Table 2-4), have agreater variety of glycerides, giving the fat ahigher melting point (about 450C) and awider melting range, and a greasy and softappearance.
Brockerhoff et al. (1966) studied the fattyacid distribution in the 1-, 2-, and 3-posi-tions of the triglycerides of animal depot fatsby stereospecific analysis. The distributionamong the three positions was nonrandom.The distribution of fatty acids seems to begoverned by chain length and unsaturation.In most fats a short chain and unsaturationdirect a fatty acid toward position 2. Thedepot fat of pigs is an exception, palmiticacid being predominant in position 2. In thefats of marine animals, chain length is thedirecting factor, with polyunsaturated andshort-chain fatty acids accumulated in the 2-position and long chains in the 1- and 3-positions. In the fats of birds, unsaturationseems to be the only directing factor andthese acids accumulate in the 2-position.
The positional distribution of fatty acids inpig fat (lard) and cocoa butter is shown inTable 2-15. Most of the unsaturation in lardis located in the 1- and 3-positions, whereasin cocoa butter the major portion of theunsaturation is located in the 2-position. This
Figure 2-9 Calculated Values for GlycerideTypes in Random Distribution (Solid Lines) andEven Distribution (Dotted Lines). Source: FromRD. Gunstone, An Introduction to the Chemis-try of Fats and Fatty Acids, 1967, Chapman andHall.
Acid (A)(TL mot}
Gly
ceri
de
(%fn
oD
difference accounts for the difference inphysical properties of the two fats (deMan etal. 1987).
Milk fat, with its great variety of fattyacids, also has a very large number of glyc-erides. It is possible, by, for example, frac-tional crystallization from solvents, to sep-arate milk fat in a number of fractions withdifferent melting points (Chen and deMan1966). Milk fat is peculiar in some respects.Its short-chain fatty acids are classifiedchemically as saturated compounds but
behave physically like unsaturated fattyacids. One of the unsaturated fatty acids, theso-called oleic acid, is partly trans and has amuch higher melting point than the cis iso-mers. In the highest melting fraction frommilk fat, there is very little short-chain fattyacid and little unsaturation, mostly in thetrans configuration (Woodrow and deMan1968). The low melting fractions are high inshort-chain fatty acids and unsaturation (cis).The general distribution of major fatty acidsin whole milk fat is as follows (Morrison
Figure 2-10 Fatty Acid Distribution in the Triacylglycerols of Vegetable Oils
2-random
1,3-random
unsaturated
saturated+
long-chainmono-
unsaturated
Table 2-14 Comparison of the Glyceride Composition of Some Natural Fats as DeterminedExperimentally and as Calculated by 1,3 Random, 2 Random Hypothesis
Molecular Species
Fat
LardLardChicken fatChicken fatCocoa butterCocoa butter
Method
ExperimentCalculatedExperimentCalculatedExperimentCalculated
SSS(Mole %)
863355
SUS(Mole %)
O210106669
SSU(Mole %)
292991072
USU(Mole %)
36361293O
UUS(Mole %)
151238362022
UUU(Mole %)
1215283212
Source: From RJ. Vander WaI, Triglyceride Structure, Adv. LIpId Res., Vol. 2, pp. 1-16, 1964.
1970): 4:0 and 6:0 are located largely in pri-mary positions; 18:0 and 18:1 are preferen-tially in primary positions; 10:0, 12:0, and16:0 are distributed randomly or with aslight preference for the secondary position;and 14:0 is predominantly in the secondaryposition. The distribution of milk fat tria-cylglycerols according to carbon numberand unsaturation has been reported byJensen and Newburg (1995) and is presentedin Table 2-16.
PHOSPHOLIPIDS
All fats and oils and fat-containing foodscontain a number of phospholipids. Thelowest amounts of phospholipid are presentin pure animal fats such as lard and beef tal-low. In some crude vegetable oils, such ascottonseed, corn, and soybean oils, phos-pholipids may be present at levels of 2 to 3percent. Fish, Crustacea, and mollusks con-tain approximately 0.7 percent of phospho-lipids in the muscle tissue. Phospholipidsare surface active, because they contain alipophilic and hydrophilic portion. Sincethey can easily be hydrated, they can beremoved from fats and oils during the refin-
ing process. In some cases they may beremoved by separation of two phases; forexample, if butter is melted and filtered, thepure oil thus obtained is free from phospho-lipids. The structure of the most importantphospholipids is given in Figure 2-11. Afterrefining of oils, neutralization, bleaching,and deodorization, the phospholipid contentis reduced to virtually zero. The phospholip-ids removed from soybean oil are used asemulsifiers in certain foods, such as choco-late. Soybean phospholipids contain about35 percent lecithin and 65 percent cephalin.The fatty acid composition of phospholipidsis usually different from that of the oil inwhich they are present. The acyl groups areusually more unsaturated than those of thetriglycerides. Phospholipids of many vegeta-ble oils contain two oleic acid residues. Thephospholipids of milk do not contain theshort-chain fatty acids found in milk fat tri-glycerides, and they contain more long-chain polyunsaturated fatty acids than thetriglycerides. The composition of cow'smilk phospholipids has been reported byJensen (1973), as shown in Table 2-17. Thedifference in composition of triglyceridesand phospholipids in mackerel is demon-
Table 2-15 Positional Distribution Fatty Acids in Pig Fat and Cocoa Butter
Fatty Acid (Mole %)
Fat
Pig fat
Cocoa butter
Position
123123
14:0
0.94.1O
16:0
9.572.30.4
34.01.7
36.5
16:1
2.44.81.50.60.20.3
18:0
29.52.17.4
50.42.1
52.8
18:1
51.313.472.712.387.48.6
18:2
6.43.3
18.21.38.60.4
7s w" C
o55'
gQ.
^-*
Tl
M 2?
% P
I?
-O Oi
I
J° i. <& P CO (D § CO o" *<" W. CO O^ H 05' ^ CQ *< 8 O W 5' S 3 I=. 0> a Q I $ >
strated by the data reported by Ackman andEaton (1971), as shown in Table 2-18. Thephospholipids of flesh and liver in mackerelare considerably more unsaturated than thetriglycerides.
The distribution of fatty acids in phospho-lipids is not random, with saturated fattyacids preferentially occupying position 1 andunsaturated fatty acids position 2.
UNSAPONIFIABLES
The unsaponifiable fraction of fats consistsof sterols, terpenic alcohols, aliphatic alco-hols, squalene, and hydrocarbons. The distri-bution of the various components of theunsaponifiable fraction in some fats and oilsis given in Table 2-19. In most fats the majorcomponents of the unsaponifiable fractionare sterols. Animal fats contain cholesterol
and, in some cases, minor amounts of othersterols such as lanosterol. Plant fats and oilscontain phytosterols, usually at least three,and sometimes four (Fedeli and Jacini 1971).They contain no or only trace amounts ofcholesterol. The predominant phytosterol is(3-sitosterol; the others are campesterol andstigmasterol. In rapeseed oil, brassicasteroltakes the place of stigmasterol. Sterols arecompounds containing the perhydrocyclo-penteno-phenanthrene nucleus, which theyhave in common with many other naturalcompounds, including bile acids, hormones,and vitamin D. The nucleus and the descrip-tion of the four rings, as well as the systemof numbering of the carbon atoms, areshown in Figure 2-12A. The sterols are sol-ids with high melting points. Stereochemi-cally they are relatively flat molecules,usually with all trans linkages, as shown in
Table 2-16 Distribution (wt %) of Milk Fat Triacylglycerols According to Carbon Number and Unsaturation
Number of Double Bonds
Carbon Number
3436384042444648505254Total
O
4.85.04.62.01.51.01.31.62.62.72.2
29.3
1
1.44.96.94.62.42.82.12.23.45.71.4
37.8
2
2.62.93.12.12.92.22.22.71.90.3
22.9
3
3.11.21.21.01.01.00.80.4
9.7
Source: Reprinted with permission from R.G. Jensen and D. S. Newburg, Milk Lipids, in Handbook of Milk Compo-sition, R.G. Jensen, ed., p. 550, © 1995, Academic Press.
Figure 2-11 Structure of the Major Phospholipids
Table 2-17 Composition of the Phospholipids of Cow's Milk
Source: From R.G. Jensen, Composition of Bovine Milk Lipids, J. Am. OH Chem. Soc., Vol. 50, pp. 186-192,1973.
Phosphatidylcholine(lecithin)
Phosphatidylethanolamine(cephalin)
Phosphatidylserine
Phosphoinositides
Phospholipid
PhosphatidylcholinePhosphatidylethanolaminePhosphatidylserinePhosphatidylinositolSphingomyelinLysophosphatidylcholineLysophosphatidylethanolamineTotal choline phospholipidsPlasmalogensDiphosphatidyl glycerolCeramidesCerebrosides
Mole (%)
34.531.83.14.725.2TraceTrace59.7
3TraceTraceTrace
Figure 2-12B. The ring junction betweenrings A and B is trans in some steroids, cis inothers. The junctions between B and C and Cand D are normally trans. Substituents thatlie above the plane, as drawn in Figure 2 -12C, are named p, those below the plane, a.The 3-OH group in cholesterol (Figure 2 -12C) is the p-configuration, and it is thisgroup that may form ester linkages. Thecomposition of the plant sterols is given inFigure 2-13. Part of the sterols in natural
fats are present as esters of fatty acids; forexample, in milk fat, about 10 percent of thecholesterol occurs in the form of cholesterolesters.
The sterols provide a method of distin-guishing between animal and vegetable fatsby means of their acetates. Cholesterol ace-tate has a melting point of 1140C, whereasphytosterol acetates melt in the range of 126to 1370C. This provides a way to detect adul-teration of animal fats with vegetable fats.
Table 2-19 Composition of the Unsaponifiable Fraction of Some Fats and Oils
Oils
OliveLinseedTeaseedSoybeanRapeseedCornLardTallow
Hydrocarbons
2.8-3.53.7-14.0
3.43.88.71.4
23.811.8
Squalene
32-501.0-3.9
2.62.54.32.24.61.2
Aliphatic Alcohols
0.52.5-5.9
4.97.25.02.12.4
Terpenic Alcohols
20-2629-30
23.29.26.77.15.5
Sterols
20-3034.5-52
22.758.463.681.347.064.0
Source: From G. Jacini, E. Fedeli, and A. Lanzani, Research in the Nonglyceride Substances of Vegetable Oils,J. Assoc. Off. Anal. Chem., Vol. 50, pp. 84-90, 1967.
Table 2-18 Triglycerides and Phospholipids of Mackerel Lipids and Calculated Iodine Values for MethylEsters of Fatty Acid from Lipids
Triglycerides
Light fleshDark fleshLiver
In Lipid(%)
89.574.279.5
In Tissue(%)
9.110.714.4
Ester IodineValue
152.3144.3130.9
Phospholipids
In Lipid(%)
4.711.39.3
In Tissue(%)
0.51.61.7
Ester IodineValue
242.9208.1242.1
Source: From R.G. Ackman and C.A. Eaton, Mackerel Lipids and Fatty Acids, Can. lnst. Food ScL Technol. J.,Vol.4, pp. 169-174,1971.
The sterol content of some fats and oils isgiven in Table 2-20. Cholesterol is the mainsterol of animal, fish, and marine fats and oils.
The hydrocarbons of the unsaponifiableoils are members of the it-paraffins as well asof the branched-chain paraffins of the iso andante-iso configuration. The composition ofhydrocarbon constituents of some vegetableoils has been reported by Jacini et al. (1967)and is listed in Table 2-21.
The structure of squalene is presented inFigure 2-14, which also gives the structureof one of the terpenic alcohols, geranyl
geraniol; this alcohol has been reported to bea component of the nonglyceride fraction ofvegetable oils (Fedeli et al. 1966).
AUTOXIDATION
The unsaturated bonds present in all fatsand oils represent active centers that, amongother things, may react with oxygen. Thisreaction leads to the formation of primary,secondary, and tertiary oxidation productsthat may make the fat or fat-containing foodsunsuitable for consumption.
Figure 2-12 Sterols. (A) Structure of the Steroid Nucleus, (B) Stereochemical Representation, and (C)Cholesterol
C
B
A
Me
Me R
The process of autoxidation and the result-ing deterioration in flavor of fats and fattyfoods are often described by the term rancid-ity. Usually rancidity refers to oxidative dete-rioration but, in the field of dairy science,rancidity refers usually to hydrolytic changesresulting from enzyme activity. Lundberg(1961) distinguishes several types of rancid-ity. In fats such as lard, common oxidativerancidity results from exposure to oxygen;this is characterized by a sweet but undesir-able odor and flavor that become progres-sively more intense and unpleasant asoxidation progresses. Flavor reversion is theterm used for the objectionable flavors thatdevelop in oils containing linolenic acid.This type of oxidation is produced with con-siderably less oxygen than with commonoxidation. A type of oxidation similar toreversion may take place in dairy products,where a very small amount of oxygen may
result in intense oxidation off-flavors. It isinteresting to note that the linolenic acid con-tent of milk fat is quite low.
Among the many factors that affect the rateof oxidation are the following:
Table 2-20 Sterol Content of Fats and Oils
Fat Sterol (%)
L a r d 0 / 1 2Beef tallow 0.08Milk fat 0.3Herring 0.2-0.6Cottonseed 1.4Soybean 0.7Corn 1.0Rapeseed 0.4Coconut 0.08Cocoa butter 0.2
/3-Sitosterol
Brassicasterol
Stigmasterol
Campesterol
Figure 2-13 Structures of the Plant Sterols
• amount of oxygen present• degree of unsaturation of the lipids• presence of antioxidants• presence of prooxidants, especially cop-
per, and some organic compounds such asheme-containing molecules and lipoxi-dase
• nature of packaging material• light exposure• temperature of storage
The autoxidation reaction can be dividedinto the following three parts: initiation,propagation, and termination. In the initia-tion part, hydrogen is abstracted from an ole-fmic compound to yield a free radical.
RH -> R* + H"
The removal of hydrogen takes place at thecarbon atom next to the double bond and can
be brought about by the action of, forinstance, light or metals. The dissociationenergy of hydrogen in various olefinic com-pounds has been listed by Ohloff (1973) andis shown in Table 2-22. Once a free radicalhas been formed, it will combine with oxy-gen to form a peroxy-free radical, which canin turn abstract hydrogen from another unsat-urated molecule to yield a peroxide and anew free radical, thus starting the propaga-tion reaction. This reaction may be repeatedup to several thousand times and has thenature of a chain reaction.
Table 2-21 Hydrocarbon Composition of Some Vegetable Oils
Oils
CornPeanutRapeseedLinseedOlive
n-Paraffins
Cl 1-31C-1 1-30Cl 1-31Cl 1-35
Cl 1» GI 3-30
iso- and/or ante-isoParaffins
Cl 1-21G! 1-23
C-1 1-17> 1̂9-21Cl 1-21
Unidentified
87676
TotalHydrocarbons
404036
43-4529
Source: From G. Jacini, E. Fedeli, and A. Lanzani, Research in the Nonglyceride Substances of Vegetable Oils, J.Assoc. Off. Anal. Chem., Vol. 50, pp. 84-90, 1967.
Figure 2-14 Structure of Squalene and Geranyl GeraniolGeranyl geraniol
Squalene
Source: From G. Ohloff, Fats as Precursors, in Func-tional Properties of Fats in Foods, J. Solms, ed., 1973,Forster Publishing.
The propagation can be followed by termi-nation if the free radicals react with them-selves to yield nonactive products, as shownhere:
carbonyls, which are the most important. Theperoxides have no importance to flavor dete-rioration, which is wholly caused by the sec-ondary oxidation products. The nature of theprocess can be represented by the curves ofFigure 2-15 (Pokorny 1971). In the initialstages of the reaction, the amount of hydro-peroxides increases slowly; this stage istermed the induction period. At the end ofthe induction period, there is a suddenincrease in peroxide content. Because perox-ides are easily determined in fats, the perox-ide value is frequently used to measure theprogress of oxidation. Organoleptic changesare more closely related to the secondaryoxidation products, which can be measuredby various procedures, including the benzi-dine value, which is related to aldehydedecomposition products. As the aldehydesare themselves oxidized, fatty acids areformed; these free fatty acids may be consid-ered tertiary oxidation products. The lengthof the induction period, therefore, depends
Table 2-22 Dissociation Energy for theAbstraction of Hydrogen from OlefinicCompounds and Peroxides
CompoundAE (tea//
mole)
The hydroperoxides formed in the propaga-tion part of the reaction are the primary oxi-dation products. The hydroperoxide mech-anism of autoxidation was first proposed byFarmer (1946). These oxidation productsare generally unstable and decompose intothe secondary oxidation products, whichinclude a variety of compounds, including
VA
LUE
Figure 2-15 Autoxidation of Lard. (A) peroxidevalue, (B) benzidine value, (C) acid value.Source: From J. Pokorny, Stabilization of Fats byPhenolic Antioxidants, Can. Inst. Food ScL Tech-nol J., Vol. 4, pp. 68-74, 1971.
TIME
R-R
RO2R
(R02)n
R" + R"
R' + RO2*
nR02 '
In addition to the changes in double bondposition, there is isomerization from cis totrans, and 90 percent of the peroxidesformed may be in the trans configuration(Lundberg 1961).
From linoleic acid (a'si-c/5'-9,12-octadeca-dienoic acid), three isomeric hydroperoxidescan be formed as shown in the next formula.In this mixture of 9, 11, and 13 hydroperox-ides, the conjugated ones occur in greatest
quantity because they are the more stableforms. The hydroperoxides occur in the cis-trans and trans-trans configurations, thecontent of the latter being greater with highertemperature and greater extent of oxidation.From the oxidation of linolenic acid (cis, cis,ds-9,12,15-octadecatrienoic acid), six iso-metric hydroperoxides can be expectedaccording to theory, as shown:
on the method used to determine oxidationproducts.
Although even saturated fatty acids may beoxidized, the rate of oxidation greatly de-pends on the degree of unsaturation. In theseries of 18-carbon-atom fatty acids 18:0,18:1, 18:2, 18:3, the relative rate of oxidationhas been reported to be in the ratio of1:100:1200:2500. The reaction of unsatur-ated compounds proceeds by the abstractionof hydrogen from the a carbon, and the
resulting free radical is stabilized by reso-nance as follows:
1 2 3 1 2 3
-CH-CH=CH- ^=^ -CH=CH-CH-• •
If oleic acid is taken as example of a mono-ethenoid compound (c/.s-9-octadecenoicacid), the reaction will proceed by abstrac-tion of hydrogen from carbons 8 or 11,resulting in two pairs of resonance hybrids.
This leads to the formation of the followingfour isomeric hydroperoxides:
Hydroperoxides of linolenate decomposemore readily than those of oleate andlinoleate because active methylene groupsare present. The active methylene groups arethe ones located between a single doublebond and a conjugated diene group. Thehydrogen at this methylene group couldreadily be abstracted to form dihydroperox-ides. The possibilities here for decomposi-tion products are obviously more abundantthan with oleate oxidation.
The decomposition of hydroperoxides hasbeen outlined by Keeney (1962). The firststep involves decomposition to the alkoxyand hydroxy free radicals.
R-CH(OOH)-R » R—CH-R + "OH
O*
The alkoxy radical can react to form alde-hydes.
R — C H - R » R' + RCHO
O*
This reaction involves fission of the chainand can occur on either side of the free radi-cal. The aldehyde that is formed can be ashort-chain volatile compound, or it can beattached to the glyceride part of the mole-cule; in this case, the compound is nonvola-
tile. The volatile aldehydes are in great partresponsible for the oxidized flavor of fats.
The alkoxy radical may also abstract ahydrogen atom from another molecule toyield an alcohol and a new free radical, asshown:
R—CH-R + R1H » R—CH—R + R1'
O" OH
The new free radicals formed may partici-pate in propagation of the chain reaction.Some of the free radicals may interact withthemselves to terminate the chain, and thiscould lead to the formation of ketones as fol-lows:
R — C H - R + R1' > R—C—R + R1HI Il
O' O
As indicated, a variety of aldehydes havebeen demonstrated in oxidized fats. Alcoholshave also been identified, but the presence ofketones is not as certain. Keeney (1962) haslisted the aldehydes that may be formed frombreakdown of hydroperoxides of oxidizedoleic, linoleic, linolenic, and arachidonicacids (Table 2-23). The aldehydes are pow-erful flavor compounds and have very lowflavor thresholds; for example, 2,4-decadie-nal has a flavor threshold of less than onepart per billion. The presence of a doublebond in an aldehyde generally lowers the fla-vor threshold considerably. The aldehydescan be further oxidized to carboxylic acids orother tertiary oxidation products.
When chain fission of the alkoxy radicaloccurs on the other side of the free radicalgroup, the reaction will not yield volatilealdehydes but will instead form nonvolatilealdehydo-glycerides. Volatile oxidation prod-ucts can be removed in the refining process
during deodorization, but the nonvolatileproducts remain; this can result in a loweroxidative stability of oils that have alreadyoxidized before refining.
The rate and course of autoxidation dependprimarily on the composition of the fat—itsdegree of unsaturation and the types ofunsaturated fatty acids present. The absence,or at least a low value, of peroxides does not
necessarily indicate that an oil is not oxi-dized. As Figure 2-16 indicates, peroxidesare labile and may be transformed into sec-ondary oxidation products. A combinedindex of primary and secondary oxidationproducts gives a better evaluation of the stateof oxidation of an oil. This is expressed asTotox value: Totox value = 2 x peroxidevalue + anisidine value. (Anisidine value is a
"Only the most active methylene groups in each acid are considered.
Source: From M. Keeney, Secondary Degradation Products, in Lipids and Their Oxidation, H.W. Schultz et al.,eds., 1962, AVI Publishing Co.
Table 2-23 Hydroperoxides and Aldehydes (with Single Oxygen Function) That May Be Formed inAutoxidation of Some Unsaturated Fatty Acids
Fatty Acid
Oleic
Llnoleic
Linolenic
Arachidonic
MethyleneGroup
Involved"
11
8
11
14
11
13
10
7
lsomeric Hydroperoxides Formed fromthe Structures Contributing to the
Intermediate Free Radical ResonanceHybrid
1 1-hydroperoxy-9-ene9-hydroperoxy-1 0-ene8-hydroperoxy-9-ene1 0-hydroperoxy-8-ene1 3-hydroperoxy-9,1 1 -diene1 1-hydroperoxy-9,12-diene9-hydroperoxy- 1 0, 1 2-diene1 6-hydroperoxy-9, 1 2, 1 4-triene1 4-hydroperoxy-9, 1 2, 1 5-triene1 2-hydroperoxy-9, 1 3, 1 5-triene13-hydroperoxy-9, 11,1 5-triene1 1 -hydroperoxy-9,1 2,1 5-triene9-hydroperoxy- 1 0,1 2,1 5-triene1 5-hydroperoxy-5,8,1 1 ,1 3-tetraene1 3-hydroperoxy-5,8, 11,1 4-tetraene1 1 -hydroperoxy-5,8, 12,1 4-tetraene1 2-hydroperoxy-5,8, 1 0, 1 4-tetraene1 0-hydroperoxy-5,8, 11,1 4-tetraene8-hydroperoxy-5,9,1 1 ,1 4-tetraene9-hydroperoxy-5,7, 11,1 4-tetraene7-hydroperoxy-5,8, 11,1 4-tetraene5-hydroperoxy-6,8,1 1 ,1 4-tetraene
Aldehydes Formed by Decom-position of the Hydroperoxides
octanal2-decenal2-undecenalnonanalhexanal2-octenal2,4-decadienalpropanal2-pentenal2,4-heptadienal3-hexenal2,5-octadienal2,4,7-decatrienalhexanal2-octenal2,4-decadienal3-nonenal2,5-undecadienal2,4,7-tridecatrienal3,6-dodecadienal2,5,8-tetradecatrienal2,4,7,1 0-hexadecatetraenal
measure of secondary oxidation products.)Removal of oxygen from foods will preventoxidation, but, in practice, this is not easy toaccomplish in many cases. At high tempera-tures (100 to 14O0C) such as those used inthe accelerated tests for oil stability (activeoxygen method), formic acid is produced,which can be used to indicate the end of theinduction period. The formation of formicacid results from aldehyde decomposition.Peroxidation of aldehydes establishes a reso-nance equilibrium between two limitingforms.
The second hybrid ties up oxygen at the acarbon to yield the cc-hydroperoxy aldehydeas follows:
Breakdown of oxygen and carbon bondsyields formic acid and a new aldehyde.
Per
oxid
e V
alue
InductionPeriod
Free RadicalInitiationPhase
Free RadicalPropagation
"PeroxideFormationPhase"
Free RadicalTermination Phase
PeroxideStabi I ization Perox ide
DecompositionPhase
Time (hours)
Figure 2-16 Peroxide Formation and Decomposition as a Function of Time
deMan et al. (1987) investigated this reactionwith a variety of oils and found that althoughformic acid was the main reaction product,other short-chain acids from acetic to caproicwere also formed. Trace metals, especiallycopper, and to a lesser extent iron, will cata-lyze fat oxidation; metal deactivators such ascitric acid can be used to reduce the effect.Lipoxygenase (lipoxidase) and heme com-pounds act as catalysts of lipid oxidation.Antioxidants can be very effective in slowingdown oxidation and increasing the inductionperiod. Many foods contain natural antioxi-dants; the tocopherols are the most importantof these. They are present in greater amountsin vegetable oils than in animal fats, whichmay explain the former's greater stability.
Antioxidants such as tocopherols may benaturally present; they may be induced byprocesses such as smoking or roasting, oradded as synthetic antioxidants. Antioxi-dants act by reacting with free radicals, thusterminating the chain. The antioxidant AHmay react with the fatty acid free radical orwith the peroxy free radical,
AH+R--»RH+A-AH+R02'->RO2H + A'
The antioxidant free radical deactivated byfurther oxidation to quinones, thus terminat-ing the chain. Only phenolic compounds thatcan easily produce quinones are active asantioxidants (Pokorny 1971). At high con-centrations antioxidants may have a prooxi-dant effect and one of the reactions may beas follows:
A- + RH->AH+R-
Tocopherols in natural fats are usuallypresent at optimum levels. Addition of anti-
oxidant beyond optimum amounts may resultin increasing the extent of prooxidant action.Lard is an example of a fat with very low nat-ural antioxidant activity and antioxidant mustbe added to it, to provide protection. Theeffect of antioxidants can be expressed interms of protection factor, as shown in Figure2-17 (Pokorny 1971). The highly active anti-oxidants that are used in the food industry areactive at about 10 to 50 parts per million(ppm). Chemical structure of the antioxidantsis the most important factor affecting theiractivity. The number of synthetic antioxi-dants permitted in foods is limited, and thestructure of the most widely used compoundsis shown in Figure 2-18. Propyl gallate ismore soluble in water than in fats. The octyland dodecyl esters are more fat soluble. Theyare heat resistant and nonvolatile with steam,making them useful for frying oils and inbaked products. These are considered to havecarry-through properties. Butylated hydroxy-anisole (BHA) has carry-through propertiesbut butylated hydroxy toluene (BHT) doesnot, because it is volatile with steam. Thecompound tert-butyl hydroquinine (TBHQ)is used for its effectiveness in increasing oxi-dative stability of polyunsaturated oils andfats. It also provides carry-through protectionfor fried foods. Antioxidants are frequentlyused in combination or together with syner-gists. The latter are frequently metal deactiva-tors that have the ability to chelate metal ions.An example of the combined effect of antiox-idants is shown in Figure 2-19. It has beenpointed out (Zambiazi and Przybylski 1998)that fatty acid composition can explain onlyabout half of the oxidative stability of a vege-table oil. The other half can be contributed tominor components including tocopherols,metals, pigments, free fatty acids, phenols,phospholipids, and sterols.
PHOTOOXIDATION
Oxidation of lipids, in addition to the freeradical process, can be brought about by atleast two other mechanisms—photooxida-tion and enzymic oxidation by lipoxygenase.The latter is dealt with in Chapter 10. Light-
induced oxidation or photooxidation resultsfrom the reactivity of an excited state of oxy-gen, known as singlet oxygen (1O2). Ground-state or normal oxygen is triplet oxygen(3O2). The activation energy for the reactionof normal oxygen with an unsaturated fattyacid is very high, of the order of 146 to 273
Figure 2-18 Structure of Propyl Gallate (PG), Butylated Hydroxyanisole (BHA), Butylated HydroxyToluene (BHT), and Tert-Butyl Hydroquinone (TBHQ)
PG BHA BHT TBHQ
Figure 2-17 Determination of Protection Factor. (A) lard, (B) lard + antioxidant. Source: From J.Pokorny, Stabilization of Fats by Phenolic Antioxidants, Can. Inst. Food ScL TechnoL /., Vol. 4, pp.68-74, 1971.
TIME
PVPF=S^St1
kJ/mole. When oxygen is converted from theground state to the singlet state, energy istaken up amounting to 92 kJ/mole, and inthis state the oxygen is much more reactive.Singlet-state oxygen production requires thepresence of a sensitizer. The sensitizer isactivated by light, and can then either reactdirectly with the substrate (type I sensitizer)or activate oxygen to the singlet state (type IIsensitizer). In both cases unsaturated fattyacid residues are converted into hydroperox-ides. The light can be from the visible orultraviolet region of the spectrum.
Singlet oxygen is short-lived and revertsback to the ground state with the emission oflight. This light is fluorescent, which meansthat the wavelength of the emitted light ishigher than that of the light that wasabsorbed for the excitation. The reactivity ofsinglet oxygen is 1,500 greater than that ofground-state oxygen. Compounds that canact as sensitizers are widely occurring foodcomponents, including chlorophyll, myoglo-bin, riboflavin, and heavy metals. Most of
these compounds promote type II oxidationreactions. In these reactions the sensitizer istransformed into the activated state by light.The activated sensitizer then reacts with oxy-gen to produce singlet oxygen.
hvsen ^- sen*
sen* + O 2 *- sen + 1O2
The singlet oxygen can react directly withunsaturated fatty acids.
1O2 + RH ^ ROOH
The singlet oxygen reacts directly with thedouble bond by addition, and shifts the dou-ble bond one carbon away. The singlet oxy-gen attack on linoleate produces fourhydroperoxides as shown in Figure 2-20.Photooxidation has no induction period, butthe reaction can be quenched by carotenoids
Figure 2-19 Effect of Copper Concentration on Protective Effect of Antioxidants in Lard. (A) lard +0.01% BHT, (B) lard + 0.01% ascorbyl palmitate, (C) lard + 0.005% BHT and 0.05% ascorbyl palmi-tate. Source: From J. Pokorny, Stabilization of Fats by Phenolic Antioxidants, Can. Inst. Food ScLTechnol J., Vol. 4, pp. 68-74, 1971.
L O G CCu
PV
A B C
that effectively compete for the singlet oxy-gen and bring it back to the ground state.
Phenolic antioxidants do not protect fatsfrom oxidation by singlet oxidation (Yasaeiet al. 1996). However, the antioxidant ascor-byl palmitate is an effective singlet oxygenquencher (Lee et al. 1997). Carotenoids arewidely used as quenchers. Rahmani andCsallany (1998) reported that in the photoox-idation of virgin olive oil, pheophytin Afunctioned as sensitizer, while p-caroteneacted as a quencher.
The combination of light and sensitizers ispresent in many foods displayed in transpar-ent containers in brightly lit supermarkets.The light-induced deterioration of milk hasbeen studied extensively. Sattar et al. (1976)
reported on the light-induced flavor deterio-ration of several oils and fats. Of the five fatsexamined, milk fat and soybean oil weremost susceptible and corn oil least suscepti-ble to singlet oxygen attack. The effect oftemperature on the rate of oxidation of illu-minated corn oil was reported by Chahineand deMan (1971) (Figure 2-21). Theyfound that temperature has an importanteffect on photooxidation rates, but evenfreezing does not completely prevent oxida-tion.
HEATED FATS—FRYING
Fats and oils are heated during commercialprocessing and during frying. Heating during
Figure 2-20 Photooxidation. Singlet-oxygen attack on oleate produces two hydroperoxides; linoleateyields four hydroperoxides
Methyl linoleate
Methyl linoleate
Methyl oleate
Methyl oleate
processing mainly involves hydrogenation,physical refining, and deodorization. Tem-perature used in these processes may rangefrom 12O0C to 27O0C. The oil is not in con-tact with air, which eliminates the possibilityof oxidation. At the high temperatures usedin physical refining and deodorization, sev-eral chemical changes may take place. Theseinclude randomization of the glyceride struc-ture, dimer formation, cis-trans isomeriza-tion, and formation of conjugated fatty acids(positional isomerization) of polyunsaturatedfatty acids (Hoffmann 1989). The trans iso-mer formation in sunflower oil as a result ofhigh temperature deodorization is shown inFigure 2-22 (Ackman 1994).
Conditions prevailing during frying areless favorable than those encountered in theabove-mentioned processes. Deep frying,where the food is heated by immersion in hot
oil, is practiced in commercial frying as wellas in food service operations. The tempera-tures used are in the range of 16O0C to1950C. At lower temperatures frying takeslonger, and at higher temperatures deteriora-tion of the oil is the limiting factor. Deep fry-ing is a complex process involving both theoil and the food to be fried. The reactionstaking place are schematically presented inFigure 2-23. Steam is given off during thefrying, which removes volatile antioxidants,free fatty acids, and other volatiles. Contactwith the air leads to autoxidation and the for-mation of a large number of degradationproducts. The presence of steam results inhydrolysis, with the production of free fattyacids and partial glycerides. At lower fryingtemperatures the food has to be fried longerto reach the desirable color, and this resultsin higher oil uptake. Oil absorption by fried
STORAGE TIME (HOURS)
Figure 2-21 Effect of Temperature on Rate of Oxidation of Illuminated Corn Oil. Source: From M.H.Chahine and J.M. deMan, Autoxidation of Corn Oil under the Influence of Fluorescent Light, Can.Inst. Food ScI Technol J.t Vol. 4, pp. 24-28, 1971.
PE
RO
XID
E
VA
LU
E
(meq
uiv.
/Kg)
foods may range from 10 to 40 percent,depending on conditions of frying and thenature and size of the food.
Oils used in deep frying must be of highquality because of the harsh conditions dur-ing deep frying and to provide satisfactoryshelf life in fried foods. The suitability of anoil for frying is directly related to its contentof unsaturated fatty acids, especially lino-lenic acid. This has been described by Erick-son (1996) as "inherent stability" calculatedfrom the level of each of the unsaturatedfatty acids (oleic, linoleic, and linolenic) andtheir relative reaction rate with oxygen. Theinherent stability calculated for a number ofoils is given in Table 2-24. The higher theinherent stability, the less suitable the oil isfor frying. The liquid seed oils, such as soy-bean and sunflower oil, are not suitable fordeep frying and are usually partially hydro-genated for this purpose. Such hydrogenatedoils can take the form of shortenings, which
may be plastic solids or pourable suspen-sions. Through plant breeding and geneticengineering, oils with higher inherent stabil-ity can be obtained, such as high-oleic sun-flower oil, low-linolenic canola oil, and low-linolenic soybean oil.
The stability of frying oils and fats is usu-ally measured by an accelerated test knownas the active oxygen method (AOM). In thistest, air is bubbled through an oil samplemaintained at 950C and the peroxide value ismeasured at intervals. At the end point theperoxide value shows a sharp increase, andthis represents the AOM value in hours. Typ-ical AOM values for liquid seed oils rangefrom 10 to 30 hours; heavy-duty fryingshortenings range from 200 to 300 hours.AOM values of some oils and fats deter-mined by measuring the peroxide value andusing an automatic recording of volatileacids produced during the test are given inTable 2-25 (deMan et al. 1987).
Figure 2-22 Trans Isomer Formation in Sunflower Oil as a Function of Deodorization Temperature.Source: Reprinted from R.G. Ackman, Animal and Marine Lipids, in Improved and TechnologicalAdvances in Alternative Sources of Lipids, B. Kamel and Y. Kakuda, eds., p. 301, 1994, Aspen Publish-ers, Inc.
Hours
trans c
onte
nt
(w/w
%)
As shown in Figure 2-23, oil breakdownduring frying can be caused by oxidation andthermal alteration. Oxidation can result in theformation of oxidized monomeric, dimeric,and oligomeric triglycerides as well as vola-tile compounds including aldehydes, ketones,alcohols, and hydrocarbons. In addition, oxi-dized sterols may be formed. Thermal degra-
dation can result in cyclic monomeric tri-glycerides and nonpolar dimeric and oligo-meric triglycerides. The polymerization reac-tion may take place by conversion of part ofthe cw-cw-1,4 diene system of linoleates tothe trans-trans conjugated diene. The 1,4 and1,3 dienes can combine in a Diels-Alder typeaddition reaction to produce a dimer as
Figure 2-23 Summary of Chemical Reactions Occurring During Deep Frying. Source: Reprinted withpermission from FT. Orthoefer, S. Gurkin, and K. Lui, Dynamics of Frying in Deep Frying, in Chem-istry, Nutrition and Practical Applications, E.G. Perkins and M.D. Erickson, eds., p. 224. © 1996,AOCS Press.
dlmerscyclic compounds
HEATING
acids hydrocarbons
dlmerst rimersepoxldesalcoholshydrocarbons
ketonesalcoholsaldehydes
DEHYDRATION FREE RADICALSFISSION
hydroperoxides(conjugated dlenes)
colored compoundsfood llpids
SOLUBILIZATION
free fatty acidsdiglyceridesmonoglyceridesglycerine
HYOROLYSIS
steam
FOODOXIDATION
oxygen
AERATION ABSORPTION VAPORIZATION
steamvollflles (smoke!antfoxldants
Table 2-25 Active Oxygen Method (AOM) Time ofSeveral Oils and Fats as Determined by PeroxideValue and Conductivity Measurements
AOM Time AOM TimeOil (POVf (Conductivity)*
Sunflower 6.2 7.1Canola 14.0 15.8Olive 17.8 17.8Corn 12.4 13.8Peanut 21.1 21.5Soybean 11.0 10.4Triolein 8.1 7.4Lard 42.7 43.2Butterfat 2.8 2.0
3At peroxide value 100.bAt intercept of conductivity curve and time axis.
Source: Reprinted with permission from J.M. deMan,et al., Formation of Short Chain Volatile Organic Acidsin the Automated AOM Method, J.A.O.C.S., Vol. 64, p.996, © 1987, American Oil Chemists' Society.
shown in Figure 2-24. Other possible routesfor dimer formation are through free radicalreactions. As shown in Figure 2-25, this mayinvolve combination of radicals, intermolecu-lar addition, and intramolecular addition.From dimers, higher oligomers can be pro-duced; the structure of these is still relativelyunknown.
Another class of compounds formed duringfrying is cyclic monomers of fatty acids.Linoleic acid can react at either the C9 or C12double bonds to give rings between carbons 5and 9, 5 and 10, 8 and 12, 12 and 17, and 13and 17. Cyclic monomers with a cyclopente-nyl ring have been isolated from heated sun-flower oil, and their structure is illustrated inFigure 2-26 (Le Quere and Sebedio 1996).
Some countries such as France require thatfrying oils contain less than 2 percent lino-lenic acid. Several European countries haveset maximum limits for the level of polar
Table 2-24 Inherent Stability of Oils for Use in Frying
Oil
SoybeanSunflowerHigh-oleic sunflowerCornCottonseedCanolaPeanutLardOlivePalmPahn oleinPalm stearinTallowPalm kernelCoconut
Iodine Value
1301209011.09811092608855583550179
Inherent Stability*
7.47.72.06.25.25.44.51.41.81.41.61.00.70.50.4
Inherent stability calculated from decimal fraction of fatty acids multiplied by relative reaction rates with oxygen,assuming rate for oleic acid = 1 , linoleic acid = 10, and linolenic acid = 25.
compounds or for the level of free fatty acidsbeyond which the fat is considered unfit forhuman consumption. In continuous indus-trial frying, oil is constantly being removedfrom the fryer with the fried food and replen-ished with fresh oil so that the quality of theoil can remain satisfactory. This is more diffi-cult in intermittent frying operations.
FLAVOR REVERSION
Soybean oil and other fats and oils contain-ing linolenic acid show the reversion phe-nomenon when exposed to air. Reversionflavor is a particular type of oxidized flavorthat develops at comparatively low levels ofoxidation. The off-flavors may develop in oils
Figure 2-24 Polymerization of Diene Systems To Form Dimers
a) Combination of radicals:
b) lntermolecular addition:
c) Intramolecular addition:
Figure 2-25 Nonpolar Dimer Formation Through Free Radical Reactions
Next page
INTRODUCTION
Proteins are polymers of some 21 differentamino acids joined together by peptidebonds. Because of the variety of side chainsthat occur when these amino acids are linkedtogether, the different proteins may have dif-ferent chemical properties and widely differ-ent secondary and tertiary structures. Thevarious amino acids joined in a peptide chainare shown in Figure 3-1. The amino acidsare grouped on the basis of the chemicalnature of the side chains (Krull and Wall1969). The side chains may be polar or non-polar. High levels of polar amino acid resi-dues in a protein increase water solubility.The most polar side chains are those of thebasic and acidic amino acids. These aminoacids are present at high levels in the solublealbumins and globulins. In contrast, the wheatproteins, gliadin and glutenin, have low levelsof polar side chains and are quite insoluble inwater. The acidic amino acids may also bepresent in proteins in the form of theiramides, glutamine and asparagine. Thisincreases the nitrogen content of the protein.Hydroxyl groups in the side chains maybecome involved in ester linkages with phos-phoric acid and phosphates. Sulfur aminoacids may form disulfide cross-links betweenneighboring peptide chains or between dif-
ferent parts of the same chain. Proline andhydroxyproline impose significant structurallimitations on the geometry of the peptidechain.
Proteins occur in animal as well as vegeta-ble products in important quantities. In thedeveloped countries, people obtain much oftheir protein from animal products. In otherparts of the world, the major portion ofdietary protein is derived from plant prod-ucts. Many plant proteins are deficient in oneor more of the essential amino acids. Theprotein content of some selected foods islisted in Table 3-1.
AMINO ACID COMPOSITION
Amino acids joined together by peptidebonds form the primary structure of proteins.The amino acid composition establishes thenature of secondary and tertiary structures.These, in turn, significantly influence thefunctional properties of food proteins andtheir behavior during processing. Of the 20amino acids, only about half are essential forhuman nutrition. The amounts of these essen-tial amino acids present in a protein and theiravailability determine the nutritional qualityof the protein. In general, animal proteins areof higher quality than plant proteins. Plant
Proteins
CHAPTER 3
Figure 3-1 Component Amino Acids of ProteinsJoined by Peptide Bonds and Character of SideChains. Source: From Northern Regional Re-search Laboratory, U.S. Department of Agricul-ture.
proteins can be upgraded nutritionally byjudicious blending or by genetic modificationthrough plant breeding. The amino acid com-position of some selected animal and vegeta-ble proteins is given in Table 3—2.
Egg protein is one of the best quality pro-teins and is considered to have a biologicalvalue of 100. It is widely used as a standard,and protein efficiency ratio (PER) valuessometimes use egg white as a standard.Cereal proteins are generally deficient inlysine and threonine, as indicated in Table
Table 3-1 Protein Content of Some SelectedFoods
Product Protein (g/1 OO g)
Meat: beef 16.5pork 10.2
Chicken (light meat) 23.4Fish: haddock 18.3
cod 17.6Milk 3.6Egg 12.9Wheat 13.3Bread 8.7Soybeans: dry, raw 34.1
cooked 11.0Peas 6.3Beans: dry, raw 22.3
cooked 7.8Rice: white, raw 6.7
cooked 2.0Cassava 1.6Potato 2.0Corn 10.0
3-3. Soybean is a good source of Iysine butis deficient in methionine. Cottonseed pro-tein is deficient in lysine and peanut proteinin methionine and lysine. The protein ofpotato although present in small quantity(Table 3-1) is of excellent quality and isequivalent to that of whole egg.
Table 3-3 Limiting Essential Amino Acids ofSome Grain Proteins
First SecondLimiting Limiting
Grain Amino Acid Amino Acid
Wheat Lysine ThreonineCorn Lysine TryptophanRice Lysine ThreonineSorghum Lysine ThreonineMillet Lysine Threonine
PROTEIN CLASSIFICATION
Proteins are complex molecules, and classi-fication has been based mostly on solubility indifferent solvents. Increasingly, however, asmore knowledge about molecular composi-tion and structure is obtained, other criteriaare being used for classification. Theseinclude behavior in the ultracentrifuge andelectrophoretic properties. Proteins are di-vided into the following main groups: simple,conjugated, and derived proteins.
Simple Proteins
Simple proteins yield only amino acids onhydrolysis and include the following classes:
• Albumins. Soluble in neutral, salt-freewater. Usually these are proteins of rela-tively low molecular weight. Examples
Table 3-2 Amino AcJd Content of Some Selected Foods (mg/g Total Nitrogen)
Amino Acid
lsoleucineLeucineLysineMethlonineCystinePhenylalanineTyroslneThreonineValineArginineHistidineAlanineAspartic acidGlutamic acidGlycineProlineSerine
Meat (Beef)
30150755616980
275225287313395213365562955304236252
Milk
399782450156
434396278463160214255424
1151144514342
Egg
393551436210152358260320428381152370601796207260478
Wheat
20441717994
159282187183276288143226308
1866245621281
Peas
2674254705770
287171254294595143255685
1009253244271
Com
23078316712097
305239225303262170471392
1184231559311
are egg albumin, lactalbumin, and serumalbumin in the whey proteins of milk,leucosin of cereals, and legumelin inlegume seeds.
• Globulins. Soluble in neutral salt solu-tions and almost insoluble in water.Examples are serum globulins and (3-lac-toglobulin in milk, myosin and actin inmeat, and glycinin in soybeans.
• Glutelins. Soluble in very dilute acid orbase and insoluble in neutral solvents.These proteins occur in cereals, such asglutenin in wheat and oryzenin in rice.
• Prolamins. Soluble in 50 to 90 percentethanol and insoluble in water. Theseproteins have large amounts of prolineand glutamic acid and occur in cereals.Examples are zein in corn, gliadin inwheat, and hordein in barley.
• Scleroproteins. Insoluble in water andneutral solvents and resistant to enzymichydrolysis. These are fibrous proteinsserving structural and binding purposes.Collagen of muscle tissue is included inthis group, as is gelatin, which is derivedfrom it. Other examples include elastin,a component of tendons, and keratin, acomponent of hair and hoofs.
• Histories. Basic proteins, as defined bytheir high content of lysine and arginine.Soluble in water and precipitated byammonia.
• Protamines. Strongly basic proteins oflow molecular weight (4,000 to 8,000).They are rich in arginine. Examples areclupein from herring and scombrin frommackerel.
Conjugated Proteins
Conjugated proteins contain an aminoacid part combined with a nonprotein mate-rial such as a lipid, nucleic acid, or carbohy-
drate. Some of the major conjugatedproteins are as follows:
• Phosphoproteins. An important groupthat includes many major food proteins.Phosphate groups are linked to thehydroxyl groups of serine and threonine.This group includes casein of milk andthe phosphoproteins of egg yolk.
• Lipoproteins. These are combinations oflipids with protein and have excellentemulsifying capacity. Lipoproteins occurin milk and egg yolk.
• Nucleoproteins. These are combinationsof nucleic acids with protein. Thesecompounds are found in cell nuclei.
• Glycoproteins. These are combinationsof carbohydrates with protein. Usuallythe amount of carbohydrate is small, butsome glycoproteins have carbohydratecontents of 8 to 20 percent. An exampleof such a mucoprotein is ovomucin ofegg white.
• Chromopmteins. These are proteins witha colored prosthetic group. There aremany compounds of this type, includinghemoglobin and myoglobin, chlorophyll,and flavoproteins.
Derived Proteins
These are compounds obtained by chemi-cal or enzymatic methods and are dividedinto primary and secondary derivatives, de-pending on the extent of change that hastaken place. Primary derivatives are slightlymodified and are insoluble in water; rennet-coagulated casein is an example of a primaryderivative. Secondary derivatives are moreextensively changed and include proteoses,peptones, and peptides. The differencebetween these breakdown products is in sizeand solubility. All are soluble in water and
not coagulated by heat, but proteoses can beprecipitated with saturated ammonium sul-fate solution. Peptides contain two or moreamino acid residues. These breakdown prod-ucts are formed during the processing ofmany foods, for example, during ripening ofcheese.
PROTEIN STRUCTURE
Proteins are macromolecules with differentlevels of structural organization. The primarystructure of proteins relates to the peptidebonds between component amino acids andalso to the amino acid sequence in the mole-cule. Researchers have elucidated the aminoacid sequence in many proteins. For exam-ple, the amino acid composition and se-quence for several milk proteins is now wellestablished (Swaisgood 1982).
Some proteolytic enzymes have quite spe-cific actions; they attack only a limited num-ber of bonds, involving only particular aminoacid residues in a particular sequence. Thismay lead to the accumulation of well-definedpeptides during some enzymic proteolyticreactions in foods.
The secondary structure of proteins in-volves folding the primary structure. Hydro-gen bonds between amide nitrogen and car-bonyl oxygen are the major stabilizing force.These bonds may be formed between differ-ent areas of the same polypeptide chain orbetween adjacent chains. In aqueous media,the hydrogen bonds may be less significant,and van der Waals forces and hydrophobicinteraction between apolar side chains maycontribute to the stability of the secondarystructure. The secondary structure may beeither the oc-helix or the sheet structure, asshown in Figure 3-2. The helical structuresare stabilized by intramolecular hydrogenbonds, the sheet structures by intermolecular
hydrogen bonds. The requirements for maxi-mum stability of the helix structure wereestablished by Pauling et al. (1951). Thehelix model involves a translation of 0.54 nmper turn along the central axis. A completeturn is made for every 3.6 amino acid resi-dues. Proteins do not necessarily have tooccur in a complete a-helix configuration;rather, only parts of the peptide chains maybe helical, with other areas of the chain in amore or less unordered configuration. Pro-teins with a-helix structure may be eitherglobular or fibrous. In the parallel sheetstructure, the polypeptide chains are almostfully extended and can form hydrogen bondsbetween adjacent chains. Such structures aregenerally insoluble in aqueous solvents andare fibrous in nature.
The tertiary structure of proteins involves apattern of folding of the chains into a com-pact unit that is stabilized by hydrogen bonds,van der Waals forces, disulfide bridges, andhydrophobic interactions. The tertiary struc-ture results in the formation of a tightlypacked unit with most of the polar amino acidresidues located on the outside and hydrated.This leaves the internal part with most of theapolar side chains and virtually no hydration.Certain amino acids, such as proline, disruptthe a-helix, and this causes fold regions withrandom structure (Kinsella 1982). The natureof the tertiary structure varies among proteinsas does the ratio of a-helix and random coil.Insulin is loosely folded, and its tertiary struc-ture is stabilized by disulfide bridges. Lyso-zyme and glycinin have disulfide bridges butare compactly folded.
Large molecules of molecular weightsabove about 50,000 may form quaternarystructures by association of subunits. Thesestructures may be stabilized by hydrogenbonds, disulfide bridges, and hydrophobicinteractions. The bond energies involved in
Figure 3-2 Secondary Structures of Proteins, (A) Alpha Helix, (B) Antiparallel Sheet
3rdturn
2ndturn
1stturn
Rise perresidue
forming these structures are listed in Table3-4.
The term subunit denotes a protein chainpossessing an internal covalent and noncova-lent structure that is capable of joining withother similar subunits through noncovalentforces or disulfide bonds to form an oligo-meric macromolecule (Stanley and Yada1992). Many food proteins are oligomericand consist of a number of subunits, usually2 or 4, but occasionally as many as 24. A list-ing of some oligomeric food proteins isgiven in Table 3-5. The subunits of proteinsare held together by various types of bonds:electrostatic bonds involving carboxyl,amino, imidazole, and guanido groups; hy-drogen bonds involving hydroxyl, amide,and phenol groups; hydrophobic bonds in-volving long-chain aliphatic residues or aro-matic groups; and covalent disulfide bondsinvolving cystine residues. Hydrophobicbonds are not true bonds but have beendescribed as interactions of nonpolar groups.These nonpolar groups or areas have a ten-dency to orient themselves to the interior ofthe protein molecule. This tendency dependson the relative number of nonpolar amino
Table 3-4 Bond Energies of the Bonds Involvedin Protein Structure
Bond Energy*Bond (kcal/mole)Covalent C-C 83Covalent S-S 50Hydrogen bond 3-7Ionic electrostatic bond 3-7Hydrophobic bond 3-5Van der Waals bond 1 -2
These refer to free energy required to break thebonds: in the case of a hydrophobic bond, the freeenergy required to unfold a nonpolar side chain fromthe interior of the molecule into the aqueous medium.
acid residues and their location in the peptidechain. Many food proteins, especially plantstorage proteins, are highly hydrophobic—somuch so that not all of the hydrophobic areascan be oriented toward the inside and have tobe located on the surface. This is a possiblefactor in subunits association and in somecases may result in aggregation. The hydro-phobicity values of some food proteins asreported by Stanley and Yada (1992) arelisted in Table 3-6.
The well-defined secondary, tertiary, andquaternary structures are thought to arisedirectly from the primary structure. Thismeans that a given combination of aminoacids will automatically assume the type ofstructure that is most stable and possiblegiven the considerations described by Paul-ing etal. (1951).
Table 3-5 Oligomeric Food Proteins
MolecularProtein Weight (d) Subunits
Lactoglobulin 35,000 2Hemoglobin 64,500 4Avidin 68,300 4Lipoxygenase 108,000 2Tyrosinase 128,000 4Lactate 140,000 4
dehydrogenase7S soy protein 200,000 9Invertase 210,000 4Catalase 232,000 4Collagen 300,000 311S soy protein 350,000 12Legumin 360,000 6Myosin 475,000 6
Source: Reprinted with permission from D.W. Stanleyand R.Y. Yada, Thermal Reactions in Food Protein Sys-tems, Physical Chemistry of Foods, H.G. Schwartzbergand R.H. Hartel, eds., p. 676, 1992, by courtesy of Mar-cel Dekker, Inc.
DENATURATION
Denaturation is a process that changes themolecular structure without breaking any ofthe peptide bonds of a protein. The process ispeculiar to proteins and affects different pro-teins to different degrees, depending on thestructure of a protein. Denaturation can bebrought about by a variety of agents, ofwhich the most important are heat, pH, salts,and surface effects. Considering the com-plexity of many food systems, it is not sur-prising that denaturation is a complex pro-cess that cannot easily be described in simpleterms. Denaturation usually involves loss ofbiological activity and significant changes insome physical or functional properties suchas solubility. The destruction of enzymeactivity by heat is an important operation infood processing. In most cases, denaturationis nonreversible; however, there are some
Table 3-6 Hydrophobicity Values of Some FoodProteins
HydrophobicityProtein cal/residue
Gliadin 1300Bovine serum albumin 1120-1000oc-Lactalbumin 1050
(3-Lactoglobulin 1050Actin 1000Ovalbumin 980Collagen 880Myosin 880Casein 725Whey protein 387Gluten 349
Source: Reprinted with permission from D.W. Stanleyand R.Y. Yada, Thermal Reactions in Food Protein Sys-tems, Physical Chemistry of Foods, H.G. Schwartzbergand R.H. Hartel, eds., p. 677, 1992, by courtesy of Mar-cel Dekker, Inc.
exceptions, such as the recovery of sometypes of enzyme activity after heating. Heatdenaturation is sometimes desirable—forexample, the denaturation of whey proteinsfor the production of milk powder used inbaking. The relationship among temperature,heating time, and the extent of whey proteindenaturation in skim milk is demonstrated inFigure 3-3 (Harland et al. 1952).
The proteins of egg white are readily dena-tured by heat and by surface forces when eggwhite is whipped to a foam. Meat proteinsare denatured in the temperature range 57 to750C, which has a profound effect on tex-ture, water holding capacity, and shrinkage.
Denaturation may sometimes result in theflocculation of globular proteins but mayalso lead to the formation of gels. Foods maybe denatured, and their proteins destabilized,during freezing and frozen storage. Fish pro-teins are particularly susceptible to destabili-zation. After freezing, fish may becometough and rubbery and lose moisture. Thecaseinate micelles of milk, which are quitestable to heat, may be destabilized by freez-ing. On frozen storage of milk, the stabilityof the caseinate progressively decreases, andthis may lead to complete coagulation.
Protein denaturation and coagulation areaspects of heat stability that can be related tothe amino acid composition and sequence ofthe protein. Denaturation can be defined as amajor change in the native structure thatdoes not involve alteration of the amino acidsequence. The effect of heat usually involvesa change in the tertiary structure, leading to aless ordered arrangement of the polypeptidechains. The temperature range in whichdenaturation and coagulation of most pro-teins take place is about 55 to 750C, as indi-cated in Table 3-7. There are some notableexceptions to this general pattern. Casein andgelatin are examples of proteins that can be
boiled without apparent change in stability.The exceptional stability of casein makes itpossible to boil, sterilize, and concentratemilk, without coagulation. The reasons forthis exceptional stability have been discussed
by Kirchmeier (1962). In the first place,restricted formation of disulfide bonds due tolow content of cystine and cysteine results inincreased stability. The relationship betweencoagulation temperature as a measure of sta-
Figure 3-3 Time-Temperature Relationships for the Heat Denaturation of Whey Proteins in SkimMilk. Source: From H.A. Harland, S.T. Coulter, and R. Jenness, The Effects of Various Steps in theManufacture on the Extent of Serum Protein Denaturation in Nonfat Dry Milk Solids. /. Dairy ScL 35:363-368, 1952.
TCNPCKATURf
PER
C
ENT
DeN
ATU
MTK
)N
TIM
E O
F
HCAT
INO
IN
M
INU
TCS
bility and sulfur amino acid content is shownin Tables 3-7 and 3-8. Peptides, which arelow in these particular amino acids, are lesslikely to become involved in the type of sulf-hydryl agglomeration shown in Figure 3-4.Casein, with its extremely low content ofsulfur amino acids, exemplifies this behav-ior. The heat stability of casein is alsoexplained by the restraints against forming afolded tertiary structure. These restraints aredue to the relatively high content of prolineand hydroxyproline in the heat stable pro-teins (Table 3-9). In a peptide chain free ofproline, the possibility of forming inter- andintramolecular hydrogen bonds is better thanin a chain containing many proline residues(Figure 3-5). These considerations showhow amino acid composition directly relatesto secondary and tertiary structure of pro-teins; these structures are, in turn, responsi-ble for some of the physical properties of theprotein and the food of which it is a part.
NONENZYMIC BROWNING
The nonenzymic browning or Maillardreaction is of great importance in food man-ufacturing and its results can be either desir-
Table 3-7 Heat Coagulation Temperatures ofSome Albumins and Globulins and Casein
CoagulationProtein Temp. (0C)
Egg albumin 56Serum albumin (bovine) 67Milk albumin (bovine) 72Legumelin (pea) 60Serum globulin (human) 75p-Lactoglobulin (bovine) 70-75Fibrinogen (human) 56-64Myosin (rabbit) 47-56Casein (bovine) 160-200
able or undesirable. For example, the browncrust formation on bread is desirable; thebrown discoloration of evaporated and steril-ized milk is undesirable. For products inwhich the browning reaction is favorable, theresulting color and flavor characteristics aregenerally experienced as pleasant. In otherproducts, color and flavor may become quiteunpleasant.
The browning reaction can be defined as thesequence of events that begins with the reac-tion of the amino group of amino acids, pep-tides, or proteins with a glycosidic hydroxylgroup of sugars; the sequence terminates withthe formation of brown nitrogenous polymersor melanoidins (Ellis 1959).
The reaction velocity and pattern are influ-enced by the nature of the reacting aminoacid or protein and the carbohydrate. Thismeans that each kind of food may show adifferent browning pattern. Generally, Iysineis the most reactive amino acid because ofthe free £-amino group. Since lysine is thelimiting essential amino acid in many foodproteins, its destruction can substantiallyreduce the nutritional value of the protein.Foods that are rich in reducing sugars arevery reactive, and this explains why lysine inmilk is destroyed more easily than in other
Table 3-8 Cysteine and Cystine Content ofSome Proteins (g Amino Acid/100 g Protein)
Cysteine CystineProtein (%) (%)Egg albumin 1.4 0.5Serum albumin 0.3 5.7(bovine)Milk albumin 6.4 —p-Lactoglobulin 1.1 2.3Fibrinogen 0.4 2.3Casein — 0.3
Figure 3-4 Reactions Involved in SulfhydrylPolymerization of Proteins. Source: From O.Kirchmeier, The Physical-Chemical Causes ofthe Heat Stability of Milk Proteins, Milchwis-senschaft (German), Vol. 17, pp. 408-412, 1962.
foods (Figure 3-6). Other factors that influ-ence the browning reaction are temperature,pH, moisture level, oxygen, metals, phos-phates, sulfur dioxide, and other inhibitors.
The browning reaction involves a numberof steps. An outline of the total pathway ofmelanoidin formation has been given byHodge (1953) and is shown in Figure 3-7.According to Hurst (1972), the followingfive steps are involved in the process:
1. The production of an Af-substitutedglycosylamine from an aldose orketose reacting with a primary aminogroup of an amino acid, peptide, orprotein.
2. Rearrangement of the glycosylamineby an Amadori rearrangement type ofreaction to yield an aldoseamine orketoseamine.
3. A second rearrangement of the ketose-amine with a second mole of aldose toform a diketoseamine, or the reaction
Table 3-9 Amino Acid Composition of Serum Albumin, Casein, and Gelatin (g Amino Acid/100 g Protein)
Amino Acid
GlycineAlanineValineLeucineisoleucineSerineThreonineCystine 1/2MethioninePhenylalanineTyrosineProlineHydroxyprolineAspartic acidGlutamic acidLysineArginineHistidine
Serum Albumin
1.86.35.9
12.32.64.25.86.00.86.65.14.8
10.916.512.85.94.0
Casein
1.93.16.89.25.65.34.40.31.85.35.7
13.5
7.624.58.93.33.8
Gelatin
27.511.02.63.31.74.22.20.00.92.20.3
16.414.16.7
11.44.58.80.8
of an aldoseamine with a second moleof amino acid to yield a diamino sugar.
4. Degradation of the amino sugars withloss of one or more molecules of waterto give amino or nonamino com-pounds.
5. Condensation of the compoundsformed in Step 4 with each other orwith amino compounds to form brownpigments and polymers.
The formation of glycosylamines from thereaction of amino groups and sugars is
reversible (Figure 3-8) and the equilibriumis highly dependent on the moisture level.The mechanism as shown is thought toinvolve addition of the amine to the carbonylgroup of the open-chain form of the sugar,elimination of a molecule of water, and clo-sure of the ring. The rate is high at low watercontent; this explains the ease of browning indried and concentrated foods.
The Amadori rearrangement of the glyco-sylamines involves the presence of an acidcatalyst and leads to the formation of ketose-amine or 1-amino-1-deoxyketose according
Figure 3-5 Effect of Proline Residues on Possible Hydrogen Bond Formation in Peptide Chains. (A)Proline-free chain; (B) proline-containing chain; (C) hydrogen bond formation in proline-free and pro-line-containing chains. Source: From O. Kirchmeier, The Physical-Chemical Causes of the Heat Sta-bility of Milk Proteins, Milchwissenschaft (German), Vol. 17, pp. 408-412, 1962.
A
B
C
to the scheme shown in Figure 3-9. In thereaction of D-glucose with glycine, the aminoacid reacts as the catalyst and the compoundproduced is 1-deoxy-l-glycino-p-D-fructose(Figure 3-10). The ketoseamines are rela-tively stable compounds, which are formed inmaximum yield in systems with 18 percentwater content. A second type of rearangementreaction is the Heyns rearrangement, which isan alternative to the Amadori rearrangementand leads to the same type of transformation.The mechanism of the Amadori rearrange-ment (Figure 3-9) involves protonation of thenitrogen atom at carbon 1. The Heyns rear-rangement (Figure 3-11) involves protona-tion of the oxygen at carbon 6.
Secondary reactions lead to the formationof diketoseamines and diamino sugars. Theformation of these compounds involves com-plex reactions and, in contrast to the forma-tion of the primary products, does not occuron a mole-for-mole basis.
In Step 4, the ketoseamines are decom-posed by 1,2-enolization or 2,3-enolization.The former pathway appears to be the moreimportant one for the formation of browncolor, whereas the latter results in the forma-tion of flavor products. According to Hurst(1972), the 1,2-enolization pathway appearsmainly to lead to browning but also contrib-utes to formation of off-flavors throughhydroxymethylfurfural, which may be a fac-
Figure 3-6 Loss of Lysine Occurring as a Result of Heating of Several Foods. Source: From J. Adrian,The Maillard Reaction. IV. Study on the Behavior of Some Amino Acids During Roasting of Proteina-ceous Foods, Ann. Nutr. Aliment. (French), Vol. 21, pp. 129-147, 1967.
Heating at 150° (minutes)
Cotton
Peanut
Wheat
Milk
Loss of lysine
tor in causing off-flavors in stored, over-heated, or dehydrated food products. Themechanism of this reaction is shown in Fig-ure 3-12 (Hurst 1972). The ketoseamine (1)is protonated in acid medium to yield (2).This is changed in a reversible reaction into
the 1,2-enolamine (3) and this is assisted bythe N substituent on carbon 1. The followingsteps involve the p-elimination of the hy-droxyl group on carbon 3. In (4) the enola-mine is in the free base form and converts tothe Schiff base (5). The Schiff base may
Figure 3-7 Reaction Pattern of the Formation of Melanoidins from Aldose Sugars and Amino Com-pounds. Source: From I.E. Hodge, Chemistry of Browning Reactions in Model Systems, Agr. FoodChem., Vol. 1, pp. 928-943, 1953.
Figure 3-8 Reversible Formation of Glycosylamines in the Browning Reaction. Source: From D.T.Hurst, Recent Development in the Study ofNonenzymic Browning and Its Inhibition by Sulpher Diox-ide, BFMIRA Scientific and Technical Surveys, No. 75, Leatherhead, England, 1972.
Amadori Rearrangement
a-D-Glucopyrano-sylamine
1-Amino-l-deoxy-a-D-fructopyranose
Figure 3-9 Amadori Rearrangement. Source: From MJ. Kort, Reactions of Free Sugars with AqueousAmmonia, Adv. Carbohydrate Chem. Biochem., Vol. 25, pp 311-349, 1970.
Figure 3-10 Structure of 1-Deoxy-l-Glycino-p-D-Fructose
undergo hydrolysis and form the enolform (7)of 3-deoxyosulose (8). In another step theSchiff base (5) may lose a proton and thehydroxyl from carbon 4 to yield a new Schiffbase (6). Both this compound and the 3-deox-
yosulose may be transformed into an unsatur-ated osulose (9), and by elimination of aproton and a hydroxyl group, hydroxymeth-y!furfural (10) is formed.
Following the production of 1,2-enol formsof aldose and ketose amines, a series of deg-radations and condensations results in the for-mation of melanoidins. The oc-p-dicarbonylcompounds enter into aldol type condensa-tions, which lead to the formation of poly-mers, initially of small size, highly hydrated,and in colloidal form. These initial productsof condensation are fluorescent, and continu-ation of the reaction results in the formationof the brown melanoidins. These polymersare of nondistinct composition and contain
cr-D-Fructopyrano-sylamine
2-Amino-2-deoxy-o?-D-glucopyranose
Figure 3-11 Heyns Rearrangement. Source: From MJ. Kort, Reactions of Free Sugars with AqueousAmmonia, Adv. Carbohydrate Chem. Biochem., Vol. 25, pp. 311-349, 1970.
varying levels of nitrogen. The compositionvaries with the nature of the reaction partners,pH, temperature, and other conditions.
The flavors produced by the Maillard reac-tion also vary widely. In some cases, the fla-vor is reminiscent of caramelization. TheStrecker degradation of a-amino acids is areaction that also significantly contributes tothe formation of flavor compounds. The
dicarbonyl compounds formed in the previ-ously described schemes react in the follow-ing manner with a-amino acids:
Figure 3-12 1,2-Enolization Mechanism of the Browning Reaction. Source: From D.T. Hurst, RecentDevelopments in the Study of Nonenzymic Browning and Its Inhibition by Sulphur Dioxide,BFMIRA Scientific and Technical Surveys, No. 75, Leatherhead, England, 1972.
The amino acid is converted into an aldehydewith one less carbon atom (Schonberg andMoubacher 1952). Some of the compoundsof browning flavor have been described byHodge et al. (1972). Corny, nutty, bready,and crackery aroma compounds consist ofplanar unsaturated heterocyclic compoundswith one or two nitrogen atoms in the ring.Other important members of this group arepartially saturated Af-heterocyclics with alkylor acetyl group substituents. Compounds thatcontribute to pungent, burnt aromas arelisted in Table 3-10. These are mostly vici-nal polycarbonyl compounds and oc,p-unsat-urated aldehydes. They condense rapidly toform melanoidins. The Strecker degradationaldehydes contribute to the aroma of bread,peanuts, cocoa, and other roasted foods. Al-though acetic, phenylacetic, isobutyric, andisovaleric aldehydes are prominent in thearomas of bread, malt, peanuts, and cocoa,they are not really characteristic of thesefoods (Hodge et al. 1972).
A somewhat different mechanism for thebrowning reaction has been proposed byBurton and McWeeney (1964) and is shownin Figure 3-13. After formation of the aldo-sylamine, dehydration reactions result in theproduction of 4- to 6-membered ring com-pounds. When the reaction proceeds underconditions of moderate heating, fluorescentnitrogenous compounds are formed. Thesereact rapidly with glycine to yield melanoi-dins.
The influence of reaction components andreaction conditions results in a wide varietyof reaction patterns. Many of these condi-tions are interdependent. Increasing tempera-ture results in a rapidly increasing rate ofbrowning; not only reaction rate, but also thepattern of the reaction may change with tem-perature. In model systems, the rate ofbrowning increases two to three times for
each 10° rise in temperature. In foods con-taining fructose, the increase may be 5 to 10times for each 10° rise. At high sugar con-tents, the rate may be even more rapid. Tem-perature also affects the composition of thepigment formed. At higher temperatures, thecarbon content of the pigment increases andmore pigment is formed per mole of carbondioxide released. Color intensity of the pig-ment increases with increasing temperature.The effect of temperature on the reaction rateof D-glucose with DL-leucine is illustratedin Figure 3-14.
In the Maillard reaction, the basic aminogroup disappears; therefore, the initial pH orthe presence of a buffer has an importanteffect on the reaction. The browning reactionis slowed down by decreasing pH, and thebrowning reaction can be said to be self-inhibitory since the pH decreases with theloss of the basic amino group. The effect ofpH on the reaction rate of D-glucose withDL-leucine is demonstrated in Figure 3-15.The effect of pH on the browning reaction ishighly dependent on moisture content. Whena large amount of water is present, most ofthe browning is caused by caramelization,but at low water levels and at pH greater than6, the Maillard reaction is predominant.
The nature of the sugars in a nonenzymicbrowning reaction determines their reactiv-ity. Reactivity is related to their conforma-tional stability or to the amount of open-chain structure present in solution. Pentosesare more reactive than hexoses, and hexosesmore than reducing disaccharides. Nonre-ducing disaccharides only react after hydrol-ysis has taken place. The order of reactivityof some of the aldohexoses is: mannose ismore reactive than galactose, which is morereactive than glucose.
The effect of the type of amino acid can besummarized as follows. In the a-amino acid
Figure 3-13 Proposed Browning Reaction Mechanism According to Burton and McWeeney. Source:From H.S. Burton and DJ. McWeeney, Non-Enzymatic Browning: Routes to the Production of MeI-anoidins from Aldoses and Amino Compounds, Chem. Ind., Vol. 11, pp. 462-463, 1964.
Table 3-10 Aroma and Structure Classification of Browned Flavor Compounds
Aromas:Structure:
Examples ofcompounds:
Burnt (pungent, empyreumatic)Polycarbonyls(a,p-Unsat'daldehydes-C:O-C:0-=C-CHO)
IGlyoxalPyruvaldehydeDiacetylMesoxalic dialdehyde
AcroleinCrotonaldehyde
Variable (aldehydic, ketonic)Monocarbonyls(R-CHO, R-C:0-CH3)
Strecker aldehydeslsobutyricIsovalericMethiona!
2-Furaldehydes2-Pyrrole aldehydesC3-C6 Methyl ketones
Source: From J.E. Hodge, FD. Mills, and B.E. Fisher, Compounds of Browned Flavor from Sugar-Amine Reac-tions, Cereal Sd. Today, Vol. 17, pp. 34-40, 1972.
( MELANOIDINS )PolymerCo - PolymerPolymer
AV-containingcompounds
-H2O( Somecyciisotco)
Furfurals
Unsaturatcdosones
D«oxyoson«sother carbonyt
compounds
Monofcttose -ammocompound
ALOOSE AHINO COMPOUND
Atdosylominocompounds Unsoturated
car bony Icompounds
Dikctosc-amino compound
series, glycine is the most reactive. Longerand more complex substituent groups reducethe rate of browning. In the co-amino acidseries, browning rate increases with increas-ing chain length. Ornithine browns more rap-idly than lysine. When the reactant is aprotein, particular sites in the molecule mayreact faster than others. In proteins, the e-amino group of lysine is particularly vulnera-ble to attack by aldoses and ketoses.
Moisture content is an important factor ininfluencing the rate of the browning reaction.Browning occurs at low temperatures andintermediate moisture content; the rate in-creases with increasing water content. Therate is extremely low below the glass transi-
tion temperature, probably because of lim-ited diffusion (Roos and Himberg 1994;Roos et al. 1996a, b).
Methods of preventing browning couldconsist of measures intended to slow reactionrates, such as control of moisture, tempera-ture, or pH, or removal of an active interme-diate. Generally, it is easier to use an in-hibitor. One of the most effective inhibitorsof browning is sulfur dioxide. The action ofsulfur dioxide is unique and no other suitableinhibitor has been found. It is known thatsulfite can combine with the carbonyl groupof an aldose to give an addition compound:
NaHSO3 + RCHO -> RCHOHSO3Na
Time (minutes).
Figure 3-14 Effect of Temperature on the Reaction Rate of D-Glucose with DL-Leucine. Source: FromG. Haugard, L. Tumerman, and A. Sylvestri, A Study on the Reaction of Aldoses and Amino Acids, /.Am. Chem. Soc., Vol. 73, pp. 4594-4600, 1951.
Milh
mol
e o
f DL
- l
eucm
e p
er m
l
However, this reaction cannot possiblyaccount for the inhibitory effect of sulfite. Itis thought that sulfur dioxide reacts with thedegradation products of the amino sugars,thus preventing these compounds from con-densing into melanoidins. A serious draw-back of the use of sulfur dioxide is that itreacts with thiamine and proteins, therebyreducing the nutritional value of foods. Sul-fur dioxide destroys thiamine and is there-fore not permitted for use in foods containingthis vitamin.
CHEMICAL CHANGES
During processing and storage of foods, anumber of chemical changes involving pro-teins may occur (Hurrell 1984). Some ofthese may be desirable, others undesirable.
Such chemical changes may lead to com-pounds that are not hydrolyzable by intesti-nal enzymes or to modifications of thepeptide side chains that render certain aminoacids unavailable. Mild heat treatments in thepresence of water can significantly improvethe protein's nutritional value in some cases.Sulfur-containing amino acids may becomemore available and certain antinutritional fac-tors such as the trypsin inhibitors of soybeansmay be deactivated. Excessive heat in theabsence of water can be detrimental to pro-tein quality; for example, in fish proteins,tryptophan, arginine, methionine, and lysinemay be damaged. A number of chemicalreactions may take place during heat treat-ment including decomposition, dehydrationof serine and threonine, loss of sulfur fromcysteine, oxidation of cysteine and methio-
Time (minutes).Figure 3-15 Effect of pH on the Reaction Rate of D-Glucose with DL-Leucine. Source: From G.Haugard, L. Tumerman, and A. Sylvestri, A Study on the Reaction of Aldoses and Amino Acids, J.Am. Chem. Soc., Vol. 73, pp. 4594-4600, 1951.
Milli
mol
e o
f O
L-le
ucm
e pe
r ml.
nine, cyclization of glutamic and asparticacids and threonine (Mauron 1970; 1983).
The nonenzymic browning, or Maillard,reaction causes the decomposition of certainamino acids. For this reaction, the presenceof a reducing sugar is required. Heat damagemay also occur in the absence of sugars.Bjarnason and Carpenter (1970) demon-strated that the heating of bovine plasmaalbumin for 27 hours at 1150C resulted in a50 percent loss of cystine and a 4 percentloss of lysine. These authors suggest thatamide-type bonds are formed by reactionbetween the e-amino group of lysine and theamide groups of asparagine or glutamine,with the reacting units present either in thesame peptide chain or in neighboring ones(Figure 3-16). The Maillard reaction leads tothe formation of brown pigments, or mel-anoidins, which are not well defined andmay result in numerous flavor and odor com-pounds. The browning reaction may alsoresult in the blocking of lysine. Lysinebecomes unavailable when it is involved inthe Amadori reaction, the first stage ofbrowning. Blockage of lysine is nonexistentin pasteurization of milk products, and is at Oto 2 percent in UHT sterilization, 10 to 15percent in conventional sterilization, and 20to 50 percent in roller drying (Hurrell 1984).
Some amino acids may be oxidized byreacting with free radicals formed by lipidoxidation. Methionine can react with a lipidperoxide to yield methionine sulfoxide. Cys-teine can be decomposed by a lipid free radi-cal according to the following scheme:
The decomposition of unsaturated fatty acidsproduces reactive carbonyl compounds thatmay lead to reactions similar to thoseinvolved in nonenzymic browning. Methio-nine can be oxidized under aerobic condi-tions in the presence of SO2 as follows:
R-S-CH3 + 2SO3= -> R-SO-CH3 + 2SO4=
This reaction is catalyzed by manganese ionsat pH values from 6 to 7.5. SO2 can alsoreact with cystine to yield a series of oxida-tion products. Some of the possible reactionproducts resulting from the oxidation of sul-fur amino acids are listed in Table 3-11.Nielsen et al. (1985) studied the reactionsbetween protein-bound amino acids and oxi-dizing lipids. Significant losses occurred ofthe amino acids lysine, tryptophan, and histi-dine. Methionine was extensively oxidizedto its sulfoxide. Increasing water activity in-creased losses of lysine and tryptophan buthad no effect on methionine oxidation.
Alkali treatment of proteins is becomingmore common in the food industry and mayresult in several undesirable reactions. Whencystine is treated with calcium hydroxide, itis transformed into amino-acrylic acid, hy-drogen sulfide, free sulfur, and 2-methyl thia-zolidine-2, 4-dicarboxylic acid as follows:
AlanineThis can also occur under alkaline condi-tions, when cystine is changed into amino-
Cysteine •*- Pyruvic acid
(Thiazolidinc)
Figure 3-16 Formation of Amide-Type Bondsfrom the Reaction Between £-amine Groups ofLysine and Amide Groups of Asparagine (n = 1)Glutamine (n = 2). Source: From J. Bjarnason andK. J. Carpenter, Mechanisms of Heat Damage inProteins. 2 Chemical Changes in Pure Proteins,Brit. J. Nutr., Vol. 24, pp. 313-329, 1970.
acrylic acid and thiocysteine by a (3-elimina-tion mechanism, as follows:
group of lysine to yield lysmoalanine (Zieg-ler 1964) as shown:
NH2 - C H - ( C H 2 )4 - N H 2 + CH2 = C-COOH »•COOH NH2
NH2 - C H - ( C H 2 )4 - N H - C H 2 —CH-COOHCOOH NH2
Lysinoalanine formation is not restricted toalkaline conditions—it can also be formedby prolonged heat treatment. Any factorfavoring lower pH and less drastic heat treat-ment will reduce the formation of lysinoala-nine. Hurrell (1984) found that dried wholemilk and UHT milk contained no lysmoala-nine and that evaporated and sterilized milkcontained 1,000 ppm. More severe treatmentwith alkali can decompose arginine into orni-thine and urea. Ornithine can combine withdehydroalanine in a reaction similar to theone giving lysinoalanine and, in this case,ornithinoalanme is formed.
Treatment of proteins with ammonia canresult in addition of ammonia to dehydroala-nine to yield (3-amino-alanine as follows:
CH 2 =C-COOH-HNH 3
NH2
> NH2 - C H 2 —CH-COOHI
NH2
Light-induced oxidation of proteins hasbeen shown to lead to off-flavors and destruc-tion of essential amino acids in milk. Patton(1954) demonstrated that sunlight attacksmethionine and converts it into methional ((3-methylmercaptopropionaldehyde), which cancause a typical sunlight off-flavor at a level of0.1 ppm. It was later demonstrated by Finleyand Shipe (1971) that the source of the light-induced off-flavor in milk resides in a low-density lipoprotein fraction.
Ammo-acrylic acid (dehydroalanine) is veryreactive and can combine with the E-amino
Table 3-11 Oxidation Products of the Sulfur-Containing Amino Acids
Name Formula
Methionine R-S-CH3
Sulfoxide R-SO-CH3
Sulfone R-SO2-CH3
Cystine R-S-S-RDisulfoxide R-SO-SO-RDisulfone R-SO2-SO2-R
Cysteine R-SHSulfenic R-SOHSulfinic R-SO2HSulfonic (or R-SO3Hcysteic acid)
Proteins react with polyphenols such asphenolic acids, flavonoids, and tannins,which occur widely in plant products. Thesereactions may result in the lowering of avail-able lysine, protein digestibility, and biolog-ical value (Hurrell 1984).
Racemization is the result of heat and alka-line treatment of food proteins. The aminoacids present in proteins are of the L-series.The racemization reaction starts with theabstraction of an a-proton from an aminoacid residue to give a negatively charged pla-nar carbanion. When a proton is added backto this optically inactive intermediate, eithera D- or L-enantiomer may be formed (Mas-ters and Friedman 1980). Racemization leadsto reduced digestibility and protein quality.
FUNCTIONAL PROPERTIES
Increasing emphasis is being placed onisolating proteins from various sources andusing them as food ingredients. In manyapplications functional properties are of
great importance. Functional propertieshave been defined as those physical andchemical properties that affect the behaviorof proteins in food systems during process-ing, storage, preparation, and consumption(Kinsella 1982). A summary of these proper-ties is given in Table 3-12.
Even when protein ingredients are addedto food in relatively small amounts, they maysignificantly influence some of the physicalproperties of the food. Hermansson (1973)found that addition of 4 percent of a soybeanprotein isolate to processed meat signifi-cantly affected firmness, as measured byextrusion force, compression work, and sen-sory evaluation.
The emulsifying and foaming properties ofproteins relate to their adsorption at inter-faces and to the structure of the protein filmformed there (Mitchell 1986). The emulsify-ing and emulsion stabilizing capacity of pro-tein meat additives is important to theproduction of sausages. The emulsifyingproperties of proteins are also involved in theproduction of whipped toppings and coffeewhiteners. The whipping properties of pro-teins are essential in the production ofwhipped toppings. Paulsen and Horan (1965)determined the functional characteristics ofedible soya flours, especially in relation totheir use in bakery products. They evaluatedthe measurable parameters of functionalproperties such as water dispersibility, wetta-bility, solubility, and foaming characteristicsas those properties affected the quality ofbaked products containing added soya flour.
Some typical functional properties of foodproteins are listed in Table 3-13.
Surface Activity of Proteins
Proteins can act as surfactants in stabiliz-ing emulsions and foams. To perform thisfunction proteins must be amphiphilic just
Next page
INTRODUCTION
Carbohydrates occur in plant and animaltissues as well as in microorganisms in manydifferent forms and levels. In animal organ-isms, the main sugar is glucose and the stor-age carbohydrate is glycogen; in milk, themain sugar is almost exclusively the disac-charide lactose. In plant organisms, a widevariety of monosaccharides and oligosaccha-rides occur, and the storage carbohydrate isstarch. The structural polysaccharide ofplants is cellulose. The gums are a variedgroup of polysaccharides obtained fromplants, seaweeds, and microorganisms.Because of their useful physical properties,the gums have found widespread applicationin food processing. The carbohydrates thatoccur in a number of food products are listedin Table 4-1.
MONOSACCHARIDES
D-glucose is the most important monosac-charide and is derived from the simplestsugar, D-glyceraldehyde, which is classed asan aldotriose. The designation of aldose andketose sugars indicates the chemical charac-ter of the reducing form of a sugar and can beindicated by the simple or open-chain for-mula of Fischer, as shown in Figure 4-1. This
type of formula shows the free aldehydegroup and four optically active secondaryhydroxyls. Since the chemical reactions ofthe sugars do not correspond to this structure,a ring configuration involving a hemiacetalbetween carbons 1 and 5 more accuratelyrepresents the structure of the monosaccha-rides. The five-membered ring structure iscalled furanose; the six-membered ring, pyra-nose. Such rings are heterocyclic because onemember is an oxygen atom. When the reduc-ing group becomes involved in a hemiacetalring structure, carbon 1 becomes asymmetricand two isomers are possible; these are calledanomers.
Most natural sugars are members of the Dseries. The designation D or L refers to twoseries of sugars. In the D series, the highestnumbered asymmetric carbon has the OHgroup directed to the right, in the Fischerprojection formula. In the L series, thishydroxyl points to the left. This originatesfrom the simplest sugars, D- and L-glyceral-dehyde (Figure 4-2).
After the introduction of the Fischer for-mulas came the use of the Haworth represen-tation, which was an attempt to give a moreaccurate spatial view of the molecule. Be-cause the Haworth formula does not accountfor the actual bond angles, the modern con-
Carbohydrates
CHAPTER 4
Table 4-1 Carbohydrates in Some Foods and Food Products
Product
Fruits
Apple
Grape
Strawberry
Vegetables
Carrot
Onion
PeanutsPotato
Sweet corn
Sweet potato
Turnip
Others
Honey
Maple syrup
MeatMilkSugarbeetSugar cane juice
Total Sugar(%)
14.5
17.3
8.4
9.7
8.7
18.617.1
22.1
26.3
6.6
82.3
65.5
4.918-2014-28
Mono- and Disaccharides (%)
glucose 1.17; fructose 6.04;sucrose 3.78; mannose trace
glucose 5.35; fructose 5.33;sucrose 1.32; mannose 2.19
glucose 2.09; fructose 2.40;sucrose 1 .03; mannose 0.07
glucose 0.85; fructose 0.85;sucrose 4.25
glucose 2.07; fructose 1.09;sucrose 0.89
sucrose 4-1 2
sucrose 12-17
glucose 0.87; sucrose 2-3
glucose 1.5; fructose 1.18;sucrose 0.42
glucose 28-35; fructose 34-41 ;sucrose 1-5
sucrose 58.2-65.5;hexoses 0.0-7.9
glucose 0.01lactose 4.9sucrose 18-20glucose + fructose 4-8;
sucrose 10-20
Polysaccharides(%)
starch 1 .5;cellulose 1 .0
cellulose 0.6
cellulose 1 .3
starch 7.8;cellulose 1.0
cellulose 0.71
cellulose 2.4starch 14;
cellulose 0.5cellulose 0.7;
cellulose 60starch 14.65;
cellulose 0.7cellulose 0.9
glycogen 0.10
formational formulas (Figure 4-1) moreaccurately represent the sugar molecule. Anumber of chair conformations of pyranosesugars are possible (Shallenberger and Birch1975) and the two most important ones for
glucose are shown in Figure 4—1. These arenamed the CI D and the IC D forms (alsodescribed as O-outside and O-inside, respec-tively). In the CID form of (3-D-gluco-pyra-nose, all hydroxyls are in the equatorial
position, which represents the highest ther-modynamic stability.
The two possible anomeric forms ofmonosaccharides are designated by Greekletter prefix a or p. In the oc-anomer thehydroxyl group points to the right, accordingto the Fischer projection formula; thehydroxyl group points to the left in the p-anomer. In Figure 4-1 the structure markedCl D represents the oc-anomer, and 1C Drepresents the p-anomer. The anomericforms of the sugars are in tautomeric equilib-rium in solution; and this causes the changein optical rotation when a sugar is placed in
CHO CHO
HCOH HOCHI I
C H 2 O H C H 2 O H
Figure 4-2 Structure of D- and L-Glyceralde-hyde. Source: From R.S. Shallenberger and G.G.Birch, Sugar Chemistry, 1975, AVI PublishingCo.
solution. Under normal conditions, it maytake several hours or longer before the equi-librium is established and the optical rotationreaches its equilibrium value. At room tem-perature an aqueous solution of glucose canexist in four tautomeric forms (Angyal1984): P-furanoside—0.14 percent, acyclicaldehyde—0.0026 percent, p-pyranoside—62 percent, and oc-pyranoside—38 percent(Figure 4-3). Fructose under the same con-ditions also exists in four tautomeric formsas follows: oc-pyranoside—trace, p-pyrano-side—75 percent, oc-furanoside—4 percent,and p-furanoside—21 percent (Figure 4-4)(Angyal 1976).
When the monosaccharides becomeinvolved in condensation into di-, oligo-, andpolysaccharides, the conformation of thebond on the number 1 carbon becomes fixedand the different compounds have either anall-a or all-p structure at this position.
Naturally occurring sugars are mostly hex-oses, but sugars with different numbers ofcarbons are also present in many products.There are also sugars with different func-
Figure 4-1 Methods of Representation of D-Glucose. Source: From M.L. Wolfrom, Physical andChemical Structures of Carbohydrates, in Symposium on Foods: Carbohydrates and Their Roles, H. W.Schultz, R.F. Cain, and R.W. Wrolstad, eds., 1969, AVI Publishing Co.
GLUCOSE(deKtrose)Aldose (oldohexose)
Howorth
ConformotionolGlucopyronoseGlucose
FischerFischer
Figure 4-3 Tautomeric Forms of Glucose in Aqueous Solution at Room Temperature
Figure 4-4 Tautomeric Forms of Fructose in Aqueous Solution at Room Temperature
tional groups or substituents; these lead tosuch diverse compounds as aldoses, ketoses,amino sugars, deoxy sugars, sugar acids,sugar alcohols, acetylated or methylated sug-ars, anhydro sugars, oligo- and polysaccha-rides, and glycosides. Fructose is the mostwidely occurring ketose and is shown in itsvarious representations in Figure 4-5. It isthe sweetest known sugar and occurs boundto glucose in sucrose or common sugar. Ofall the other possible hexoses only two occurwidely—D-mannose and D-galactose. Theirformulas and relationship to D-glucose aregiven in Figure 4-6.
RELATED COMPOUNDS
Amino sugars usually contain D-glu-cosamine (2-deoxy-2-amino glucose). Theyoccur as components of high molecularweight compounds such as the chitin of crus-taceans and mollusks, as well as in certainmushrooms and in combination with theovomucin of egg white.
Glycosides are sugars in which the hydro-gen of an anomeric hydroxy group has beenreplaced by an alkyl or aryl group to form amixed acetal. Glycosides are hydrolyzed byacid or enzymes but are stable to alkali. For-mation of the full acetal means that glyco-sides have no reducing power. Hydrolysis ofglycosides yields sugar and the aglycone.Amygdalin is an example of one of the cyan-ogetic glycosides and is a component of bit-ter almonds. The glycone moiety of thiscompound is gentiobiose, and completehydrolysis yields benzaldehyde, hydrocyanicacid, and glucose (Figure 4-7). Other impor-tant glycosides are the flavonone glycosidesof citrus rind, which include hesperidin andnaringin, and the mustard oil glycosides,such as sinigrin, which is a component ofmustard and horseradish. Deoxy sugarsoccur as components of nucleotides; forexample, 2-deoxyribose constitutes part ofdeoxyribonucleic acid.
Sugar alcohols occur in some fruits and areproduced industrially as food ingredients.
Figure 4-5 Methods of Representation of D-Fructose. Source: From M.L. Wolfrom, Physical andChemical Structures of Carbohydrates, in Symposium on Foods: Carbohydrates and Their Roles,H.W. Schultz, R.F. Cain, and R.W. Wrolstad, eds., 1969, AVI Publishing Co.
They can be made by reduction of free sug-ars with sodium amalgam and lithium alumi-num hydride or by catalytic hydrogenation.The resulting compounds are sweet as sug-ars, but are only slowly absorbed and can,therefore, be used as sweeteners in diabeticfoods. Reduction of glucose yields glucitol(Figure 4-8), which has the trivial name sor-bitol. Another commercially produced sugar
alcohol is xylitol, a five-carbon compound,which is also used for diabetic foods (Figure4-8). Pentitols and hexitols are widely dis-tributed in many foods, especially fruits andvegetables (Washiittl et al. 1973), as is indi-cated in Table 4-2.
Anhydro sugars occur as components ofseaweed polysaccharides such as alginateand agar. Sugar acids occur in the pectic sub-
Figure 4-6 Relationship of D-Aldehyde Sugars. Source: From M.L. Wolfrom, Physical and ChemicalStructures of Carbohydrates, in Symposium on Foods: Carbohydrates and Their Roles, H.W. Schultz,R.F Cain, and R.W. Wrolstad, eds., 1969, AVI Publishing Co.
Figure 4-7 Hydrolysis of the Glycoside Amygdalin
Benzaldehyde o-Glucose
Amygdalin
Gentiobiose
stances. When some of the carboxyl groupsare esterified with methanol, the compoundsare known as pectins. By far the largestgroup of saccharides occurs as oligo- andpolysaccharides.
OLIGOSACCHARIDES
Polymers of monosaccharides may beeither of the homo- or hetero-type. When thenumber of units in a glycosidic chain is inthe range of 2 to 10, the resulting compoundis an oligosaccharide. More than 10 units areusually considered to constitute a polysac-
CH 2OH
HCOH CH2OHI I
HOCH HCOHI I
HCOH HOCHI I
HCOH HCOHI ICH2OH CH2OH
Figure 4-8 Structure of Sorbitol and Xylitol
charide. The number of possible oligosac-charides is very large, but only a few arefound in large quantities in foods; these arelisted in Table 4-3. They are composed ofthe monosaccharides D-glucose, D-galac-tose, and D-fructose, and they are closelyrelated to one another, as shown in Figure4-9.
Sucrose or ordinary sugar occurs in abun-dant quantities in many plants and is com-mercially obtained from sugar cane or sugarbeets. Since the reducing groups of themonosaccharides are linked in the glycosidicbond, this constitutes one of the few nonre-ducing disaccharides. Sucrose, therefore,does not reduce Fehling solution or formosazones and it does not undergo mutarota-tion in solution. Because of the unique car-bonyl-to-carbonyl linkage, sucrose is highlylabile in acid medium, and acid hydrolysis ismore rapid than with other oligosaccharides.The structure of sucrose is shown in Figure4-10. When sucrose is heated to 21O0C, par-tial decomposition takes place and caramel isformed. An important reaction of sucrose,
Table 4-2 Occurrence of Sugar-Alcohols in Some Foods (Expressed as mg/100g of Dry Food)
Source: From J. Washiittl, R Reiderer, and E. Bancher, A Qualitative and Quantitative Study of Sugar-Alcohols inSeveral Foods: A Research Role, J. Food ScL, Vol. 38, pp. 1262-1263,1973.
Product
BananasPearsRaspberriesStrawberriesPeachesCeleryCauliflowerWhite mushrooms
Arabitol
340
Xylitol
21
268362
300128
Mannitol
4050
476
Sorbitol
4600
960
Galactitol
48
which it has in common with other sugars, isthe formation of insoluble compounds withcalcium hydroxide. This reaction results inthe formation of tricalcium compoundsC12H22O11-S Ca(OH)2 and is useful for
recovering sucrose from molasses. When thecalcium saccharate is treated with CO2, thesugar is liberated.
Hydrolysis of sucrose results in the forma-tion of equal quantities of D-glucose and D-
MANNlNOTRlOSE
GALACTOBIOSE
MEUBIOSE
GAlACTOSE GAtACTOSE GlUCOSE FRUCTOSE
SUCROSE
RAFFINOSE
STACHYOSE
Figure 4-9 Composition of Some Major Oligosaccharides Occurring in Foods. Source: From R.S.Shallenberger and G.G. Birch, Sugar Chemistry, 1975, AVI Publishing Co.
Table 4-3 Common Oligosaccharides Occurring in Foods
SucroseLactoseMaltosea,oc-TrehaloseRaffinose
Stachyose
Verbascose
(a-D-glucopyranosyl p-D-fructofuranoside)(4-O-p-D-galactopyranosyl-D-glucopyranose)(4-O-a-D-glucopyranosyl-D-glucopyranose)(a-D-glucopyranosyl-a-D-glycopyranoside)[O-a-D-galactopyranosyl-(1 ->6)-O-cc-D-glucopyranosyl-(1 ->2)-
p-D-fructofuranoside][O-a-D-galactopyranosyl-(1^6)-O-a-D-galactopyranosyl-
(1 -»6)-O-a-D-glucopyranosyl-(1 -»2)-p-D-fructofuranoside][O-a-D-galactopyranosyl-(1^6)-O-a-D-galactopyranosyl-
(1 -»6)-O-cc-D-galactopyranosyl-(1 ->6)-O-oc-D-glucopyrano-syl-(1 ->2)-p-D-fructofuranoside]
Source: From R.S. Shallenberger and G. G. Birch, Sugar Chemistry, 1975, AVI Publishing Co.
fructose. Since the specific rotation ofsucrose is +66.5°, of D-glucose +52.2°, andof D-fructose -93°, the resulting invert sugarhas a specific rotation of -20.4°. The nameinvert sugar refers to the inversion of thedirection of rotation.
Sucrose is highly soluble over a widetemperature range, as is indicated in Figure4-11. This property makes sucrose anexcellent ingredient for syrups and othersugar-containing foods.
The characteristic carbohydrate of milk islactose or milk sugar. With a few minorexceptions, lactose is the only sugar in themilk of all species and does not occur else-where. Lactose is the major constituent ofthe dry matter of cow's milk, as it representsclose to 50 percent of the total solids. Thelactose content of cow's milk ranges from4.4 to 5.2 percent, with an average of 4.8
percent expressed as anhydrous lactose. Thelactose content of human milk is higher,about 7.0 percent.
Lactose is a disaccharide of D-glucose andD-galactose and is designated as 4-0-p-D-galactopyranosyl-D-glucopyranose (Figure4-10). It is hydrolyzed by the enzyme (3-D-galactosidase (lactase) and by dilute solu-tions of strong acids. Organic acids such ascitric acid, which easily hydrolyze sucrose,are unable to hydrolyze lactose. This differ-ence is the basis of the determination of thetwo sugars in mixtures.
Maltose is 4-a-D-glucopyranosyl-f5-D-glucopyranose. It is the major end product ofthe enzymic degradation of starch and glyco-gen by p-amylase and has a characteristicflavor of malt. Maltose is a reducing disac-charide, shows mutarotation, is fermentable,and is easily soluble in water.
LactoseCellobTose
MaltoseSucrose
Figure 4-10 Structure of Some Important Disaccharides
TEMPERATURE
Figure 4-11 Approximate Solubility of SomeSugars at Different Temperatures. Source: FromR.S. Shallenberger and G.G. Birch, SugarChemistry, 1975, AVI Publishing Co.
Cellobiose is 4-p-D-glucopyranosyl-p-D-glucopyranose, a reducing disaccharideresulting from partial hydrolysis of cellulose.
Legumes contain several oligosaccha-rides, including raffmose and stachyose.These sugars are poorly absorbed wheningested, which results in their fermentationin the large intestine. This leads to gas pro-duction and flatulence, which present a bar-rier to wider food use of such legumes.deMan et al. (1975 and 1987) analyzed alarge number of soybean varieties and foundan average content of 1.21 percent stachyose,0.38 percent raffinose, 3.47 percent sucrose,and very small amounts of melibose. In soymilk, total reducing sugars after inversionamounted to 11.1 percent calculated on drybasis.
Cow's milk contains traces of oligosaccha-rides other than lactose. They are made up oftwo, three, or four units of lactose, glucose,galactose, neuraminic acid, mannose, andacetyl glucosamine. Human milk contains
about 1 g/L of these oligosaccharides, whichare referred to as the bifidus factor. The oli-gosaccharides have a beneficial effect on theintestinal flora of infants.
Fructooligosaccharides (FOSs) are oligo-mers of sucrose where an additional one,two, or three fructose units have been addedby a p-(2-l)-glucosidic linkage to the fruc-tose unit of sucrose. The resulting FOSs,therefore, contain two, three, or four fruc-tose units. The FOSs occur naturally ascomponents of edible plants includingbanana, tomato, and onion (Spiegel et al.1994). FOSs are also manufactured com-mercially by the action of a fungal enzymefrom Aspergillus niger, p-fructofuranosi-dase, on sucrose. The three possible FOSsare !^(l-p-fructofuranosyl)^ sucrose oli-gomers with abbreviated and commonnames as follows: GF2 (1-kestose), GF3
(nystose), and GF4 (lF-p-fructofuranosyl-nystose). The commercially manufacturedproduct is a mixture of all three FOSs withsucrose, glucose, and fructose. FOSs arenondigestible by humans and are suggestedto have some dietary fiber-like function.
Chemical Reactions
Mutarotation
When a crystalline reducing sugar is placedin water, an equilibrium is establishedbetween isomers, as is evidenced by a rela-tively slow change in specific rotation thateventually reaches the final equilibrium val-ue. The working hypothesis for the occur-rence of mutarotation has been described byShallenberger and Birch (1975). It is assumedthat five structural isomers are possible forany given reducing sugar (Figure 4-12), withpyranose and furanose ring structures beinggenerated from a central straight-chain inter-
7. SU
GAR
mediate. When all of these forms are present,the mutarotation is complex. When only thepyranose forms are present, the mutarotationis simple. Aldoses that have the gluco,manno, gulo, and allo configurations (Figure4-6) exhibit simple mutarotation. D-glucose,for example, shows simple mutarotation andin aqueous solution only two forms arepresent, 36 percent oc-D-glucopyranose and64 percent (3-D-glucopyranose. The amountof aldehyde form of glucose in solution hasbeen estimated at 0.003 percent. The distribu-tion of isomers in some mutarotated mono-saccharides at 2O0C is shown in Table 4-4,The distribution of a- and p-anomers in solu-tions of lactose and maltose is nearly thesame as in glucose, about 32 percent a- and64 percent (3-anomer. Simple mutarotation is
a first-order reaction characterized by uni-form values of the reaction constants k1 andk2 in the equation
*ioc-D-glucopyranose ̂ p-D-glucopyranose
k2The velocity of the reaction is greatly
accelerated by acid or base. The rate is at aminimum for pyranose-pyranose intercon-versions in the pH range 2.5 to 6.5. Bothacids and bases accelerate mutarotation rate,with bases being more effective. This wasexpressed by Hudson (1907) in the followingequation:
K250 = 0.0096 + 0.258 [H+] + 9.750 [OH~]
Figure 4-12 Equilibria Involved in Mutarotation. Source: From R.S. Shallenberger and G.G. Birch,Sugar Chemistry, 1975, AVI Publishing Co.
OC- PYRANOSE /3- PYRANOSE
ALDEHYDOOR
KETO FORM
OL- FURANOSE B- FURANOSE
Table 4-4 Percentage Distribution of Isomers of Mutarotated Sugars at 2O0C
Sugar
D-GlucoseD-GalactoseD-MannoseD-Fructose
a-Pyranose
31.1-37.429.6-35.064.0-68.9
4.0?
$~Pyranose
64.0-67.963.9-70.431.1-36.068.4-76.0
a-Furanose
1.0
fi-Furanose
3.1
28.0-31.6
Source: From R.S. Shallenberger and G.G. Birch, Sugar Chemistry, AVI Publishing Co.
This indicates that the effect of thehydroxyl ion is about 40,000 times greaterthan that of the hydrogen ion. The rate ofmutarotation is also temperature dependent;increases from 1.5 to 3 times occur for every1O0C rise in temperature.
Other Reactions
Sugars in solution are unstable andundergo a number of reactions. In addition tomutarotation, which is the first reaction tooccur when a sugar is dissolved, enolizationand isomerization, dehydration and fragmen-tation, anhydride formation and polymeriza-tion may all take place. These reactions areoutlined in Figure 4-13, using glucose as anexample. Compounds (1) and (2) are the aand (3 forms in equilibrium during mutarota-tion with the aldehyde form (5). Heatingresults in dehydration of the IC conforma-tion of (3-D glucopyranose (3) and formationof levoglucosan (4), followed by the
sequence of reactions described under cara-melization. Enolization is the formation ofan enediol (6). These enediols are unstableand can rearrange in several ways. Since thereactions are reversible, the starting materialcan be regenerated. Other possibilitiesinclude formation of keto-D-fructose (10)and (3-D-fructopyranose (11), and aldehydo-D-mannose (8) and oc-D-mannopyranose (9).Another possibility is for the double bond tomove down the carbon chain to form anotherenediol (7). This compound can give rise tosaccharinic acids (containing one carboxylgroup) and to 5-(hydroxy)-methylfurfural(13). All these reactions are greatly influ-enced by pH. Mutarotation, enolization, andformation of succharic acid (containing twocarboxyl groups) are favored by alkaline pH,formation of anhydrides, and furaldehydesby acid pH.
It appears from the aforementioned reac-tions that on holding a glucose solution atalkaline pH, a mixture of glucose, mannose,
Figure 4-13 Reactions of Reducing Sugars in Solution. Source: From R.S. Shallenberger and G.G.Birch, Sugar Chemistry, 1975, AVI Publishing Co.
POLYMERIZATION
SACCHARINICACIDS
POLYMERIZATION
DIMERIZATION
HYDROLYSIS
and fructose will be formed and, in general,any one sugar will yield a mixture of sugars.When an acid solution of sugar of high con-centration is left at ambient temperature,reversion takes place. This is the formationof disaccharides. The predominant linkagesin the newly formed disaccharides are a-D-1—>6, and (3-D-l—»6. A list of reversion di-saccharides observed by Thompson et al.(1954) in a 0.082N hydrochloric acid solu-tion or in D-glucose is shown in Table 4-5.
Caramelization
The formation of the caramel pigment canbe considered a nonenzymatic browningreaction in the absence of nitrogenous com-pounds. When sugars are subjected to heat inthe absence of water or are heated in concen-trated solution, a series of reactions occursthat finally leads to caramel formation. Theinitial stage is the formation of anhydro sug-ars (Shallenberger and Birch 1975). Glucoseyields glucosan (1,2-anhydro-oc-D-glucose)and levoglucosan (1,6-anhydro-p-D-glu-cose); these have widely differing specificrotation, +69° and -67°, respectively. Thesecompounds may dimerize to form a numberof reversion disaccharides, including gentio-
biose and sophorose, which are also formedwhen glucose is melted.
Caramelization of sucrose requires a tem-perature of about 20O0C. At 16O0C, sucrosemelts and forms glucose and fructose anhy-dride (levulosan). At 20O0C, the reactionsequence consists of three distinct stages wellseparated in time. The first step requires 35minutes of heating and involves a weight lossof 4.5 percent, corresponding to a loss of onemolecule of water per molecule of sucrose.This could involve formation of compoundssuch as isosacchrosan. Pictet and Strieker(1924) showed that the composition of thiscompound is 1,3'; 2,2'-dianhydro-a-D-gluco-pyranosyl-p-D-glucopyranosyl-(3-D-fructo-furanose (Figure 4-14). After an additional 55minutes of heating, the weight loss amounts to9 percent and the pigment formed is namedcaramelan. This corresponds approximately tothe following equation:
2C12H22O11 - 4H2O —> C24H36O18
The pigment caramelan is soluble in waterand ethanol and has a bitter taste. Its meltingpoint is 1380C. A further 55 minutes of heat-ing leads to the formation of caramelen. Thiscompound corresponds to a weight loss of
Source: From A. Thompson et al., Acid Reversion Products from D-Glucose, J. Am. Chem. Soc., Vol. 76, pp. 1309-1311, 1954.
Table 4-5 Reversion Disaccharides of Glucose in 0.082A/ HCI
p, p-trehalose (p-D-glucopyranosyl p-D-glucopyranoside)p-sophorose (2-O-p-D-glucopyranosyl-p-D-glucopyranose)p-maltose (4-O-oc-D-glycopyranosyl-p-D-glycopyranose)oc-cellobiose (4-O-p-D-glucopyranosyl-oc-D-glucopyranose)p-cellobiose (4-O-p-D-glucopyranosyl-p-D-glucopyranose)p-isomaltose (6-O-a-D-glucopyranosyl-p-D-glucopyranose)oc-gentiobiose (6-O-p-D-glucopyranosyl-oc-D-glucopyranose)p-gentiobiose(G-O-p-D-glucopyranosyl-p-D-glucopyranose)
0.1%0.2%0.4%0.1%0.3%4.2%0.1%3.4%
Figure 4-14 Structure of Isosacchrosan. Source:From R.S. Shallenberger and G.G. Birch, SugarChemistry, 1975, AVI Publishing Co.
about 14 percent, which is about eight mole-cules of water from three molecules ofsucrose, as follows:
3C12H22O11 - 8H2O —» C36H50O25
Caramelen is soluble in water only andmelts at 1540C. Additional heating results inthe formation of a very dark, nearly insolublepigment of average molecular compositionC125H188O80. This material is called humin orcaramelin.
The typical caramel flavor is the result of anumber of sugar fragmentation and dehydra-tion products, including diacetyl, acetic acid,formic acid, and two degradation productsreported to have typical caramel flavor bylurch and Tatum (1970), namely, acetylfor-moin (4-hydroxy-2,3,5-hexane-trione) and 4-hydroxy-2,5-dimethyl-3(2H)-furanone.
Crystallization
An important characteristic of sugars istheir ability to form crystals. In the commer-cial production of sugars, crystallization isan important step in the purification of sugar.The purer a solution of a sugar, the easier itwill crystallize. Nonreducing oligosaccha-rides crystallize relatively easily. The fact
that certain reducing sugars crystallize withmore difficulty has been ascribed to the pres-ence of anomers and ring isomers in solu-tion, which makes these sugars intrinsically"impure" (Shallenberger and Birch 1975).Mixtures of sugars crystallize less easily thansingle sugars. In certain foods, crystallizationis undesirable, such as the crystallization oflactose in sweetened condensed milk or icecream.
Factors that influence growth of sucrosecrystals have been listed by Smythe (1971).They include supersaturation of the solution,temperature, relative velocity of crystal andsolution, nature and concentration of impuri-ties, and nature of the crystal surface. Crystalgrowth of sucrose consists of two steps: (1)the mass transfer of sucrose molecules to thesurface of the crystal, which is a first-orderprocess; and (2) the incorporation of the mol-ecules in the crystal surface, a second-orderprocess. Under usual conditions, overallgrowth rate is a function of the rate of bothprocesses, with neither being rate-control-ling. The effect of impurities can be of twokinds. Viscosity can increase, thus reducingthe rate of mass transfer, or impurities caninvolve adsorption on specific surfaces of thecrystal, thereby reducing the rate of surfaceincorporation.
The crystal structure of sucrose has beenestablished by X-ray diffraction and neutrondiffraction studies. The packing of sucrosemolecules in the crystal lattice is determinedmainly by hydrogen bond formation betweenhydroxyl groups of the fructose moiety. Asan example of the type of packing of mole-cules in a sucrose crystal, a projection of thecrystal structure along the a axis is shown inFigure 4-15. The dotted square representsone unit cell. The crystal faces indicated inthis figure follow planes between adjacentsucrose molecules in such a way that the
furanose and pyranose rings are not inter-sected.
Lactose can occur in two crystallineforms, the a-hydrate and the p-anhydrousforms and can occur in an amorphous orglassy state. The most common form is thea-hydrate (C12H22O11-H2O), which can beobtained by crystallization from a supersatu-rated solution below 93.50C. When crystalli-zation is carried out above 93.50C, thecrystals formed are of p-anhydrous type.Some properties of these forms have beenlisted by Jenness and Patton (1959) (Table4-6). Under normal conditions the oc-
hydrate form is the stable one, and othersolid forms spontaneously change to thatform provided sufficient water is present. Atequilibrium and at room temperature, the P-form is much more soluble and the amountof a-form is small. However, because of itslower solubility, the a-hydrate crystallizesout and the equilibrium shifts to convert p-into a-hydrate. The solubility of the twoforms and the equilibrium mixture is repre-sented in Figure 4-16.
The solubility of lactose is less than that ofmost other sugars, which may present prob-lems in a number of foods containing lac-
Table 4-6 Some Physical Properties of the Two Common Forms of Lactose
Property
Melting point1
Specific rotation2 [a]^°Solubility (g/100 mL) Water at 2O0CWater at 10O0CSpecific gravity (2O0C)Specific heatHeat of combustion (cal/g~1)
a-Hydrate
2020C (dec.)+89.4°
870
1.540.2993761.6
ft- An hydride
2520C (dec.)+35°5595
1.590.2853932.7
1 Values vary with rate of heating, a-hydrate losses H2O (12O0C).2 Values on anhydrous basis, both forms mutarotate to +55.4°.
Source: From R. Jenness and S. Patton, Principles of Dairy Chemistry, 1959, John Wiley and Sons.
Figure 4-15 Projection of a Sucrose Crystal Along the a Axis. Source: From B.M. Smythe, SucroseCrystal Growth, Sugar Technol Rev., Vol. 1, pp. 191-231, 1971.
tose. When milk is concentrated 3:1, theconcentration of lactose approaches its finalsolubility. When this product is cooled orwhen sucrose is added, crystals of a-hydratemay develop. Such lactose crystals are veryhard and sharp; when left undisturbed theymay develop to a large size, causing a sensa-tion of grittiness or sandiness in the mouth.This same phenomenon limits the amount ofmilk solids that can be incorporated into icecream.
The crystals of a-hydrate lactose usuallyoccur in a prism or tomahawk shape. Thelatter is the basic shape and all other shapesare derived from it by different relativegrowth rates of the various faces. The shapeof an a-hydrate lactose crystal is shown inFigure 4-17. The crystal has been character-
ized by X-ray diffraction, and the followingconstants for the dimensions of the unit celland one of the axial angles have been estab-lished: a = 0.798 nm, b = 2.168 nm, c =0.4836 nm, and (3 = 109° 47'. The crystallo-graphic description of the crystal faces isindicated in Figure 4-17. These faces growat different rates; the more a face is orientedtoward the (3 direction, the slower it growsand the (OTO) face does not grow at all.
Amorphous or glassy lactose is formedwhen lactose-containing solutions are driedquickly. The dry lactose is noncrystalline andcontains the same ratio of alpha/beta as the
Figure 4-16 Solubility of Lactose in Water.Source: From E.O. Whittier, Lactose and ItsUtilization: A Review, /. Dairy ScL, Vol. 27, p.505, 1944.
Figure 4-17 Crystallographic Representation ofa Tomahawk Crystal of a-Lactose Monohy-drate. Source: From A. Van Kreveld and A.S.Michaels, Measurement of Crystal Growth ofa=Lactose, J. Dairy ScL, Vol. 48, pp. 259-265,1965.
Temperature (0C.)
Initial solubilityof Of-form
Final solubilityat equilibrium
Initial solubilityof P-form
x Direct determinationso Calculated assumingequilibrium constantsand no interferenceby other form
Usual rangeof super-saturation
Solubi
lity (g.
anhydr
ous lac
tose/1
00 g. w
ater)
original product. This holds true for spray orroller drying of milk products and also dur-ing drying for moisture determination. Theglassy lactose is extremely hygroscopic andtakes up moisture from the atmosphere.When the moisture content reaches about 8percent, the lactose molecules recrystallizeand form a-hydrate crystals. As these crys-tals grow, powdered products may cake andbecome lumpy.
Both lactose and sucrose have been shownto crystallize in an amorphous form at mois-ture contents close to the glass transitiontemperature (Roos and Karel 1991a,b; Roosand Karel 1992). When amorphous lactose isheld at constant water content, crystallizationreleases water to the remaining amorphousmaterial, which depresses the glass transitiontemperature and accelerates crystallization.These authors have done extensive studies onthe glass transition of amorphous carbohy-drate solutions (Roos 1993; Roos and Karel199Id).
Seeding is a commonly used procedure toprevent the slow crystallization of lactoseand the resulting sandiness in some dairyproducts. Finely ground lactose crystals areintroduced into the concentrated product,and these provide numerous crystal nuclei.Many small crystals are formed rapidly;therefore, there is no opportunity for crystalsto slowly grow in the supersaturated solutionuntil they would become noticeable in themouth.
Starch Hydrolyzates—Corn Sweeteners
Starch can be hydrolyzed by acid orenzymes or by a combination of acid andenzyme treatments. A large variety of prod-ucts can be obtained from starch hydrolysis
using various starches such as corn, wheat,potato, and cassava (tapioca) starch. Glucosesyrups, known in the United States as cornsyrup, are hydrolysis products of starch withvarying amounts of glucose monomer,dimer, oligosaccharides, and polysaccha-rides. Depending on the method of hydroly-sis used, different compositions with a broadrange of functional properties can beobtained. The degree of hydrolysis isexpressed as dextrose equivalent (DE),defined as the amount of reducing sugarspresent as dextrose and calculated as a per-centage of the total dry matter. Glucose syr-ups have a DE greater than 20 and less than80. Below DE 20 the products are referred toas maltodextrins and above DE 80 as hydro-lyzates. The properties of maltodextrins areinfluenced by the nature of the starch used;those of hydrolyzates are not affected by thetype of starch.
The initial step in starch hydrolysisinvolves the use of a heat-stable endo-ot-amylase. This enzyme randomly attacks a-1,4 glycosidic bonds resulting in rapid decreasein viscosity. These enzymes can be used attemperatures as high as 1050C. This reactionproduces maltodextrins (Figure 4-18), whichcan be used as important functional foodingredients—fillers, stabilizers, and thicken-ers. The next step is saccharification by usinga series of enzymes that hydrolyze either thea-1,4 linkages of amylose or the a-1,6 link-ages of the branched amylopectin. The actionof the various starch-degrading enzymes isshown in Figure 4-19 (Olsen 1995). In addi-tion to products containing high levels of glu-cose (95 to 97 percent), sweeteners with DEof 40 to 45 (maltose), 50 to 55 (high mal-tose), and 55 to 70 (high conversion syrup)can be produced. High dextrose syrups canbe obtained by saccharification with amylo-glucosidase. At the beginning of the reaction
dextrose formation is rapid but graduallyslows down. This slowdown is caused by for-mation of branched dextrins and because athigh dextrose level the repolymerization ofdextrose into isomaltose occurs.
The isomerization of glucose to fructoseopened the way for starch hydrolyzates toreplace cane or beet sugar (Dziezak 1987).This process is done with glucose isomerasein immobilized enzyme reactors. The con-version is reversible and the equilibrium is at50 percent conversion. High-fructose cornsyrups are produced with 42 or 55 percentfructose. These sweeteners have taken overone-third of the sugar market in the UnitedStates (Olsen 1995).
The acid conversion process has a practicallimit of 55 DE; above this value, dark colorand bitter taste become prominent. Depend-ing on the process used and the reaction con-
ditions employed, a variety of products canbe obtained as shown in Table 4-7 (Com-merford 1974). There is a fairly constantrelationship between the composition ofacid-converted corn syrup and its DE. Thecomposition of syrups made by acid-enzymeor dual-enzyme processes cannot be as easilypredicted from DE.
Maltodextrins (DE below 20) have compo-sitions that reflect the nature of the starchused. This depends on the amylose/amy-lopectin ratio of the starch. A maltodextrinwith DE 12 shows retrogradation in solution,producing cloudiness. A maltodextrin fromwaxy corn at the same DE does not show ret-rogradation because of the higher level of oc-1, 6 branches. As the DE decreases, the dif-ferences become more pronounced. A vari-ety of maltodextrins with different functionalproperties, such as gel formation, can be
MALTODEXTRIN
STARCH SLURRY
cc-AMYLASE LIQUEFACTION MALTODEXTRIN
GLUCOAMYLASE/PULLULANASE
SACCHARIFICATION
GLUCOSE/ISOMERASE
PURIFICATION
ISOMERIZATION
REFINING
MALTOSESYRUPSGLUCOSESYRUPSMIXED SYRUPS
FRUCTOSESYRUPS
Figure 4-18 Major Steps in Enzymic Starch Conversion. Source: Reprinted from H.S. Olsen, EnzymicProduction of Glucose Syrups, in Handbook of Starch Hydrolysis Products and Their Derivatives,M.W. Kearsley and S.Z. Dziedzic, eds., p. 30, © 1995, Aspen Publishers, Inc.
obtained by using different starch raw mate-rials.
Maltodextrins of varying molecular weightsare plasticized by water and decrease the glasstransition temperature. Maltodextrins retardthe crystallization of amorphous sucrose andat high concentrations totally inhibit sucrosecrystallization (Roos and Karel 199Ic).
Maltodextrins with low DE and with littleor no remaining polysaccharide can be pro-duced by using two enzymes. Alpha-amylaserandomly hydrolyzes 1 —» 4 linkages toreduce the viscosity of the suspension. Pullu-
lanase is specific for 1 —» 6 linkages and actsas a debranching enzyme. The application ofthese two enzymes makes it possible to pro-duce maltodextrins in high yield (Kennedy etal. 1985).
Polyols
Polyols or sugar alcohols occur in natureand are produced industrially from the corre-sponding saccharides by catalytic hydroge-nation. Sorbitol, the most widely distributednatural polyol, is found in many fruits such
Figure 4-19 Schematic Representation of the Action of Starch-Degrading Enzymes. (A) Amylose andamylopectin, (B) action of a-amylase on amylose and amylopectin, (C) action of a debranchingenzyme on amylose and amylopectin, (D) action of amyloglucosidase and debranching enzyme onamylose and amylopectin. Source: Reprinted from H.S. Olsen, Enzymic Production of Glucose Syr-ups, in Handbook of Starch Hydrolysis Products and Their Derivatives, M. W. Kearsley and S.Z.Dziedzic, eds., p. 36, © 1995, Aspen Publishers, Inc.
C D
Amylose
AmylopectinBA
as plums, berries, cherries, apples, and pears.It is a component of fruit juices, fruit wines,and other fruit products. It is commerciallyproduced by catalytic hydrogenation of D-glucose. Mannitol, the reduced form of D-mannose, is found as a component of mush-rooms, celery, and olives. Xylitol is obtainedfrom saccharification of xylan-containingplant materials; it is a pentitol, being thereduced form of xylose. Sorbitol, mannitol,and xylitol are monosaccharide-derivedpolyols with properties that make them valu-able for specific applications in foods: theyare suitable for diabetics, they are noncario-genic, they possess reduced physiologicalcaloric value, and they are useful as sweeten-ers that are nonfermentable by yeasts.
In recent years disaccharide alcohols havebecome important. These include isomalt,maltitol, lactitol, and hydrogenated starchhydrolyzates (HSH). Maltitol is hydroge-nated maltose with the structure shown inFigure 4-20. It has the highest sweetness ofthe disaccharidepolyols compared to sugar
(Table 4-8) (Heume and Rapaille 1996). Ithas a low negative heat of solution and,therefore, gives no cooling effect in contrastto sorbitol and xylitol. It also has a very highviscosity in solution. Sorbitol and maltitolare derived from starch by the productionprocess illustrated in Figure 4-21. Lactitolis a disaccharide alcohol, 1,4-galactosyl-glucitol, produced by hydrogenation of lac-tose. It has low sweetness and a lowerenergy value than other polyols. It has a cal-orie value of 2 kcal/g and is noncariogenic(Blankers 1995). It can be used in combina-tion with intense sweeteners like aspartameor acesulfame-K to produce sweetening
Table 4-7 Composition of Representative Corn Syrups
Saccharines (%)
Type of Conversion
AcidAcidAcid-enzymeAcidAcidAcid-enzymeAcid-enzyme
DextroseEquivalent
30424354606371
Mono-
10.418.55.529.736.238.843.7
Di-
9.313.946.217.819.528.136.7
Tr/-
8.611.612.313.213.213.73.7
Tetra-
8.29.93.29.68.74.13.2
Penta-
7.28.41.87.36.34.50.8
Hexa-
6.06.61.55.34.42.64.3
Hepta-
5.25.7
4.33.2
Higher
45.125.429.51
12.88.58.21
7.61
1 Includes heptasaccharides.
Source: From J.D. Commerford, Corn Sweetener Industry, in Symposium: Sweeteners, I.E. lnglett, ed., 1974, AVIPublishing Co.
Figure 4-20 Structure of Maltitol
Table 4-8 Relative Sweetness of Polyols andSucrose Solutions at 2O0C
Compound Relative Sweetness
Xylitol 80-100Sorbitol 50-60Mannitol 50-60Maltitol 80-90Lactitol 30-40lsomalt 50-60Sucrose 100
Source: Reprinted from H. Schiweck and S.C. Zies-enitz, Physiological Properties of Polyols in Comparisonwith Easily Metabolizable Saccharides, in Advances inSweeteners, T.H. Grenby, ed., p. 87, ©1996, AspenPublishers, Inc.
power similar to sucrose. These combina-tions provide a milky, sweet taste that allowsgood perception of other flavors. lsomalt,also known as hydrogenated isomaltulose orhydrogenated palatinose, is manufactured ina two-step process: (1) the enzymatic trans-glycosylation of the nonreducing sucrose tothe reducing sugar isomaltulose; and (2)hydrogenation, which produces isomalt—anequimolar mixture of D-glucopyranosyl-oc-(l-l)-D-mannitol and D-glucopyranosyl-oc-(l-6)-D-sorbitol. Isomalt is extremely stableand has a pure, sweet taste. Because it is onlyhalf as sweet as sucrose, it can be used as aversatile bulk sweetener (Ziesenitz 1996).
POLYSACCHARIDES
Starch
Starch is a polymer of D-glucose and isfound as a storage carbohydrate in plants. Itoccurs as small granules with the size rangeand appearance characteristic to each plantspecies. The granules can be shown by ordi-
Figure 4-21 Production Process for the Conver-sion of Starch to Sorbitol and Maltitol. Source:Reprinted from H. Schiweck and S.C. Ziesenitz,Physiological Properties of Polyols in Compari-son with Easily Metabolizable Saccharides,Advances in Sweeteners, T.H. Grenby, ed., p. 90,© 1996, Aspen Publishers, Inc.
Figure 4-22 Appearance of Starch Granules asSeen in the Microscope
CORN POTATO RICE
SAGO TAPIOCA WHEAT
SORBITOL MALTITOL
crystallization or solidification
MALTITOL SYRUPSORBITOL SYRUP
hydrogenation/filtration/ion exchange/evaporation
DEXTROSEGLUCOSE SYRUP
MALTOSE SYRUP
enzymatic hydrolysis
STARCH
Next page
INTRODUCTION
In addition to the major components, allfoods contain varying amounts of minerals.The mineral material may be present as inor-ganic or organic salts or may be combinedwith organic material, as the phosphorus iscombined with phosphoproteins and metalsare combined with enzymes. More than 60elements may be present in foods. It is cus-tomary to divide the minerals into twogroups, the major salt components and thetrace elements. The major salt componentsinclude potassium, sodium, calcium, magne-sium, chloride, sulfate, phosphate, and bicar-bonate. Trace elements are all others and areusually present in amounts below 50 partsper million (ppm). The trace elements can bedivided into the following three groups:
1. essential nutritive elements, whichinclude Fe, Cu, I, Co, Mn, Zn, Cr, Ni,Si, F, Mo, and Se.
2. nonnutritive, nontoxic elements, in-cluding Al, B, and Sn
3. nonnutritive, toxic elements, includingHg, Pb, As, Cd, and Sb
The minerals in foods are usually deter-mined by ashing or incineration. Thisdestroys the organic compounds and leavesthe minerals behind. However, determined in
this way, the ash does not include the nitrogencontained in proteins and is in several otherrespects different from the real mineral con-tent. Organic anions disappear during inciner-ation, and metals are changed to their oxides.Carbonates in ash may be the result ofdecomposition of organic material. The phos-phorus and sulfur of proteins and the phos-phorus of lipids are also part of ash. Some ofthe trace elements and some salts may be lostby volatilization during the ashing. Sodiumchloride will be lost from the ash if the incin-eration temperature is over 60O0C. Clearly,when we compare data on mineral composi-tion of foods, we must pay great attention tothe methods of analysis used.
Some elements appear in plant and animalproducts at relatively constant levels, but in anumber of cases an abundance of a certainelement in the environment may result in agreatly increased level of that mineral inplant or animal products. Enrichment of ele-ments in a biological chain may occur; note,for instance, the high mercury levels re-ported in some large predatory fish speciessuch as swordfish and tuna.
MAJORMINERALS
Some of the major mineral constituents,especially monovalent species, are present in
Minerals
CHAPTER 5
foods as soluble salts and mostly in ionizedform. This applies, for example, to the cat-ions sodium and potassium and the anionschloride and sulfate. Some of the polyvalentions, however, are usually present in the formof an equilibrium between ionic, dissolvednonionic, and colloidal species. Such equilib-ria exist, for instance, in milk and in meat.Metals are often present in the form of che-lates. Chelates are metal complexes formedby coordinate covalent bonds between aligand and a metal cation; the ligand in a che-late has two or more coordinate covalentbonds to the metal. The name chelate isderived from the claw-like manner in whichthe metal is held by the coordinate covalentbonds of the ligand. In the formation of a che-late, the ligand functions as a Lewis base, andthe metal ion acts as a Lewis acid. The stabil-ity constant of a chelate is influenced by anumber of factors. The chelate is more stablewhen the ligand is relatively more basic. Thechelate's stability depends on the nature ofthe metal ion and is related to the electroneg-ative character of the metal. The stability of achelate normally decreases with decreasingpH. In a chelate the donor atoms can be N, O,P, S, and Cl; some common donor groups are-NH2, =C=O, =NH, -COOH, and -OH-O-PO(OH)2. Many metal ions, especially thetransition metals, can serve as acceptors toform chelates with these donor groups. For-mation of chelates can involve ring systemswith four, five, or six members. Some exam-ples of four- and five-membered ring struc-tures are given in Figure 5-1. An example ofa six-membered chelate ring system is chlo-rophyll. Other examples of food componentsthat can be considered metal chelates arehemoglobin and myoglobin, vitamin B12, andcalcium casemate (Pfeilsticker 1970). It hasalso been proposed that the gelation of certainpolysaccharides, such as alginates and pec-
tates, with metal ions occurs through chela-tion involving both hydroxyl and carboxylgroups (Schweiger 1966). A requirement forthe formation of chelates by these polysac-charides is that the OH groups be present invicinal pairs.
Concerns about the role of sodium inhuman hypertension have drawn attention tothe levels of sodium and potassium in foodsand to measures intended to lower oursodium intake. The total daily intake byAmericans of salt is 10 to 12 g, or 4 to 5 g ofsodium. This is distributed as 3 g occurringnaturally in food, 3 g added during foodpreparation and at the table, and 4 to 6 gadded during commercial processing. Thisamount is far greater than the daily require-ment, estimated at 0.5 g (Marsh 1983). Salthas an important effect on the flavor andacceptability of a variety of foods. In addi-tion to lowering the level of added salt infood, researchers have suggested replacingsalt with a mixture of sodium chloride andpotassium chloride (Maurer 1983; Dunaifand Khoo 1986). It has been suggested thatcalcium also plays an important role in regu-lating blood pressure.
Interactions with Other FoodComponents
The behavior of minerals is often influ-enced by the presence of other food constit-uents. The recent interest in the beneficialeffect of dietary fiber has led to studies ofthe role fiber plays in the absorption of min-erals. It has been shown (Toma and Curtis1986) that mineral absorption is decreasedby fiber. A study of the behavior of iron,zinc, and calcium showed that interactionsoccur with phytate, which is present in fiber.Phytates can form insoluble complexes withiron and zinc and may interfere with the
absorption of calcium by causing formationof fiber-bound calcium in the intestines.
Iron bioavailability may be increased in thepresence of meat (Politz and Clydesdale1988). This is the so-called meat factor. Theexact mechanism of this effect is not known,but it has been suggested that amino acids orpolypeptides that result from digestion areable to chelate nonheme iron. These com-plexes would facilitate the absorption of iron.In nitrite-cured meats some factors promoteiron bioavailability (the meat factor), particu-larly heme iron and ascorbic acid or erythor-bic acid. Negative factors may in-cludenitrite and nitrosated heme (Lee and Greger1983).
Minerals in Milk
The normal levels of the major mineralconstituents of cow's milk are listed in Table5-1. These are average values; there is aconsiderable natural variation in the levels ofthese constituents. A number of factorsinfluence the variations in salt composition,such as feed, season, breed and individualityof the cow, stage of lactation, and udderinfections. In all but the last case, the varia-tions in individual mineral constituents donot affect the milk's osmotic pressure. Theash content of milk is relatively constant atabout 0.7 percent. An important differencebetween milk and blood plasma is the rela-
Figure 5-1 Examples of Metal Chelates. Only the relevant portions of the molecules are shown. Thechelate formers are: (A) thiocarbamate, (B) phosphate, (C) thioacid, (D) diamine, (E) 0-phenantrolin, (F)oc-aminoacid, (G) 0-diphenol, (H) oxalic acid. Source: From K. Pfeilsticker, Food Components as MetalChelates, Food Sd. Technol., Vol. 3, pp. 45-51, 1970.
5-Ring
4-Ring
Table 5-1 Average Values for Major MineralContent of Cow's MIIk (Skim Milk)
Normal LevelConstituent (mg/100 mL)Sodium 50Potassium 145Calcium 120Magnesium 13Phosphorus (total) 95Phosphorus (inorganic) 75Chloride 100Sulfate 10Carbonate (as CO2) 20Citrate (as citric acid) 175
tive levels of sodium and potassium. Bloodplasma contains 330 mg/100 mL of sodiumand only 20 mg/100 mL of potassium. Incontrast, the potassium level in milk is aboutthree times as high as that of sodium. Someof the mineral salts of milk are present atlevels exceeding their solubility and there-fore occur in the colloidal form. Colloidalparticles in milk contain calcium, magne-sium, phosphate, and citrate. These colloidalparticles precipitate with the curd when milkis coagulated with rennin. Dialysis and ultra-filtration are other methods used to obtain aserum free from these colloidal particles. Inmilk the salts of the weak acids (phosphates,citrates, and carbonates) are distributedamong the various possible ionic forms. Asindicated by Jenness and Patton (1959), theratios of the ionic species can be calculatedby using the Henderson-Hasselbach equa-tion,
[salt]pU=pKa + log [^id]
The values for the dissociation constants ofthe three acids are listed in Table 5-2. Whenthese values are substituted in the Henderson-Hasselbach equation for a sample of milk atpH 6.6, the following ratios will be obtained:
Citrate" Citrate=
^.. . T-J = J,IHJU = ILCitric acid C i t r a t e -
Citrate= . ,~ = 1°
Citrate"
From these ratios we can conclude that inmilk at pH 6.6 no appreciable free citric acidor monocitrate ion is present and that trici-trate and dicitrate are the predominant ions,present in a ratio of about 16 to 1. For phos-phates, the following ratios are obtained:
H2PO4 ' HP04=
o^prT = 4 3 ' 6 0 0 ~- = 0 3 °W3F U4 H2PO4
PO4"—_ = 0.000002
HPO4 '
This indicates that mono- and diphosphateions are the predominant species. For car-bonates the ratios are as follows:
HCO3"
H^CO3- = L ?
C03=
- _ = 0.0002HCO3
Table 5-2 Dissociation Constants of Weak Acids
Acid PK1 pK2 pK3
Citric 3 ~ 0 8 4 7 4 5̂ 40Phosphoric 1.96 7.12 10.32Carbonic 6.37 10.25 —
The predominant forms are bicarbonates andthe free acid.
Note that milk contains considerably morecations than anions; Jenness and Patton(1959) have suggested that this can beexplained by assuming the formation ofcomplex ions of calcium and magnesiumwith the weak acids. In the case of citrate(symbol ©~) the following equilibria exist:
H©= ^ ©s + H+
©s + Ca++ ^ Ca ©"
Ca©- + H + ^ CaH ©
2Ca©~ + Ca++ ^ Ca3 ©2
Soluble complex ions such as Ca ©~ canaccount for a considerable portion of the cal-cium and magnesium in milk, and analogouscomplex ions can be formed with phosphateand possibly with bicarbonate.
The equilibria described here are repre-sented schematically in Figure 5-2, and thelevels of total and soluble calcium and phos-phorus are listed in Table 5-3. The mineralequilibria in milk have been extensivelystudied because the ratio of ionic and totalcalcium exerts a profound effect on the sta-bility of the caseinate particles in milk. Pro-
cessing conditions such as heating andevaporation change the salt equilibria andtherefore the protein stability. When milk isheated, calcium and phosphate change fromthe soluble to the colloidal phase. Changes inpH result in profound changes of all of thesalt equilibria in milk. Decreasing the pHresults in changing calcium and phosphatefrom the colloidal to the soluble form. At pH5.2, all of the calcium and phosphate of milkbecomes soluble. An equilibrium changeresults from the removal of CO2 as milkleaves the cow's udder. This loss of CO2 bystirring or heating results in an increased pH.Concentration of milk results in a dual effect.The reduction in volume leads to a change ofcalcium and phosphate to the colloidalphase, but this also liberates hydrogen ions,which tend to dissolve some of the colloidalcalcium phosphate. The net result dependson initial salt balance of the milk and thenature of the heat treatment.
The stability of the caseinate particles inmilk can be measured by a test such as the heatstability test, rennet coagulation test, or alco-hol stability test. Addition of various phos-phates—especially polyphosphates, which areeffective calcium complexing agents—canincrease the caseinate stability of milk. Addi-tion of calcium ions has the opposite effect anddecreases the stability of milk. Calcium isbound by polyphosphates in the form of a che-late, as shown in Figure 5-3.
Minerals in Meat
The major mineral constituents of meat arelisted in Table 5-4. Sodium, potassium, andphosphorus are present in relatively highamounts. Muscle tissue contains much morepotassium than sodium. Meat also containsconsiderably more magnesium than cal-cium. Table 5—4 also provides information
about the distribution of these mineralsbetween the soluble and nonsoluble forms.The nonsoluble minerals are associated withthe proteins. Since the minerals are mainlyassociated with the nonfatty portion of meat,the leaner meats usually have a higher min-eral or ash content. When liquid is lost frommeat (drip loss), the major element lost issodium and, to a lesser extent, calcium,
Table 5-3 Total and Soluble Calcium andPhosphorus Content of Milk
Constituent mg/1 OO mL
Total calcium 112.5Soluble calcium 35.2Ionic calcium 27.0Total phosphorus 69.6Soluble phosphorus 33.3
phosphorus, and potassium. Muscle tissueconsists of about 40 percent intracellularfluid, 20 percent extracellular fluid, and 40percent solids. The potassium is foundalmost entirely in the intracellular fluid, asare magnesium, phosphate, and sulfate.Sodium is mainly present in the extracellular
Figure 5-3 Calcium Chelate of a Polyphosphate
Figure 5-2 Equilibrium Among Milk Salts. Source: Reprinted with permission from R. Jenness and S.Patton, Principles of Dairy Chemistry, © 1959, John Wiley & Sons.
Colloidal ComplexCasein CalciumPhosphate MagnesiumCitrate
Table 5-4 Mineral Constituents in Meat (Beef)
Constituent mg/100g
Total calcium 8.6Soluble calcium 3.8Total magnesium 24.4Soluble magnesium 17.7Total citrate 8.2Soluble citrate 6.6Total inorganic phosphorus 233.0Soluble inorganic phosphorus 95.2Sodium 168Potassium 244Chloride 48
fluid in association with chloride and bicar-bonate. During cooking, sodium may be lost,but the other minerals are well retained. Pro-cessing does not usually reduce the mineralcontent of meat. Many processed meats arecured in a brine that contains mostly sodium
chloride. As a result, the sodium content ofcured meats may be increased.
Ionic equilibria play an important role inthe water-binding capacity of meat (Hamm1971). The normal pH of rigor or post-rigormuscle (pH 5.5) is close to the isoelectricpoint of actomyosin. At this point the netcharge on the protein is at a minimum. Byaddition of an acid or base, a cleavage of saltcross-linkages occurs, which increases theelectrostatic repulsion (Figure 5-4), loosensthe protein network, and thus permits morewater to be taken up. Addition of neutral saltssuch as sodium chloride to meat increaseswater-holding capacity and swelling. Theswelling effect has been attributed mainly tothe chloride ion. The existence of intra- andextracellular fluid components has been de-scribed by Merkel (1971) and may explainthe effect of salts such as sodium chloride.The proteins inside the cell membrane arenondiffusible, whereas the inorganic ionsmay move across this semipermeable mem-brane. If a solution of the sodium salt of a
Figure 5-4 Schematic Representation of the Addition of Acid (HA) or Base (B ) to an Isoelectric Pro-tein. The isoelectric protein has equal numbers of positive and negative charges. The acid HA donatesprotons, the base B~ accepts protons. Source: Reprinted with permission from R. Hamm, Colloid Chem-istry of Meat, © 1972, Paul Parey (in German).
Acid:
Base:
protein is on one side of the membrane andsodium chloride on the other side, diffusionwill occur until equilibrium has beenreached. This can be represented as follows:
3Na+ 3Na+ 4Na+ 2Na+
3 P r 3 c r 3 P r 2 c r
i c r
At start At equilibrium
At equilibrium the product of the concen-trations of diffusable ions on the left side ofthe membrane must be equal to the producton the right side, shown as follows:
[Na+]L[Cr]L = [Na+]R[Cl-]R
In addition, the sum of the cations on oneside must equal the sum of anions on theother side and vice versa:
[Na+]L = [Pr]L + [C1-]L and [Na+]R = [CT|R
This is called the Gibbs-Donnan equilib-rium and provides an insight into the reasonsfor the higher concentration of sodium ionsin the intracellular fluid.
Struvite
Occasionally, phosphates can form unde-sirable crystals in foods. The most commonexample is struvite, a magnesium-am-monium phosphate of the compositionMg.(NH4)PO4.6H2O. Struvite crystals areeasily mistaken by consumers for brokenpieces of glass. Most reports of struvite for-mation have been related to canned seafood,but occasionally the presence of struvite inother foods has been reported. It is assumedthat in canned seafood, the struvite is formedfrom the magnesium of sea water and ammo-
nia generated by the effect of heat on the fishor shellfish muscle protein.
Minerals in Plant Products
Plants generally have a higher content ofpotassium than of sodium. The major miner-als in wheat are listed in Table 5-5 andinclude potassium, phosphorus, calcium,magnesium, and sulfur (Schrenk 1964).Sodium in wheat is present at a level of onlyabout 80 ppm and is considered a trace ele-ment in this case. The minerals in a wheatkernel are not uniformly distributed; rather,they are concentrated in the areas close to thebran coat and in the bran itself. The variousfractions resulting from the milling processhave quite different ash contents. The ashcontent of flour is considered to be related toquality, and the degree of extraction of wheatin milling can be judged from the ash contentof the flour. Wheat flour with high ash con-tent is darker in color; generally, the lowerthe ash content, the whiter the flour. Thisgeneral principle applies, but the ash contentof wheat may vary within wide limits and isinfluenced by rainfall, soil conditions, fertil-izers, and other factors. The distribution ofmineral components in the various parts ofthe wheat kernel is shown in Table 5-6.
Table 5-5 Major Mineral Element Componentsin Wheat Grain
Element Average (%) Range (%)
Potassium 0.40 0.20-0.60Phosphorus 0.40 0.15-0.55Calcium 0.05 0.03-0.12Magnesium 0.15 0.08-0.30Sulfur 0.20 0.12-0.30
Source: Reprinted with permission from W.G.Schrenk, Minerals in Wheat Grain, Technical Bulletin136, © 1964, Kansas State University AgriculturalExperimental Station.
High-grade patent flour, which is pureendosperm, has an ash content of 0.30 to0.35 percent, whereas whole wheat mealmay have an ash content from 1.35 to 1.80percent.
The ash content of soybeans is relativelyhigh, close to 5 percent. The ash and majormineral levels in soybeans are listed in Table5-7. Potassium and phosphorus are the ele-ments present in greatest abundance. About70 to 80 percent of the phosphorus in soy-beans is present in the form of phytic acid,the phosphoric acid ester of inositol (Figure5-5). Phytin is the calcium-magnesium-potassium salt of inositol hexaphosphoricacid or phytic acid. The phytates are impor-tant because of their effect on protein solu-bility and because they may interfere withabsorption of calcium from the diet. Phyticacid is present in many foods of plant origin.
A major study of the mineral compositionof fruits was conducted by Zook and Leh-mann (1968). Some of their findings for themajor minerals in fruits are listed in Table5-8. Fruits are generally not as rich in min-erals as vegetables are. Apples have the low-
est mineral content of the fruits analyzed.The mineral levels of all fruits show greatvariation depending on growing region.
The rate of senescence of fruits and vege-tables is influenced by the calcium content ofthe tissue (Poovaiah 1986.) When fruits andvegetables are treated with calcium solu-tions, the quality and storage life of the prod-ucts can be extended.
TRACE ELEMENTS
Because trace metals are ubiquitous in ourenvironment, they are found in all of thefoods we eat. In general, the abundance oftrace elements in foods is related to theirabundance in the environment, although thisrelationship is not absolute, as has been indi-cated by Warren (1972b). Table 5-9 presentsthe order of abundance of some trace ele-ments in soil, sea water, vegetables, andhumans and the order of our intake. Traceelements may be present in foods as a resultof uptake from soil or feeds or from contami-nation during and subsequent to processing
Table 5-6 Mineral Components in Endosperm and Bran Fractions of Red Winter Wheat
Totalendosperm
Total branWheat kernelCenter sec-
tionGerm endBrush endEntire kernel
P(%)
0.10
0.38
0.35
0.550.410.44
K(%)
0.13
0.35
0.34
0.520.410.42
Na(%)
0.0029
0.0067
0.0051
0.00360.00570.0064
Ca(%)
0.017
0.032
0.025
0.0510.0360.037
Mg(%)
0.016
0.11
0.086
0.130.130.11
Mn (ppm)
2.4
32
29
774449
Fe (ppm)
13
31
40
814654
Cu (ppm)
8
11
7
8128
Source: From V.H. Morris et al., Studies on the Composition of the Wheat Kernel. II. Distribution of Certain Inor-ganic Elements in Center Sections, Cereal Chem., Vol. 22, pp. 361-372, 1945.
of foods. For example, the level of sometrace elements in milk depends on the levelin the feed; for other trace elements,increases in levels in the feed are notreflected in increased levels in the milk.Crustacea and mollusks accumulate metalions from the ambient sea water. As a result,
concentrations of 8,000 ppm of copper and28,000 ppm of zinc have been recorded(Meranger and Somers 1968). Contamina-tion of food products with metal can occur asa result of pickup of metals from equipmentor from packaging materials, especially tincans. The nickel found in milk comes almost
Table 5-7 Mineral Content of Soybeans (Dry Basis)
Mineral
AshPotassiumCalciumMagnesiumPhosphorusSulfurChlorineSodium
No. of Analyses
2997
37626
Range (%)
3.30-6.350.81-2.390.19-0.300.24-0.340.50-1.080.10-0.450.03-0.040.14-0.61
Mean (%)
4.601.830.240.310.780.240.030.24
Source: Reprinted with permission from A.K. Smith and SJ. Circle, Soybeans: Chemistry and Technology, © 1972,AVI Publishing Co.
Figure 5-5 Inositol and Phytic Acid
INOSITOL
PHYTIC ACID
exclusively from stainless steel in processingequipment. Milk coming from the udder hasno detectable nickel content. On the otherhand, nutritionists are concerned about thelow iron intake levels for large numbers ofthe population; this low intake can in part beexplained by the disappearance of ironequipment and utensils from processing andfood preparation.
Originally, nine of the trace elements wereconsidered to be essential to humans: cobalt,copper, fluorine, iodine, iron, manganese,molybdenum, selenium, and zinc. Recently,chromium, silicon, and nickel have beenadded to this list (Reilly 1996). These aremostly metals; some are metalloids. In addi-tion to essential trace elements, several traceelements have no known essentiality and
Table 5-8 Mineral Content of Some Fruits
Minerals (mg/100 g)
Fruit
Orange (California navel)Apple (Mclntosh)Grape (Thompson)Cherry (Bing)Pear (Bartlett)Banana (Ecuador)Pineapple (Puerto Rico)
N
16230
12119463
16871
Ca
23.72.46.29.64.82.72.2
Mg
10.23.65.8
16.26.5
25.43.9
P
15.85.4
12.813.39.3
16.43.0
K
17596
200250129373142
Source: From E.G. Zook and J. Lehmann, Mineral Composition of Fruits, J. Am. Dietetic Assoc., Vol. 52, pp. 225-231,1968.
Table 5-9 Order of Abundance of Some Trace Elements in Various Media
Element
IronManganeseNickelZincCopperCobaltLeadMolybdenumCadmiumMercury
Soil
1243576899
Sea Water
14723856?9
Vegetables
1362485799
Man
156238479
10
Man's Intake
1352486799
Source: From H.V. Warren, Geology and Medicine, Western Miner, pp. 34-37, 1972.
some are toxic (such as lead, mercury, andcadmium). These toxic trace elements, whichare classified as contaminants, are dealt within Chapter 11.
Trace elements get into foods by differentpathways. The most important source is fromthe soil, by absorption of elements in aque-ous solution through the roots. Another,minor, source is foliar penetration. This isusually associated with industrial air pollu-tion and vehicle emissions. Other possiblesources are fertilizers, agricultural chemi-cals, and sewage sludge. Sewage sludge is agood source of nitrogen and phosphate butmay contain high levels of trace minerals,many of these originating from industrialactivities such as electroplating. Trace min-erals may also originate from food process-ing and handling equipment, food packagingmaterials, and food additives.
Cobalt
Cobalt is an integral part of the only metalcontaining vitamin B12. The level of cobaltin foods varies widely, from as little as 0.01ppm in corn and cereals to 1 ppm in somelegumes. The human requirement is verysmall and deficiencies do not occur.
Copper
Copper is present in foods as part of sev-eral copper-containing enzymes, includingthe polyphenolases. Copper is a very power-ful prooxidant and catalyzes the oxidation ofunsaturated fats and oils as well as ascorbicacid. The normal daily diet contains from 2to 5 mg of copper, more than ample to coverthe daily requirement of 0.6 to 2 mg.
Iron
Iron is a component of the heme pigmentsand of some enzymes. In spite of the fact thatsome foods have high iron levels, much ofthe population has frequently been found tobe deficient in this element. Animal foodproducts may have high levels that are wellabsorbed; liver may contain several thousandppm of iron. The iron from other foods suchas vegetables and eggs is more poorly ab-sorbed. In the case of eggs the uptake is poorbecause the ferric iron is closely bound to thephosphate of the yolk phosphoproteins. Ironis used as a food additive to enrich flour andcereal products. The form of iron used sig-nificantly determines how well it will betaken up by the body. Ferrous sulfate is verywell absorbed, but will easily discolor or oxi-dize the food to which it is added. Elementaliron is also well absorbed and is less likely tochange the food. For these reasons, it is thepreferred form of iron for the enrichment offlour.
Zinc
Zinc is the second most important of theessential trace elements for humans. It is aconstituent of some enzymes, such as car-bonic anhydrase. Zinc is sufficiently abun-dant that deficiencies of zinc are unknown.The highest levels of zinc are found in shell-fish, which may contain 400 ppm. The levelof zinc in cereal grains is 30 to 40 ppm.When acid foods such as fruit juices arestored in galvanized containers, sufficientzinc may be dissolved to cause zinc poison-ing. The zinc in meat is tightly bound to themyofibrils and has been speculated to influ-ence meat's water-binding capacity (Hamm1972).
Manganese
Manganese is present in a wide range offoods but is not easily absorbed. This metalis associated with the activation of a numberof enzymes. In wheat, a manganese contentof 49 ppm has been reported (Schrenk 1964).This is mostly concentrated in the germ andbran; the level in the endosperm is only 2.4ppm. Information on the manganese contentof seafoods has been supplied by Merangerand Somers (1968). Values range from a lowof 1.1 ppm in salmon to a high of 42 ppm inoyster.
Molybdenum
Molybdenum plays a role in severalenzyme reactions. Some of the molybdenum-containing enzymes are aldehyde oxidase,sulfite oxidase, xanthine dehydrogenase, andxanthine oxidase. This metal is found incereal grains and legumes; leafy vegetables,especially those rich in chlorophyll; animalorgans; and in relatively small amounts, lessthan 0.1 ppm, in fruits. The molybdenum con-tent of foods is subject to large variations.
Selenium
Selenium has recently been found to pro-tect against liver necrosis. It usually occursbound to organic molecules. Different sele-nium compounds have greater or lesser pro-tective effect. The most active form ofselenium is selenite, which is also the leaststable chemically. Many selenium com-pounds are volatile and can be lost by cook-ing or processing. Kiermeier and Wigand(1969) found about a 5 percent loss of sele-nium as a result of drying of skim milk. Thevariation in selenium content of milk is wide
and undoubtedly associated with the sele-nium content of the soil. The same authorsreport figures for selenium in milk in variousparts of the world ranging from 5 to 1,270|ig/kg. The selenium in milk is virtually allbound to the proteins. Morris and Levander(1970) determined the selenium content of awide variety of foods. Most fruits and vege-tables contain less than 0.01 |iig/g. Grainproducts range from 0.025 to 0.66 |ig/g,dried skim milk from 0.095 to 0.24 |Hg/g,meat from 0.1 to 1.9 M-g/g, and seafood from0.4 to 0.7 |iig/g.
Fluorine
Fluorine is a constituent of skeletal boneand helps reduce the incidence of dental car-ies. The fluorine content of drinking water isusually below 0.2 mg/L but in some loca-tions may be as high as 5 mg/L. The optimalconcentration for dental health is 1 mg/L.The fluoride content of vegetables is low,with the exception of spinach, which con-tains 280 |0,g/100 g. Milk contains 20 [Ig/100 g and beef about 100 |Lig/100 g. Fishfoods may contain up to 700 |ng/100 g andtea about 100 |Hg/g.
Iodine
Iodine is not present in sufficient amountsin the diet in several areas of the world; aniodine deficiency results in goiter. The addi-tion of iodine to table salt has been extremelyeffective in reducing the incidence of goiter.The iodine content of most foods is in thearea of a few mg/100 g and is subject to greatlocal variations. Fish and shellfish havehigher levels. Saltwater fish have levels ofabout 50 to 150 mg/100 g and shellfish mayhave levels as high as 400 mg/100 g.
Nickel
Foods with a relatively high nickel contentinclude nuts, legumes, cocoa products, shell-fish, and hydrogenated fats. The source ofnickel in the latter results from the use ofnickel catalyst in the hydrogenation process.Animal products are generally low in nickel,plant products high (Table 5-10). The intakeof nickel from the diet depends, therefore, onthe origin and amounts of various foods con-sumed. Dietary nickel intake has been esti-mated to be in the range of 150 to 700 |Lig/day(Nielsen 1988), and the suggested dietarynickel requirement is about 35 |ig/day.
Finished hydrogenated vegetable oils con-tain less than 1 mg/kg nickel. Treatment ofthe finished oil with citric or phosphoric acidfollowed by bleaching should result in nickellevels of less than 0.2 mg/kg.
Chromium
Recent well-controlled studies (Anderson1988) have found that dietary intake of chro-
Table 5-10 Nickel Content of Some Foods
Nickel ContentFood ([ig/g Fresh Weight)
Cashew nuts 5.1Peanuts 1.6Cocoa powder 9.8Bittersweet chocolate 2.6Milk chocolate 1.2Red kidney beans 0.45Peas, frozen 0.35Spinach 0.39Shortening 0.59-2.78
Source: Reprinted with permission from RH.Nielsen, The Ultratrace Elements, in Trace Minerals inFoods, KT. Smith, ed., p. 385, 1988, by courtesy ofMarcel Dekker, Inc.
mium is in the order of 50 |ig/day. Refiningand processing of foods may lead to loss ofchromium. As an example, in the milling offlour, recovery of chromium in white flour isonly 35 to 44 percent of that of the parentwheat (Zook et al. 1970). On the other hand,the widespread use of stainless steel equip-ment in food processing results in leachingof chromium into the food products (Offen-bacher and Pi-Sunyer 1983). No foods areknown to contain higher-than-average levelsof chromium. The average daily intake ofchromium from various food groups isshown in Table 5-11. It has been suggestedthat the dietary intake of chromium in mostnormal individuals is suboptimal and canlead to nutritional problems (Anderson1988).
Silicon
Silicon is ubiquitous in the environmentand present in many foods. Foods of animalorigin are relatively low in silicon; foods ofplant origin are relatively high. Good plantsources are unrefined grains, cereal products,and root crops. The dietary intake of siliconis poorly known but appears to be in therange of 20 to 50 |iig/day. Although silicon isnow regarded as an essential mineral forhumans, a minimum requirement has notbeen established.
Additional Information on TraceElements
The variations in trace elements in vegeta-bles may be considerable (Warren 1972a)and may depend to a large extent on thenature of the soil in which the vegetables aregrown. Table 5-12 illustrates the extent ofthe variability in the content of copper, zinc,lead, and molybdenum of a number of vege-
tables. The range of concentrations of thesemetals frequently covers one order of magni-tude and occasionally as much as two ordersof magnitude. Unusually high concentrationsof certain metals may be associated with theincidence of diseases such as multiple sclero-sis and cancer in humans.
Aluminum, which has been assumed to benonnutritious and nontoxic, has come underincreasing scrutiny. Its presence has beensuggested to be involved in several seriousconditions, including Alzheimer's disease(Greger 1985). Since aluminum is widelyused in utensils and packaging materials,there is great interest in the aluminum con-tent of foods. Several aluminum salts areused as food additives, for example, sodiumaluminum phosphate as a leavening agentand aluminum sulfate for pH control. Theestimated average daily intake of aluminumis 26.5 mg, with 70 percent coming fromgrain products (Greger 1985).
Fruits contain relatively high levels oforganic acids, which may combine withmetal ions. It is now generally agreed that
these compounds may form chelates of thegeneral formula MyHpLm(OR)x, where Mand L represent the metal and the ligand,respectively. According to Pollard and Tim-berlake (1971), cupric ions form strong com-plexes with acids containing oc-hydroxylgroups. The major fruit acids, citric, malic,and tartaric, are multidendate ligands capa-ble of forming polynuclear chelates. Cupricand ferric ions form stronger complexes thanferrous ions. The strongest complexes areformed by citrate, followed by malate andthen tartrate.
METAL UPTAKE IN CANNED FOODS
Canned foods may take up metals from thecontainer, tin and iron from the tin plate, andtin and lead from the solder. There are sev-eral types of internal can corrosion. Rapiddetinning is one of the most serious problemsof can corrosion. With most acid foods,when canned in the absence of oxygen, tinforms the anode of the tin-iron couple. Thetin under these conditions goes into solution
Table 5-11 Chromium Intake from Various Food Groups
Food Group
Cereal productsMeat
Fish and seafoodFruits, vegetables, nutsDairy products, eggs, margarineBeverages, confectionery, sugar, and
condimentsTotal
Average DailyIntake (\ig)
3.75.2
0.66.86.26.6
29.1
Co/?7A77ente
55% from wheat55% from pork25% from beef
70% from fruits and berries85% from milk45% from beer, wine, and soft
drinks
Source: Reprinted with permission from R. A. Anderson, Chromium, in Trace Minerals in Foods, KT. Smith, ed., p.238, 1988, by courtesy of Marcel Dekker, Inc.
at an extremely slow rate and can provideproduct protection for two years or longer.There are, however, conditions where ironforms the anode, and in the presence ofdepolarizing or oxidizing agents the dissolu-tion of tin is greatly accelerated. The food is
protected until most of the tin is dissolved;thereafter, hydrogen is produced and the canswells and becomes a springer. Some foodsare more likely to involve rapid detinning,including spinach, green beans, tomato prod-ucts, potatoes, carrots, vegetable soups, and
Table 5-12 Extreme Variation in the Content of Copper, Zinc, Lead, and Molybdenum in SomeVegetables
CopperLettuceCabbagePotatoBean (except broad)CarrotBeet
ZincLettuceCabbagePotatoBean (except broad)CarrotBeet
LeadLettuceCabbagePotatoBean (except broad)CarrotBeet
MolybdenumLettuceCabbagePotatoBean (except broad)CarrotBeet
"Normal" Contentin ppm Wet
Weight
0.740.260.920.560.520.78
4.91.92.93.63.44.1
0.250.100.400.240.220.20
0.060.200.150.480.220.04
Minimum asFraction of"Normal"
1/151/61/9%VQ1/9
1/61/21/21/21/21/4
1/10Vs1/101/51/3VQ
VB
!£o1/16
%01/41/30
Maximum asMultiple of"Normal"
82.542.52.52.5
156528
12
302.5
1549
11
1287.573.5
10
Extreme Range
1-1201-151-361-221-221-20
1-901-121-101-41-481-16
1-3001-201-1501-201-271-66
1-961-2401-1201-2101-141-300
Source: From H.V. Warren, Variations in the Trace Element Contents of Some Vegetables, J. Roy. Coll. Gen. Prac-tit.,Vo\. 22, pp. 56-60, 1972.
certain fruit juices such as prune and grape-fruit juice.
Another corrosion problem of cans is sul-fide staining. This may happen when thefood contains the sulfur-containing aminoacids cysteine, cystine, or methionine. Whenthe food is heated or aged, reduction mayresult in the formation of sulfide ions, whichcan then react with tin and iron to form SnSand FeS. The compound SnS is the majorcomponent of the sulfide stain. This type ofcorrosion may occur with foods such aspork, fish, and peas (Seiler 1968). Corrosionof tin cans depends on the nature of thecanned food as well as on the type of tinplate used. Formerly, hot dipped tin platewas used, but this has been mostly replacedby electrolytically coated plate. It has beenshown (McKirahan et al. 1959) that the sizeof the crystals in the tin coating has animportant effect on corrosion resistance. Tinplate with small tin crystals easily developshydrogen swell, whereas tin plate containinglarge crystals is quite resistant. Seiler (1968)found that the orientation of the differentcrystal planes also significantly affected theease of forming sulfide stains.
The influence of processing techniques forgrapefruit juice on the rate of can corrosionwas studied by Bakal and Mannheim (1966).They found that the dissolved tin content canserve as a corrosion indicator. In Israel themaximum prescribed limit for tin content ofcanned food is 250 ppm. Deaeration of thejuice significantly lowers tin dissolution. In astudy of the in-can shelf life of tomato paste,Vander Merwe and Knock (1968) found that,depending on maturity and variety, 1 g oftomato paste stored at 220C could corrode tinat rates ranging from 9 x 10~6 g/month to 68x 10~6 g/month. The useful shelf life couldvary from 24 months to as few as 3 months.Up to 95 percent of the variation could be
related to effects of maturity and variety andthe associated differences in contents ofwater-insoluble solids and nitrate.
Severe detinning has often been observedwith applesauce packed in plain cans withenameled ends. This is usually characterizedby detinning at the headspace interface.Stevenson and Wilson (1968) found thatsteam flow closure reduced the detinningproblem, but the best results were obtainedby complete removal of oxygen throughnitrogen closure. Detinning by canned spin-ach was studied by Lambeth et al. (1969) andwas found to be significantly related to theoxalic acid content of the fresh leaves andthe pH of the canned product. High-oxalatespinach caused detinning in excess of 60 per-cent after 9 months' storage.
In some cases the dissolution of tin into afood may have a beneficial effect on food
Table 5-13 Iron and Tin Content of Fruit Juices
Product Iron (ppm) Tin (ppm)
Fresh orange 0.5 7.5juice
Bottled orange 2.5 25juice
Bottled orange 2.0 50juice
Bottled pineapple 15.0 50juice
Canned orange 2.5 60juice
Canned orange 0.5 115juice
Canned orange 2.5 120juice
Canned pineapple 17.5 135juiceSource: From WJ. Price and J.T.H. Roos, Analysis
of Fruit Juice by Atomic Absorption Spectrophotometry.I. The Determination of Iron and Tin in Canned Juice,J. Sd. FoodAgric., Vol. 20, pp. 427-439, 1969.
color, with iron having the opposite effect.This is the case for canned wax beans (VanBuren and Downing 1969). Stannous ionswere effective in preserving the light colorof the beans, whereas small amounts of ironresulted in considerable darkening. A blackdiscoloration has sometimes been observedin canned all-green asparagus after openingof the can. This has been attributed (Lueck1970) to the formation of a black, water-insoluble coordination compound of ironand rutin. The iron is dissolved from the can,
REFERENCES
Anderson, R.A. 1988. Chromium. In Trace minerals infoods, ed. K.T. Smith. New York: Marcel Dekker.
Bakal, A., and H.C. Mannheim. 1966. The influence ofprocessing variants of grapefruit juice on the rate ofcan corrosion and product quality. Israel J. Technol.4: 262-267.
Dunaif, G.D., and C.-S. Khoo. 1986. Developing lowand reduced-sodium products: An industrial per-spective. Food Technol. 40, no. 12: 105-107.
Greger, J.L. 1985. Aluminum content of the Americandiet. Food Technol. 39, no. 5: 73-80.
Hamm, R. 1971. Interactions between phosphates andmeat proteins. In Phosphates in food processing, ed.J.M. deMan and P. Melnychyn. Westport, CT: AVIPublishing Co.
Hamm, R. 1972. Colloid chemistry of meat (in Ger-man). Berlin: Paul Parey.
Jenness, R., and S. Patton. 1959. Principles of dairychemistry. New York: John Wiley & Sons.
Kiermeier, F, and W. Wigand. 1969. Selenium contentof milk and milk powder (in German). Z Lebensm.Unters. Forsch. 139: 205-211.
Lambeth, V.N., et al. 1969. Detinning by canned spin-ach as related to oxalic acid, nitrates and mineralcomposition. Food Technol. 23, no. 6: 132-134.
Lee, K., and J.L. Greger. 1983. Bioavailability andchemistry of iron from nitrite-cured meats. FoodTechnol. 37, no. 10: 139-144.
Lueck, R.H. 1970. Black discoloration in cannedasparagus. Interrelations of iron, tin, oxygen, andrutin. Agr. Food Chem. 18: 607-612.
and the rutin is extracted from the asparagusduring the sterilization. Rutin is a flavonol,the 3-rutinoside of quercetin. The black dis-coloration occurs only after the iron hasbeen oxidized to the ferric state. Tin forms ayellow, water-soluble complex with rutin,which does not present a color problem. Theuptake of iron and tin from canned foods is acommon occurrence, as is demonstrated byPrice and Roos (1969), who studied thepresence of iron and tin in fruit juice (Table5-13).
Marsh, A.C. 1983. Processes and formulations thataffect the sodium content of foods. Food Technol.37, no. 7: 45-49.
Maurer, AJ. 1983. Reduced sodium usage in poultrymuscle foods. Food Technol. 37, no. 7: 60-65.
McKirahan, R.D., et al. 1959. Application of differen-tially coated tin plate for food containers. FoodTechnol. 13: 228-232.
Meranger, J.C., and E. Somers. 1968. Determination ofthe heavy metal content of seafoods by atomicabsorption spectrophotometry. Bull. Environ. Con-tamination Toxicol. 3: 360-365.
Merkel, R.A. 1971. Inorganic constituents. In The sci-ence of meat and meat products, ed. J.F. Price andB.S. Schweigert. San Francisco: W.H. Freeman andCo.
Morris, V.C., and O.A. Levander. 1970. Selenium con-tent of foods. J. Nutr. 100: 1383-1388.
Nielsen, RH. 1988. The ultratrace elements. In Traceminerals in foods, ed. K.T. Smith. New York: MarcelDekker.
Offenbacher, E.G., and F.X. Pi-Sunyer. 1983. Tempera-ture and pH effects on the release of chromium fromstainless steel into water and fruit juices. J. Agr.Food Chem. 31: 89-92.
Pfeilsticker, K. 1970. Food components as metal che-lates. Food Sd. Technol. 3: 45-51.
Politz, M.L., and RM. Clydesdale. 1988. Effect ofenzymatic digestion, pH and molecular weight onthe iron solubilizing properties of chicken muscle. J.Food ScL 52: 1081-1085, 1090.
Pollard, A., and C.F. Timberlake. 1971. Fruit juices. InThe biochemistry of fruits and their products, Vol. 2,ed. A.C. Hulme. New York: Academic Press.
Poovaiah, B.W. 1986. Role of calcium in prolongingstorage life of fruits and vegetables. Food Technol40, no. 5: 86-89.
Price, W.J., and J.T.H. Roos. 1969. Analysis of fruitjuice by atomic absorption spectrophotometry I. Thedetermination of iron and tin in canned juice. J. ScLFoodAgric. 20: 427-439.
Reilly, C. 1996. Selenium in food and health. London:Blackie Academic and Professional.
Schrenk, W.G. 1964. Minerals in wheat grain. Techni-cal Bulletin 136. Manhattan, KS: Kansas State Uni-versity Agricultural Experimental Station.
Schweiger, R.G. 1966. Metal chelates of pectate andcomparison with alginate. KolloidZ. 208: 28-31.
Seiler, B.C. 1968. The mechanism of sulflde stainingof tin foodpacks. Food Technol 22: 1425-1429.
Stevenson, C.A., and C.H. Wilson. 1968. Nitrogenenclosure of canned applesauce. Food Technol. 33:1143-1145.
Toma, R.B., and DJ. Curtis. 1986. Dietary fiber: Effecton mineral bioavailability. Food Technol. 40, no. 2:111-116.
Van Buren, J.P., and D.L. Downing. 1969. Can charac-teristics, metal additives, and chelating agents:Effect on the color of canned wax beans. Food Tech-nol 23: 800-802.
Vander Merwe, H.B., and G.G. Knock. 1968. In-canshelf life of tomato paste as affected by tomato vari-ety and maturity. J. Food Technol 3: 249-262.
Warren, H.V. 1972a. Variations in the trace elementcontents of some vegetables. J. Roy. Coll Gen.Practit. 22: 56-60.
Warren, H.V. 1972b. Geology and medicine. WesternMiner, Sept., 34-37.
Zook, E.G. et al. 1970. Nutrient composition ofselected wheat and wheat products. Cereal Chem.47: 720-727.
Zook, E.G., and J. Lehmann. 1968. Mineral composi-tion of fruits. J. Am. Dietetic Assoc. 52: 225-231.
INTRODUCTION
Color is important to many foods, boththose that are unprocessed and those that aremanufactured. Together with flavor and tex-ture, color plays an important role in foodacceptability. In addition, color may providean indication of chemical changes in a food,such as browning and caramelization. For afew clear liquid foods, such as oils and bev-erages, color is mainly a matter of transmis-sion of light. Other foods are opaque—theyderive their color mostly from reflection.
Color is the general name for all sensationsarising from the activity of the retina of theeye. When light reaches the retina, the eye'sneural mechanism responds, signaling coloramong other things. Light is the radiantenergy in the wavelength range of about 400to 800 nm. According to this definition, color(like flavor and texture) cannot be studiedwithout considering the human sensory sys-tem. The color perceived when the eye viewsan illuminated object is related to the follow-ing three factors: the spectral composition ofthe light source, the chemical and physicalcharacteristics of the object, and the spectralsensitivity properties of the eye. To evaluatethe properties of the object, we must stan-dardize the other two factors. Fortunately,the characteristics of different people's eyes
for viewing colors are fairly uniform; it is nottoo difficult to replace the eye by someinstrumental sensor or photocell that can pro-vide consistent results. There are several sys-tems of color classification; the mostimportant is the CIE system (CommissionInternational de 1'Eclairage—InternationalCommission on Illumination). Other systemsused to describe food color are the Munsell,Hunter, and Lovibond systems.
When the reflectance of different coloredobjects is determined by means of spectro-photometry, curves of the type shown in Fig-ure 6-1 are obtained. White materials reflectequally over the whole visible wavelengthrange, at a high level. Gray and black materi-als also reflect equally over this range but toa lower degree. Red materials reflect in thehigher wavelength range and absorb theother wavelengths. Blue materials reflect inthe low-wavelength range and absorb thehigh-wavelength light.
CIE SYSTEM
The spectral energy distribution of CIElight sources A and C is shown in Figure 6-2.CIE illuminant A is an incandescent lightoperated at 28540K, and illuminant C is thesame light modified by filters to result in a
Color
CHAPTER 6
Figure 6-1 Spectrophotometric Curves of Col-ored Objects. Source: From Hunter AssociatesLab., Inc.
spectral composition that approximates thatof normal daylight. Figure 6-2 also showsthe luminosity curve of the standard observeras specified by CIE. This curve indicates
how the eyes of normal observers respond tothe various spectral light types in the visibleportion of the spectrum. By breaking downthe spectrum, complex light types are re-duced to their component spectral lighttypes. Each spectral light type is completelydetermined by its wavelength. In some lightsources, a great deal of radiant energy is con-centrated in a single spectral light type. Anexample of this is the sodium lamp shown inFigure 6-3, which produces monochromaticlight. Other light sources, such as incandes-cent lamps, give off a continuous spectrum.A fluorescent lamp gives off a continuousspectrum on which is superimposed a linespectrum of the primary radiation producedby the gas discharge (Figure 6-3).
In the description of light sources, refer-ence is sometimes made to the black body.This is a radiating surface inside a hollowspace, and the light source's radiation comesout through a small opening. The radiation isindependent of the type of material the lightsource is made of. When the temperature isvery high, about 600O0K the maximum ofthe energy distribution will fall about in themiddle of the visible spectrum. Such energydistribution corresponds with that of daylighton a cloudy day. At lower temperatures, themaximum of the energy distribution shifts tolonger wavelengths. At 3000° K, the spectralenergy distribution is similar to that of anincandescent lamp; at this temperature theenergy at 380 nm is only one-sixteenth ofthat at 780 nm, and most of the energy isconcentrated at higher wavelengths (Figure6-3). The uneven spectral distribution ofincandescent light makes red objects lookattractive and blue ones unattractive. This iscalled color rendition. The human eye hasthe ability to adjust for this effect.
The CIE system is a trichromatic system;its basis is the fact that any color can be
REFLE
CTAN
CE (%
)
WAVELENGTH
WAVE LENGTH nmFigure 6-2 Spectral Energy Distribution ofLight Sources A and C, the CIE, and RelativeLuminosity Function y for the CIE StandardObserver
RELA
TIVE
LUM
INOS
ITY
(y)
RELA
TIVE
ENER
GY (
A AN
D C)
C.I.E. STANDARDOBSERVER
matched by a suitable mixture of three pri-mary colors. The three primary colors, or pri-maries, are red, green, and blue. Any possiblecolor can be represented as a point in a trian-gle. The triangle in Figure 6-4 shows howcolors can be designated as a ratio of the threeprimaries. If the red, green, and blue values ofa given light type are represented by a, b, andc, then the ratios of each to the total light aregiven by a/(a + b + c), bl(a + b + c), and cl(a+ b + c), respectively. Since the sum of theseis one, then only two have to be known toknow all three. Color, therefore, is deter-mined by two, not three, of these mutuallydependent quantities. In Figure 6-4, a colorpoint is represented by P. By determining thedistance of P from the right angle, the quanti-ties al(a + b + c) and bl(a + b + c) are found.The quantity cl(a + b + c) is then found, byfirst extending the horizontal dotted linethrough P until it crosses the hypotenuse at Qand by then constructing another right angletriangle with Q at the top. All combinations
of a, b, and c will be points inside the trian-gle.
The relative amounts of the three primariesrequired to match a given color are called the
WAVELENGTH NM
Figure 6-3 Spectral Energy Distribution of Sunlight (S), CIE Illuminant (A), Cool White FluorescentLamp (B), and Sodium Light (N)
REL
ATI
VE E
NER
GY
Figure 6-4 Representation of a Color as a Pointin a Color Triangle
COLOR
tristimulus values of the color. The CIE pri-maries are imaginary, because there are noreal primaries that can be combined to matchthe highly saturated hues of the spectrum.
In the CIE system the red, green, and blueprimaries are indicated by X, Y9 and Z. Theamount of each primary at any particularwavelength is given by the values J, y, and z.These are called the distribution coefficientsor the red, green, and blue factors. They rep-resent the tristimulus values for each chosenwavelength. The distribution coefficients forthe visible spectrum are presented in Figure6-5. The values of y correspond with theluminosity curve of the standard observer(Figure 6-2). The distribution coefficientsare dimensionless because they are the num-bers by which radiation energy at each wave-length must be multiplied to arrive at the X,
y, and Z content. The amounts of X, Y, and Zprimaries required to produce a given colorare calculated as follows:
780
X=Ix IRdh
380
780
XY = J y IRdh
380
780
XZ = J z IRdh
380where
/ = spectral energy distribution of illu-minant
R = spectral reflectance of sampledh = small wavelength intervaljc, y, ~z = red, green, and blue factors
The ratios of the primaries can beexpressed as
_ X
*"x+y+z
_ yy"x+y+z
_ zZ~X+Y+Z
The quantities x and y are called the chroma-ticity coordinates and can be calculated foreach wavelength from
Figure 6-5 Distribution Coefficients JC, y, and zfor the Visible Spectrum. Source: From HunterAssociates Lab., Inc.
WAVELENGTH (NANOMETERS)
RELA
TIVE A
MOUN
T
jc = xf(x + y + z)
y= y/(x + y + z)
z=l-(x + y)
A plot of jc versus y results in the CIE chro-maticity diagram (Figure 6-6). When thechromaticities of all of the spectral colors areplaced in this graph, they form a line calledthe locus. Within this locus and the line con-necting the ends, represented by 400 and 700nm, every point represents a color that can bemade by mixing the three primaries. Thepoint at which exactly equal amounts of each
of the primaries are present is called theequal point and is white. This white pointrepresents the chromaticity coordinates ofilluminant C. The red primary is located at jc= 1 and y = O; the green primary at x = O andy = 1; and the blue primary at x = O and y = O.The line connecting the ends of the locusrepresents purples, which are nonspectralcolors resulting from mixing various amountsof red and blue. All points within the locusrepresent real colors. All points outside thelocus are unreal, including the imaginary pri-maries X, Y, and Z. At the red end of thelocus, there is only one point to represent thewavelength interval of 700 to 780 nm. This
Figure 6-6 CIE Chromaticity Diagram
means that all colors in this range can besimply matched by adjustment of luminosity.In the range of 540 to 700 nm, the spectrumlocus is almost straight; mixtures of twospectral light types along this line segmentwill closely match intervening colors withlittle loss of purity. In contrast, the spectrumlocus below 540 nm is curved, indicating thata combination of two spectral lights alongthis portion of the locus results in colors ofdecreased purity.
A pure spectral color is gradually dilutedwith white when moving from a point on thespectrum locus to the white point P. Such astraight line with purity decreasing from 100to O percent is known as a line of constantdominant wavelength. Each color, except thepurples, has a dominant wavelength. Theposition of a color on the line connecting thelocus and P is called excitation purity (pe)and is calculated as follows:
_ *-xw _ y-y*f — —— -̂— — _———.
XP ~~xw yp~~ y\vwhere
jc and y are the chromaticity coordinates ofa color
xw and yw are the chromaticity coordinatesof the achromatic source
xp and yp are the chromaticity coordinatesof the pure spectral color
Achromatic colors are white, black, andgray. Black and gray differ from white onlyin their relative reflection of incident light.The purples are nonspectral chromatic col-ors. All other colors are chromatic; for exam-ple, brown is a yellow of low lightness andlow saturation. It has a dominant wavelengthin the yellow or orange range.
A color can be specified in terms of the tri-stimulus value Y and the chromaticity coor-
dinates x and y. The Y value is a measure ofluminous reflectance or transmittance and isexpressed in percent simply as 7/1000.
Another method of expressing color is interms of luminance, dominant wavelength,and excitation purity. These latter are roughlyequivalent to the three recognizable attrib-utes of color: lightness, hue, and saturation.Lightness is associated with the relativeluminous flux, reflected or transmitted. Hueis associated with the sense of redness, yel-lowness, blueness, and so forth. Saturation isassociated with the strength of hue or the rel-ative admixture with white. The combinationof hue and saturation can be described aschromaticity.
Complementary colors (Table 6-1) areobtained when a straight line is drawnthrough the equal energy point P. When thisis done for the ends of the spectrum locus,the wavelength complementary to the 700 to780 point is at 492.5 nm, and for the 380 to410 point is at 567 nm. All of the wave-lengths between 492.5 and 567 nm are com-plementary to purple. The purples can bedescribed in terms of dominant wavelengthby using the wavelength complementary toeach purple, and purity can be expressed in amanner similar to that of spectral colors.
Table 6-1 Complementary Colors
Wavelength Complementary(nm) Color Color
~400 Violet ~450 Blue Y e " ° W
^ Orange500 Green *550 Yellow 6
600 Orange650 Red ^ UG
700 G r e e n
An example of the application of the CIEsystem for color description is shown in Fig-ure 6-7. The curved, dotted line originatingfrom C represents the locus of the chromatic-ity coordinates of caramel and glycerol solu-tions. The chromaticity coordinates of maplesyrup and honey follow the same locus. Threetriangles on this curve represent the chroma-ticity coordinates of U.S. Department of Agri-culture (USDA) glass color standards for
maple syrup. These are described as lightamber, medium amber, and dark amber. Thesix squares are chromaticity coordinates ofhoney, designated by USDA as water white,extra white, white, extra light amber, lightamber, and amber. Such specifications areuseful in describing color standards for a vari-ety of products. In the case of the light amberstandard for maple syrup, the following valuesapply: x = 0.486, y = 0.447, and T = 38.9 per-
Figure 6-7 CIE Chromaticity Diagram with Color Points for Maple Syrup and Honey Glass ColorStandards
X
y
GLASS COLOR STANDARDSA FOR MAPLE SYRUP• FOR HONEY
cent. In this way, x and y provide a specifica-tion for chromaticity and T for luminoustransmittance or lightness. This is easilyexpressed as the mixture of primaries underilluminant C as follows: 48.6 percent of redprimary, 44.7 percent of green primary, and6.7 percent of blue primary. The light trans-mittance is 38.9 percent.
The importance of the light source andother conditions that affect viewing of sam-ples cannot be overemphasized. Many sub-stances are metameric; that is, they may haveequal transmittance or reflectance at a certainwavelength but possess noticeably differentcolors when viewed under illuminant C.
MUNSELL SYSTEM
In the Munsell system of color classifica-tion, all colors are described by the threeattributes of hue, value, and chroma. Thiscan be envisaged as a three-dimensional sys-tem (Figure 6-8). The hue scale is based onten hues which are distributed on the circum-ference of the hue circle. There are five hues:red, yellow, green, blue, and purple; they arewritten as R, Y, G, B, and P. There are alsofive intermediate hues, YR, GY, BG, PB, andRP. Each of the ten hues is at the midpoint ofa scale from 1 to 10. The value scale is alightness scale ranging from O (black) to 10(white). This scale is distributed on a lineperpendicular to the plane of the hue circleand intersecting its center. Chroma is a mea-sure of the difference of a color from a grayof same lightness. It is a measure of purity.The chroma scale is of irregular length, andbegins with O for the central gray. The scaleextends outward in steps to the limit of purityobtainable by available pigments. The shapeof the complete Munsell color space is indi-cated in Figure 6-9. The description of acolor in the Munsell system is given as //,VIC. For example, a color indicated as 5R
Figure 6-8 The Munsell System of Color Clas-sification
2.8/3.7 means a color with a red hue of 5R, avalue of 2.8, and a chroma of 3.7. All colorsthat can be made with available pigments arelaid down as color chips in the Munsell bookof color.
Figure 6-9 The Munsell Color Space
Black
Whte
Purple
Black
Yellow
Green
White
Blue
Red
SaturationChroma
Lightn
ess
Value
HUNTER SYSTEM
The CIE system of color measurement isbased on the principle of color sensing by thehuman eye. This accepts that the eyes containthree light-sensitive receptors—the red, green,and blue receptors. One problem with thissystem is that the X, Y, and Z values have norelationship to color as perceived, though acolor is completely defined. To overcome thisproblem, other color systems have been sug-gested. One of these, widely used for foodcolorimetry, is the Hunter L, a, fo, system. Theso-called uniform-color, opponent-colors colorscales are based on the opponent-colorstheory of color vision. In this theory, it isassumed that there is an intermediate signal-switching stage between the light receptors inthe retina and the optic nerve, which trans-mits color signals to the brain. In this switch-ing mechanism, red responses are comparedwith green and result in a red-to-green colordimension. The green response is comparedwith blue to give a yellow-to-blue colordimension. These two color dimensions are
represented by the symbols a and b. The thirdcolor dimension is lightness L, which is non-linear and usually indicated as the square orcube root of K This system can be repre-sented by the color space shown in Figure6-10. The L, a, b, color solid is similar to theMunsell color space. The lightness scale iscommon to both. The chromatic spacing isdifferent. In the Munsell system, there are thepolar hue and chroma coordinates, whereas inthe L, a, b, color space, chromaticity isdefined by rectangular a and b coordinates.CIE values can be converted to color valuesby the equations shown in Table 6-2 into L, a,b, values and vice versa (MacKinney and Lit-tle 1962; Clydesdale and Francis 1970). Thisis not the case with Munsell values. These areobtained from visual comparison with colorchips (called Munsell renotations) or frominstrumental measurements (called Munsellrenotations), and conversion is difficult andtedious.
The Hunter tristimulus data, L (value), a(redness or greenness), and b (yellowness orblueness), can be converted to a single color
Figure 6-10 The Hunter L, a, b Color Space. Source: From Hunter Associates Lab., Inc.
L=O
BLACK
BLUE
RED
YELLOW
GRAYGREEN
L--100
WHITE
function called color difference (AE) byusing the following relationship:
AE = (AL)2 + (Afl)2 + (Ab)2
The color difference is a measure of the dis-tance in color space between two colors. Itdoes not indicate the direction in which thecolors differ.
LOVIBOND SYSTEM
The Lovibond system is widely used forthe determination of the color of vegetableoils. The method involves the visual compar-ison of light transmitted through a glass
cuvette filled with oil at one side of aninspection field; at the other side, coloredglass filters are placed between the lightsource and the observer. When the colors oneach side of the field are matched, the nomi-nal value of the filters is used to define thecolor of the oil. Four series of filters areused—red, yellow, blue, and gray filters. Thegray filters are used to compensate for inten-sity when measuring samples with intensechroma (color purity) and are used in thelight path going through the sample. The red,yellow, and blue filters of increasing inten-sity are placed in the light path until a matchwith the sample is obtained. Vegetable oilcolors are usually expressed in terms of red
Source: From Hunter Associates Lab., Inc.
Table 6-2 Mathematical Relationship Between Color Scales
To Convert To L, a, b To X%, Y, Z% ToY,x,y
From
From
From
and yellow; a typical example of the Lovi-bond color of an oil would be Rl.7 Y17. Thevisual determination of oil color by the Lovi-bond method is widely used in industry andis an official method of the American OilChemists' Society. Visual methods of thistype are subject to a number of errors, andthe results obtained are highly variable. Astudy has been reported (Maes et al., 1997)to calculate CIE and Lovibond color valuesof oils based on their visible light transmis-sion spectra as measured by a spectropho-tometer. A computer software has beendeveloped that can easily convert light trans-mission spectra into CIE and Lovibond colorindexes.
GLOSS
In addition to color, there is another impor-tant aspect of appearance, namely gloss.Gloss can be characterized as the reflectingproperty of a material. Reflection of light canbe diffused or undiffused (specular). In spec-ular reflection, the surface of the object actsas a mirror, and the light is reflected in ahighly directional manner. Surfaces can rangefrom a perfect mirror with completely specu-lar reflection to a surface reflecting in a com-pletely diffuse manner. In the latter, the lightfrom an incident beam is scattered in alldirections and the surface is called matte.
FOOD COLORANTS
The colors of foods are the result of naturalpigments or of added colorants. The naturalpigments are a group of substances present inanimal and vegetable products. The addedcolorants are regulated as food additives, butsome of the synthetic colors, especially ca-rotenoids, are considered "nature identical"
and therefore are not subject to stringent tox-icological evaluation as are other additives(Dziezak 1987).
The naturally occurring pigments embracethose already present in foods as well asthose that are formed on heating, storage, orprocessing. With few exceptions, these pig-ments can be divided into the following fourgroups:
1. tetrapyrrole compounds: chlorophylls,hemes, and bilins
2. isoprenoid derivatives: carotenoids3. benzopyran derivatives: anthocyanins
and flavonoids4. artefacts: melanoidins, caramels
The chlorophylls are characteristic of greenvegetables and leaves. The heme pigmentsare found in meat and fish. The carotenoidsare a large group of compounds that arewidely distributed in animal and vegetableproducts; they are found in fish and crusta-ceans, vegetables and fruits, eggs, dairyproducts, and cereals. Anthocyanins and fla-vonoids are found in root vegetables andfruits such as berries and grapes. Caramelsand melanoidins are found in syrups andcereal products, especially if these productshave been subjected to heat treatment.
Tetrapyrrole Pigments
The basic unit from which the tetrapyrrolepigments are derived is pyrrole.
The basic structure of the heme pigmentsconsists of four pyrrole units joined togetherinto a porphyrin ring as shown in Figure 6-11.
Figure 6-11 Schematic Representation of theHeme Complex of Myoglobin. M = methyl, P =propyl, V = vinyl. Source: From C.E. Bodwelland RE. McClain, Proteins, in The Sciences ofMeat Products, 2nd ed., I.E. Price and B.S. Sch-weigert, eds., 1971, W.H. Freeman & Co.
In the heme pigments, the nitrogen atoms arelinked to a central iron atom. The color ofmeat is the result of the presence of two pig-ments, myoglobin and hemoglobin. Bothpigments have globin as the protein portion,and the heme group is composed of the por-phyrin ring system and the central iron atom.In myoglobin, the protein portion has amolecular weight of about 17,000. In hemo-globin, this is about 67,000—equivalent tofour times the size of the myoglobin protein.The central iron in Figure 6-11 has six coor-dination bonds; each bond represents anelectron pair accepted by the iron from fivenitrogen atoms, four from the porphyrin ringand one from a histidyl residue of the globin.The sixth bond is available for joining withany atom that has an electron pair to donate.The ease with which an electron pair isdonated determines the nature of the bondformed and the color of the complex. Other
factors playing a role in color formation arethe oxidation state of the iron atom and thephysical state of the globin.
In fresh meat and in the presence of oxy-gen, there is a dynamic system of three pig-ments, oxymyoglobin, myoglobin, and met-myoglobin. The reversible reaction with oxy-gen is
Mb + O2 ̂ MbO2
In both pigments, the iron is in the ferrousform; upon oxidation to the ferric state, thecompound becomes metmyoglobin. Thebright red color of fresh meat is due to thepresence of oxymyoglobin; discoloration tobrown occurs in two stages, as follows:
MbO2 ^ Mb ^ MetMbRed Purplish red Brownish
Oxymyoglobin represents a ferrous covalentcomplex of myoglobin and oxygen. Theabsorption spectra of the three pigments areshown in Figure 6-12 (Bodwell and McClain1971). Myoglobin forms an ionic complexwith water in the absence of strong electronpair donors that can form covalent com-plexes. It shows a diffuse absorption band inthe green area of the spectrum at about 555nm and has a purple color. In metmyoglobin,the major absorption peak is shifted towardthe blue portion of the spectrum at about 505nm with a smaller peak at 627 nm. The com-pound appears brown.
As indicated above, oxymyoglobin andmyoglobin exist in a state of equilibriumwith oxygen; therefore, the ratio of the pig-ments is dependent on oxygen pressure. Theoxidized form of myoglobin, the metmyo-globin, cannot bind oxygen. In meat, there isa slow and continuous oxidation of the heme
Globin
pigments to the metmyoglobin state. Reduc-ing substances in the tissue reduce the met-myoglobin to the ferrous form. The oxygenpressure, which is so important for the stateof the equilibrium, is greatly affected bypackaging materials used for meats. Themaximum rate of conversion to metmyoglo-bin occurs at partial pressures of 1 to 20 nmof mercury, depending on pigment, pH, andtemperature (Fox 1966). When a packagingfilm with low oxygen permeability is used,the oxygen pressure drops to the point whereoxidation is favored. To prevent this, Lan-drock and Wallace (1955) established thatoxygen permeability of the packaging filmmust be at least 5 liters of oxygen/squaremeter/day/atm.
Fresh meat open to the air displays thebright red color of oxymyoglobin on the sur-face. In the interior, the myoglobin is in thereduced state and the meat has a dark purplecolor. As long as reducing substances arepresent in the meat, the myoglobin willremain in the reduced form; when they areused up, the brown color of metmyoglobinwill predominate. According to Solberg(1970), there is a thin layer a few nanome-ters below the bright red surface and justbefore the myoglobin region, where a defi-nite brown color is visible. This is the areawhere the oxygen partial pressure is about1.4 nm and the brown pigment dominates.The growth of bacteria at the meat surfacemay reduce the partial oxygen pressure to
Oxymyoglobin
Metmyoglolmi
Myoglobin
Wavelength (in/x)
Ext
inct
ion
coef
fici
ent
(cn
v/m
g^
Figure 6-12 Absorption Spectra of Myoglobin, Oxymyoglobin, and Metmyoglobin. Source: FromC.E. Bodwell and RE. McClain, Proteins, in The Sciences of Meat Products, 2nd ed., I.E. Price andB.S. Schweigert, eds., 1971, W.H. Freeman & Co.
below the critical level of 4 nm. Microor-ganisms entering the logarithmic growthphase may change the surface color to thatof the purplish-red myoglobin (Solberg1968).
In the presence of sulfhydryl as a reducingagent, myoglobin may form a green pigment,called sulfmyoglobin. The pigment is greenbecause of a strong absorption band in thered region of the spectrum at 616 nm. In thepresence of other reducing agents, such asascorbate, cholemyoglobin is formed. In thispigment, the porphyrin ring is oxidized. Theconversion into sulfmyoglobin is reversible;cholemyoglobin formation is irreversible,and this compound is rapidly oxidized toyield globin, iron, and tetrapyrrole. Accord-ing to Fox (1966), this reaction may happenin the pH range of 5 to 7.
Heating of meat results in the formation ofa number of pigments. The globin is dena-tured. In addition, the iron is oxidized to theferric state. The pigment of cooked meat isbrown and called hemichrome. In the pres-ence of reducing substances such as thosethat occur in the interior of cooked meat, theiron may be reduced to the ferrous form; theresulting pigment is pink hemochrome.
In the curing of meat, the heme reacts withnitrite of the curing mixture. The nitrite-heme complex is called nitrosomyoglobin,which has a red color but is not particularlystable. On heating the more stable nitrosohe-mochrome, the major cured meat pigment isformed, and the globin portion of the mole-cule is denatured. This requires a tempera-ture of 650C. This molecule has been callednitrosomyoglobin and nitrosylmyoglobin,but Mohler (1974) has pointed out that theonly correct name is nitric oxide myoglobin.The first reaction of nitrite with myoglobin isoxidation of the ferrous iron to the ferricform and formation of MetMb. At the same
time, nitrate is formed according to the fol-lowing reaction (Mohler 1974):
4MbO2 + 4NO2- + 2H2O -»4MetMbOH + 4NO3" + O2
During the formation of the curing pigment,the nitrite content is gradually lowered; thereare no definite theories to account for thisloss.
The reactions of the heme pigments inmeat and meat products have been summa-rized in the scheme presented in Figure 6-13(Fox 1966). Bilin-type structures are formedwhen the porphyrin ring system is broken.
Chlorophylls
The chlorophylls are green pigmentsresponsible for the color of leafy vegetablesand some fruits. In green leaves, the chloro-phyll is broken down during senescence andthe green color tends to disappear. In manyfruits, chlorophyll is present in the unripestate and gradually disappears as the yellowand red carotenoids take over during ripen-ing. In plants, chlorophyll is isolated in thechloroplastids. These are microscopic parti-cles consisting of even smaller units, calledgrana, which are usually less than one micro-meter in size and at the limit of resolution ofthe light microscope. The grana are highlystructured and contain laminae betweenwhich the chlorophyll molecules are posi-tioned.
The chlorophylls are tetrapyrrole pigmentsin which the porphyrin ring is in the dihydroform and the central metal atom is magne-sium. There are two chlorophylls, a and b,which occur together in a ratio of about 1:25.Chlorophyll b differs from chlorophyll a inthat the methyl group on carbon 3 is replacedwith an aldehyde group. The structural for-
mula of chlorophyll a is given in Figure 6-14. Chlorophyll is a diester of a dicarboxylicacid (chlorophyllin); one group is esterifiedwith methanol, the other with phytyl alcohol.The magnesium is removed very easily byacids, giving pheophytins a and b. The actionof acid is especially important for fruits thatare naturally high in acid. However, itappears that the chlorophyll in plant tissuesis bound to lipoproteins and is protectedfrom the effect of acid. Heating coagulatesthe protein and lowers the protective effect.The color of the pheophytins is olive-brown.Chlorophyll is stable in alkaline medium.The phytol chain confers insolubility inwater on the chlorophyll molecule. Uponhydrolysis of the phytol group, the water-sol-
uble methyl chlorophyllides are formed. Thisreaction can be catalyzed by the enzymechlorophyllase. In the presence of copper orzinc ions, it is possible to replace the magne-sium, and the resulting zinc or copper com-plexes are very stable. Removal of the phytolgroup and the magnesium results inpheophorbides. All of these reactions aresummarized in the scheme presented in Fig-ure 6-15.
In addition to those reactions describedabove, it appears that chlorophyll can bedegraded by yet another pathway. Chichesterand McFeeters (1971) reported on chloro-phyll degradation in frozen beans, whichthey related to fat peroxidation. In this reac-tion, lipoxidase may play a role, and no
Figure 6-13 Heme Pigment Reactions in Meat and Meat Products. ChMb, cholemyoglobin (oxidizedporphyrin ring); O2Mb, oxymyoglobin (Fe+2); MMb metmyoglobin (Fe+3); Mb, myoglobin (Fe+2);MMb-NO2, metmyoglobin nitrate; NOMMb, nitrosylmetmyoglobin; NOMb, nitrosylmyoglobin;NMMb, nitrimetmyoglobin; NMb, nitrimyoglobin, the latter two being reaction products of nitrous acidand the heme portion of the molecule; R, reductants; O, strong oxidizing conditions. Source: FromJ.B. Fox, The Chemistry of Meat Pigments, J. Agr. Food Chem., Vol. 14, no. 3, pp. 207-210, 1966,American Chemical Society.
Nitrosyl-hemochrome
Denatured GlobinHemichrome
HeminTetrapyrroles Nitrihemin
heat acid heat acidacid
BilePigment
FRESH CURED
pheophytins, chlorophyllides, or pheophor-bides are detected. The reaction requiresoxygen and is inhibited by antioxidants.
Carotenoids
The naturally occurring carotenoids, withthe exception of crocetin and bixin, are tet-raterpenoids. They have a basic structure ofeight isoprenoid residues arranged as if two20-carbon units, formed by head-to-tail con-densation of four isoprenoid units, hadjoined tail to tail. There are two possibleways of classifying the carotenoids. The firstsystem recognizes two main classes, the car-
otenes, which are hydrocarbons, and thexanthophylls, which contain oxygen in theform of hydroxyl, methoxyl, carboxyl, keto,or epoxy groups. The second system dividesthe carotenoids into three types (Figure 6-16),acyclic, monocyclic, and bicyclic. Examplesare lycopene (I)—acyclic, y-carotene (II)—monocyclic, and a-carotene (III) and p-car-otene (IV)—bicyclic.
The carotenoids take their name from themajor pigments of carrot (Daucus car old).The color is the result of the presence of asystem of conjugated double bonds. Thegreater the number of conjugated doublebonds present in the molecule, the further themajor absorption bands will be shifted to the
Figure 6-14 Structure of Chlorophyll a. (Chlorophyll b differs in having a formyl group at carbon 3).Source: Reprinted with permission from J.R. Whitaker, Principles of Enzymology for the Food Sciences,1972, by courtesy of Marcel Dekker, Inc.
region of longer wavelength; as a result, thehue will become more red. A minimum ofseven conjugated double bonds are requiredbefore a perceptible yellow color appears.Each double bond may occur in either cis ortrans configuration. The carotenoids in foodsare usually of the all-trans type and onlyoccasionally a mono-cis or di-cis compoundoccurs. The prefix neo- is used for stereoiso-mex.1 with at least one cis double bond. Theprefix pro- is for poly-a's carotenoids. Theeffect of the presence of cis double bonds onthe absorption spectrum of p-carotene isshown in Figure 6-17. The configuration hasan effect on color. The all-trans compoundshave the deepest color; increasing numbersof cis bonds result in gradual lightening ofthe color. Factors that cause change of bondsfrom trans to cis are light, heat, and acid.
In the narrower sense, the carotenoids arethe four compounds shown in Figure 6-16—a-, p-, and y-carotene and lycopene—poly-ene hydrocarbons of overall compositionC40H56. The relation between these and caro-tenoids with fewer than 40 carbon atoms isshown in Figure 6-18. The prefix apo- isused to designate a carotenoid that is derivedfrom another one by loss of a structural ele-ment through degradation. It has been sug-gested that some of these smaller carotenoidmolecules are formed in nature by oxidativedegradation of C40 carotenoids (Grob 1963).
Several examples of this possible relation-ship are found in nature. One of the bestknown is the formation of retinin and vita-min A from p-carotene (Figure 6-19).Another obvious relationship is that of lyco-pene and bixin (Figure 6-20). Bixin is a food
Figure 6-15 Reactions of Chlorophylls
chlorinpurpurins
alkaliO2
alkaliO2
acidalkali°2
pheophytinphytol
acidpheophorbide acid methyl
chlorophyllide
phytol
chlorophyllasechlorophyll
strongacid
acid
phytol
Figure 6-16 The Carotenoids: (I) Lycopene, (II)y-carotene, (III) a-Carotene, and (IV) p-Caro-tene. Source: From E.G. Grob, The Biogenesis ofCarotenes and Carotenoids, in Carotenes andCarotenoids, K. Lang, ed., 1963, Steinkopff Ver-lag.
color additive obtained from the seed coat ofthe fruit of a tropical brush, Bixa orellana.The pigment bixin is a dicarboxylic acidesterified with one methanol molecule. Apigment named crocin has been isolatedfrom saffron. Crocin is a glycoside contain-ing two molecules of gentiobiose. Whenthese are removed, the dicarboxylic acid cro-cetin is formed (Figure 6-21). It has thesame general structure as the aliphatic chainof the carotenes. Also obtained from saffronis the bitter compound picrocrocin. It is aglycoside and, after removal of the glucose,yields saffronal. It is possible to imagine acombination of two molecules of picrocrocinand one of crocin; this would yield protocro-cin. Protocrocin, which is directly related tozeaxanthin, has been found in saffron (Grob1963).
The structure of a number of importantxanthophylls as they relate to the structure ofP-carotene is given in Figure 6-22. Carot-enoids may occur in foods as relatively sim-ple mixtures of only a few compounds or asvery complex mixtures of large numbers ofcarotenoids. The simplest mixtures usuallyexist in animal products because the animalorganism has a limited ability to absorb anddeposit carotenoids. Some of the most com-plex mixtures are found in citrus fruits.
Beta-carotene as determined in fruits andvegetables is used as a measure of the provi-tamin A content of foods. The column chro-matographic procedure, which determinesthis content, does not separate cc-carotene, p-carotene, and cryptoxanthin. Provitamin Avalues of some foods are given in Table 6-3.Carotenoids are not synthesized by animals,but they may change ingested carotenoidsinto animal carotenoids—as in, for example,salmon, eggs, and crustaceans. Usually carot-enoid content of foods does not exceed 0.1percent on a dry weight basis.
In ripening fruit, carotenoids increase atthe same time chlorophylls decrease. Theratio of carotenes to xanthophylls also in-creases. Common carotenoids in fruits are oc-and y-carotene and lycopene. Fruit xantho-phylls are usually present in esterified form.Oxygen, but not light, is required for caro-tenoid synthesis and the temperature range iscritical. The relative amounts of differentcarotenoids are related to the characteristiccolor of some fruits. In the sequence ofpeach, apricot, and tomato, there is an in-creasing proportion of lycopene and increas-ing redness. Many peach varieties are devoidof lycopene. Apricots may have about 10percent and tomatoes up to 90 percent. Thelycopene content of tomatoes increases dur-ing ripening. As the chlorophyll breaks downduring ripening, large amounts of carot-
enoids are formed (Table 6-4). Color is animportant attribute of citrus juice and isaffected by variety, maturity, and processingmethods. The carotenoid content of orangesis used as a measure of total color. Curl andBailey (1956) showed that the 5,6-epoxidesof fresh orange juice isomerize completely to5,8-epoxides during storage of canned juice.This change amounts to the loss of one dou-ble bond from the conjugated double bondsystem and causes a shift in the wavelengthof maximum absorption as well as a decrease
in molar absorbance. In one year's storage at7O0F, an apparent carotenoid loss of 20 to 30percent occurs.
Peaches contain violaxanthin, cryptoxan-thin, p-carotene, and persicaxanthin as wellas 25 other carotenoids, including neoxan-thin. Apricots contain mainly p- and ycaro-tene, lycopene, and little if any xanthophyll.Carrots have been found to have an averageof 54 ppm of total carotene (Borenstein andBunnell 1967), consisting mainly of a-, p,and ^-carotene and some lycopene and xan-
Figure 6-17 Absorption Spectra of the Three Stereoisomers of Beta Carotene. B = neo-p-carotene; U =neo-p-carotene-U; T = all-trans-p-carotene. a, b, c, and d indicate the location of the mercury arc lines334.1, 404.7, 435.8 and 491.6 nm, respectively. Source: From F Stitt et al., Spectrophotometric Deter-mination of Beta Carotene Stereoisomers in Alfalfa, /. Assoc. Off. Agric. Chem. Vol. 34, pp. 460-471,1951.
Wavelength ( n m)
Abs
orpt
ivity
(l
/g-c
m)
Figure 6-18 Relationship Between the Caroteneand Carotenoids with Fewer than 40 Carbons
thophyll. Canning of carrots resulted in a 7 to12 percent loss of provitamin A activitybecause of cis-trans isomerization of a- andp-carotene (Weckel et al. 1962). In dehy-
drated carrots, carotene oxidation and off-flavor development have been correlated(Falconer et al. 1964). Corn contains aboutone-third of the total carotenoids as car-otenes and two-thirds xanthophylls. Com-pounds found in corn include zeaxanthin,cryptoxanthin, p-carotene, and lutein.
One of the highest known concentrationsof carotenoids occurs in crude palm oil. Itcontains about 15 to 300 times more retinolequivalent than carrots, green leafy vegeta-bles, and tomatoes. All of the carotenoids incrude palm oil are destroyed by the normalprocessing and refining operations. Recently,improved gentler processes have been devel-oped that result in a "red palm oil" thatretains most of the carotenoids. The compo-sition of the carotenes in crude palm oil witha total carotene concentration of 673 mg/kgis shown in Table 6-5.
Milkfat contains carotenoids with sea-sonal variation (related to feed conditions)ranging from 2 to 13 ppm.
Figure 6-19 Formation of Retinin and Vitamin A from p-Carotene. Source: From B.C. Grob, The Bio-genesis of Carotenes and Carotenoids, in Carotenes and Carotenoids, K. Lang, ed., 1963, SteinkopffVerlag.
Vilomin A
Retinin
ft-Corottn*
AZAFRIN IONONE
13C27C
PlCROCROClN CROCIN PICROCROCIN
1OC2OC1OC
BIXINMETHYLHEPTENONE8C24C
METHYLHEPTENONE8C
VITAMIN A
2OC
VITAMINA
2OC
CAROTENES
4OC
lycopene
Bixin
Figure 6-20 Relationship Between Lycopene and Bixin. Source: From E.G. Grob, The Biogenesis ofCarotenes and Carotenoids, in Carotenes and Carotenoids, K. Lang, ed., 1963, Steinkopff Verlag.
Zcaxanthin
Protocrocin
PicrocrocinCf OC in Picrocrocin
Glucose
Gentiobiote Genttobiose
Gluco»e
Crocetin
S of fronal SaHronol
Figure 6-21 Relationship Between Crocin and Picrocrocin and the Carotenoids. Source: From B.C.Grob, The Biogenesis of Carotenes and Carotenoids, in Carotenes and Carotenoids, K. Lang, ed., 1963,Steinkopff Verlag.
Figure 6-22 Structure of Some of the Important Carotenoids. Source: From B. Borenstein and R.H.Bunnell, Carotenoids: Properties, Occurrence, and Utilization in Foods, in Advances in Food Research,Vol. 15, C.O. Chichester et al., eds., 1967, Academic Press.
Astaxanthin
Torularhodin
Canthaxanthin
Physalien
Zeaxanthin
Isozeaxanthin
Lutein
Cryptoxanthin
p-Apo-8'-carotenal
p-Carotene
Capsorubin
Capsanthin
Table 6-3 Provitamin A Value of Some Fruits andVegetables
Product IU/100g
Carrots, mature 20,000Carrots, young 10,000Spinach 13,000Sweet potato 6,000Broccoli 3,500Apricots 2,000Lettuce 2,000Tomato 1,200Asparagus 1,000Bean, trench 1,000Cabbage 500Peach 800Brussels sprouts 700Watermelon 550Banana 400Orange juice 200
Source: From B. Borenstein and R.H. Bunnell, Caro-tenoids: Properties, Occurrence, and Utilization inFoods, in Advances in Food Research, Vol. 15, C.O.Chichester et al., eds., 1967, Academic Press.
Egg yolk contains lutein, zeaxanthin, andcryptoxanthin. The total carotenoid contentranges from 3 to 89 ppm.
Crustaceans contain carotenoids bound toprotein resulting in a blue or blue-gray color.When the animal is immersed in boilingwater, the carotenoid-protein bond is brokenand the orange-red color of the free car-
otenoid appears. Widely distributed in crus-taceans is astaxanthin. Red fish containastaxanthin, lutein, and taraxanthin.
Common unit operations of food process-ing are reported to have only minor effectson the carotenoids (Borenstein and Bunnell1967). The carotenoid-protein complexes aregenerally more stable than the free car-otenoids. Because carotenoids are highly un-saturated, oxygen and light are major factorsin their breakdown. Blanching destroysenzymes that cause carotenoid destruction.Carotenoids in frozen or heat-sterilized foodsare quite stable. The stability of carotenoidsin dehydrated foods is poor, unless the foodis packaged in inert gas. A notable exceptionis dried apricots, which keep their color well.Dehydrated carrots fade rapidly.
Several of the carotenoids are now com-mercially synthesized and used as food col-ors. A possible method of synthesis isdescribed by Borenstein and Bunnell (1967).Beta-ionone is obtained from lemon grass oiland converted into a C14 aldehyde. The C14aldehyde is changed to a C16 aldehyde, thento a C19 aldehyde. Two moles of the C19aldehyde are condensed with acetylene di-magnesium bromide and, after a series ofreactions, yield p-carotene.
Three synthetically produced carotenoidsare used as food colorants, p-carotene, p-apo-8'-carotenal (apocarotenal), and can-thaxanthin. Because of their high tinctorialpower, they are used at levels of 1 to 25 ppm
Pigment
LycopeneCaroteneXanthophyllXanthophyll ester
Green(mg/100g)
0.110.160.02O
Half-ripe(mg/100g)
0.840.430.030.02
Ripe(mg/100g)
7.850.730.060.10
Table 6-4 Development of Pigments in the Ripening Tomato
in foods (Dziezak 1987). They are unstablein light but otherwise exhibit good stabilityin food applications. Although they are fatsoluble, water-dispersible forms have beendeveloped for use in a variety of foods. Beta-carotene imparts a light yellow to orangecolor, apocarotenal a light orange to reddish-orange, and canthaxanthin, orange-red tored. The application of these compounds in avariety of foods has been described by Coun-sell (1985). Natural carotenoid food colorsare annatto, oleoresin of paprika, and unre-fined palm oil.
Anthocyanins and Flavonoids
The anthocyanin pigments are present in
the sap of plant cells; they take the form of
glycosides and are responsible for the red,
blue, and violet colors of many fruits and
vegetables. When the sugar moiety is re-
Table 6-5 Composition of the Carotenes in
Crude Palm Oil
% of TotalCarotene Carotenes
Phytoene 1.27Cis-p-carotene 0.68Phytofluene 0.06p-carotene 56.02cc-carotene 35.06^-carotene 0.69y-carotene 0.336-carotene 0.83Neurosporene 0.29p-zeacarotene 0.74oc-zeacarotene 0.23Lycopene 1.30
Source: Reprinted with permission from Choo YuenMay, Carotenoids from Palm Oil, Palm Oil Develop-ments, Vol. 22, pp. 1-6, Palm Oil Research Institute ofMalaysia.
moved by hydrolysis, the aglucone remainsand is called anthocyanidin. The sugar partusually consists of one or two molecules ofglucose, galactose, and rhamnose. The basicstructure consists of 2-phenyl-benzopyry-lium or flavylium with a number of hydroxyand methoxy substituents. Most of the antho-cyanidins are derived from 3,5,7-trihydroxy-flavylium chloride (Figure 6-23} and thesugar moiety is usually attached to thehydroxyl group on carbon 3. The anthocya-nins are highly colored, and their names arederived from those of flowers. The structureof some of the more important anthocyani-dins is shown in Figure 6-24, and the occur-rence of anthocyanidins in some fruits andvegetables is listed in Table 6-6. Recentstudies have indicated that some anthocya-nins contain additional components such asorganic acids and metals (Fe, Al, Mg).
Substitution of hydroxyl and methoxylgroups influences the color of the anthocya-nins. This effect has been shown by Braver-man (1963) (Figure 6-25). Increase in thenumber of hydroxyl groups tends to deepenthe color to a more bluish shade. Increase inthe number of methoxyl groups increasesredness. The anthocyanins can occur in dif-ferent forms. In solution, there is an equilib-rium between the colored cation R+ oroxonium salt and the colorless pseudobaseROH, which is dependent on pH.
R+ + H2O ̂ ROH + H+
As the pH is raised, more pseudobase isformed and the color becomes weaker. How-ever, in addition to pH, other factors influ-ence the color of anthocyanins, includingmetal chelation and combination with otherflavonoids and tannins.
Anthocyanidins are highly colored instrongly acid medium. They have two ab-sorption maxima—one in the visible spec-
trum at 500-550 nm, which is responsiblefor the color, and a second in the ultraviolet(UV) spectrum at 280 nm. The absorptionmaxima relate to color. For example, therelationship in 0.01 percent HCl in methanolis as follows: at 520 nm pelargonidin is scar-let, at 535 nm cyanidin is crimson, and at 546nm delphinidin is blue-mauve (Macheix etal. 1990).
About 16 anthocyanidins have been identi-fied in natural products, but only the followingsix of these occur frequently and in many dif-ferent products: pelargonidin, cyanidin, del-phinidin, peonidin, malvidin, and petunidin.The anthocyanin pigments of Red Deliciousapples were found to contain mostly cyanidin-3-galactoside, cyanidin-3-arabinoside, and cya-nidin-7-arabinoside (Sun and Francis 1968).Bing cherries contain primarily cyanidin-3-rutinoside, cyanidin-3-glucoside, and smallamounts of the pigments cyanidin, peonidin,peonidin-3-glucoside, and peonidin-3-ruti-
noside (Lynn and Luh 1964). Cranberryanthocyanins were identified as cyanidin-3-monogalactoside, peonidin-3-monogalacto-side, cyanidin monoarabinoside, and peonidin-3-monoarabinoside (Zapsalis and Francis1965). Cabernet Sauvignon grapes containfour major anthocyanins: delphinidin-3-monoglucoside, petunidin-3-monoglucoside,malvidin-3-monoglucoside, and malvidin-3-monoglucoside acetylated with chlorogenicacid. One of the major pigments is petunidin(Somaatmadja and Powers 1963).
Anthocyanin pigments can easily bedestroyed when fruits and vegetables are pro-cessed. High temperature, increased sugarlevel, pH, and ascorbic acid can affect therate of destruction (Daravingas and Cain1965). These authors studied the change inanthocyanin pigments during the processingand storage of raspberries. During storage,the absorption maximum of the pigmentsshifted, indicating a change in color. The
Figure 6-23 Chemical Structure of Fruit Anthocyanidins
R, = H
R1 =OH
R1 = OH
R, = OCH3
R1 = OCH,
R1 = OCH3
R2 = H
R2 = H
R2 = OH
R2=H
R2 = OH
R2 = OCH3
PELARGONIDIN
CYANIDIN
DELPHINIDIN
PEONIDIN
PETUNIDIN
MALVIDIN
Table 6-6 Anthocyanidins Occurring in SomeFruits and Vegetables
Fruit or Vegetable Anthocyanidin
Apple CyanidinBlack currant Cyanidin and delphinidinBlueberry Cyanidin, delphinidin, mal-
vidin, petunidin, andpeonidin
Cabbage (red) CyanidinCherry Cyanidin and peonidinGrape Malvidin, peonidin, delphini-
din, cyanidin, petunidin,and pelargonidin
Orange Cyanidin and delphinidinPeach CyanidinPlum Cyanidin and peonidinRadish PelargonidinRaspberry CyanidinStrawberry Pelargonidin and a little
cyanidin
Source: From P. Markakis, Anthocyanins, in Encyclo-pedia of Food Technology, A.M. Johnson and M.S.Peterson, eds., 1974, AVI Publishing Co.
level of pigments was lowered by prolongedtimes and higher temperatures of storage.Higher concentration of the ingoing sugarsyrup and the presence of oxygen resulted ingreater pigment destruction.
The stability of anthocyanins is increasedby acylation (Dougall et al. 1997). Theseacylated anthocyanins may occur naturallyas in the case of an anthocyanin from thepurple yam (Yoshida et al. 1991). This antho-cyanin has one sinapic residue attachedthrough a disaccharide and was found to bestable at pH 6.0 compared to other anthocya-nins without acylation. Dougall et al. (1977)were able to produce stable anthocyanins byacylation of carrot anthocyanins in cell cul-tures. They found that a wide range of aro-matic acids could be incorporated into theanthocyanin.
Anthocyanins can form purplish or slate-gray pigments with metals, which are calledlakes. This can happen when canned foodstake up tin from the container. Anthocyanins
Figure 6-24 Structure of Some Important Anthocyanidins
Delphinidin Peonidin
Cyanidin
can be bleached by sulfur dioxide. Accord-ing to Jurd (1964), this is a reversible processthat does not involve hydrolysis of the glyco-sidic linkage, reduction of the pigment, oraddition of bisulfite to a ketonic, chalconederivative. The reactive species was found tobe the anthocyanin carbonium ion (R+),which reacts with a bisulfite ion to form acolorless chromen-2(or 4)-sulfonic acid(R-SO3H), similar in structure and proper-ties to an anthocyanin carbinol base (R-OH).This reaction is shown in Figure 6-26.
The colors of the anthocyanins at acid pHvalues correspond to those of the oxoniumsalts. In slightly alkaline solutions (pH 8 to10), highly colored ionized anhydro basesare formed. At pH 12, these hydrolyze rap-idly to fully ionized chalcones (Figure 6-27).Leuco bases are the reduced form of theanthocyanins. They are usually withoutmuch color but are widely distributed infruits and vegetables. Under the influence ofoxygen and acid hydrolysis, they maydevelop the characteristic color of the car-
Figure 6-25 Effect of Substituents on the Color of Anthocyanidins. Source: Reprinted with permissionfrom J.B.S. Braverman, Introduction to the Biochemistry of Foods, © 1963, Elsevier Publishing Co.
pelargonidin cyanidin delphinidin
petunidin
malvidin
shade of blue
shade of red
Figure 6-26 Reaction of Bisulfite with the Anthocyanin Carbonium Ion
bonium ion. Canned pears, for example, mayshow "pinking"—a change from the leucobase to the anthocyanin.
The flavonoids or anthoxanthins are glyco-sides with a benzopyrone nucleus. The fla-vones have a double bond between carbons 2and 3. The flavonols have an additionalhydroyxl group at carbon 3, and the fla-vanones are saturated at carbons 2 and 3(Figure 6-28). The flavonoids have low col-
oring power but may be involved in discolor-ations; for example, they can impart blue andgreen colors when combined with iron.Some of these compounds are also potentialsubstrates for enzymic browning and cancause undesirable discoloration through thismechanism. The most ubiquitous flavonoidis quercetin, a 3,5,7,3',4'-pentahydroxy fla-vone (Figure 6-29). Many flavonoids containthe sugar rutinose, a disaccharide of glucose
Figure 6-27 Structure of Anhydro Base (I) and Chalcone (II)
I I I
(1) flavones (positions 2:3 unsaturated)
(2) flavonols (an additional OH at position 3)
(3) flavanones (saturated at positions 2:3)
(4) flavanonols (position 3 saturated and extrahydroxyl group)
(5) isoflavones (phenol ring B at position 3)
Figure 6-28 Structure of Flavones, Flavonals, Flavanones, Flavanonols, and Isoflavones
Figure 6-29 Structure of Quercetin
and rhamnose. Hesperidin is a flavanoneoccurring in citrus fruits and, at pH 12, theinner ring opens to form a chalcone in a sim-ilar way as shown for the anthocyanins. Thechalcones are yellow to brown in color.
Tannins
Tannins are polyphenolic compounds pres-ent in many fruits. They are important ascolor compounds and also for their effect ontaste as a factor in astringency (see Chapter7). Tannins can be divided into two classes—hydrolyzable tannins and nonhydrolyzable orcondensed tannins. The tannins are charac-terized by the presence of a large number ofhydroxyl groups, which provide the ability toform reversible bonds with other macromol-ecules, polysaccharides, and proteins, as wellas other substances such as alkaloids. Thisbond formation may occur during the devel-opment of the fruit or during the mechanicaldamage that takes place during processing.
Hydrolyzable tannins are composed ofphenolic acids and sugars that can be brokendown by acid, alkaline, or enzymic hydroly-
sis. They are polyesters based on gallic acidand/or hexahydroxydiphenic acid (Figure6-30). The usual sugar is D-glucose andmolecular weights are in the range of 500 to2,800. Gallotannins release gallic acid onhydrolysis, and ellagitannins produce ellagicacid. Ellagic acid is the lactone form ofhexahydroxydiphenic acid, which is thecompound originally present in the tannin(Figure 6-30).
Nonhydrolyzable or condensed tanninsare also named proanthocyanidins. These arepolymers of flavan-3-ols, with the flavanbonds most commonly between C4 and C8or C6 (Figure 6-23) (Macheix et al. 1990).Many plants contain tannins that are poly-mers of (-f)-catechin or (-)-epicatechin.These are hydrogenated forms of flavonoidsor anthocyanidins. Other monomers occupy-ing places in condensed fruit tannins havetrihydroxylation in the B-ring: (+)-gallocat-echin and (-)-epigallocatechin. Oligomericand polymeric procyanidins are formed byaddition of more flavan-3-ol units and resultin the formation of helical structures. Thesestructures can form bonds with proteins.
Tannins are present in the skins of redgrapes and play an important part in the fla-vor profile of red wine. Tannins in grapes areusually estimated in terms of the content ofgallic acid (Amerine and Joslyn 1970).
Oxidation and polymerization of phenoliccompounds as a result of enzymic activity ofphenoloxidases or peroxidases may result in
ELLAGIC ACIDHEXAHYDROXYDIPHENIC ACIDGALLIC ACID
Figure 6-30 Structure of Components of Hydrolyzable Tannins
the formation of brown pigments. This cantake place during the growth of fruits (e.g., indates) or during mechanical damage in pro-cessing.
Betalains
Table beets are a good source of red pig-ments; these have been increasingly used forfood coloring. The red and yellow pigments
obtained from beets are known as betalainsand consist of the red betacyanins and theyellow betaxanthins (Von Elbe and Maing1973). The structures of the betacyanins areshown in Figure 6-31. The major betacyaninis betanin, which accounts for 75 to 95 per-cent of the total pigments of beets. Theremaining pigments contain isobetanin, pre-betanin, and isoprebetanin. The latter two aresulfate monoesters of betanin and isobetanin,
III BETANINIV ISOBETANIN, C-15
EPIMER OF BETANIN
I BETANIDINII ISOBETANIDIN, C-15
EPIMER OF BETANIDIN
V VULGAXANTHIN-I Vl VULGAXANTHIN-II
Figure 6-31 Structure of Naturally Occurring Betalains in Red Beets. Source: From J.H. Von Elbe andL-Y. Maing, Betalains as Possible Food Colorants of Meat Substitutes, Cereal ScL Today, Vol. 18, pp.263-264,316-317, 1973.
respectively. The major yellow pigments arevulgaxanthin I and vulgaxanthin II. Betaninis the glucoside of betanidin, and isobetaninis the C-15 epimer of betanin.
Betanidin has three carboxyl groups (pka =3.4), two phenol groups (pHa = 8.5), andasymmetric carbons at positions 2 and 15.The 15-position is easily isomerized underacid or basic conditions in the absence ofoxygen to yield isobetanidin. Under alkalineconditions and in the presence of glutamineor glutamic acid, betanin can be converted tovulgaxanthin (Mabry 1970).
The color of betanin solutions is influ-enced by pH. In the range of 3.5 to 7.0, thespectrum shows a maximum of 537 nm (Fig-ure 6-32). Below pH 3, the intensity of thismaximum decreases and a slight increase inthe region of 570 to 640 nm occurs and thecolor shifts toward violet. At pH values over
7, a shift of the maximum occurs to longerwavelength. At pH 9, the maximum is about544 nm and the color shifts toward blue. VonElbe et al. (1974) found that the color of bet-anin is most stable between pH 4.0 and 6.0.The thermostability is greatest between pH4.0 and 5.0. Light and air have a degradingeffect on betanin, and the effect is cumula-tive.
Caramel
Caramel color can be produced from a vari-ety of carbohydrate sources, but usually cornsugar syrup is used. Corn starch is first hydro-lyzed with acid to a DE of 8 to 9, followed byhydrolysis with bacterial oc-amylase to a DEof 12 to 14, then with fungal amyloglucosi-dase up to a DE of 90 to 95. Several types ofcaramel are produced. The largest amount is
Figure 6-32 Visible Spectra of Betanin at pH Values of 2.0, 5.0, and 9.0. Source: From J.H. Von Elbe,L-Y. Maing, and C.H. Amundson, Color Stability of Betanin, Journal of Food Science, Vol. 39, pp. 334-337, 1974, Institute of Food Technologists.
A ( nm )
electropositive or positive caramel, which ismade with ammonia. Electronegative or neg-ative caramel is made with ammonium salts.A slightly electronegative caramel is solublein alcohol and is used for coloring beverages(Greenshields 1973). The composition andcoloring power of caramel depends on thetype of raw materials and the process used.Both Maillard-type reactions and pure cara-melizing reactions are thought to be involved,and the commercial product is extremelycomplex in composition. Caramels containhigh and low molecular weight colored com-pounds, as well as a variety of volatile com-ponents.
Other Colorants
Synthetic colorants, used commercially,are also known as certified color additives.There are two types, FD&C dyes and FD&Clakes. FD&C indicates substances approvedfor use in food, drug, and cosmetic use byU.S. federal regulations. Dyes are water-solu-ble compounds that produce color in solu-tion. They are manufactured in the form ofpowders, granules, pastes, and dispersions.They are used in foods at concentrations ofless than 300 ppm (Institute of Food Technol-ogists 1986). Lakes are made by combiningdyes with alumina to form insoluble colo-rants, which have dye contents in the range of20 to 25 percent (Pearce 1985). The lakesproduce color in dispersion and can be usedin oil-based foods when insufficient water ispresent for the solubilization of the dye. Thelist of approved water-soluble colorants haschanged frequently; the current list is givenin Chapter 11.
The uncertified color additives (Institute ofFood Technologists 1986) include a numberof natural extracts as well as inorganic sub-stances such as titanium dioxide. Some ofthese can be used only with certain restric-tions (Table 6-7). The consumer demand formore natural colorants has provided an impe-tus for examining many natural coloring sub-stances. These have been described in detailby Francis (1987). The possibility of usingplant tissue culture for the production of nat-ural pigments has also been considered (Ilker1987).
Table 6-7 Color Additives Not RequiringCertification
Colorant Restriction
Annatto extract —Beta-apo-8'-carotenal 33 mg/kgBeta-carotene —Beet powder —Canthaxanthin 66 mg/kgCaramel —Carrot oil —Cochineal extract —
(carmine)Ferrous gluconate Ripe olives onlyFruit juice —Grape color extract Nonbeverage
foods onlyGrape skin extract Beverages
(enocianina)Paprika and its oleoresin —Riboflavin —Saffron —Titanium dioxide 1 %Turmeric and its oleo- —
resinVegetable juice —
REFERENCES
Amerine, M.A., and M.A. Joslyn. 1970. Table wines.The technology of their production. Berkeley, CA:University of California Press.
Bodwell, C.E., and RE. McClain. 1971. Proteins. InThe sciences of meat products, ed. J. Price and B. S.Schweigert. San Francisco: W.H. Freeman and Co.
Borenstein, B., and R.H. Bunnell. 1967. Carotenoids:Properties, occurrence and utilization in foods. InAdvances in food research, Vol. 15, ed. C.O. Chich-ester, E.M. Mrak, and G.F. Stewart. New York: Aca-demic Press.
Braverman, J.B.S. 1963. Introduction to the biochemis-try of foods. New York: Elsevier Publishing Co.
Chichester, C.O., and R. McFeeters. 1971. Pigmentdegeneration during processing and storage. In Thebiochemistry of fruits and their products, ed. A.C.Hulme. New York: Academic Press.
Clydesdale, F.M., and FJ. Francis. 1970. Color scales.Food Prod. Dev. 3: 117-125.
Counsell, J.N. 1985. Uses of carotenoids in foods.IFST Proceedings 18: 156-162.
Curl, A.L., and G.F. Bailey. 1956. Carotenoids of agedcanned Valencia orange juice. J. Agr. Food Chem. 4:159-162.
Daravingas, G., and R.F. Cain. 1965. Changes in theanthocyanin pigments of raspberries during process-ing and storage. /. Food ScL 30: 400-405.
Dougall, D.K., et al. 1997. Biosynthesis and stability ofmonoacylated anthocyanins. Food TechnoL 51, no.11:69-71.
Dziezak, J.D. 1987. Applications of food colorants.Food TechnoL 41, no. 4: 78-88.
Falconer, M.E., et al. 1964. Carotene oxidation and off-flavor development in dehydrated carrot. J. ScL FoodAgr. 15: 897-901.
Fox, J.B. 1966. The chemistry of meat pigments. /.Agr. Food Chem. 14: 207-210.
Francis, FJ. 1987. Lesser known food colorants. FoodTechnoL 41, no. 4: 62-68.
Greenshields, R.N. 1973. Caramel—Part 2. Manufac-ture, composition and properties. Process Biochem.8, no. 4: 17-20.
Grob, E.C. 1963. The biogenesis of carotenes and caro-tenoids. In Carotenes and carotenoids, ed. K. Lang.Darmstadt, Germany: Steinkopff Verlag.
Ilker, R. 1987. In-vitro pigment production: An alter-native to color synthesis. Food TechnoL 41, no. 4:70-72.
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Jurd, L. 1964. Reactions involved in sulfite bleachingof anthocyanins. J. Food ScL 29: 16-19.
Landrock, A.H., and G.A. Wallace. 1955. Discolora-tion of fresh red meat and its relationship to filmoxygen permeability. Food TechnoL 9: 194-196.
Lynn, D.Y.C., and B.S. Luh. 1964. Anthocyanin pig-ments in Bing cherries. J. Food ScL 29: 735-743.
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Macheix, JJ., et al. 1990. Fruit phenolics. Boca Raton,FL: CRC Press.
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INTRODUCTION
Flavor has been defined by Hall (1968) asfollows: "Flavor is the sensation produced bya material taken in the mouth, perceivedprincipally by the senses of taste and smell,and also by the general pain, tactile and tem-perature receptors in the mouth. Flavor alsodenotes the sum of the characteristics of thematerial which produce that sensation."
This definition makes clear that flavor is aproperty of a material (a food) as well as ofthe receptor mechanism of the personingesting the food. The study of flavorincludes the composition of food com-pounds having taste or smell, as well as theinteraction of these compounds with thereceptors in the taste and smell sensoryorgans. Following an interaction, the organsproduce signals that are carried to the cen-tral nervous system, thus creating what weunderstand as flavor. This process is proba-bly less well understood than the processesoccurring in other organs (O'Mahony1984). Beidler (1957) has represented thetaste process schematically (Figure 7-1).
Although flavor is composed mainly oftaste and odor, other qualities contribute tothe overall sensation. Texture has a verydefinite effect. Smoothness, roughness,granularity, and viscosity can all influence
Figure 7-1 Schematic Representation of theTaste Process. Source: From LM. Beidler, Factsand Theory on the Mechanism of Taste and OdorPerception, in Chemistry of Natural Food Fla-vors, 1957, Quartermaster Food and ContainerInstitute for the Armed Forces.
flavor, as can hotness of spices, coolness ofmenthol, brothiness or fullness of certainamino acids, and the tastes described asmetallic and alkaline.
TASTE
It is generally agreed that there are onlyfour basic, or true, tastes: sweet, bitter, sour,
Flavor
CHAPTER 7
TASTE SENSATIONS
BRAIN
NEURAL PATTERNS OF ACTIVITY
TACTILE TONGUE PAIN
WARM
TASTE
COLD
and salty. The sensitivity to taste is located intaste buds of the tongue. The taste buds aregrouped in papillae, which appear to be sen-sitive to more than one taste. There isundoubtedly a regional distribution of thefour kinds of receptors at the tongue, creat-ing areas of sensitivity—the sweet taste atthe tip of the tongue, bitter at the back, sourat the edges, and salty at both edges and tip(Figure 7-2). The question of how the fourtypes of receptors are able to respond thisspecifically has not been resolved. Accord-ing to Teranishi et al. (1971), perception ofthe basic taste qualities results from a patternof nerve activity coming from many tastecells; specific receptors for sweet, sour, bit-ter, and salty do not exist. It may be envi-sioned that a single taste cell possessesmultiple receptor sites, each of which mayhave specificity.
The mechanism of the interaction betweenthe taste substance and the taste receptor isnot well understood. It has been suggested
that the taste compounds interact with spe-cific proteins in the receptor cells. Sweet-and bitter-sensitive proteins have beenreported. Dastoli and Price (1966) isolated aprotein from bovine tongue epithelium thatshowed the properties of a sweet taste recep-tor molecule. Dastoli et al. (1968) reportedisolating a protein that had the properties of abitter receptor.
We know that binding between stimulusand receptor is a weak one because no irre-versible effects have been observed. A mech-anism of taste stimulation with electrolyteshas been proposed by Beidler (1957); it isshown in Figure 7-3. The time required fortaste response to take place is in the order of25 milliseconds. The taste molecule isweakly adsorbed, thereby creating a distur-bance in the molecular geography of the sur-face and allowing an interchange of ionsacross the surface. This reaction is followedby an electrical depolarization that initiates anerve impulse.
The taste receptor mechanism has beenmore fully described by Kurihara (1987). Theprocess from chemical stimulation to trans-mitter release is schematically presented inFigure 7-4. The receptor membranes containvoltage-dependent calcium channels. Tastecompounds contact the taste cells and depo-larize the receptor membrane; this depolar-ization spreads to the synaptic area, activatingthe voltage-dependent calcium channels.Influx of calcium triggers the release of thetransmitter norepinephrine.
The relationship between stimulus concen-tration and neural response is not a simpleone. As the stimulus concentration increases,the response increases at a decreasing rateuntil a point is reached where further in-crease in stimulus concentration does notproduce a further increase in response.Beidler (1954) proposed the following equa-
B I T T E R
Figure 7-2 Areas of Taste Sensitivity of theTongue
S W E E T
Figure 7-4 Diagram of a Taste Cell and the Mechanism of Chemical Stimulation and TransmitterRelease. Source: Reprinted with permission from Y. Kawamura and M.R. Kare, Umami: A Basic Tale,© 1987, Marcel Dekker, Inc.
Release of transmitter(norepinephrine)
Tastenerve
Ca influx
Activation ofvoltage-dependent Ca channel
Receptor potential
Adsorption
Receptormembrane
Electriccurrent
Synapse
Figure 7-3 Mechanism of Taste Stimulation as Proposed by Beidler. Source: From L.M. Beidler, Factsand Theory on the Mechanism of Taste and Odor Perception, in Chemistry of Natural Food Flavors,1957, Quartermaster Food and Container Institute for the Armed Forces.
NERVE ACTION POTENTIALS
SENSE CELL DEPOLARIZATION
CELLULAR CHANGESSTRUCTURAL
CHEMICAL
PHYSICOCHEMICAL CHANGESSPATIAL ARRANGEMENTS
CHARGE DENSITIES
BINDING SITESPROTEINS
LIPIDS
HYDRATEDIONS
tion relating magnitude of response and stim-ulus concentration:
£ - — —R " Ws
+ KR s
whereC = stimulus concentrationR = response magnitudeRs = maximum responseK = equilibrium constant for the stimulus-
receptor reaction
K values reported by Beidler for many sub-stances are in the range of 5 to 15.
It appears that the initial step in the stimu-lus-receptor reaction is the formation of aweak complex, as evidenced by the smallvalues of K. The complex formation resultsin the initiation of the nerve impulse. Tasteresponses are relatively insensitive to changesin pH and temperature. Because of thedecreasing rate of response, we know that thenumber of receptor sites is finite. The tasteresponse is a function of the proportion ofsites occupied by the stimulus compound.
According to Beidler (1957), the thresholdvalue of a substance depends on the equilib-rium constant and the maximum response.Since K and Rx both vary from one substanceto another and from one species to another,the threshold also varies between substancesand species. The concentration of the stimu-lus can be increased in steps just large enoughto elicit an increase in response. This amountis called the just noticeable difference (JND).
There appear to be no significant age- orsex-related differences in taste sensitivity(Fisher 1971), but heavy smoking (more than20 cigarettes per day) results in a deteriora-tion in taste responsiveness with age.
Differences in taste perception betweenindividuals seem to be common. Peryam
(1963) found that sweet and salt are usuallywell recognized. However, with sour and bit-ter taste some difficulty is experienced.Some tasters ascribe a bitter quality to citricacid and a sour quality to caffeine.
Chemical Structure and Taste
A first requirement for a substance to pro-duce a taste is that it be water soluble. Therelationship between the chemical structureof a compound and its taste is more easilyestablished than that between structure andsmell. In general, all acid substances aresour. Sodium chloride and other salts aresalty, but as constituent atoms get bigger, abitter taste develops. Potassium bromide isboth salty and bitter, and potassium iodide ispredominantly bitter. Sweetness is a propertyof sugars and related compounds but also oflead acetate, beryllium salts, and many othersubstances such as the artificial sweetenerssaccharin and cyclamate. Bitterness is exhib-ited by alkaloids such as quinine, picric acid,and heavy metal salts.
Minor changes in chemical structure maychange the taste of a compound from sweetto bitter or tasteless. For example, Beidler(1966) has examined saccharin and its sub-stitution compounds. Saccharin is 500 timessweeter than sugar (Figure 7-5). Introduc-tion of a methyl group or of chloride in thepara position reduces the sweetness by half.Placing a nitro group in the meta positionmakes the compound very bitter. Introduc-tion of an amino group in the para positionretains the sweetness. Substitutions at theimino group by methyl, ethyl, or bromoethylgroups all result in tasteless compounds.However, introduction of sodium at this loca-tion yields sodium saccharin, which is verysweet.
The compound 5-nitro-otoluidine is sweet.The positional isomers 3-nitro-o-toluidineand 3-nitro-p-toluidine are both tasteless(Figure 7-6). Teranishi et al. (1971) pro-vided another example of change in tasteresulting from the position of substituentgroup: 2-amino-4-nitro-propoxybenzene is4,000 times sweeter than sugar, 2-nitro-4-amino-propoxybenzene is tasteless, and 2,4-dinitro-propoxybenzene is bitter (Figure 7-7).Dulcin (p-ethoxyphenylurea) is extremelysweet, the thiourea analog is bitter, and the0-ethoxyphenylurea is tasteless (Figure 7-8).
Just as positional isomers affect taste, sodo different stereoisomers. There are eightamino acids that are practically tasteless. Agroup of three has varying tastes; except forglutamic acid, these are probably derivedfrom sulfur-containing decomposition prod-
ucts. Seven amino acids have a bitter taste inthe L form or a sweet taste in the D form,except for L-alanine, which has a sweet taste(Table 7-1). Solms et al. (1965) reported onthe taste intensity, especially of aromaticamino acids. L-tryptophan is about half asbitter as caffeine; D-tryptophan is 35 timessweeter than sucrose and 1.7 times sweeterthan calcium cyclamate. L-phenylalanine isabout one-fourth as bitter as caffeine; the Dform is about seven times sweeter thansucrose. L-tyrosine is about one-twentieth asbitter as caffeine, but D-tyrosine is still 5.5times sweeter than sucrose.
Some researchers claim that differencesexist between the L and D forms of some sug-ars. They propose that L-glucose is slightlysalty and not sweet, whereas D-glucose issweet. There is even a difference in taste
Figure 7-5 The Effect of Substitutions in Saccharin on Sweetness. Source: From L.M. Beidler, Chem-ical Excitation of Taste and Odor Receptors, in Flavor Chemistry, I. Hornstein, ed., 1966, AmericanChemistry Society.
Sweet Tasteless Tasteless Tasteless Sweet
Sweet Sweet Sweet Bitter Sweet
Figure 7-6 Taste of Nitrotoluidine Isomers
SWEET TASTELESS TASTELESS
between the two anomers of D-mannose. Thea form is sweet as sugar, and the (3 form is bit-ter as quinine.
Optical isomers of carvone have totallydifferent flavors. The D+ form is characteris-tic of caraway; the L- form is characteristicof spearmint.
The ability to taste certain substances isgenetically determined and has been studiedwith phenylthiourea. At low concentrations,about 25 percent of subjects tested do nottaste this compound; for the other 75 percent,the taste is bitter. The inability to taste phen-ylthiourea is probably due to a recessivegene. The compounds by which tasters andnontasters can be differentiated all containthe following isothiocyanate group:
SIi
- C - N -
These compounds—phenylthiourea, thio-urea, and thiouracil—are illustrated in Figure
7-9. The corresponding compounds thatcontain the group,
OIl
- C - N -
phenylurea, urea, and uracil, do not showthis phenomenon. Another compound con-taining the isothiocyanate group has beenfound in many species of the Cruciferae fam-ily; this family includes cabbage, turnips,and rapeseed and is well known for itsgoitrogenic effect. The compound is goitrin,5-vinyloxazolidine-2-thione (Figure 7-10).
Sweet Taste
Many investigators have attempted to relatethe chemical structure of sweet tasting com-pounds to the taste effect, and a series of theo-ries have been proposed (Shallenberger 1971).Shallenberger and Acree (1967, 1969) pro-
TASTELESSBITTERSWEET
Figure 7-8 Taste of Substituted Ethoxybenzenes
BITTERTASTELESSSWEETFigure 7-7 Taste of Substituted Propoxybenzenes
posed a theory that can be considered a refine-ment of some of the ideas incorporated inprevious theories. According to this theory,called the AH,B theory, all compounds thatbring about a sweet taste response possess anelectronegative atom A, such as oxygen ornitrogen. This atom also possesses a protonattached to it by a single covalent bond; there-fore, AH can represent a hydroxyl group, animine or amine group, or a methine group.
Within a distance of about 0.3 nm from theAH proton, there must be a second electrone-gative atom B, which again can be oxygen ornitrogen (Figure 7-11). Investigators haverecognized that sugars that occur in a favoredchair conformation yield a glycol unit confor-mation with the proton of one hydroxyl groupat a distance of about 0.3 nm from the oxygenof the next hydroxyl group; this unit can beconsidered as an AH,B system. It was alsofound that the K bonding cloud of the benzenering could serve as a B moiety. This explainsthe sweetness of benzyl alcohol and thesweetness of the anti isomer of anisaldehydeoxime, as well as the lack of sweetness of thesyn isomer. The structure of these compoundsis given in Figure 7-12. The AH,B systempresent in sweet compounds is, according toShallenberger, able to react with a similarAH,B unit that exists at the taste bud receptorsite through the formation of simultaneoushydrogen bonds. The relatively strong natureof such bonds could explain why the sense ofsweetness is a lingering sensation. Accordingto the AH,B theory, there should not be a dif-ference in sweetness between the L and D iso-mers of sugars. Experiments by Shallenberger(1971) indicated that a panel could not distin-guish among the sweet taste of the enantio-morphic forms of glucose, galactose, man-nose, arabinose, xylose, rhamnose, and gluco-heptulose. This suggests that the notion that Lsugars are tasteless is a myth.
Phenylthiourea Thiourea Thiouracil
Figure 7-9 Compounds Containing theDifferentiated
Group by Which Tasters and Nontasters Can Be
SIl
- C - N -
Table 7-1 Difference in Taste Between the L-and D-Forms of Amino Acids
Amino Acid
AsparagineGlutamic
acidPhenylala-
nineLeucine
Valine
Serine
Histidine
lsoleucineMethionineTryptophane
Taste ofL lsomer
InsipidUnique
Faintly bitter
Flat, faintlybitter
Slightlysweet,bitter
Faintly sweet,stale after-taste
Tasteless tobitter
BitterFlatBitter
Taste ofD lsomer
SweetAlmost taste-
lessSweet, bitter
aftertasteStrikingly
sweetStrikingly
sweet
Strikinglysweet
Sweet
SweetSweetVery sweet
Figure 7-10 5-Vinyloxazolidine-2-thione
Spillane (1996) has pointed out that theAH,B theory appears to work quite well,although spatial, hydrophobic/hydrophilic,and electronic effects are also important.Shallenberger (1998) describes the initiationof sweetness as being due to a concertedintermolecular, antiparallel hydrogen-bond-ing interaction between the glycophore(Greek glyks, sweet; phoros, to carry) andreceptor dipoles. The difficulty in explainingthe sweetness of compounds with differentchemical structures is also covered by Shal-lenberger (1998) and how this has resulted inalternative taste theories. The application ofsweetness theory is shown to have importantapplications in the food industry.
Extensive experiments with a large num-ber of sugars by Birch and Lee (1971) sup-port Shallenberger's theory of sweetnessand indicate that the fourth hydroxyl groupof glucopyranosides is of unique impor-tance in determining sweetness, possibly bydonating the proton as the AH group. Ap-
parently the primary alcohol group is of lit-tle importance for sweetness. Substitutionof acetyl or azide groups confers intensebitterness to sugars, whereas substitution ofbenzoyl groups causes tastelessness.
As the molecular weight of saccharidesincreases, their sweetness decreases. This isbest explained by the decrease in solubilityand increase in size of the molecule. Appar-ently, only one sugar residue in each oli-gosaccharide is involved in the interaction atthe taste bud receptor site.
The relative sweetness of a number of sug-ars and other sweeteners has been reportedby Solms (1971) and is given in Table 7-2.These figures apply to compounds tasted sin-gly and do not necessarily apply to sugars infoods, except in a general sense. The relativesweetness of mixtures of sugars changeswith the concentration of the components.Synergistic effects may increase the sweet-ness by as much as 20 to 30 percent in suchmixtures (Stone and Oliver 1969).
Sour Taste
Although it is generally recognized thatsour taste is a property of the hydrogen ion,there is no simple relationship between sour-ness and acid concentration. Acids have dif-ferent tastes; the sourness as experienced inthe mouth may depend on the nature of theacid group, pH, titratable acidity, buffering
S W E E T
COMPOUND
RECEPTOR
S I T E
Figure 7-11 The AH,B Theory of Sweet Taste Perception
effects and the presence of other compounds,especially sugars. Organic acids have agreater taste effect than inorganic acids (suchas hydrochloric acid) at the same pH. Infor-mation on a number of the most commonacids found in foods and phosphoric acid(which is also used in soft drinks) has beencollected by Solms (1971) and comparedwith hydrochloric acid. This information ispresented in Table 7-3.
According to Beatty and Cragg (1935), rel-ative sourness in unbuffered solutions ofacids is not a function of molarity but is pro-portional to the amount of phosphate bufferrequired to bring the pH to 4.4. Ough (1963)determined relative sourness of four organicacids added to wine and also preference forthese acids. Citric acid was judged the mostsour, fumaric and tartaric about equal, andadipic least sour. The tastes of citric and tar-taric acids were preferred over those offumaric and adipic acids.
Pangborn (1963) determined the relativesourness of lactic, tartaric, acetic, and citricacid and found no relation between pH, totalacidity, and relative sourness. It was alsofound that there may be considerable differ-ences in taste effects between sugars andacids when they are tested in aqueous solu-tions and in actual food products.
Table 7-2 Relative Sweetness of Sugars andOther Sweeteners
Compound Relative Sweetness
Sucrose 1
Lactose 0.27
Maltose 0.5
Sorbitol 0.5
Galactose 0.6
Glucose 0.5-0.7
Mannitol 0.7
Glycerol 0.8
Fructose 1.1-1.5
Cyclamate 30-80
Glycyrrhizin 50
Aspartyl-phenylalanine 100-200
methylester
Stevioside 300
Naringin dihydrochal- 300
cone
Saccharin 500-700
Neohesperidin 1000-1500dihydrochalcone
Source: From J. Solms, Nonvolatile Compounds andthe Flavor of Foods, in Gustation and Olfaction, G.Ohloff and A.F. Thomas, eds., 1971, Academic Press.
SWEET TASTELESS
Figure 7-12 Anfr'-Anisaldehyde Oxime, Sweet; and Syrc-Anisaldehyde Oxime, Tasteless
Buffering action appears to help determinethe sourness of various acids; this mayexplain why weak organic acids taste moresour than mineral acids of the same pH. It issuggested that the buffering capacity of salivamay play a role, and foods contain many sub-stances that could have a buffering capacity.
Wucherpfennig (1969) examined the sourtaste in wine and found that alcohol maydecrease the sourness of organic acids. Heexamined the relative sourness of 17 organicacids and found that the acids tasted at thesame level of undissociated acid have greatlydifferent intensities of sourness. Partiallyneutralized acids taste more sour than pureacids containing the same amount of undis-sociated acids. The change of malic into lac-tic acid during the malolactic fermentation ofwines leads to a decrease in sourness, thusmaking the flavor of the wine milder.
Salty Taste
The salty taste is best exhibited by sodiumchloride. It is sometimes claimed that thetaste of salt by itself is unpleasant and thatthe main purpose of salt as a food componentis to act as a flavor enhancer or flavor poten-tiator. The taste of salts depends on thenature of both cation and anion. As themolecular weight of either cation or anion—or both—increases, salts are likely to tastebitter. The lead and beryllium salts of aceticacid have a sweet taste. The taste of a num-ber of salts is presented in Table 7-4.
The current trend of reducing sodiumintake in the diet has resulted in the formula-tion of low-sodium or reduced-sodium foods.It has been shown (Gillette 1985) thatsodium chloride enhances mouthfeel, sweet-ness, balance, and saltiness, and also masks
Table 7-3 Properties of Some Acids, Arranged in Order of Decreasing Acid Taste and with Tartaric Acidas Reference
Properties ofO.OSN Solutions
Acid
HydrochloricTartaricMalic
PhosphoricAceticLacticCitric
Propionic
Taste
+1.43O-0.43
-1.14-1.14-1.14-1.28
-1.85
TotalAcidg/L
1.853.753.35
1.653.004.503.50
3.70
pH
1.702.452.65
2.252.952.602.60
2.90
lonizationConstant
1.04 x 10~3
3.9X10"4
7.52 x 1Q-3
1.75 x 10~5
1.26 x 1Q-4
8.4 x 1Q-4
1.34 x 10~5
TasteSensation
HardGreen
IntenseVinegarSour, tartFresh
Sour,cheesy
Found In
GrapeApple, pear, prune,
grape, cherry,apricot
Orange, grapefruit
Berries, citrus,pineapple
Source: From J. Solms, Nonvolatile Compounds and the Flavor of Foods, in Gustation and Olfaction, G. Ohloffand A.F. Thomas, eds., 1971, Academic Press.
or decreases off-notes. Salt substitutes basedon potassium chloride do not enhancemouthfeel or balance and increase bitter ormetallic off-notes.
Bitter Taste
Bitter taste is characteristic of many foodsand can be attributed to a great variety ofinorganic and organic compounds. Manysubstances of plant origin are bitter. Al-though bitter taste by itself is usually consid-ered to be unpleasant, it is a component ofthe taste of many foods, usually those foodsthat are sweet or sour. Inorganic salts canhave a bitter taste (Table 7-4). Some aminoacids may be bitter (Table 7-1). Bitter pep-tides may be formed during the partial enzy-mic hydrolysis of proteins—for example,during the ripening of cheese. Solms (1969)has given a list of peptides with differenttaste sensations (Table 7-5).
The compounds best known for their bit-ter taste belong to the alkaloids and glyco-sides. Alkaloids are basic nitrogen-containingorganic compounds that are derived frompyridine, pyrrolidine, quinoline, isoquino-line, or purine. Quinine is often used as astandard for testing bitterness (Figure 7-13).
The bitterness of quinine hydrochloride isdetectable in a solution as dilute as 0.00004molar, or 0.0016 percent. If 5 mL of thissolution is tasted, the amount of substance aperson detects would be 0.08 mg (Moncri-eff 1951). Our sensitivity to bitterness ismore extreme than our sensitivity to othertastes; the order of sensitivity is from bitterto sour to salty and our least sensitivity is tosweet taste. Threshold values reported byMoncrieff are as follows: sour—0.007 per-cent HCl; salt—0.25 percent NaCl; andsweet—0.5 percent sucrose. If the artificialsweeteners such as saccharine are consid-ered, the sweet sensitivity is second to bit-ter. Quinine is used as a component of somesoft drinks to produce bitterness. Otheralkaloids occurring as natural bitter constit-uents of foods are caffeine and theobromine(Figure 7-14), which are derivatives ofpurine. Another naturally occurring bittersubstance is the glycoside naringin, whichoccurs in grapefruit and some other citrusfruits. Naringin in pure form is more bitterthan quinine and can be detected in concen-
Table 7-4 Taste Sensations of Salts
Taste
Salty
Salty and bitterBitterSweet
1 Extremely toxic
Salts
LiCI, LiBr, LiI, NaNO3, NaCI,NaBr, NaI, KNO3, KCIKBr, NH4ICsCI, CsBr, Kl, MgSO4
Lead acetate,1 berylliumacetate1
Table 7-5 Taste of Some Selected Peptides
Taste
Flat
Sour
Bitter
Sweet
Biting
Composition of Peptides
L-Lys-L-Glu, L-PhE-L-Phe, GIy-GIy-GIy-GIyL-Ala-L-Asp, y-L-Glu-L-Glu, GIy-L-Asp-L-Ser-GlyL-Leu-L-Leu, L-Arg-L-Pro, L-VaI-L-VaI-L-VaIL-Asp-L-Phe-OMe, L-Asp-L-Met-OMey-L-Glutamyl-S-(prop-1 -enyl)-L-cystein
Source: From J. Solms, Nonvolatile Compounds andthe Flavor of Foods, in Gustation and Ol faction, G.Ohloff and A.F. Thomas, eds., 1971, Academic Press.
trations of less than 0.002 percent. Naringin(Figure 7-15) contains the sugar moietyrutinose (L-rhamnose-D-glucose), whichcan be removed by hydrolysis with boilingmineral acid. The aglucose is called narin-genin, and it lacks the bitterness of narin-gin. Since naringin is only slightly solublein water (0.05 percent at 2O0C), it may crys-tallize out when grapefruit is subjected tobelow-freezing temperatures. Hesperidin(Figure 7-15) occurs widely in citrus fruitsand is also a rutinose glycoside. It occurs inoranges and lemons. Dried orange peel maycontain as much as 8 percent hesperidin.The aglycone of hesperidin is called hes-peretin. The sugar moiety is attached to car-bon 7. Horowitz and Gentili (1969) havestudied the relationship between bitternessand the structure of 7-rhamnoglycosides ofcitrus fruits; they found that the structure ofthe disaccharide moiety plays an importantrole in bitterness. The point of attachmentof rhamnose to glucose determines whetherthe substance will be bitter or tasteless.Thus, neohesperidin contains the disaccha-ride neohesporidose, which contains rham-
nose linked l->2 to glucose; therefore, thesugar moiety is 2-0-oc-L-rhamnopyranosyl-D-glucose. Glycosides containing this sugar,including neohesperidin, have a bitter taste.When the linkage between rhamnose andglucose is 1—>6, the compound is tastelessas in hesperidin, where the sugar part, ruti-nose, is 6-O-a-L-rhamnopyranosyl-D-glu-cose.
Bitterness occurs as a defect in dairy prod-ucts as a result of casein proteolysis byenzymes that produce bitter peptides. Bitterpeptides are produced in cheese because of anundesirable pattern of hydrolysis of milkcasein (Habibi-Najafi and Lee 1996). Ac-cording to Ney (1979), bitterness in aminoacids and peptides is related to hydrophobic-ity. Each amino acid has a hydrophobicityvalue (Af), which is defined as the free energyof transfer of the side chains and is based onsolubility properties (Table 7-6). The averagehydrophobicity of a peptide, Q, is obtained asthe sum of the Af of component amino acidsdivided by the number of amino acid resi-dues. Ney (1979) reported that bitterness isfound only in peptides with molecular weights
A B
Figure 7-14 (A) Caffeine and (B) Theobromine
Figure 7-13 Structure of Quinine. This has an intensely bitter taste.
below 6,000 Da when their Q value is greaterthan 1,400. These findings indicate the im-portance of molecular weight and hydropho-bicity. In a more detailed study of the compo-sition of bitter peptides, Kanehisa (1984)reported that at least six amino acids arerequired for strong bitterness. A bitter peptiderequires the presence of a basic amino acid atthe N-terminal position and a hydrophobicone at the C-terminal position. It appears thatat least two hydrophobic amino acids arerequired in the C-terminal area of the peptideto produce intense bitterness. The high hydro-phobicity of leucine and the number of leu-
Table 7-6 Hydrophoblcity Values (Af) of the SideChains of Amino Acids
Abbrevia- Af (cal/Amino Acid tion mol)
Glycine GIy OSerine Ser 40Threonine Thr 440Histidine His 500Aspartic acid Asp 540Glutamic acid GIu 550Arginine Arg 730Alanine Ala 730Methionine Met 1,300Lysine Lys 1,500Valine VaI 1,690Leucine Leu 2,420Proline Pro 2,620Phenylalanine Phe 2,650Tyrosine Tyr 2,870lsoleucine lie 2,970Tryptophan Trp 3,000
Source: Reprinted with permission from K.H. Ney,Bitterness of Peptides: Amino Acid Composition andChain Length, in Food Taste Chemistry, J.C. Boudreau,ed., ACS Symp. Ser. 115, © 1979, American ChemicalSociety.
cine and possibly proline residues in the pep-tide probably play a role in the bitterness.
Other Aspects of Taste
The basic sensations—sweet, sour, salty,and bitter—account for the major part of thetaste response. However, it is generallyagreed that these basic tastes alone cannotcompletely describe taste. In addition to thefour individual tastes, there are importantinterrelationships among them. One of themost important in foods is the interrelation-ship between sweet and sour. The sugar-acid
Figure 7-15 (A) Naringin; (B) Hesperidin; (C)Rutinose, 6-0-oc-L-Rhamnopyranosyl-D-Glu-copyranose
C
Br ut i nose
Arutinose
ratio plays an important part in many foods,especially fruits. Kushman and Ballinger(1968) have demonstrated the change insugar-acid ratio in ripening blueberries (Table7-7). Sugar-acid ratios play an importantrole in the flavor quality of fruit juices andwines (Ough 1963). Alkaline taste has beenattributed to the hydroxyl ion. Caustic com-pounds can be detected in solutions contain-ing only 0.01 percent of the alkali. Probablythe major effect of alkali is irritation of thegeneral nerve endings in the mouth. Anothereffect that is difficult to describe is astrin-gency. Borax is known for its ability to pro-duce this effect, as are the tannins present infoods, especially those that occur in tea.Even if astringency is not considered a partof the taste sense, it must still be considereda feature of food flavor.
Another important taste sensation is cool-ness, which is a characteristic of menthol.The cooling effect of menthol is part of themint flavor complex and is exhibited by onlysome of the possible isomeric forms. Only (-)and (+) menthol show the cooling effect, theformer to a higher degree than the latter, but
Table 7-7 Change in Sugar-Acid Ratio DuringRipening of Blueberries*
Unripe Ripe Overripe
Total sugar (%) 5̂ 8 7J9 12̂ 4pH 2.83 3.91 3.76Titr acidity 23.9 12.9 7.5(mEq/100g)Sugar-acid ratio 3.8 9.5 25.8
*The sugars are mainly glucose and fructose, andthe acidity is expressed as citric acid.
Source: From LJ. Kushman and W.E. Ballinger,Acid and Sugar Changes During Ripening in WolcottBlueberries, Proc. Amer. Soc. Hort. Soc., Vol. 92, pp.290-295,1968.
the isomers isomenthol, neomenthol, andneoisomenthol do not give a cooling effect(Figure 7-16) (Kulka 1967). Hotness is aproperty associated with spices and is alsoreferred to as pungency. The compound pri-marily responsible for the hotness of blackpepper is pipeline (Figure 7-17). In red pep-per or capsicum, nonvolatile amides areresponsible for the heat effect. The heat effectof spices and their constituents can be mea-sured by an organoleptic threshold method(Rogers 1966) and expressed in heat units.The pungent principle of capsicum is capsai-cin. The structure of capsaicin is given in Fig-ure 7-18. Capsaicin shows similarity to thecompound zingerone, the pungent principleof ginger (Figure 7-19).
Govindarajan (1979) has described therelationship between pungency and chemicalstructure of pungent compounds. There arethree groups of natural pungent compounds—the capsaicinoids, piperine, and the gin-gerols. These have some common structuralaspects, including an aromatic ring and analkyl side chain with a carbonyl function(Figures 7-18 and 7-19). Structural varia-tions in these compounds affect the intensityof the pungent response. These structuralvariations include the length of the alkyl sidechain, the position of the amide group nearthe polar aromatic end, the nature of thegroupings at the alkyl end, and the unsatura-tion of the alkyl chain.
The metallic taste has been described byMoncrieff (1964). There are no receptor sitesfor this taste or for the alkaline and meatytastes. However, according to Moncrieff,there is no doubt that the metallic taste is areal one. It is observable over a wide area ofthe surface of the tongue and mouth and, likeirritation and pain, appears to be a modalityof the common chemical sense. The metallictaste can be generated by salts of metals such
Figure 7-16 Isomeric Forms of Menthol
as mercury and silver (which are mostpotent) but normally by salts of iron, copper,and tin. The threshold concentration is in theorder of 20 to 30 ppm of the metal ion. Incanned foods, considerable metal uptakemay occur and the threshold could be
exceeded in such cases. Moncrieff (1964)also mentions the possibility of metallic ionexchange between the food and the con-tainer. The threshold concentration of copperis increased by salt, sugar, citric acid, andalcohol. Tannin, on the other hand, lowersthe threshold value and makes the coppertaste more noticeable. The metallic taste isfrequently observed as an aftertaste. The leadsalt of saccharin gives an impression ofintense sweetness, followed by a metallicaftertaste. Interestingly, the metallic taste isfrequently associated with oxidized prod-ucts. Tressler and Joslyn (1954) indicate that20 ppm of copper is detectable by taste inorange juice. Copper is well known for itsability to catalyze oxidation reactions. Starkand Forss (1962) have isolated and identifiedoct-l-en-3-one as the compound responsiblefor the metallic flavor in dairy products.
Taste Inhibition and Modification
Some substances have the ability to modifyour perception of taste qualities. Two suchcompounds are gymnemagenin, which isable to suppress the ability to taste sweet-ness, and the protein from miracle fruit,which changes the perception of sour tosweet. Both compounds are obtained fromtropical plants.
The leaves of the tropical plant Gymnemasylvestre, when chewed, suppress the abilityto taste sweetness. The effect lasts for hours,and sugar seems like sand in the mouth. Theability to taste other sweeteners such as sac-charin is equally suppressed. There is also adecrease in the ability to taste bitterness. Theactive principle of leaves has been namedgymnemic acid and has been found (Stocklinet al. 1967) to consist of four components,designated as gymnemic acids, A1, A2, A3,and A4. These are D-glucuronides of acety-
(db)-Neomenthol (± )-Neoisomenthol
(rt)-Menthol (dr)-Isomenthol
Figure 7-17 Piperine, Responsible for the Hot-ness of Pepper
lated gymnemagenins. The unacetylatedgymnemagenin is a hexahydroxy pentacy-clic triterpene; its structure is given in Figure7-20.
The berries of a West African shrub (Syn-sepalum dulcificum) contain a substance thathas the ability to make sour substances tastesweet. The berry, also known as miraclefruit, has been shown to contain a taste-mod-ifying protein (Kurihara and Beidler 1968;1969). The protein is a basic glycoproteinwith a molecular weight of 44,000. It is sug-gested that the protein binds to the receptormembrane near the sweet receptor site. Thelow pH changes the conformation of themembrane so that the sugar part of the pro-tein fits into the sweet receptor site. Thetaste-modifying protein was found to con-tain 6.7 percent of arabinose and xylose.
These taste-modifying substances providean insight into the mechanism of the produc-tion of taste sensations and, therefore, are avaluable tool in the study of the interrelation-ship between taste and chemical structure.
Flavor Enhancement—Umami
A number of compounds have the abilityto enhance or improve the flavor of foods. Ithas often been suggested that these com-pounds do not have a particular taste of theirown. Evidence now suggests that there is abasic taste response to amino acids, espe-cially glutamic acid. This taste is sometimesdescribed by the word umami, derived fromthe Japanese for deliciousness (Kawamuraand Kare 1987). It is suggested that a pri-mary taste has the following characteristics:
• The receptor site for a primary tastechemical is different from those of otherprimary tastes.
• The taste quality is different from others.• The taste cannot be reproduced by a
mixture of chemicals of different pri-mary tastes.
From these criteria, we can deduce that theglutamic acid taste is a primary taste for thefollowing reasons:
• The receptor for glutamic acid is differ-ent from the receptors for sweet, sour,salty, and bitter.
• Glutamic acid does not affect the taste ofthe four primary tastes.
• The taste quality of glutamic acid is dif-ferent from that of the four primarytastes.
Figure 7-18 Capsaicin, the Pungent Principle of Red Pepper
Figure 7-19 Zingerone, the Pungent Principle ofGinger
• Umami cannot be reproduced by mixingany of the four primary tastes.
Monosodium glutamate has long been rec-ognized as a flavor enhancer and is nowbeing considered a primary taste, umami.The flavor potentiation capacity of monoso-dium glutamate in foods is not the result ofan intensifying effect of the four primarytastes. Glutamate may exist in the L and Dforms and as a racemic mixture. The L formis the naturally occurring isomer that has aflavor-enhancing property. The D form isinert. Although glutamic acid was first iso-lated in 1866, the flavor-enhancing proper-ties of the sodium salt were not discovereduntil 1909 by the Japanese chemist Ikeda.Almost immediately,' commercial produc-tion of the compound started and total pro-duction for the year 1954 was estimated at13,000,000 pounds. The product as firstdescribed by Ikeda was made by neutralizinga hydrolysate of the seaweed Laminariajaponica with soda. Monosodium glutamateis now produced from wheat gluten, beetsugar waste, and soy protein and is used inthe form of the pure crystallized compound.It can also be used in the form of protein
hydrolysates derived from proteins that con-tain 16 percent or more of glutamic acid.Wheat gluten, casein, and soy flour are goodsources of glutamic acid and are used to pro-duce protein hydrolysates. The glutamic acidcontent of some proteins is listed in Table7-8 (Hall 1948). The protein is hydrolyzedwith hydrochloric acid, and the neutralizedhydrolysate is used in liquid form or as a drypowder. Soy sauce, which is similar to thesehydrolysates, is produced wholly or partiallyby enzymic hydrolysis. This results in theformation of ammonia from acid amides; soysauce contains ammonium complexes ofamino acids, including ammonium gluta-mate.
The flavor of glutamate is difficult todescribe. It has sometimes been suggestedthat glutamate has a meaty or chickeny taste,but it is now generally agreed that glutamateflavor is unique and has no similarity tomeat. Pure sodium glutamate is detectable inconcentrations as low as 0.03 percent; at 0.05percent the taste is very strong and does notincrease at higher concentrations. The tastehas been described (Crocker 1948) as a mix-ture of the four tastes. At about 2 thresholdvalues of glutamate concentration, it could
Figure 7-20 Structure of Gymnemagenin
Table 7-8 Glutamic Acid Content of SomeProteins
Glutamic AcidProtein Source (%)
Wheat gluten 36.0Corn gluten 24.5Zein 36.0Peanut flour 19.5Cottonseed flour 17.6Soybean flour 21.0Casein 22.0Rice 24.1Egg albumin 16.0Yeast 18.5
Source: From L.A. Hall, Protein Hydrolysates as aSource of Glutamate Flavors, in Monosodium Glut-amate—A Symposium, 1948, Quartermaster Food andContainer Institute for the Armed Forces.
be well matched by a solution containing 0.6threshold of sweet, 0.7 of salty, 0.3 of sour,and 0.9 of bitter. In addition, glutamate issaid to cause a tingling feeling and a markedpersistency of taste sensation. This feeling ispresent in the whole of the mouth and pro-vides a feeling of satisfaction or fullness.Apparently glutamate stimulates our tactilesense as well as our taste receptors. The pres-ence of salt is required to produce theglutamate effect. Glutamate taste is mosteffective in the pH range of 6 to 8 anddecreases at lower pH values. Sugar contentalso affects glutamate taste. The taste in acomplex food, therefore, depends on a com-plex interaction of sweet, sour, and salty, aswell as the added glutamate.
Monosodium glutamate improves the fla-vor of many food products and is thereforewidely used in processed foods. Productsbenefiting from the addition of glutamate
include meat and poultry, soups, vegetables,and seafood.
For many years glutamate was the onlyknown flavor enhancer, but recently a num-ber of compounds that act similarly havebeen discovered. The 5'-nucleotides, espe-cially 5'-inosinate and 5'-guanylate, haveenhancement properties and also show a syn-ergistic effect in the presence of glutamate.This synergistic effect has been demonstratedby determining the threshold levels of thecompounds alone and in mixtures. The datain Table 7-9 are quoted from Kuninaka(1966). The 5'-nucleotides were discoveredmany years ago in Japan as components ofdried bonito (a kind of fish). However, theywere not produced commercially and used asflavor enhancers until recently, when techni-cal problems in their production were solved.The general structure of the nucleotides withflavor activity is presented in Figure 7-21.There are three types of inosinic acid, 2'-, 3'-,and 5'-isomers; only the 5'-isomer has flavoractivity. Both riboside and S'-phosphomon-oester linkages are required for flavor activ-ity, which is also the case for the OH group atthe 6-position of the ring. Replacing the OHgroup with other groups, such as an aminogroup, sharply reduces flavor activity but thisis not true for the group at the 2-position.Hydrogen at the 2-position corresponds withinosinate and an amino group with guany-late; both have comparable flavor activity,and the effect of the two compounds is addi-tive.
The synergistic effect of umami substancesis exceptional. The subjective taste intensityof a blend of monosodium glutamate and di-sodium 5'-inosinate was found to be 16 timesstronger than that of the glutamate by itself atthe same total concentration (Yamaguchi1979).
S'-nucleotides can be produced by degra-dation of ribonucleic acid. The problem isthat most enzymes split the molecule at the3'-phosphodiester linkages, resulting innucleotides without flavor acitivity. Suitableenzymes were found in strains of Penicilliumand Streptomyces. With the aid of theseenzymes, the 5'-nucleotides can be manufac-tured industrially from yeast ribonucleic
acid. Another process produces the nucleo-side inosine by fermentation, followed bychemical phosphorylation to 5'-inosinic acid(Kuninaka 1966).
The search for other flavor enhancers hasbrought to light two new amino acids, tri-cholomic acid and ibotenic acid, obtainedfrom fungi (Figure 7-22). These aminoacids have flavor activities similar to that ofmonosodium glutamate. Apparently, the fla-vor enhancers can be divided into twogroups; the first consists of 5'-inosinate and5'-guanylate with the same kind of activityand an additive relationship. The othergroup consists of glutamate, tricholomic,and ibotenic acid, which are additive inaction. Between the members of the twogroups, the activity is synergistic.
A different type of flavor enhancer is mal-tol, which has the ability to enhance sweet-ness produced by sugars. Maltol is formedduring roasting of malt, coffee, cacao, andgrains. During the baking process, maltol isformed in the crust of bread. It is also foundin many dairy products that have beenheated, as a product of decomposition of thecasein-lactose system. Maltol (Figure 7-23)is formed from di-, tri-, and tetrasaccharidesincluding isomaltose, maltotretraose, and
Table 7-9 Threshold Levels of Flavor Enhancers Alone and in Mixtures in Aqueous Solution
Threshold Level (%)
Solvent
Water0.1%glutamate0.01%inosinate
Disodium 5'-lnosinate
0.0120.0001
Disodium5'-Guanylate
0.00350.00003
MonosodiumL-Glutamate
0.03
0.002
Source: From A. Kuninaka, Recent Studies of S'-Nucleotides as New Flavor Enhancers, In Flavor Chemistry, I.Hornstein, ed., 1966, American Chemical Society.
Figure 7-21 Structure of Nucleotides with Fla-vor Activity
panose but not from maltotriose. Formationof maltol is brought about by high tempera-tures and is catalyzed by metals such as iron,nickel, and calcium.
Maltol has antioxidant properties. It hasbeen found to prolong storage life of coffeeand roasted cereal products. Maltol is used asa flavor enhancer in chocolate and candies,ice cream, baked products, instant coffee andtea, liqueurs, and flavorings. It is used inconcentrations of 50 to 250 ppm and is com-mercially produced by a fermentation pro-cess.
ODOR
The olfactory mechanism is both morecomplex and more sensitive than the processof gustation. There are thousands of odors,and the sensitivity of the smell organ is about10,000 times greater than that of the tasteorgan. Our understanding of the odor recep-tor's mechanism is very limited, and there isno single, generally accepted theory account-ing for the relationship between molecularstructure and odor. The odorous substancearrives at the olfactory tissue in the nasalcavity, contained in a stream of air. Thismethod of sensing requires that the odorouscompound be volatile. Most odorous com-pounds are soluble in a variety of solvents,but it appears that solubility is less importantthan type of molecular arrangement, whichconfers both solubility and chemical reactiv-
ity (Moncrieff 1951). The number of volatilecompounds occurring in foods is very high.Maarse (1991) has given the following num-bers for some foods: beef (boiled, cooked)—486; beer—562; butter—257; coffee—790;grape—466; orange—203; tea—541; toma-to—387; and wine (white)—644. Not all ofthese substances may be essential in deter-mining the odor of a product. Usually, therelative amounts of a limited number of thesevolatile compounds are important in estab-lishing the characteristic odor and flavor of afood product.
The sensitivity of the human olfactoryorgan is inferior to that of many animals.Dogs and rats can detect odorous compoundsat threshold concentrations 100 times lowerthan man. When air is breathed in, only asmall part of it is likely to flow over theolfactory epithelium in the upper nasal cav-ity. When a smell is perceived, sniffing mayincrease the amount reaching the olfactorytissue. When foods are eaten, the passage ofbreath during exhalation reaches the nasalcavity from the back. Doving (1967) hasquoted the threshold concentrations of odor-ous substances listed in Table 7-10. Appar-ently, it is possible to change odor thresholdsby a factor of 100 or more by stimulating thesympathetic nervous system so that moreodor can reach the olfactory tissue. What isremarkable about the olfactory mechanism isnot only that thousands of odors can be rec-ognized, but that it is possible to store the
A B
Figure 7-22 (A) Tricholomic and (B) Ibotenic Acid
information in the brain for retrieval afterlong periods of time. The ability to smell isaffected by several conditions, such as colds,menstrual cycle, and drugs such as penicillin.Odors are usually the result of the presenceof mixtures of several, sometimes many, dif-ferent odorous compounds. The combinedeffect creates an impression that may be very
Table 7-10 Odor Threshold Concentrations ofOdorous Substances Perceived During NormalInspiration
Threshold ConcentrationCompound (Molecules/cc)
AIIyI mercaptan 6 x 107
Sec. butyl mercaptan 1 x 108
lsopropyl mercaptan 1 x 108
lsobutyl mercaptan 4 x 108
Tert. butyl mercaptan 6 x 108
Thiophenol 8x108
Ethyl mercaptan 1 x 109
1,3-Xylen-4-ol 2x1012
ji-Xylene 2x1012
Acetone 6x1013
Source: From K.B. Doving, Problems in the Physiol-ogy of Olfaction, in Symposium on Foods: The Chem-istry and Physiology of Flavors, H.W. Schultz et al.,eds., 1967, AVI Publishing.
different from that of the individual compo-nents. Many food flavors, natural as well asartificial, are of this compound nature.
Odor and Molecular Structure
M. Stoll wrote in 1957: "The whole sub-ject of the relation between molecular struc-ture and odor is very perplexing, as there isno doubt that there exist as many relation-ships of structure and odor as there are struc-tures of odorous substances." In 1971(referring to Stoll 1957), Teranishi wrote:"The relation between molecular structureand odor was perplexing then. It is now." Wecan observe a number of similarities betweenthe chemical structure of compounds andtheir odors. However, the field of food fla-vors, as is the field of perfumery, is still verymuch an art, albeit one greatly supported byscientists' advancing ability to classify struc-tures and identify the effect of certain molec-ular configurations. The odor potency of va-rious compounds ranges widely. Table 7-11indicates a range of about eight orders ofmagnitude (Teranishi 1971). This indicatesthat volatile flavor compounds may be presentin greatly differing quantities, from traces torelatively large amounts.
The musks are a common illustration ofcompounds with different structures that all
Figure 7-23 Some Furanones (1,2,3), Isomaltol (4), and Maltol (5)
i 2 3
4 5
Table 7-11 Odor Thresholds of CompoundsCovering a Wide Range of Intensity
Odorant Threshold (\ig/L of Water)
Ethanol 100,000Butyric acid 240Nootkatone 170Humulene 160Myrcene 15n-Amyl acetate 5A7-Decanal 0.0a- and p-Sinensal 0.05Methyl mercaptan 0.02p-lonone 0.0072-methoxy-3- 0.002
isobutylpyra-zine
Source: From R. Teranishi, Odor and MolecularStructure, in Gustation and Olfaction, G. Ohloff and A.F.Thomas, eds., 1971, Academic Press.
give similar odors. These may include tricy-clic compounds, macrocyclic ketones andlactones, steroids, nitrocyclohexanes, indanes,tetrahydronaphthalenes, and acetophenoses.Small changes in the structure of these mol-ecules may significantly change in potencybut will not affect quality, since all aremusky. There are also some compounds thathave similar structures and very differentodors, such as nootkatone and related com-pounds (Teranishi 1971). Nootkatone is aflavor compound from grapefruit oil. Thiscompound and 1,10-dihydronootkatone havea grapefruity flavor (Figure 7-24). Severalother related compounds have a woody fla-vor. The odor character of stereoisomersmay be quite different. The case of mentholhas already been described. Only mentholisomers have peppermint aroma. The iso-,neo-, and neoisomenthols have an unpleas-ant musty flavor. Naves (1957) describes the
difference between the cis- and trans- forms of3-hexenol (CH2OH-CH2-CH=CH-CH2CH3).The ds-isomer has a fresh green odor,whereas the frans'-isomer has a scent remi-niscent of chrysanthemum. The 2-trans-6-cisnonadienal smells of cucumber and is quitedifferent from the smell of the 2-trans-6-trans isomer (nonadienal, CHO-CH=CH-(CH2)2-CH=CH-CH2-CH3). Lengtheningof the carbon chain may affect odorousproperties. The odor of saturated acidschanges remarkably as chain length in-creases. The lower fatty acids, especiallybutyric, have very intense and unpleasantflavors, because an increased chain lengthchanges flavor character (Table 7-12) andlessens intensity. The fatty acids with 16 or18 carbon atoms have only a faint flavor.
Another example is given by Kulka (1967).Gamma-nonalactone has a strong coconut-like flavor; y-undecalactone has a peacharoma. As the chain length is increased byone more carbon atom, the flavor characterbecomes peach-musk. The lactones are com-pounds of widely differing structure andodor quality and are found as components ofmany food flavors. Gamma- and 5-lactoneswith 10 to 16 carbon atoms have beenreported (Juriens and OeIe 1965) as flavorcomponents of butter, contributing to the but-ter flavor in concentrations of only parts permillion. The flavor character and chemicalstructure of some y-lactones as reported byTeranishi (1971) are shown in Figure 7-25.One of these, the y-lactone with a total chainlength of 10 carbons, has peach flavor. Thea-hydroxy-p-methyl-y-carboxy-A^-y-hex-eno-lactone occurs in protein hydrolysateand has very strong odor and flavor of beefbouillon. Gold and Wilson (1963) found thatthe volatile flavor compounds of celery con-tain a number of phthalides (phthalides arelactones of phthalic acid, lactones are inter-
nal esters of hydroxy acids). These includethe following:
• 3-isobutyliden-3a,4-dihydrophthalide(Figure 7-26)
• 3-isovalidene-3a,4-dihydrophthalide• 3-isobutylidene phthalide• 3-isovalidene phthalide
These compounds exhibit celery-like odorsat levels of 0.1 ppm in water. Pyrazines havebeen identified as the compounds giving thecharacteristic intense odor of green peppers(Seifert et al. 1970). A number of pyrazinederivatives were tested and, within this singleclass of compounds, odor potencies showed arange of eight orders of magnitude equal tothat of the widely varying compounds listed inTable 7-11. The compounds examined bySeifert et al. (1970) are listed in Table 7-13.2-methoxy-3-isobutylpyrazine appears to bethe compound responsible for the green pep-per odor. Removal of the methoxy- or alkyl-
groups reduces the odor potency by 105 to 106
times, as is the case with 2-methoxypyrazine,2-iosbutylpyrazine, and 2,5-dimethylpyrazine.Thus, small changes in molecular structuremay greatly affect flavor potency. The odorsof isobutyl, propyl, and hexyl methoxypyra-zines are similar to that of green peppers. Theisopropyl compound is moderately similar topeppers and its odor is somewhat similar toraw potato. The ethyl compound is even moresimilar to raw potato and less to pepper. Infact, this compound can be isolated from pota-toes. The methyl compound has an odor likeroasted peanuts. The structure of some of thepyrazines is shown in Figure 7-27. Pyrazineshave been identified as flavor components in anumber of foods that are normally heated dur-ing processing. Rizzi (1967) demonstrated thepresence of seven alkyl-substituted pyrazinesin chocolate aroma. These were isolated bysteam distillation, separated by gas-liquidchromatography, and identified by mass spec-trometry. The components are methyl pyra-
Figure 7-24 Odor Character of Nootkatone and Related Compounds
Uonootkotone Eremophilone
4—Epinootkotone Tetrohydronootk atone 1U2-Dihydronootkotone
WOODY1,10—DihvdronootlcafoneNootkatone
GRAPEFRUITY
Table 7-12 Flavor Character of Some N-Carboxylic Acids
Acid Flavor Character
Formic Acid, pungentAcetic Acid, vinegary, pungentPropionic Acid, pungent, rancid,
cheesyButyric Acid, rancidHexanoic Sweaty, goatyOctanoic RancidDecanoic WaxyLaurie TallowyMyristic Soapy, cardboardPalmitic Soapy
zine; 2,3-dimethylpyrazine; 2-ethyl-5-methyl-pyrazine; trimethylpyrazine; 2,5-dimethyl-3-ethylpyrazine; 2,6-dimethyl-3-ethylpyrazine;and tetramethylpyrazine. Other researchers(Flament et al. 1967; Marion et al. 1967) haveisolated these and other pyrazines from thearoma components of cocoa. Pyrazines arealso aroma constituents of coffee. Goldman etal. (1967) isolated and identified 24 pyra-zines and pyridines and revealed the pres-ence of possibly 10 more. Bondarovich et al.(1967) isolated and identified a large numberof pyrazines from coffee aroma and drew
attention to the importance of pyrazines anddihydropyrazines to the flavor of roasted orotherwise cooked foods. These authors alsodrew attention to the instability of the dihy-dropyrazines. This instability not only makestheir detection and isolation difficult, but mayhelp explain why flavors such as that ofroasted coffee rapidly change with time.Another roasted product from which pyra-zines have been isolated is peanuts. Mason etal. (1966) found methylpyrazine; 2,5-dimeth-ylpyrazine; trimethylpyrazine; methylethyl-pyrazine; and dimethylethylpyrazine in theflavor of roasted peanuts. The pyrazinesappear to be present in unprocessed as well asin heated foods.
Another group of compounds that havebeen related to the aroma of heated foods isthe furanones. Teranishi (1971) summarizedthe findings on several of the furanones (seeFigure 7-23). The 4-hydroxy-2,5-dimethyl-3-dihydrofuranone (1) has a caramel or burntpineapple odor. The 4-hydroxy-5-methyl-3-dihydrofuranone (2) has a roasted chicoryroot odor. Both compounds may contributeto beef broth flavor. The 2,5-dimethyl-3-dihydrofuranone (3) has the odor of freshlybaked bread. Isomaltol (4) and maltol (5) areproducts of the caramelization and pyrolysisof carbohydrates.
Beef bouillon
Figure 7-25 Flavor Character of Some Lactones. Source: From R. Teranishi, Odor and MolecularStructure, in Gustation and Olfaction, G. Ohloff and A.F. Thomas, eds., 1971, Academic Press.
R = 05!-! J1 (coconut)R = CgH13 (peach)R = C7H15 (peach)R = C8H17 (peach-musk)
Figure 7-26 Phthalides of Celery Volatiles
Theories of Olfaction
When an odoriferous compound, or odor-
ivector, arrives at the olfactory organ, a reac-
tion takes place between the odor molecules
and the chemoreceptors; this reaction pro-
Table 7-13 Odor Threshold of Pyrazine and
Derivatives
duces a neural pulse, which eventuallyreaches the brain. The exact nature of theinteraction between odorivector and chemo-receptor is not well known. The number ofolfactory receptors in the smell organs is inthe order of 100 million, and Moncrieff(1951) has calculated that the number ofmolecules at the threshold concentration ofone of the powerful mercaptans in a sniff(about 20 mL) of air would be 1 x 1010 mole-cules. Obviously, only a fraction of thesewould interact with the receptors, butundoubtedly numerous interactions are re-quired to produce a neural response.Dravnieks (1966) has indicated that accord-ing to information theory, 13 types of sensorsare needed to distinguish 10,000 odors on ayes-or-no basis, but more than 20 might berequired to respond rapidly and withouterror. Many attempts have been made to clas-sify odors into a relatively small number ofgroups of related odors. These so-called pri-mary odors have been used in olfaction theo-ries to explain odor quality. One theory, thestereochemical site theory (Amoore et al.1964; Amoore 1967), is based on molecularsize and shape. Amoore compared the vari-ous odor qualities that have been used tocharacterize odors and concluded that sevenprimary odors would suffice to cover themall: camphoraceous, pungent, ethereal, floral,
Compound
2-methoxy-3-hexylpyrazine2-methoxy-3-isobutylpyra-
zine2-methoxy-3-propylpyra-
zine2-methoxy-3-isopropylpyra-
zine2-methoxy-3-ethylpyrazine2-methoxy-3-methylpyra-
zine2-methoxypyrazine2-isobutylpyrazine2-5-dimethylpyrazinepyrazine
Odor Threshold(Parts perl O12
Parts of Water)
12
6
2
4004000
700,000400,000
1,800,000175,000,000
Source: From R.M. Seifert et al., Synthesis of Some2-Methoxy-3-Alkylpyrazines with Strong Bell Pepper-Like Odors, J. Agr. Food Chem., Vol. 18, pp. 246-249,1 970, American Chemical Society.
pepperminty, musky, and putrid. Table 7-14lists some of the chemical compounds thatcan be used to demonstrate these primaryodors. The theory is based on the assumptionthat all odorous compounds have a distinc-tive molecular shape and size that fit into asocket on the receptor site. This would besimilar to the "lock-and-key" concept ofenzyme action. Five of the receptor siteswould accept the flavor compound accordingto shape and size, and two (pungent andputrid) on the basis of electronic status (Fig-ure 7-28). The site-fitting concept as initiallyproposed was inadequate because it assessedonly one-half of the molecule; subsequentrefinements considered all aspects of molec-ular surface in a "shadow-matching" tech-nique (Amoore 1967). It was also suggestedthat there may be more than seven primaries.The primary odors may have to be split intosubgroups and others added as new prima-ries. Molecular model silhouettes as devel-oped for five primary odors are reproducedin Figure 7-29.
A membrane-puncturing theory has beenproposed by Davis (Dravnieks 1967). Ac-cording to this theory, the odorous substancemolecules are adsorbed across the interfaceof the thin lipid membrane, which forms partof the cylindrical wall of the neuron in thechemoreceptor and the aqueous phase thatsurrounds the neuron. Adsorbed moleculesorient themselves with the hydrophilic endtoward the aqueous phase. When theadsorbed molecules are desorbed, they moveinto the aqueous phase, leaving a defect. Ions
may adsorb into this puncture and cause aneural response. This theory could be con-sidered a thermodynamic form of the profilefunctional group concept, since the freeenergy of adsorption of the odor substance atthe interface is related to shape, size, func-tional groups and their distribution, and posi-tion. The adsorption is a dynamic processwith a free energy of adsorption of about 1 to8 kcal/mole for different substances. Daviesprepared a plot of molecular cross-sectionalarea versus free energy of adsorption andobtained a diagram (Figure 7-30) in whichgroups of related odors occupy distinct areas.
The suggestion that odorous character isrelated to vibrational specificity of odor mol-ecules has led to the vibrational theory ofolfaction (Wright 1957). Vibrational energylevels can be derived from the infrared orRaman spectra. The spectral area of greatestinterest is that below 700 cm'1, which isrelated to vibrations of chains and flexing ortwisting of bonds between groups of atomsin the molecule. Wright and others havedemonstrated that correlations exist betweenspectral properties and odor quality in anumber of cases, but inconsistencies in othercases have yet to be explained.
Obviously, none of the many theories ofolfaction proposed so far have been entirelysatisfactory. It might be better to speak ofhypotheses rather than of theories. Most ofthese theories deal with the explanation ofodor quality and do not account for the quan-titative aspects of the mechanism of olfac-tion. The classification of odor and the
A B C
Figure 7-27 (A) Pyrazine, (B) 2-Methoxypyrazine, and (C) 2-Methoxy-3-Hexylpyrazine
correlation of chemical structure and odorremain difficult to resolve.
Odor Description
An odor can be described by the combina-tion of threshold value and odor quality. Thethreshold value, the lowest concentration thatcreates an odor impression, can be consid-ered the intensity factor, whereas the odorquality describes the character of the aroma.As has been mentioned under olfactory theo-ries, attempts at reducing the number ofcharacteristic odor qualities to a small num-ber have not been successful. In many cases,the aroma and flavor of a food can be relatedto the presence of one or a few compoundsthat create an impression of a particular foodwhen smelled alone. Such compounds havebeen named contributory flavor compoundsby Jennings and Sevenants (1964). Somesuch compounds are the pyrazines, whichgive the odor quality of green bell peppers;nootkatone for grapefruit; esters for fruits;
and nona-2-frans-6-a's-dienal for cucum-bers (Forss et al. 1962). In a great number ofother cases, there are no easily recognizablecontributory flavor compounds, but the fla-vor seems to be the integrated impression ofa large number of compounds.
Determining the threshold value is difficultbecause subthreshold levels of one com-pound may affect the threshold levels ofanother. Also, the flavor quality of a com-pound may be different at threshold level andat suprathreshold levels. The total range ofperception can be divided into units that rep-resent the smallest additional amount thatcan be perceived. This amount is called justnoticeable difference (JND). The wholeintensity scale of odor perception coversabout 25 JNDs; this is similar to the numberof JNDs that comprise the scale of tasteintensity. Flavor thresholds for some com-pounds depend on the medium in which thecompound is dispersed or dissolved. Patton(1964) found large differences in the thresh-old values of saturated fatty acids dissolvedin water and in oil.
Table 7-14 Primary Odors for Humans and Compounds Eliciting These Odors
Primary Odor
CamphoraceousPungentEthereal
FloralPepperminty
Musky
Putrid
Odor Compounds
Borneol, terf-butyl alcohol d-camphor, cineol, pentamethyl ethyl alcoholAIIyI alcohol, cyanogen, formaldehyde, formic acid, methylisothiocyanateAcetylene, carbon tetrachloride, chloroform, ethylene dichloride, propyl
alcoholBenzyl acetate, geraniol, a-ionone, phenylethyl alcohol, terpineolte/t-butylcarbinol, cyclohexanone, menthone, piperitol, 1,1,3-trimethyl-
cyclo-5-hexanoneAndrostan-3oc-ol (strong), cyclohexadecanone, ethylene cebacate, 17-
methylandrostan-3a-ol, pentadecanolactoneAmylmercaptan, cadaverine, hydrogen sulfide, indole (when concentrated,
floral when dilute), skatole
Source: From J.E. Amoore et al., The Stereochemical Theory of Odor, Sc/. Am., Vol. 210, No. 2, pp. 42-49, 1964.
PEPPERMINTY ETHEREAL PUTRID
PUNGENT
FLORALMUSKYCAMPHORACEOUS
Figure 7-28 Olfactory Receptor Sites According to the Stereochemical Theory of Odor
Top silhouette Front silhouette Right silhouette
1,2-Dichloroethane(ethereal)
1,8-Cineole(camphoraceous)
15-Hydroxypentadecanoicacid lactone
(musky)
d,/-p-Phenylethyl methylethylcarbinol
(floral)
d,l-Menthone(minty)
Figure 7-29 Molecular Model Silhouettes of Five Standard Odorants. Source: From J. Amoore, Stere-ochemical Theory of Olfaction, in Symposium on Foods: The Chemistry and Physiology of Flavors,H.W. Schultz et al., eds., 1967, AVI Publishing Co.
DESCRIPTION OF FOOD FLAVORS
The flavor impression of a food is influ-enced by compounds that affect both tasteand odor. The analysis and identification ofmany volatile flavor compounds in a largevariety of food products have been assistedby the development of powerful analyticaltechniques. Gas-liquid chromatography waswidely used in the early 1950s when com-mercial instruments became available. Intro-duction of the flame ionization detectorincreased sensitivity by a factor of 100 and,together with mass spectrometers, gave amethod for rapid identification of many com-ponents in complex mixtures. These methodshave been described by Teranishi et al.(1971). As a result, a great deal of informa-tion on volatile flavor components has beenobtained in recent years for a variety of foodproducts. The combination of gas chroma-tography and mass spectrometry can provideidentification and quantitation of flavor com-pounds. However, when the flavor consistsof many compounds, sometimes several hun-
dred, it is impossible to evaluate a flavorfrom this information alone. It is then possi-ble to use pattern recognition techniques tofurther describe the flavor. The pattern rec-ognition method involves the application ofcomputer analysis of complex mixtures ofcompounds. Computer multivariate analysishas been used for the detection of adultera-tion of orange juice (Page 1986) and Spanishsherries (Maarse et al. 1987).
Flavors are often described by using thehuman senses on the basis of widely recog-nized taste and smell sensations. A proposedwine aroma description system is shown inFigure 7-31 (Noble et al. 1987). Such sys-tems attempt to provide an orderly and reli-able basis for comparison of flavor descrip-tions by different tasters.
The aroma is divided into first-, second-,and third-tier terms, with the first-tier termsin the center. Examination of the descriptorsin the aroma wheel shows that they can bedivided into two types, flavors and off-fla-vors. Thus, it would be more useful to dividethe flavor wheel into two tables—one for fla-
Figure 7-30 Plot of Molecular Cross-Sectional Area Versus Free Energy of Adsorption for Davies'Theory of Olfaction
-AG0/w(CALORIES MOLE"')
(J. T. OAVlES )
MO
LECU
LAR
2C
RO
SS-S
ECTI
ON
AL A
REA
(A
.)
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INTRODUCTION
Food texture can be defined as the way inwhich the various constituents and structuralelements are arranged and combined into amicro- and macrostructure and the externalmanifestations of this structure in terms offlow and deformation.
Most of our foods are complex physico-chemical structures and, as a result, the phys-ical properties cover a wide range—fromfluid, Newtonian materials to the most com-plex disperse systems with semisolid charac-ter. There is a direct relationship between thechemical composition of a food, its physi-cal structure, and the resulting physical ormechanical properties; this relationship ispresented in Figure 8-1. Food texture can beevaluated by mechanical tests (instrumentalmethods) or by sensory analysis. In the lattercase, we use the human sense organs as ana-lytical tools. A proper understanding of tex-tural properties often requires study of thephysical structure. This is most often accom-plished by light and electron microscopy, aswell as by several other physical methods.X-ray diffraction analysis provides informa-tion about crystalline structure, differentialscanning calorimetry provides informationabout melting and solidification and otherphase transitions, and particle size analysis
and sedimentation methods provide informa-tion about particle size distribution and parti-cle shape.
In the study of food texture, attention isgiven to two interdependent areas: the flowand deformation properties and the macro-and microstructure. The study of food tex-ture is important for three reasons:
1. to evaluate the resistance of productsagainst mechanical action, such as inmechanical harvesting of fruits andvegetables
2. to determine the flow properties ofproducts during processing, handling,and storage
3. to establish the mechanical behavior ofa food when consumed
There is sometimes a tendency to restricttexture to the third area. The other two areequally important, although the first area isgenerally considered to belong in the domainof agricultural engineering.
Because most foods are complex dispersesystems, there are great difficulties in estab-lishing objective criteria for texture measure-ment. It is also difficult in many cases torelate results obtained by instrumental tech-niques of measurement to the type of re-sponse obtained by sensory panel tests.
Texture
CHAPTER 8
The terms for the textural properties offoods have a long history. Many of the termsare accepted but are often poorly defineddescriptive terms. Following are some exam-ples of such terms:
• Consistency denotes those aspects oftexture that relate to flow and deforma-tion. It can be said to encompass all ofthe rheological properties of a product.
• Hardness has been defined as resistanceto deformation.
• Firmness is essentially identical to hard-ness but is occasionally used to describethe property of a substance able to resistdeformation under its own weight.
• Brittleness is the property of fracturingbefore significant flow has occurred.
• Stickiness is a surface property related tothe adhesion between material and ad-joining surface. When the two surfacesare of identical material, we use the termcohesion.
A variety of other words and expressionsare used to describe textural characteristics,such as body, crisp, greasy, brittle, tender,juicy, mealy, flaky, crunchy, and so forth.Many of these terms have been discussed bySzczesniak (1963) and Sherman (1969);most have no objective physical meaning andcannot be expressed in units of measurementthat are universally applicable. Kokini (1985)has attempted to relate some of these ill-defined terms to the physical propertiesinvolved in their evaluation. Through the
Figure 8-1 Interrelationships in Texture Studies. Source: From P. Sherman, A Texture Profile of Food-stuffs Based upon Well-Defmed Rheological Properties, J. Food ScL, Vol. 34, pp. 458^62, 1969.
PHYSICAL
PROPERTIES
( T E X T U R E )
MECHANICAL TESTS
SENSORY ANALYSIS
MICROSCOPY(LM-TEM-SEM)X-RAY DIFFRACTIONDSC
CHEMICAL
COMPOSITION
PHYSICAL
STRUCTURECHEMICAL ANALYSIS
years, many types of instruments have beendeveloped for measuring certain aspects offood texture. Unfortunately, the instrumentsare often based on empirical procedures, andresults cannot be compared with thoseobtained with other instruments. Recently,instruments have been developed that aremore widely applicable and are based onsound physical and engineering principles.
TEXTURE PROFILE
Texture is an important aspect of foodquality, sometimes even more important thanflavor and color. Szczesniak and Kleyn(1963) conducted a consumer-awarenessstudy of texture and found that texture signif-icantly influences people's image of food.Texture was most important in bland foodsand foods that are crunchy or crisp. Thecharacteristics most often referred to werehardness, cohesiveness, and moisture con-tent. Several attempts have been made todevelop a classification system for texturalcharacteristics. Szczesniak (1963) dividedtextural characteristics into three mainclasses, as follows:
1. mechanical characteristics2. geometrical characteristics3. other characteristics, related mainly to
moisture and fat content
Mechanical characteristics include fivebasic parameters.
1. Hardness—the force necessary toattain a given deformation.
2. Cohesiveness—the strength of theinternal bonds making up the body ofthe product.
3. Viscosity—the rate of flow per unitforce.
4. Elasticity—the rate at which a de-formed material reverts to its unde-formed condition after the deformingforce is removed.
5. Adhesiveness—the work necessary toovercome the attractive forces betweenthe surface of the food and the surfaceof other materials with which the foodcomes in contact (e.g., tongue, teeth,and palate).
In addition, there are in this class the threefollowing secondary parameters:
1. Brittleness—the force with which thematerial fractures. This is related tohardness and cohesiveness. In brittlematerials, cohesiveness is low, andhardness can be either low or high.Brittle materials often create soundeffects when masticated (e.g., toast,carrots, celery).
2. Chewiness—the energy required tomasticate a solid food product to a stateready for swallowing. It is related tohardness, cohesiveness, and elasticity.
3. Gumminess—the energy required todisintegrate a semisolid food to a stateready for swallowing. It is related tohardness and cohesiveness.
Geometrical characteristics include twogeneral groups: those related to size andshape of the particles, and those related toshape and orientation. Names for geometri-cal characteristics include smooth, cellular,fibrous, and so on. The group of other char-acteristics in this system is related to mois-ture and fat content and includes qualitiessuch as moist, oily, and greasy. A summaryof this system is given in Table 8-1.
Based on the Szczesniak system of texturalcharacteristics, Brandt et al. (1963) devel-
oped a method for profiling texture so that asensory evaluation could be given that wouldassess the entire texture of a food. The tex-ture profile method was based on the earlierdevelopment of the flavor profile (Cairncrossand Sjostrom 1950).
The Szczesniak system was critically ex-amined by Sherman (1969), who proposedsome modifications. In the improved system,no distinction is drawn among analytical,geometrical, and mechanical attributes. In-stead, the only criterion is whether a charac-
teristic is a fundamental property or derivedby a combination of two or more attributes inunknown proportions. The Sherman systemcontains three groups of characteristics (Fig-ure 8-2). The primary category includes ana-lytical characteristics from which all otherattributes are derived. The basic rheologicalparameters, elasticity, viscosity, and adhe-sion form the secondary category; theremaining attributes form the tertiary cate-gory since they are a complex mixture ofthese secondary parameters. This system is
Table 8-1 Classification of Textural Characteristics
MECHANICAL CHARACTERISTICS
Primary Parameters
HardnessCohesiveness
ViscosityElasticityAdhesiveness
SecondaryParameters
BrittlenessChewinessGumminess
Popular Terms
Soft -» Firm -> HardCrumbly -» Crunchy -> BrittleTender -» Chewy -> ToughShort -> Mealy -» Pasty -> GummyThin -> ViscousPlastic -> ElasticSticky -> Tacky -> Gooey
GEOMETRICAL CHARACTERISTICS
Class
Particle size and shapeParticle shape and orientation
Examples
Gritty, Grainy, Coarse, etc.Fibrous, Cellular, Crystalline, etc.
OTHER CHARACTERISTICS
Primary Parameters
Moisture contentFat content
SecondaryParameters
OilinessGreasiness
Popular Terms
Dry -> Moist -> Wet -> WateryOilyGreasy
Source: From A.S. Szczesniak, Classification of Textural Characteristics, J. Food Sd., Vol. 28, pp. 385-389, 1963.
Figure 8-2 The Modified Texture Profile. Source: From P. Sherman, A Texture Profile of Foodstuffs Based upon Well-Defined RheologicalProperties, /. Food ScL1 Vol. 34, pp. 458-462, 1969.
Initial perception
Initial perceptionon palate
Mastication(high shearing stress)
Residual masticatoryimpression
Mechanical properties(non-masticatory)
TERTIARY CHARACTERISTICS
SECONDARY CHARACTERISTICS
PRIMARY CHARACTERISTICS
Mechanical properties(mastication)
Disintegration
Visual appearance
Sampling and slicing characteristics
Spreading, creaming characteristics, pourability
Analytical characteristics
Particle size, size distribution; particle shape
Air content, air cell size, size distribution, shape
Elasticity (cohesion)
Viscosity
Adhesion (to palate)
Hard, soft
Brittle, plastic, crisp, rubbery, spongy
Smooth, coarse, powdery, lumpy, pasty
Creamy, watery, soggy
Sticky, tacky
Greasy, gummy, stringy
Melt down properties on palate
interesting because it attempts to relate sen-sory responses with mechanical strain-timetests. Sensory panel responses associatedwith masticatory tertiary characteristics ofthe Sherman texture profile for solid, semi-solid, and liquid foods are given in Figure8-3.
OBJECTIVE MEASUREMENT OFTEXTURE
The objective measurement of texturebelongs in the area of rheology, which is thescience of flow and deformation of matter.Determining the rheological properties of afood does not necessarily mean that the com-plete texture of the product is determined.However, knowledge of some of the rheolog-ical properties of a food may give importantclues as to its acceptability and may beimportant in determining the nature anddesign of processing methods and equip-ment.
Food rheology is mainly concerned withforces and deformations. In addition, time isan important factor; many rheological phe-nomena are time-dependent. Temperature isanother important variable. Many productsshow important changes in rheological be-havior as a result of changes in temperature.In addition to flow and deformation of cohe-sive bodies, food rheology includes suchphenomena as the breakup or rupture of solidmaterials and surface phenomena such asstickiness (adhesion).
Deformation may be of one or both of twotypes, irreversible deformation, called flow,and reversible deformation, called elasticity.The energy used in irreversible deformationis dissipated as heat, and the body is perma-nently deformed. The energy used in revers-ible deformation is recovered upon release of
the deforming stress, when the body regainsits original shape.
Force and Stress
When a force acts externally on a body,several different cases may be distinguished:tension, compression, and shear. Bendinginvolves tension and compression, torqueinvolves shear, and hydrostatic compressioninvolves all three. All other cases mayinvolve one of these three factors or a combi-nation of them. In addition, the weight orinertia of a body may constitute a force lead-ing to deformation. Generally, however, theexternally applied forces are of much greatermagnitude and the effect of weight is usuallyneglected. The forces acting on a body canbe expressed in grams or in pounds. Stress isthe intensity factor of force and is expressedas force per unit area; it is similar to pres-sure. There are several types of stress: com-pressive stress (with the stress componentsdirected at right angles toward the plane onwhich they act); tensile stress (in which thestress components are directed away fromthe plane on which they act); and shearingstress (in which the stress components acttangentially to the plane on which they act).A uniaxial stress is usually designated by thesymbol a, a shearing stress by T. Shear stressis expressed in dynes/cm2 when using themetric system of measurement; in the SI sys-tem it is expressed in N/m2 or pascal (P).
Deformation and Strain
When the dimensions of a body change,we speak of deformation. Deformation canbe linear, as in a tensile test when a body oforiginal length L is subjected to a tensilestress. The linear deformation AL can then beexpressed as strain e = AL/L. Strain can be
Figure 8-3 Panel Responses Associated with Masticatory Tertiary Characteristics of the Modified Texture Profile
Thin, watery, viscousCreamy, fatty, greasySticky
Pasty, crumbly, coherentMoist, dry, sticky, soggyLumpy, smooth
Rubbery, spongy, tender, plasticMoist, dry, sticky, soggySmooth, coarse
Crisp, brittle, powderyMoist, dry, stickyTough, tender
Chocolate, cookies, frozen ice cream,frozen water ices, hard vegetables,hard fruit, corn flakes, potato crisps
Meat, cheese, bread, cake,margarine, butter, gels, JeII-O,puddings
Processed cheese, yogurt, cakebatters, mashed potato, sausage meat,jam, high-fat content cream, syntheticcream
Thawed ice cream and water ices,mayonnaise, salad dressings, sauces,fruit drinks, soups
Hard
Soft
Solid
Semisolid
Fluid
Mechanicalproperties(masticatory)TERTIARY
CHARACTERISTICS
expressed as a ratio or percent; inches perinch or centimeters per centimeter. In addi-tion to linear deformations, there are othertypes of deformation, such as in a hydrostatictest where there will be a volumetric strainAV/tf
For certain materials the deformationresulting from an applied force can be verylarge; this indicates the material is a liquid.In such cases, we deal with rate of deforma-tion, or shear rate; dy/dt or y. This is thevelocity difference per unit thickness of theliquid. Y is expressed in units of s"1.
Viscosity
Consider a liquid contained between twoparallel plates, each of area A cm2 (Figure8-^4). The plates are h cm apart and a force ofP dynes is applied on the upper plate. Thisshearing stress causes it to move with respectto the lower plate with a velocity of v cm s"1.The shearing stress T acts throughout the liq-uid contained between the plates and can bedefined as the shearing force P divided bythe area A, or PIA dynes/cm2. The deforma-tion can be expressed as the mean rate ofshear y or velocity gradient and is equal tothe velocity difference divided by the dis-tance between the plates y = v/h, expressedin units of s"1.
The relationship between shearing stressand rate of shear can be used to define theflow properties of materials. In the simplestcase, the shearing stress is directly propor-tional to the mean rate of shear T = r|y (Fig-ure 8-5). The proportionality constant T| iscalled the viscosity coefficient, or dynamicviscosity, or simply the viscosity of the liq-uid. The metric unit of viscosity is the dyne.scm"2, or Poise (P). The commonly used unit is100 times smaller and called centiPoise (cP).In the SI system, T| is expressed in N.s/m2. or
Pa.s. Therefore, 1 Pa.s = 10 P = 1000 cP.Some instruments measure kinematic viscos-ity, which is equal to dynamic viscosity xdensity and is expressed in units of Stokes.The viscosity of water at room temperature isabout 1 cP. Mohsenin (1970) has listed theviscosities of some foods; these, as well astheir SI equivalents, are given in Table 8-2.
Materials that exhibit a direct proportional-ity between shearing stress and rate of shearare called Newtonian materials. These in-clude water and aqueous solutions, simpleorganic liquids, and dilute suspensions andemulsions. Most foods are non-Newtonian incharacter, and their shearing stress-rate-of-shear curves are either not straight or do notgo through the origin, or both. This intro-duces a considerable difficulty, because theirflow behavior cannot be expressed by a sin-gle value, as is the case for Newtonian liq-uids.
The ratio of shearing stress and rate ofshear in such materials is not a constantvalue, so the value is designated apparentviscosity. To be useful, a reported value forapparent viscosity of a non-Newtonian mate-rial should be given together with the valueof rate of shear or shearing stress used in thedetermination. The relationship of shearingstress and rate of shear of non-Newtonianmaterials such as the dilatant and pseudo-plastic bodies of Figure 8-5 can be repre-sented by a power law as follows:
T = AY
Figure 8-4 Flow Between Parallel Plates
Figure 8-5 Shearing Stress-Rate of Shear Dia-grams. (A) Newtonian liquid, viscous flow, (B)dilatant flow, (C) pseudoplastic flow, (D) plasticflow.
where A and n are constants. A is the consis-tency index or apparent viscosity and n is theflow behavior index. The exponent is n = 1for Newtonian liquids; for dilatant materials,it is greater than 1; and for pseudoplastic
Table 8-2 Viscosity Coefficients of Some Foods
materials, it is less than 1. In its logarithmicform,
log T = log A + n log *Y
A plot of log T versus log y will yield astraight line with a slope of n.
For non-Newtonian materials that have ayield stress, the Casson or Hershel-Bulkleymodels can be used. The Casson model isrepresented by the equation,
*fc = J^ + A^j
where T0 = yield stress.This model has been found useful for sev-
eral food products, especially chocolate(Kleinert 1976).
The Hershel-Bulkley model describesmaterial with a yield stress and a linear rela-tionship between log shear stress and logshear rate:
T = TQ + AY"
S h e a r i n g S t r e s s
Rate
of S
hear DC
A
B
Viscosity
Product
WaterWaterSkim milkMilk, wholeMilk, wholeCream (20% fat)Cream (30% fat)Soybean oilSucrose solution (60%)Olive oilCottonseed oilMolasses
Temperature (0C)
O2025O
2044
3021301621
(CP)
1.791.001.374.282.126.20
13.7840.660.284.091.0
6600.0
(Pa-S)0.001790.001000.001370.004280.002120.006200.013780.04060.06020.08400.09106.600
Source: Reprinted with permission from N. N. Mohsenin, Physical Properties of Plant and Animal Materials, Vol. 1,Structure, Physical Characteristics and Mechanical Properties, © 1970, Gordon and Breach Science Publisher.
The value of n indicates how close the lin-ear plot of shear stress and shear rate is tobeing a straight line.
Principles of Measurement
For Newtonian fluids, it is sufficient tomeasure the ratio of shearing stress and rateof shear from which the viscosity can be cal-culated. This can be done in a viscometer,which can be one of various types, includingcapillary, rotational, falling ball, and so on.For non-Newtonian materials, such as thedilatant, pseudoplastic, and plastic bodiesshown in Figure 8-5, the problem is moredifficult. With non-Newtonian materials,several methods of measurement involve theratio of shear stress and rate of shear, therelationship of stress to time under constantstrain (relaxation), and the relationship ofstrain to time under constant stress (creep).In relaxation measurements, a material issubjected to a sudden deformation er,, whichis held constant. In many materials, the stresswill decay with time according to the curveof Figure 8-6. The point at which the stresshas decayed to G/e, or 36.7 percent of theoriginal value of C0, is called the relaxationtime. When the strain is removed at time T,the stress returns to zero. In a creep experi-ment, a material is subjected to the instanta-neous application of a constant load or stressand the strain measured as a function of time.The resulting creep curve has the shape indi-cated in Figure 8-7. At time zero, the appliedload results in a strain E0, which increaseswith time. When the load is removed at timeT, the strain immediately decreases, as indi-cated by the vertical straight portion of thecurve at T\ the strain continues to decreasethereafter with time. In many materials, thevalue of 8 never reaches zero, and we know,therefore, a permanent deformation ep has
Figure 8-6 Relaxation Curve (Relationship ofStress to Time under Constant Strain)
resulted. The ratio of strain to applied stressin a creep experiment is a function of timeand is called the creep compliance (J). Creepexperiments are sometimes plotted as graphsrelating / to time.
DIFFERENT TYPES OF BODIES
The Elastic Body
For certain solid bodies, the relationshipbetween stress and strain is represented by astraight line through the origin (Figure 8-8)
Figure 8-7 Creep Curve (Relationship of Strainto Time under Constant Stress)
up to the so-called limit of elasticity, accord-ing to the law of Hooke, a = Ez. The propor-tionality factor E for uniaxial stress is calledmodulus of elasticity, or Young's modulus.For a shear stress, the modulus is G, or Cou-lomb modulus. Note that a modulus is theratio of stress to strain, E = a/8. The behaviorof a Hookean body is further exemplified bythe stress-time and strain-time curves of Fig-ure 8-9. When a Hookean body is subjectedto a constant strain er;, the stress a willremain constant with time and will return tozero when the strain is removed at time T.The strain E will follow the same patternwhen a constant stress is applied andreleased at time T.
The Retarded Elastic Body
In bodies showing retarded elasticity, thedeformation is a function of time as well asstress. Such a stress-strain curve is shown inFigure 8-10. The upward part of the curverepresents increasing values of stress; whenthe stress is reduced, the correspondingstrains are greater on the downward part ofthe curve. When the stress reaches O, thestrain has a finite value, which will slowlyreturn to zero. There is no permanent defor-mation. The corresponding relaxation(stress-time) and creep (strain-time) curves
Figure 8-8 Stress-Strain Curve for a PerfectlyElastic Body
for this type of body are given in Figure8-11.
The Viscous Body
A viscous or Newtonian liquid is oneshowing a direct proportionality betweenstress and rate of shear, as indicated by curveA in Figure 8-5.
The Viscoelastic Body
Certain bodies combine the properties ofboth viscous and elastic materials. The elas-
Figure 8-9 (A) Stress-Time and (B) Strain-Time Curves of a Hookean Body
BA
Figure 8-10 Stress-Strain Curve of a RetardedElastic Body
tic component can be partially retarded elas-ticity. Viscoelastic bodies may flow slowlyand nonreversibly under the influence of asmall stress. Under larger stresses the elasticcomponent becomes apparent. The relax-ation curve of viscoelastic materials has theshape indicated in Figure 8-12A. The curvehas the tendency to approach the time axis.The creep curve indicates that the strainincreases for as long as the stress is applied(Figure 8-12B). The magnitude of the per-manent deformation of the body increaseswith the applied stress and with the length ofapplication.
Mechanical models can be used to visual-ize the behavior of different bodies. Thus, aspring denotes a Hookean body, and a dash-pot denotes a purely viscous body or Newto-nian fluid. These elements can be combinedin a variety of ways to represent the rheolog-ical behavior of complex substances. Twobasic viscoelastic models are the Voigt-KeIvin and the Maxwell bodies. The Voigt-Kelvin model employs a spring and dashpotin parallel, the Maxwell model a spring anddashpot in series (Figure 8-13). In the Voigt-Kelvin body, the stress is the sum of twocomponents where one is proportional to thestrain and the other to the rate of shear.Because the elements are in parallel, theymust move together. In the Maxwell modelthe deformation is composed of two parts—one purely viscous, the other purely elastic.Although both the Voigt-Kelvin and Max-well bodies represent viscoelasticity, theyreact differently in relaxation and creepexperiments. When a constant load is appliedin a creep test to a Voigt-Kelvin model, afinal steady-state deformation is obtainedbecause the compressed spring element re-sists further movement. The Maxwell modelwill give continuing flow under these condi-tions because the viscous element is not lim-ited by the spring element. When the load isremoved, the Voigt-Kelvin model recovers
BA
Figure 8-11 (A) Stress-Time and (B) Strain-Time Curves of a Retarded Elastic Body
completely, but not instantaneously. TheMaxwell body does not recover completelybut, rather, instantly. The Voigt-Kelvin body,therefore, shows no stress relaxation but theMaxwell body does. A variety of models canbe constructed to represent the rheologicalbehavior of viscoelastic materials. By plac-ing a number of Kelvin models in series, aso-called generalized Kelvin model is ob-tained. Similarly, a generalized Maxwellmodel is obtained by placing a number ofMaxwell models in parallel. The combina-tion of a Kelvin and a Maxwell model in
series (Figure 8-13C) is called a Burgersmodel.
For ideal viscoelastic materials, the initialelastic deformation at the time the load isapplied should equal the instantaneous elas-tic deformation when the load is removed(Figure 8-14). For most food products, thisis not the case. As is shown by the exampleof butter in Figure 8-14, the initial deforma-tion is greater than the elastic recovery attime t. This may result from the fact thatthese foods are plastic as well as viscoelastic,which means they have a yield value. There-
Figure 8-12 (A) Stress-Time and (B) Strain-Time Curves of a Viscoelastic Body
A B
Figure 8-13 (A) Voigt-Kelvin, (B) Maxwell, and (C) Burgers Models
A B C
fore, the initial deformation consists of bothan instantaneous elastic deformation and apermanent deformation (viscous flow com-ponent). It has also been found (deMan et al.1985) that the magnitude of the instanta-neous elastic recovery in fat products is timedependent and decreases as the time of appli-cation of the load increases. It appears thatthe fat crystal network gradually collapses asthe load remains on the sample.
The Plastic Body
A plastic material is defined as one thatdoes not undergo a permanent deformationuntil a certain yield stress has been exceeded.A perfectly plastic body showing no elastic-ity would have the stress-strain behaviordepicted in Figure 8-15. Under influence ofa small stress, no deformation occurs; whenthe stress is increased, the material will sud-denly start to flow at applied stress C0 (theyield stress). The material will then continueto flow at the same stress until this isremoved; the material retains its total defor-mation. In reality, few bodies are perfectlyplastic; rather, they are plasto-elastic orplasto-viscoelastic. The mechanical modelused to represent a plastic body, also called aSt. Venant body, is a friction element. The
model is analogous to a block of solid mate-rial that rests on a flat horizontal surface. Theblock will not move when a force is appliedto it until the force exceeds the friction exist-ing between block and surface. The modelsfor ideal plastic and plasto-elastic bodies areshown in Figure 8-16A and 8-16B.
A more common body is the plasto-vis-coelastic, or Bingham body. Its mechanicalmodel is shown in Figure 8-16C. When astress is applied that is below the yield stress,the Bingham body reacts as an elastic body.At stress values beyond the yield stress, thereare two components, one of which is con-stant and is represented by the friction ele-
Figure 8-14 (A) Creep Curve for an Ideal Viscoelastic Body and (B) Creep Curve for Butter
TIME TIME min
LOAD REMOVEDA B
STRA
IN-
G
DISP
LACE
MENT
mm
Figure 8-15 Stress-Strain Curve of an IdealPlastic Body
ment, and the other, which is proportional tothe shear rate and represents the viscous flowelement. In a creep experiment with stressnot exceeding the yield stress, the creepcurve would be similar to the one for aHookean body (Figure 8-9B). When theshear stress is greater than the yield stress,the strain increases with time, similar to thebehavior of a Maxwell body (Figure 8-17).Upon removal of the stress at time T, thestrain decreases instantaneously and remainsconstant thereafter. The decrease representsthe elastic component; the plastic deforma-tion is permanent. The relationship of rate ofshear and shear stress of a Bingham bodywould have the form shown in Figure8-18A. When flow occurs, the relationshipbetween shearing stress and rate of shear isgiven by
G-G0=UD
whereC0 = yield stressU = proportionality constantD = mean rate of shear
The constant U can be named plastic viscos-ity and its reciprocal I/U is referred to asmobility.
In reality, plastic materials are more likelyto have a curve similar to the one in Figure8-18B. The yield stress or yield value can betaken at three different points—the loweryield value at the point where the curve startson the stress axis; the upper yield value
Figure 8-16 Mechanical Models for a Plastic Body. (A) St. Venant body, (B) plasto-elastic body, and(C) plasto-viscoelastic or Bingham body.
A B C
Figure 8-17 Creep Curve of a Bingham BodySubjected to a Stress Greater Than the YieldStress
Figure 8-18 Rate-of-Shear-Shear Stress Dia-grams of Bingham Bodies. (A) Ideal case, and(B) practical case. The yield values are as fol-lows: lower yield value (1), upper yield value (2),and Bingham yield value (3).
where the curve becomes straight; and theBingham yield value, which is found byextrapolating the straight portion of the curveto the stress axis.
The Thixotropic Body
Thixotropy can be defined as an isother-mal, reversible, sol-gel transformation and isa behavior common to many foods. Thixot-ropy is an effect brought about by mechani-cal action, and it results in a lowered ap-parent viscosity. When the body is allowedsufficient time, the apparent viscosity willreturn to its original value. Such behaviorwould result in a shear stress-rate-of-sheardiagram, as given in Figure 8-19. Increasingshear rate results in increased shear stressup to a maximum; after the maximum isreached, decreasing shear rates will result insubstantially lower shear stress.
Dynamic Behavior
Viscoelastic materials are often character-ized by their dynamic behavior. Because vis-
coelastic materials are subject to structuralbreakdown when subjected to large strains, itis useful to analyze them by small amplitudesinusoidal strain. The relationship of stressand strain under these conditions can be eval-uated from Figure 8-20 (Bell 1989). Theapplied stress is alternating at a selected fre-quency and is expressed in cycles s"1, or co inradians s"1. The response of a purely elasticmaterial will show a stress and strain re-sponse that is in phase, the phase angle 8 =0°. A purely viscous material will show thestress being out of phase by 90°, and a vis-coelastic material shows intermediate behav-ior, with 8 between 0° and 90°. The visco-elastic dynamic response is composed of anin-phase component (sin cot) and an out-of-phase component (cos cot). The energy usedfor the viscous component is lost as heat; thatused for the elastic component is retained asstored energy. This results in two moduli, thestorage modulus (G') and the loss modulus(G"). The ratio of the two moduli is knownas tan 8 and is given by tan 8 = G"/G'.
A B
Figure 8-19 Shear Stress-Rate-of-Shear Dia-gram of a Thixotropic Body. Source: From J.M.deMan amd F.W. Wood, Hardness of Butter. II.Influence of Setting, J. Dairy ScL Vol. 42, pp.56-61, 1959.
Figure 8-20 Dynamic (Oscillation) Measurement of Viscoelastic Materials. As an oscillating strain isapplied, the resulting stress values are recorded. 8 is the phase angle and its value indicates whether thematerial is viscous, elastic, or viscoelastic. Source: Reprinted from A.E. Bell, Gel Structure and FoodBiopolymers, in Water and Food Quality, TM. Hardman, ed., p. 253, © 1989, Aspen Publishers, Inc.
Strain
Time
Stress
Stress
Time
Strain
VIS
CO
US
VIS
CO
ELA
ST
ICE
LAS
TIC
Strain
Time
Stress
APPLICATION TO FOODS
Many of the Theological properties of com-plex biological materials are time-dependent,and Mohsenin (1970) has suggested thatmany foods can be regarded as viscoelasticmaterials. Many foods are disperse systemsof interacting nonspherical particles andshow thixotropic behavior. Such particlesmay interact to form a three-dimensional net-work that imparts rigidity to the system. Theinteraction may be the result of ionic forces inaqueous systems or of hydrophobic or vander Waals interactions in systems that containfat crystals in liquid oil (e.g., butter, marga-rine, and shortening). Mechanical action,such as agitation, kneading, or working re-sults in disruption of the network structureand a corresponding loss in hardness. Whenthe system is then left undisturbed, the bonds
between particles will reform and hardnesswill increase with time until maximum hard-ness is reached. The nature of thixotropy wasdemonstrated with butter by deMan andWood (1959). Hardness of freshly workedbutter was determined over a period of threeweeks (Figure 8-21). The same butter wasfrozen and removed from frozen storage afterthree weeks. No thixotropic change hadoccurred with the frozen sample. The freez-ing had completely immobilized the crystalparticles. Thixotropy is important in manyfood products; great care must be exercisedthat measurements are not influenced by thix-otropic changes.
The viscosity of Newtonian liquids can bemeasured simply, by one-point determina-tions with viscometers, such as rotational,capillary, or falling ball viscometers. Fornon-Newtonian materials, measurement of
HAR
DN
ESS
kg
./4
cm2
DAYS
Figure 8-21 Thixotropic Hardness Change in Butter. (A) Freshly worked butter left undisturbed forfour weeks at 50C. (B) The same butter stored at -2O0C for three weeks then left at 50C. (C) The samebutter left at 50C for three weeks, then frozen for three weeks and again placed at 50C. Source: FromJ.M. deMan and KW. Wood, Hardness of Butter. II. Influence of Setting, J. Dairy ScL, Vol. 42, pp.56-61, 1959.
rheological properties is more difficult be-cause single-point determinations (i.e., atone single shearing stress) will yield no use-ful information. We can visualize the rate ofshear dependence of Newtonian fluids byconsidering a diagram of two fluids, asshown in Figure 8-22 (Sherman 1973). Thebehavior of these fluids is represented by twostraight lines parallel to the shear-rate axis.With non-Newtonian fluids, a situation asshown in Figure 8-23 may arise. The fluids 3and 4 have curves that intersect. Below thispoint of intersection, fluid 4 will appearmore viscous; beyond the intersection, fluid3 will appear more viscous. Fluids 5 and 6 donot intersect and the problem does not arise.In spite of the possibility of such problems,many practical applications of rheologicalmeasurements of non-Newtonian fluids arecarried out at only one rate of shear. Notethat results obtained in this way should beinterpreted with caution. Shoemaker et al.
(1987) have given an overview of the appli-cation of rheological techniques for foods.
Probably the most widely used type of vis-cometer in the food industry is the Brookfieldrotational viscometer. An example of thisinstrument's application to a non-Newtonianfood product is given in the work of Sarava-cos and Moyer (1967) on fruit purees. Vis-cometer scale readings were plotted againstrotational speed on a logarithmic scale, andthe slope of the straight line obtained wastaken as the exponent n in the followingequation for pseudoplastic materials:
T = K Y "
whereT = shearing stress (dyne/cm2)K = constantY = shear rate (s"1)
The instrument readings were converted intoshear stress by using an oil of known viscos-
Figure 8-23 Rate of Shear Dependence of theApparent Viscosity of Several Non-NewtonianFluids. Source: From P. Sherman, Structure andTextural Properties of Foods, in Texture Measur-ment of Foods, A. Kramer and A.S. Szczesniak,eds., 1973, D. Reidel Publishing Co.
RATE OF SHEAR (SEC^)
VlSC
OSlT
Y(T)
)
VISC
OSlT
Y(T)
)
RATE OF SHEAR (SEC'1)
Figure 8-22 Rate of Shear Dependence of theViscosity of Two Newtonian Fluids. Source:From P. Sherman, Structure and Textural Prop-erties of Foods, in Texture Measurement ofFoods, A. Kramer and A.S. Szczesniak, eds.,1973, D. Reidel Publishing Co.
FLUlDl
FLUID2
ity. The shear rate at a given rotational speedN was calculated from
Y = 4nN/n
When shear stress T was plotted against shearrate Y on a double logarithmic scale, theintercept of the straight line on the T axis at Y= 1 s"1 was taken as the value of the constantK. The apparent viscosity |j,app at a givenshear rate was then calculated from the equa-tion
^app=^Yn"
Apparent viscosities of fruit purees deter-mined in this manner are shown in Figure8-24.
Factors have been reported in the literature(Johnston and Brower 1966) for the conver-sion of Brookfield viscometer scale readingsto yield value or viscosity. Saravacos (1968)has also used capillary viscometers for rheo-logical measurements of fruit purees.
For products not sufficiently fluid to bestudied with viscometers, a variety of tex-ture-measuring devices is available. Theserange from simple penetrometers such as theMagness-Taylor fruit pressure tester to com-plex universal testing machines such as theInstron. All these instruments either apply aknown and constant stress and measuredeformation or cause a constant deformationand measure stress. Some of the moresophisticated instruments can do both. In theInstron Universal Testing Machine, thecrosshead moves at a speed that can beselected by changing gears. The drive is byrotating screws, and the force measurementis done with load cells. Mohsenin (1970) andcoworkers have developed a type of universaltesting machine in which the movement isachieved by air pressure. The Kramer shear
press uses a hydraulic system for movementof the crosshead.
Texture-measuring instruments can beclassified according to their use of penetra-tion, compression, shear, or flow.
Penetrometers come in a variety of types.One of the most widely used is the Precisionpenetrometer, which is used for measuringconsistency of fats. The procedure and conedimensions are standardized and described inthe Official and Tentative Methods of theAmerican Oil Chemists' Society. Accordingto this method, the results are expressed inmm/10 of penetration depth. Haighton(1959) proposed the following formula forthe conversion of depth of penetration intoyield value:
C-KWIp1'6
whereC = yield valueK = constant depending on the angle of
the conep = penetration depthW = weight of cone
Vasic and deMan (1968) suggested conver-sion of the depth of penetration readings intohardness by using the formula
H=GfA
whereH = hardnessG = total weight of cone assemblyA = area of impression
The advantage of this conversion is thatchanges in hardness are more uniform thanchanges in penetration depth. With the latter,a difference of an equal number of units at
the tip of the cone and higher up on the coneis not at all comparable.
Many penetrometers use punches of vari-ous shapes and sizes as penetrating bodies.Little was known about the relationshipbetween shape and size and penetrating forceuntil Bourne's (1966) work. He postulatedthat when a punch penetrates a food, bothcompression and shear occur. Shear, in thiscase, is defined as the movement of inter-faces in opposite directions. Bourne sug-gested that compression is proportional tothe area under the punch and to the compres-sive strength of the food and also that the
shear force is proportional to the perimeter ofthe punch and to the shear strength of thefood (Figure 8-25). The following equationwas suggested:
F = KCA + KSP+C
whereF = measured forceKc = compression coefficient of tested foodKx = shear coefficient of tested foodA = area of punchP = perimeter of punchC = constant
SHEAR RATE. SEC-*
Figure 8-24 Apparent Viscosities of Fruit Purees Determined at 860C. Source: From G.D. Saravacosand J.C. Moyer, Heating Rates of Fruit Products in an Agitated Kettle, Food Technol, Vol. 21, pp.372-376, 1967.
APP
AR
ENT
VIS
CO
SIT
Y,
CENT
IPOI
SES
APPLE SAUCE
PEACHAPRICOT
PEAR
PLUM
The relationship between penetration forceand cross-sectional area of cylindricalpunches has been established by Kamel anddeMan (1975).
Bourne did show that, for a variety offoods, the relationships between punch areaand force and between punch perimeter andforce were represented by straight lines.DeMan (1969) later showed that for certainproducts, such as butter and margarine, thepenetrating force was dependent only on areaand was not influenced by perimeter. deMansuggested that in such products flow is theonly factor affecting force readings. It ap-pears that useful conclusions can be drawnregarding the textural characteristics of afood by using penetration tests.
A variation on the penetration method isthe back extrusion technique, where the sam-ple is contained in a cylinder and the pene-trating body leaves only a small annular gap
for the product to flow. The application ofthe back extrusion method to non-Newtonianfluids has been described by Steffe and Oso-rio (1987).
Many instruments combine shear and com-pression testing. One of the most widelyused is the Kramer shear press. Based on theprinciple of the shear cell used in the pea ten-derometer, the shear press was designed tobe a versatile and widely applicable instru-ment for texture measurement of a variety ofproducts. The shear press is essentially ahydraulically driven piston, to which thestandard 10-blade shear cell or a variety ofother specialized devices can be attached.Force measurement is achieved either by adirect reading proving ring or by an elec-tronic recording device. The results obtainedwith the shear press are influenced by theweight of the sample and the speed of thecrosshead. These factors have been exhaus-
SHEARoc PERIMETER
COMPRESSIONocAREA
Figure 8-25 Compression and Shear Components in Penetration Tests. Source: From M.C. Bourne,Measure of Shear and Compression Components of Puncture Tests, J. Food Sd., Vol. 31, pp.282-291, 1966.
tively studied by Szczesniak et al. (1970).The relationship between maximum forcevalues and sample weight was found to bedifferent for different foods. Products fittedinto three categories—those having a con-stant force-to-weight ratio (e.g., white bread,sponge cake); those having a continuouslydecreasing force-to-weight ratio (e.g., rawapples, cooked white beans); and those giv-ing a constant force, independent of sampleweight beyond a certain fill level (e.g.,canned beets, canned and frozen peas). Thisis demonstrated by the curves of Figure8-26. Some of the attachments to the shearpress are the succulometer cell, the single-blade meat shear cell, and the compressioncell.
Based on the Szczesniak classification oftextural characteristics, a new instrument wasdeveloped in the General Foods ResearchLaboratories; it is called the General FoodsTexturometer. This device is an improvedversion of the MIT denture tenderometer(Proctor et al. 1956). From the reciprocatingmotion of a deforming body on the sample,which is contained in a tray provided withstrain gages, a force record called a textureprofile curve (Figure 8-27) is obtained. Fromthis texturometer curve, a variety of rheologi-cal parameters can be obtained. Hardness ismeasured from the height of the first peak.Cohesiveness is expressed as the ratio of theareas under the second and first peaks. Elas-ticity is measured as the difference between
SAMPLE WEIGHT
Figure 8-26 Effect of Sample Weight on Maximum Force Registered with the Shear Press and Usingthe 10-Blade Standard Cell. (1) White bread and sponge cake, (2) raw apples and cooked white beans,(3) canned beets and peas and frozen peas. Source: From A.S. Szczesniak, Instrumental Methods ofTexture Measurements, in Texture Measurement of Foods, A. Kramer and A.S. Szczesniak, eds., 1973,D. Reidel Publishing Co.
MA
XIM
UM
FO
RC
E
distance J3, measured from initial sample con-tact to sample contact on the second "chew,"and the same distance (distance B) measuredwith a completely inelastic material such asclay. Adhesiveness is measured as the area ofthe negative peak A3 beneath the baseline. Inaddition, other parameters can be derivedfrom the curve such as brittleness, chewiness,and gumminess.
TEXTURAL PROPERTIES OF SOMEFOODS
Meat Texture
Meat texture is usually described in termsof tenderness or the lack of it—toughness.This obviously is related to the ease withwhich a piece of meat can be cut with a knifeor with the teeth. The oldest and most widely
used device for measuring meat tenderness isthe Warner-Bratzler shear device (Bratzler1932). In this device, a cylindrical core ofcooked meat is subjected to the shearingaction of a steel blade and the maximumforce is indicated by a springloaded mecha-nism. A considerable improvement was theshear apparatus described by Voisey andHansen (1967). In this apparatus, the shear-ing force is sensed by a strain gage trans-ducer and a complete shear-force time curveis recorded on a strip chart. The Warner-Bratzler shear method has several disadvan-tages. It is very difficult to obtain uniformmeat cores. Cores from different positions inone cut of meat may vary in tenderness, andcooking method may affect tenderness.
Meat tenderness has been measured withthe shear press. This can be done with the10-blade universal cell or with the single-
COHESIVENESS « - ^AI
SPRINGINESS = C-B
C-TIME CONSTANT FOR CLAY
HARDNESS
ADHESIVENESS«A3
Figure 8-27 Typical Texturometer Curve
blade meat shear attachment. There is nostandard procedure for measuring meat ten-derness with the shear press; sample size,sample preparation, and rate of shear are fac-tors that may affect the results.
A pressure method for measuring meattenderness has been described by Sperring etal. (1959). A sample of raw meat is con-tained in a cylinder that has a small hole inits bottom. A hydraulic press forces aplunger into the cylinder, and the pressurerequired to squeeze the meat through thehole is taken as a measure of tenderness.
A portable rotating knife tenderometer hasbeen described by Bjorksten et al. (1967). Arotating blunt knife is forced into the meatsample, and a tracing of the area traversed bya recording pen is used as a measure of ten-derness.
A meat grinder technique for measuringmeat tenderness was reported by Miyada andTappel (1956); in this method, power con-sumption of the meat grinder motor was usedas a measure of meat tenderness. The elec-tronic recording food grinder described byVoisey and deMan (1970) measures thetorque exerted on a strain gage transducer.This apparatus has been used successfullyfor measuring meat tenderness.
Other methods used for meat tendernessevaluation have included measurement ofsarcomere length (Howard and Judge 1968)and determination of the amount of connec-tive tissue present.
Stoner et al. (1974) have proposed amechanical model for postmortem striatedmuscle; it is shown in Figure 8-28. Themodel is a combination of the Voigt modelwith a four-element viscoelastic model. Theformer includes a contractile element (CE),which is the force generator. The element SEis a spring that is passively elongated by theshortening of the CE and thus develops an
internal force. The parallel elastic compo-nent (PE) contributes to the resting tension ofthe muscle. The combination of elements PE,CE, and SE represents the purely elasticproperties of the muscle as the fourth compo-nent of a four-element model (of which E2,T|3, and t|2 are the other three components).
Dough
The rheological properties of dough areimportant in determining the baking qualityof flour. For many years the Farinograph wasused to measure the physical properties ofdough. The Farinograph is a dough mixerhooked up to a dynamometer for recording
Figure 8-28 Mechanical Model for PostmortemStriated Muscle. Source: From C.W. BrabenderInstruments, Inc., South Hackensack, New Jer-sey.
torque. The instrument can measure waterabsorption of flour. A typical Farinographcurve is presented in Figure 8-29. Theamount of water required to bring the middleof the curve to 500 units, added from a buret,is a measure of the water absorption of thedough. The measurement from zero to thepoint where the top of the curve first inter-sects the 500 line on the chart is calledarrival time; the measurement from zero tomaximum consistency is called peak time;the point where the top of the curve leavesthe 500 line is called departure time; the dif-ference between departure and arrival time isstability. The elasticity of dough is measuredwith the extensigraph, which records theforce required to stretch a piece of dough ofstandard dimensions.
The peculiar viscoelastic properties ofwheat dough are the result of the presence ofa three-dimensional network of gluten pro-teins. The network is formed by thiol-disul-fide exchange reactions among gluten pro-teins. Peptide disulfides can interfere in athiol-disulfide exchange system by reactingwith a protein (PR)-thiol to liberate a peptide(R)-thiol and form a mixed disulfide, as fol-lows:
PR-SH + R-SS-R -> R-H + PR-SS-R
Disulfide bonds between proteins have anenergy of 49 kcal/mole and are not broken atroom temperature except as the result of achemical reaction. The effects of oxidizingagents on the rheological properties of dough
Figure 8-29 Typical Farinograph Curve. (A) Arrival time, (B) peak time, (C) stability, (D) departure,(E) mixing tolerance index, (F) 20-minute drop. Source: From H.P. Wehrli and Y. Pomeranz, The Roleof Chemical Bonds in Dough, Baker's Digest, Vol. 43, no. 6, pp. 22-26, 1969.
MINUTES
may be quantitatively explained as the break-ing of disulfide cross-links; their reformationmay be explained as exchange reactions withsulfhydryl groups (Wehrli and Pomeranz1969). The baking quality of wheat is strong-ly influenced by protein content and the dis-ulfide/sulfhydryl ratio. A schematic diagramof the bonds within and between polypeptidechains in dough is given in Figure 8-30.
Fats
Consistency of fats is commonly deter-mined with the cone penetrometer, as speci-fied in the Official and Tentative Methods ofthe American Oil Chemists' Society (Meth-od Cc 16-60). Other methods that have fre-quently been employed involve extrusion;they include the extrusion attachment to theshear press (Vasic and deMan 1967), anextrusion rheometer used with the Instronuniversal testing machine (Scherr and Witt-nauer 1967), and the FIRA-NIRD extruder(Prentice 1954).
Other devices used for fat consistencymeasurements include wire-cutting instru-
ments (sectilometers), penetration of a probewhen the sample is contained in a small cup,and compression of cylindrical samplesbetween two parallel plates. The compres-sion method reveals detailed informationabout plastic fats (deMan et al., 1991) suchas elasticity, viscous flow, and degree of brit-tleness. These characteristics are importantin shortenings destined for cakes and puffpastries. Compression curves of a variety ofshortenings are displayed in Figure 8-31.Temperature treatment of a fat has profoundeffect on its texture. deMan and deMan(1996) studied the effect of crystallizationtemperature and tempering temperature onthe texture of palm oil and hydrogenated fatsusing the compression method and foundthat lowering the crystallization temperaturefrom 10 to O0C resulted in softer texture,especially for palm oil. Increasing the tem-pering temperature from 25° to 3O0C alsoresulted in softer texture, especially forhydrogenated fats.
The hardness or consistency of fats is theresult of the presence of a three-dimensionalnetwork of fat crystals. All fat products such
Figure 8-30 Schematic Diagram of Bonds Within and Between Polypeptide Chains in Dough. Solidlines represent covalent bonds, dotted lines other bonds. (1) Intramolecular disulfide bond, (2) freesulfhydryl group, (3) intermolecular disulfide bond, (4) ionic bond, (5) van der Waals bond, (6) inter-peptide hydrogen bond, (7) side chain hydrogen bond. Source: From A.H. Bloksma, Rheology ofWheat Flour Dough, /. Texture Studies, Vol. 3, pp. 3-17, 1972.
as margarine, shortening, and butter are mix-tures of solid fat in crystallized form and liq-uid oil. Because the individual glycerides infats have a wide range of melting points, theratio of solid to liquid fat is highly tempera-ture dependent. The crystal particles arelinked by weak van der Waals forces. Thesebonds are easily broken by mechanicalaction during processing, and the consis-tency may be greatly influenced by suchmechanical forces. After a rest period, some
of these bonds are reformed and the revers-ible sol-gel transformation taking place iscalled thixotropy. There appear to be twotypes of bonds in fats—those that arereformed after mechanical action, and thosethat do not reform. The latter result in a por-tion of the hardness loss that is irreversible.The nature of these bonds has not been estab-lished with certainty, but it is assumed tomainly involve van der Waals forces. Thehardness loss of fats as a result of working is
Figure 8-31 Examples of Compression Curves of Shortenings. (1) and (2) soy-palm; (3) soy-canola-palm; (4) soy only; (5) tallow-lard; (6) lard; (7) palm-vegetable; (8), palm-palm kernel, a = elastic non-recoverable deformation; b = viscous flow; B = breaking force; P = plateau force; distance between Band P is an indication of brittleness.
DEFORMATION mm
FO
RC
E N
called work softening and can be expressedas follows:
WS= 0^ w x 100%Ho
where H0 and Hw are the hardness before andafter working.
The work softening is influenced not onlyby the nature of the mechanical treatment butalso by temperature conditions and the sizeand quantity of fat crystals.
Tanaka et al. (1971) have used a two-ele-ment mechanical model (Figure 8-32) torepresent fats as viscoplastic materials. Themodel consists of a dashpot representing theviscous element in parallel with a frictionelement that represents the yield value.
The theory of bond formation between thecrystal particles in plastic fats needs revision.Recently, it has been proposed that a proc-ess of "sintering"—the formation of solidbridges between fat crystals—occurs during
postcrystallization hardening (Heertje et al.,1987; Johansson and Bergenstahl 1995). Theuse of the word sintering is unfortunate sinceit describes the fusion of small particles intoa solid block; this is not the case, however,because the fat crystals in fats can be sus-pended in a solvent such as isobutanol(Chawla and deMan 1990) without showingany sign of fusion into larger aggregates. Atthe solid fat level found in plastic fats (15 to35 percent), the crystals are tightly packedtogether and exist in a state of entanglement(deMan 1997). Entanglement of crystals is amore realistic description of the structureformation in fats. The sintering processdescribed above can be considered a form ofentanglement.
Many interrelated factors influence the tex-ture of plastic fats. Fatty acid and glyceridecomposition are basic factors in establishingthe properties of a fat. These factors, in turn,are related to solid fat content, crystal sizeand shape, and polymorphic behavior. Oncethe crystal network is formed, mechanicaltreatment and temperature history may influ-ence the texture.
The network systems in plastic fats differfrom those in protein or carbohydrate sys-tems. Fat crystals are embedded in liquid oiland the crystals have no ionized groups.Therefore, the interactive forces in fat crystalnetworks are low. The minimum concentra-tion of solid particles in a fat to provide ayield value is in the range of 10 to 15 per-cent.
Fruits and Vegetables
Much of the texture work with fruits andvegetables has been done with the Kramershear press. The shear press was developedbecause of the tenderometer's limitations andhas been widely used for measuring tender-
Figure 8-32 Mechanical Model for Foods asViscoplastic Materials. Source: From M.Tanaka, et al., Measurement of Textural Proper-ties of Foods with a Constant Speed Cone Pene-trometer, /. Texture Studies, Vol. 2, pp. 301-315,1971.
ness of peas for processing (Kramer 1961).The shear press is also used, for example, inthe quality method suggested for raw andcanned sweet corn (Kramer and Cooler1962). This procedure determines shearforce with the standard shear cell and amountof juice pressed out with the juice extractioncell. It is possible to relate quality of the cornto these parameters. In addition to the shearpress, the Instron universal testing machineand others based on the same principle arepopular for fruit and vegetable products. Aspecial testing machine has been developedby Mohsenin (1970). This machine uses anair motor for movement of the crosshead butis otherwise similar to other universal testingmachines. A mechanically driven test systemhas been developed by Voisey (1971). Voisey(1970) has also described a number of testcells that are simpler in design than the stan-dard shear cell of the shear press.
The texture of fruit and vegetable productsis related to the cellular structure of thesematerials. Reeve and Brown (1968a,b) stud-ied the development of cellular structure inthe green bean pod as it relates to texture andeating quality. Sterling (1968) studied theeffect of solutes and pH on the structure andfirmness of cooked carrot. Sterling alsorelated histological changes such as cellularseparation and collapse to the texture of theproduct. In fruits and vegetables the relation-ship between physical structure and physicalproperties is probably more evident than inmany other products.
Morrow and Mohsenin (1966) have stud-ied the physical properties of a variety ofvegetables; they assumed these products tobehave as viscoelastic materials and to be-have according to the three-element modelrepresented in Figure 8-33A. Such vis-coelastic materials are characterized by the
strain-time and stress-time relationships, asgiven in Figure 8-33B,C.
Starch
The texture of starch suspensions is deter-mined by the source of the starch, the chemi-cal and/or physical modification of the starchgranule, and the cooking conditions of thestarch (Kruger and Murray 1976). The tex-ture of starch suspensions is measured bymeans of the viscoamylograph. The viscosityis recorded while the temperature of the sus-pension is raised from 30° to 950C, held at950C for 30 minutes, lowered to 250C, andheld at that temperature for 30 minutes Theviscosity of a 5 percent suspension of waxycorn is shown in Figure 8-34. Initially theviscosity is low, but it increases rapidly at thegelatinization temperature of about 730C. Asthe granules swell, they become weaker andstart to disintegrate causing the viscosity todrop. When the temperature is lowered to250C, there is another increase in viscositycaused by the interaction of the broken anddeformed granules. This phenomenon isdemonstrated by the width and irregularity ofthe recorded line, which is indicative of thecohesiveness of the starch particles.
Modification of the starch has a profoundeffect on the texture of the suspensions.Introduction of as little as 1 cross-bond per100,000 glucose units slows the breakdownof the swollen granules during and aftercooking (Figure 8-35). This results in ahigher final viscosity. Increasing the cross-bonding to 1, 3, or 6 cross-bonds per 10,000glucose units will result in no breakdownduring the heating cycle (Figure 8-36). Asthe cross-bonding increases, the granule isstrengthened and does not swell much duringheating, but viscosity is decreased. Mostfood starches used at pH values of 4 to 8
have 2 to 3 cross-bonds per 10,000 glucoseunits.
Waxy corn starch contains only amylopec-tin; corn starch contains both amylopectinand amylose. This results in a different vis-cosity profile (Figure 8-37). Corn starchshows a lower peak viscosity and less break-down during heating. After cooling, the vis-cosity continues to increase, probably be-cause the amylose interlinks with the amy-lopectin. On further storage at 250C, theslurry sets to a firm gel. Tapioca starch isintermediate between corn and waxy cornstarch (Figure 8-37). This is explained by the
fact that tapioca amylose molecules arelarger than those in corn starch.
Starches can be substituted by nonionic orionic groups. The latter can be made anionicby introduction of phosphate or succinategroups. These have lower gelatinization tem-perature, higher peak viscosity, and higherfinal cold viscosity than nonionic starches(Figure 8-38).
MICROSTRUCTURE
With only a few exceptions, food productsare non-Newtonian and possess a variety of
Figure 8-33 Mechanical Model Proposed by Morrow and Mohsenin. (A) Viscoelastic foods, (B) thestrain time, (C) stress time, characteristics of the system.
Time
Stress
Time
Strain
A
B
c
Figure 8-34 Viscosity and Granule Appearancein the Viscoamylograph Test of a 5% Suspen-sion of Waxy Corn in Water. A = viscositycurve, B = granule shape and size, C = magni-fied portion of curve to indicate cohesiveness, a= unswollen granule, b = swollen granule, c =collapsed granule, d = entwined collapsed gran-ules. Source: Reprinted from L.H. Kruger andR. Murray, Starch Texture, in Rheology andTexture in Food Quality, J.M. deMan, RW. Voi-sey, V.F. Raspar, and D.W. Stanley, eds., ©1976, Aspen Publishers, Inc.
internal structures. Cellular and fibrousstructures are found in fruits and vegetables;fibrous structures are found in meat; andmany manufactured foods contain protein,carbohydrate, or fat crystal networks.
Many of these food systems are disper-sions that belong in the realm of colloids.Colloids are defined as heterogeneous or dis-persed systems that contain at least twophases—the dispersed phase and the contin-uous phase. Colloids are characterized bytheir ability to exist in either the sol or the gelform. In the former, the dispersed particles
Figure 8-35 Viscosity and Granule Appearancein the Viscoamylograph Test of a 5% Suspensionin Water of Waxy Corn with 1 Cross-Bond per100,000 Glucose Units. A = viscosity curve, B =granule appearance. Source: Reprinted from L.H.Kruger and R. Murray, Starch Texture, in Rheol-ogy and Texture in Food Quality, J.M. deMan,RW. Voisey, V.F. Raspar, and D.W. Stanley, eds.,© 1976, Aspen Publishers, Inc.
exist as independent entities; in the latter,they associate to form network structuresthat may entrap large volumes of the contin-uous phase. The isothermal reversible sol-geltransformation exhibited by many foods iscalled thixotropy. Disperse systems can beclassified on the basis of particle size. Coarsedispersions have particle size greater than 0.5jam. They can be seen in the light micro-scope, can be filtered over a paper filter, andwill sediment rapidly. Colloidal dispersionshave particles in the range of 0.5 (Jm to 1 nm.These particles remain in suspension byBrownian movement and can run through apaper filter but cannot run through a mem-brane filter. Particles smaller than these are
VISC
OSI
TY P
OISE
S
VISC
OSIT
Y PO
ISES
Figure 8-36 Viscosity and Granule Appearancein the Viscoamylograph of 5% Suspension inWater of Cross-Bonded Waxy Corn. A = Icross-bond per 10,000 glucose units, B = 3cross-bonds, C = 6 cross-bonds, and D = granuleappearance. Source: Reprinted from L.H.Kruger and R. Murray, Starch Texture, in Rheol-ogy and Texture in Food Quality, J.M. deMan,RW. Voisey, V.F. Raspar, and D.W. Stanley, eds.,© 1976, Aspen Publishers, Inc.
molecular dispersions or solutions. Depend-ing on the nature of the two phases, dispersesystems can be classified into a number oftypes. A solid dispersed in a liquid is called asol; for example, margarine, which has solidfat crystals dispersed in liquid oil, is a sol.Dispersions of liquid in liquid are emulsions;many examples of these are found amongfoods such as milk and mayonnaise. Disper-sions of gas in liquid are foams (e.g.,whipped cream). In many cases, these dis-
Figure 8-37 Viscosity and Granule Appearancein the Viscoamylograph of Suspensions in Waterof Corn and Tapioca Starch. A = 6% corn starch,B = 5% tapioca starch, C = corn granule appear-ance, and D = tapioca granule appearance.Source: Reprinted from L.H. Kruger and R. Mur-ray, Starch Texture, in Rheology and Texture inFood Quality, J.M. deMan, RW. Voisey, VF. Ras-par, and D.W. Stanley, eds., © 1976, Aspen Pub-lishers, Inc.
persions are more complex than one dispersephase. Many foods have several dispersedphases. For instance, in chocolate, solidcocoa particles as well as fat crystals are dis-persed phases.
The production of disperse systems isoften achieved by dispersion methods inwhich the disperse phase is subdivided intosmall particles by mechanical means. Liq-
AMYLOSE
VISC
OSI
TY P
OIS
ES
VISC
OSI
TY P
OIS
ES
Figure 8-38 Viscoamylograph Viscosity Curvesof Substituted Waxy Corn Starch. A = cross-bonded waxy corn, B = nonionic substitutedcross-bonded waxy corn, and C = anionic substi-tuted cross-bonded waxy corn. Source: Reprintedfrom L.H. Kruger and R. Murray, Starch Texture,in Rheology and Texture in Food Quality, J.M.deMan, RW. Voisey, V.F. Raspar, and D.W. Stan-ley, eds., © 1976, Aspen Publishers, Inc.
uids are emulsified by stirring and homoge-nization; solids are subdivided by grinding,as, for instance, roller mills are used in choc-olate making and colloid mills are used inother food preparations.
An important aspect of the subdivision ofthe disperse phase is the enormous increasein specific surface area. If a sphere with aradius R = 1 cm is dispersed into particleswith radius r = 10~6 cm, the area of the inter-face will increase by a factor of 106. The
mechanical work dA needed to increase theinterfacial area is proportional to the areaincrease, as follows:
dA = CdO
where O = total interfacial area. The propor-tionality factor a is the surface tension. In theproduction of emulsions, the surface tensionis reduced by using surface active agents (seeChapter 2).
As particle size is reduced to colloidaldimensions, the particles are subject toBrownian movement. Brownian movementis the result of the random thermal move-ment of molecules, which impact on colloi-dal particles to give them a random move-ment as well (Figure 8-39). The size of dis-persed particles has a profound effect on theproperties of dispersions (Schubert 1987).Figure 8-40 shows the qualitative relation-ship of particle size and system properties.As particle size decreases, fracture resistanceincreases. The particles become increasinglyuniform, which results in a grinding limitbelow which particles cannot be furtherreduced in size. The terminal settling rate,illustrated by a flour particle falling throughthe air, increases rapidly as a function ofincreasing particle size. According to Schu-bert (1987), a flour particle of 1 |im in sizetakes more than 6 hours to fall a distance of 1meter in still air. Wetting becomes more dif-ficult as size decreases. The specific surfacearea (the surface per unit volume) increasesrapidly with decreasing particle size.
Colloidal systems, because of their largenumber of dispersed particles, show non-Newtonian flow behavior. For a highly dilutedispersion of spherical particles, the follow-ing equation has been proposed by Einstein:
Tl= Tl0 (1+2 .5 ^)
VISC
OSI
TY P
OIS
ES
Figure 8-40 Relationship Between Particle Sizeand System Properties. D = particle deposition infibrous fillers, F = adhesion force, H = homogene-ity of a particle, Sv = surface area per unit volume,W = particle weight, Vg = terminal setting rate, as
= particle fracture resistance. Source: From H.Schubert, Food Particle Technology. Part 1: Prop-erties of Particles and Particulate Food Systems,/ Food Eng., Vol. 6, pp. 1-32, 1987, ElsevierApplied Science Publishers, Ltd.
whereT|0 = viscosity of the continuous phase<|) = ratio of volumes of disperse and con-
tinuous phases
In this equation, viscosity is independentof the degree of dispersion. As soon as theratio of disperse and continuous phasesincreases to the point where particles start tointeract, the flow behavior becomes morecomplex. The effect of increasing the con-centration of the disperse phase on the flowbehavior of a disperse system is shown inFigure 8-41. The disperse phase, as well asthe low solids dispersion (curves 1 and 2),shows Newtonian flow behavior. As the sol-ids content increases, the flow behaviorbecomes non-Newtonian (curves 3 and 4).Especially with anisotropic particles, interac-tion between them will result in the forma-tion of three-dimensional network structures.These network structures usually show non-Newtonian flow behavior and viscoelasticproperties and often have a yield value. Net-work structure formation may occur in emul-sions (Figure 8-42) as well as in particulatesystems. The forces between particles thatresult in the formation of networks may be
Figure 8-39 Flat Plane Projection of the Location of a Colloidal Particle Subject to Brownian Move-ment. Source: From H. Schubert, Food Particle Technology. Part 1: Properties of Particles and Particu-late Food Systems, J. FoodEng., Vol. 6, pp. 1-32, 1987, Elsevier Applied Science Publishers, Ltd.
PARTICLE SIZE x
flowobility(weHobility.dispersibility)
PART
ICLE
PRO
PERT
Y(lo
o sc
ale)
BULK
PRO
PERT
Y(lo
g. s
cale
)
PARTICLE SIZEx
strength
Figure 8-41 Effect of Increasing the Concentra-tion of the Disperse Phase on the Flow Behaviorof a Disperse System. 1—continuous phase, 2—low solids content, 3—medium solids content,4—high solids content.
van der Waals forces, hydrophobic interac-tions, or covalent bonds. Network formationmay result from heating or from chemicalreactions that occur spontaneously eitherfrom components already present in the foodor from added enzymes or coagulants. Theformation of networks requires a minimalfraction of particles to be present, the critical
fraction occ, and the larger the number of sitesf, used for bond formation, the sooner a net-work is formed. These two quantities arerelated as follows:
CX 0=W-I)
At particle concentrations below 10 per-cent, numerous contact points are required toform a network structure (Table 8-3). Thismeans that only certain types of molecules orparticles can form networks at this concen-tration. As the network is formed, the viscos-ity increases until, at a certain point, theproduct acquires plastic and/or viscoelasticproperties. Network formation thus dependson particle concentration, reactive sites onthe particles, and particle size and shape.Heertje et al. (1985) investigated structureformation in acid milk gels and found thatthe final texture of the products was influ-enced by many factors including heat, saltbalance, pH, culture, and thickening agents.Structure formation in soy milk, induced bycoagulants in the form of calcium or magne-sium salts, results in a semisolid food calledtofu, which has a fine internal protein net-work structure (Figure 8-43). Hermansson
Figure 8-42 Structure Formation in Particulate Systems. (A) Flocculation of an emulsion. (B) Networkformation in crystallized fat.
A
B
Table 8-3 Relationship Between Critical ParticleFraction (ccc) and Number of Bond Sites (f)
«c f_
O05 210.10 110.15 80.20 60.30 40.50 3
and Larsson (1986) reported on the structureof gluten gels and concluded that these gelsconsist of a continuous phase of denselypacked protein units.
DeMan and Beers (1987) have reviewedthe factors that influence the formation ofthree-dimensional fat crystal networks. Thefat crystal networks in plastic fats (Figure8-44) are highly thixotropic, and mechanicalaction on these products will result in a dras-tic reduction of hardness.
A variety of rheological tests can be usedto evaluate the nature and properties of dif-ferent network structures in foods. Thestrength of bonds in a fat crystal network canbe evaluated by stress relaxation and by thedecrease in elastic recovery in creep tests asa function of loading time (deMan et al.1985). Van Kleef et al. (1978) have reportedon the determination of the number of cross-links in a protein gel from its mechanical andswelling properties. Oakenfull (1984) usedshear modulus measurements to estimate thesize and thermodynamic stability of junctionzones in noncovalently cross-linked gels.
Dynamic measurements of gels can provideinformation on the extent of cross-linking(Bell 1989). Systems with a relatively highstorage modulus G' show a low value for G'7G', which indicates a highly cross-linked sys-tem such as an agar gel.
WATER ACTIVITY AND TEXTURE
Water activity (aw) and water content havea profound influence on textural propertiesof foods. The three regions of the sorptionisotherm can be used to classify foods on thebasis of their textural properties (Figure8-45). Region 3 is the high moisture area,which includes many soft foods. Foods in theintermediate moisture area (region 2) appeardry and firm. At lowest values of aw (region1), most products are hard and crisp (Bourne1987).
Katz and Labuza (1981) examined the rela-tionship between aw and crispness in a studyof the crispness of popcorn (Figure 8-46).They found a direct relationship betweencrispness and aw.
Many foods contain biopolymers and lowmolecular weight carbohydrates. These canbe present in a metastable amorphous statethat is sensitive to temperature and the state
Figure 8-43 Microstructure of Soybean Curd(Tofu) as Seen in the Scanning Electron Micro-scope
Figure 8-45 The Three Regions of the Sorption Isotherm Related to the Textural Properties of FoodSystems. Source: Reprinted with permission from M.C. Bourne, Effects of Water Activity on TexturalProperties of Food, in Water Activity: Theory and Applications to Food, L.B. Rockland and L.R.Beuchat, eds., p. 76, 1987, by courtesy of Marcel Dekker, Inc.
Region 3(high moisture)
moistsoft
flaccidswollensticky
Region 2(intermediate moisture)
dryfirm
flexible
Region 1(low moisture)
dryhardcrisp
shrunken
Wat
er C
onte
nt
Figure 8-44 Fat Crystals in a Partially Crystallized Fat as Seen in the Polarizing Microscope
aw
of water. The amorphous state can be in theform of a rubbery structure or a very vis-cous glass, as shown in Figure 8-47 (Sladeand Levine 1991; Levine and Slade 1992;Roos and Karel 1991). A more detailedanalysis of the effect of temperature on tex-tural properties expressed as modulus ispresented in Figure 8-48. Below the melt-ing temperature, the material enters a stateof rubbery flow. As the temperature is low-ered further, a leathery state is observed. In
the leathery region the modulus increasessharply, until the glass transition temper-ature (Tg) is reached and the materialchanges to a glass.
Kapsalis et al. (1970) reported on a studyof the textural properties of freeze-dried beefat different points of the moisture sorptionisotherm over the complete range of wateractivity. Important changes in textural prop-erties were observed at aw values of 0.85 andat 0.15 to 0.30.
Figure 8-46 Relationship Between Water Activity and Crispness of Popcorn. Source: Reprinted withpermission from E.E. Katz and T.P. Labuza, Effect of Water Activity on the Sensory Crispness andMechanical Deformation of Snack Food Properties, J. Food ScL, Vol. 46, p. 403, © 1981, Institute ofFood Technologists.
WATER ACTIVITY
VERY CRISP
MODERATELY CRISP
SLIGHTLY CRISP
NQT CRISP
CR
ISP
NE
SS
INT
EN
SIT
Y
Figure 8-47 Rubbery and Glassy State of Moisture-Containing Foods as Affected by Temperature. Tm
= melting point; Tg = glass transition temperature.
MOISTURE
RUBBERY
GLASSY
MELT
TE
MP
ER
AT
UR
E
log
M
OD
UL
US
GLASS
Elasticor
RubberyFlow
LiquidFlow
RUBBERYPLATEAU
LeatheryRegion
TmTg
TEMPERATURE or log FREQUENCY
Figure 8-48 Effect of Temperature on the Texture as Expressed by Modulus. Tm = melting tempera-ture, Tg = glass transition temperature.
REFERENCES
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Bjorksten, J., et al. 1967. A portable rotating knife ten-derometer. FoodTechnol 21: 84-86.
Bourne, M.C. 1966. Measure of shear and compressioncomponents of puncture tests. J. Food Sd. 31:282-291.
Bourne, M.C. 1987. Effects of water activity on tex-tural properties of food. In Water activity: Theoryand applications to food, ed. L.B. Rockland andL.R. Beuchat. New York: Marcel Dekker.
Brandt, M.A., et al. 1963. Texture profile method. J.Food ScL 28:404-409.
Bratzler, LJ. 1932. Measuring the tenderness of meatby means of a mechanical shear. Master's thesis,Kansas State College.
Cairncross, S.E., and L.B. Sjostrom. 1950. Flavor pro-files—A new approach to flavor problems. FoodTechnol. 4: 308-311.
Chawla, P., and J.M. deMan. 1990. Measurement ofthe size distribution of fat crystals using a laser parti-cle counter. J. Am. Oil Chem. Soc. 67: 329-332.
deMan, J.M. 1969. Food texture measurements withthe penetration method. J. Texture Studies 1:114-119.
deMan, J.M. 1997. Relationship between chemical,physical and textural properties of fats. Paper pre-sented at International Conference on Physical Prop-erties of Fats, Oils, Emulsifiers, September,Chicago.
deMan, J.M., and A.M. Beers. 1987. Fat crystal net-works: Structure and Theological properties. J. Tex-ture Studies 18: 303-318.
deMan, J.M., and FW. Wood. 1959. Hardness of but-ter. II. Influence of setting. J. Dairy Sd. 42: 56-61.
deMan, J.M., et al. 1985. Viscoelastic properties ofplastic fat products. /. Am. Oil Chem. Soc. 62:1672-1675.
deMan, L., et al. 1991. Physical and textural character-istics of some North American shortenings. /. Am.Oil Chem. Soc. 68: 63-69.
deMan, L., and J.M. deMan. 1996. Textural measure-ments of some plastic fats: Oils, fats, lipids. Pro-ceedings of the International Society for FatResearch (ISF), the Hague 1995, 3: 543-546.Bridgewater, UK: PJ. Barnes & Assoc.
Haighton, AJ. 1959. The measurement of hardness ofmargarine and fats with the penetrometers. J. Am.Oil Chem. Soc. 36: 345-348.
Heertje, L, et al. 1985. Structure formation in acid milkgels. Food Micro'structure 4: 267-278.
Heertje, I., et al. 1987. Product morphology of fatproducts. Food Microstructure 6: 1-8.
Hermansson, A.M., and K. Larsson. 1986. The struc-ture of gluten gels. Foo d Micro structure 5: 233-240.
Howard, R.D., and M.D. Judge. 1968. Comparison ofsarcomere length to other predictors of beef tender-ness. J. Food Sd. 33: 456-460.
Johansson, D., and B. Bergenstahl. 1995. Sintering offat crystals in oil during post-crystallization pro-cesses. J. Am. Oil Chem. Soc. 72: 911-920.
Johnston, C.W., and C.H. Brower. 1966. Rheologicaland gelation measurements of plastisols. Soc. Plas-tics Eng. J. 22, no. 11: 45-52.
Kamel, B.S., and J.M. deMan. 1975. Evaluation of gel-atin gel texture by penetration tests. Lebensm. Wiss.Technol. 8: 123-127.
Kapsalis, J.G., et al. 1970. A physico-chemical studyof the mechanical properties of low and intermediatemoisture foods. J. Texture Studies 1: 464-483.
Katz, E.E., and TP Labuza. 1981. Effect of wateractivity on the sensory crispness and mechanicaldeformation of snack food properties. J. Food ScL46: 403-409.
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Levine, H., and L. Slade. 1992. Glass transitions infoods. In Physical chemistry of foods, ed. H.G.Schwartzberg and R.W. Hartel. New York: MarcelDekker.
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INTRODUCTION
Vitamins are minor components of foodsthat play an essential role in human nutri-tion. Many vitamins are unstable under cer-tain conditions of processing and storage(Table 9-1), and their levels in processedfoods, therefore, may be considerably re-duced. Synthetic vitamins are used exten-sively to compensate for these losses and torestore vitamin levels in foods. The vitaminsare usually divided into two main groups,the water-soluble and the fat-soluble vita-mins. The occurrence of the vitamins in thevarious food groups is related to their water-or fat-solubility. The relative importance ofcertain types of foods in supplying some ofthe important vitamins is shown in Table9-2. Some vitamins function as part of acoenzyme, without which the enzyme wouldbe ineffective as a biocatalyst. Frequently,such coenzymes are phosphorylated formsof vitamins and play a role in the metabo-lism of fats, proteins, and carbohydrates.Some vitamins occur in foods as provita-mins—compounds that are not vitamins butcan be changed by the body into vitamins.Vitamers are members of the same vitaminfamily.
Lack of vitamins has long been recog-nized to result in serious deficiency diseases.
It is now also recognized that overdoses ofcertain vitamins, especially some of the fat-soluble ones, may result in serious toxiceffects. For this reason, the addition of vita-mins to foods should be carefully controlled.
The sources of vitamins in significantamounts by food groups have been listed byCombs (1992) as follows:
• Meats, poultry, fish, and beans providethiamin, riboflavin, niacin, pyridoxine,pantothenic acid, biotin, and vitaminB12.
• Milk and milk products provide vitaminsA and D, riboflavin, pyridoxine, andvitamin B12.
• Bread and cereals provide thiamin, ribo-flavin, niacin, pyridoxine, folate, pan-tothenic acid, and biotin.
• Fruits and vegetables provide vitamins Aand K, ascorbic acid, riboflavin, andfolate.
• Fats and oils provide vitamins A and E.
FAT-SOLUBLE VITAMINS
Vitamin A (Retinol)
The structural formula of vitamin A isshown in Figure 9-1. It is an alcohol thatoccurs in nature predominantly in the form
Vitamins
CHAPTER 9
Table 9-1 Stability of Vitamins under Different Conditions
Unstable To:
Vitamin
vitamin A
vitamin D
vitamin E
vitamin K
vitamin Cthiamine
riboflavinniacin
vitamin B6
biotinpantothenic
acidfolatevitamin B12
Vitamer
retinolretinalretinoic aciddehydroret.ret. estersp-caroteneD2
D3
tocopherolstocopherolesters
KMKmenadioneascorbic aciddisulfide formhydrochlorided
riboflavinnicotinic acidnicotinamidepyridoxalpyridoxol (HCI)biotinfree acid1
Ca saltyd
FH4
CN-B12
UVLight
+
++
+++
+e
+
+
++
Heat*
+++
+++
+
++
O2
++
+
++++
++
+b
++
++
+
Acid++
++++
++
+a+h
Base
++
+++++++
++
Metals?++
+++
+++++++
+V
Most Stabledark, sealsealgood stabilitysealgood stabilitysealdark, cool, sealdark, cool, sealcool, neutral pHgood stability
avoid reductants0
avoid reductantsc
avoid reductants0
seal, neutral pHneutral pHc
seal, neutral pHc
dark, pH 1.5-4C
good stabilitygood stabilitycoolgood stabilityseal, neutral pHcool, neutral pHseal, pH 6-7good stability0
good stability0
ai.e., 10O0Cbin solution with Fe+++ and Cu++
cunstable to reducing agentsdslightly hygroscopicEspecially in alkaline solutionVery hygroscopic9pH<5hpH<3jpH>9
Source: Reprinted with permission from G. F. Combs, The Vitamins: Fundamental Aspects in Nutrition and Health,p. 449, © 1992, Academic Press.
of fatty acid esters. Highest levels of vitaminA are found in certain fish liver oils, such ascod and tuna. Other important sources aremammalian liver, egg yolk, and milk andmilk products. The levels of vitamin A andits provitamin carotene in some foods arelisted in Table 9-3.
The structural formula of Figure 9-1shows the unsaturated character of vitaminA. The all-trans form is the most active bio-logically. The 13-cis isomer is known as neo-vitamin A; its biological activity is onlyabout 75 percent of that of the all-trans form.The amount of neo-vitamin A in natural vita-min A preparations is about one-third of the
total. The amount is usually much less insynthetic vitamin A. The synthetic vitamin Ais made as acetate or palmitate and marketedcommercially in the form of oil solutions,stabilized powders, or aqueous emulsions.The compounds are insoluble in water butsoluble in fats, oils, and fat solvents.
Table 9-3 Vitamin A and Carotene Content ofSome Foods
Figure 9-1 Structural Formula of Vitamin A.Acetate: R = CO-CH3. Palmitate: R =CO(CH2)14CH3.
Product
Beef (grilled sirloin)Butter (May-
November)Cheddar cheeseEggs (boiled)Herring (canned)MilkTomato (canned)PeachCabbageBroccoli (boiled)Spinach (boiled)
Vitamin A(IU/100g)
372363-3452
553-1078165-488
178110-307
OOOOO
Carotene(mg/100g)
0.040.43-0.77
0.07-0.710.01-0.15
0.070.01-0.06
0.50.340.32.56.0
Table 9-2 Contributions (%) of Various Food Groups to the Vitamin Intake of Americans
Source: Reprinted with permission from G.F. Combs, The Vitamins: Fundamental Aspects in Nutrition andHealth, p. 441, © 1992, Academic Press.
Foods
vegetableslegumesfruitsgrain productsmeatsmilk productseggsfats and oilsother
Vitamin A
39.4
8.0
22.513.25.88.22.7
Vitamin C
51.8
39.0
2.03.7
3.4
Thiamin
11.75.44.441.227.18.12.0
Riboflavin
6.9
2.222.122.239.14.9
Niacin
12.08.22.527.445.01.4
3.3
Vitamin B6
22.25.48.210.240.011.62.1
Vitamin B12
1.669.220.78.5
There are several provitamins A; thesebelong to the carotenoid pigments. The mostimportant one is p-carotene, and some of thepigments that can be derived from it are ofpractical importance. These are p-apo-8'-carotenal and p-apo-8'-carotenoic acidethyl ester (Figure 9-2). Other provitaminsare a- and y-carotene and cryptoxanthin.
Beta-carotene occurs widely in plant prod-ucts and has a high vitamin A activity. In the-ory, one molecule of p-carotene could yieldtwo molecules of vitamin A. The enzyme 15-15'-dioxygenase is able to cleave a P-caro-tene molecule symmetrically to produce twomolecules of vitamin A (Figure 9-3). Thisenzyme occurs in intestinal mucosa, but theactual conversion is much less efficient. Asshown in Figure 9-3, there are other reac-tions that may cause the yield of retinol to beless than 2. After cleavage of the p-carotene,the first reaction product is retinal, which isreduced to retinol (Rouseff and Nagy 1994).A general requirement for the conversion ofa carotenoid to vitamin A is an unsubstitutedp-ionone ring. Citrus fruits are a good sourceof provitamin A, which results mostly from
the presence of p-cryptoxanthin, p-carotene,and a-carotene. Gross (1987) reported a totalof 16 carotenoids with provitamin A activityin citrus fruits.
Vitamin A levels are frequently expressedin International Units (IU), although this unitis officially no longer accepted. One IUequals 0.344 |U,g of crystalline vitamin A ace-tate, or 0.300 |Lig vitamin A alcohol, or 0.600|ig p-carotene. The recommended dailyallowance (RDA) of vitamin A of the NationalResearch Council Food and Nutrition Boardis 5000 IU for an adult. Other sources quotethe human requirement at about 1 |uig/day.Conditions of rapid growth, pregnancy, orlactation increase the need for vitamin A.
Vitamin A, or retinol, is also known as vita-min A1. Another form, vitamin A2, is foundin fish liver oils and is 3-dehydroretinol.
The Food and Agriculture Organizationand the World Health Organization of theUnited Nations (FAOAVHO) and the NationalAcademy of Sciences of the United States(1974a) have recommended that vitamin Aactivity be reported as the equivalent weightof retinol. To calculate total retinol equiva-
A
B
Figure 9-2 Structural Formulas of Some Provitamins A. (A) p-carotene, and (B) apocarotenal (R =CHO) and apocarotenoic acid ester (R = COOC2H5).
lents, it is proposed that food analyses listretinol, carotene, and other provitamin A car-otenoids separately. It is also desirable to dis-tinguish between the cis- and trans- forms ofthe provitamins in cooked vegetables. Bydefinition, 1 retinol equivalent is equal to 1|Lig of retinol, or 6 |Hg of (3-carotene, or 12 [Lgof other provitamin A carotenoids. TheNational Academy of Sciences (1974a)states that 1 retinol equivalent is equal to 3.3IU of retinol or 10 IU of p-carotene.
Vitamin A occurs only in animals and notin plants. The A1 form occurs in all animals
and fish, the A2 form in freshwater fish andnot in land animals. The biological value ofthe A2 form is only about 40 percent of thatof A1. Good sources of provitamin A in veg-etable products are carrots, sweet potatoes,tomatoes, and broccoli. In milk and milkproducts, vitamin A and carotene levels aresubject to seasonal variations. Hartman andDry den (1965) report the levels of vitamin Ain fluid whole milk in winter at 1,083 IU/Land in summer at 1,786 IU/L. Butter containsan average of 2.7 |Ug of carotene and 5.0 |Ligof vitamin A per g during winter and 6.1 |Lig
B- CAROTENE
RETINAL
RETINOL
Figure 9-3 Conversion of Beta-Carotene to Vitamin A. Source: Reprinted with permission from R.R.Rouseff and S. Nagy, Health and Nutritional Benefits of Citrus Fruit Components, Food Technology,Vol. 48, No. 11, p. 125, © 1994, Institute of Food Technologists.
of carotene and 7.6 |Lig of vitamin A per gduring summer.
Vitamin A is used to fortify margarine andskim milk. It is added to margarine at a levelof 3,525 IU per 100 g. Some of the car-otenoids (provitamin A) are used as food col-ors.
Vitamin A is relatively stable to heat in theabsence of oxygen (Table 9-4). Because ofthe highly unsaturated character of the mole-cule, it is quite susceptible to oxidation—especially under the influence of light,whether sunlight or artificial light. VitaminA is unstable in the presence of mineral acidsbut stable in alkali. Vitamin A and the car-otenoids have good stability during various
food processing operations. Losses may occurat high temperatures in the presence of oxy-gen. These compounds are also susceptibleto oxidation by lipid peroxides, and condi-tions favoring lipid oxidation also result invitamin A breakdown. The prooxidant cop-per is especially harmful, as is iron to a lesserextent. Pasteurization of milk does not resultin vitamin A loss, but exposure to light does.It is essential, therefore, that sterilized milkbe packaged in light-impervious containers.Possible losses during storage of foods aremore affected by duration of storage than bystorage temperature. Blanching of fruits andvegetables helps prevent losses during frozenstorage.
Table 9-4 Vitamin A and Carotene Stability in Foods
Product
Vitamin AButter
Margarine
Nonfat dry milk
Fortified ready-to-eat cerealFortified potato chips
CaroteneMargarine
Lard
Dried egg yolk
Carbonated beverage
Canned juice drinks
Nutrient Content
1 7,000-30,000 lU/lb
1 5,000 lU/lb
1 0,000 lU/lb
4000 lU/oz700IUAIOOg
3 mg/lb
3.3 mg/lb
35.2mg/100g
7.6 mg/29 oz
0.6-1 .3 mg/8 fl oz
Storage Conditions
12 mo & 50C5 mo @ 280C6 mo @ 50C6 mo @ 230C3 mo @ 370C12 mo 9 230C6 mo @ 230C2 mo @ 230C
6 mo 9 50C6 mo 9 230C6 mo @ 50C6 mo 9 230C3 mo 9 370C12 mo 9 230C2 mo 9 3O0C2 mo 9 230C12 mo @23°C
Retention (%)
66-9864-6889-10083-10094-10069-8983100
98891001009480949485-100
Source: From E. deRitter, Stability Characteristics of Vitamins in Processed Foods, Food Technol., Vol. 30, pp.48-51,54, 1976.
Vitamin A added to milk is more easilydestroyed by light than the native vitamin A.This is not because natural and synthetic vita-min A are different, but because these twotypes of vitamin A are dispersed differentlyin the milk (deMan 1981). The form in whichvitamin A is added to food products mayinfluence its stability. Vitamin A in beadletform is more stable than that added as a solu-tion in oil. The beadlets are stabilized by aprotective coating. If this coating is damagedby water, the stability of the vitamin is greatlyreduced (de Man et al. 1986).
Vitamin D
This vitamin occurs in several forms; thetwo most important are vitamin D2, or ergo-calciferol, and vitamin D3, or cholecalciferol.The structural formulas of these compoundsare presented in Figure 9-4. Vitamin D does
not occur in plant products. Vitamin D2
occurs in small amounts in fish liver oils;vitamin D3 is widely distributed in animalproducts, but large amounts occur only infish liver oils. Smaller quantities of vitaminD3 occur in eggs, milk, butter, and cheese(Table 9-5).
The precursors of vitamins D2 and D3 areergosterol and 7-dehydrocholesterol, respec-tively. These precursors or provitamins canbe converted into the respective D vitaminsby irradiation with ultraviolet light. In addi-tion to the two major provitamins, there areseveral other sterols that can acquire vitaminD activity when irradiated. The provitaminscan be converted to vitamin D in the humanskin by exposure to sunlight. Because veryfew foods are good sources of vitamin D,humans have a greater likelihood of vitaminD deficiency than of any other vitamin defi-ciency. Enrichment of some foods with vita-min D has significantly helped to eradicaterickets, which is a vitamin D deficiency dis-ease. Margarine and milk are the foods com-monly used as carrier for added vitamin D.
The unit of activity of vitamin D is the IU,which is equivalent to the activity of 1 mg ofa standard preparation issued by the WHO.One IU is also equivalent to the activity of0.025 |ig of pure crystalline vitamin D2 orD3. The human requirement amounts to 400
Table 9-5 Vitamin D Content of Some Foods
Vitamin D fag/WOO gProduct Edible Portion)
Liver (beef, pork) 2-5Eggs 44Milk 0.9Butter 2-40Cheese 12-47Herring oil 2,500
Figure 9-4 Structural Formulas of (A) VitaminD2 and (B) Vitamin D3
B
A
to 500 IU but increases to 1,000 IU duringpregnancy and lactation. Adults who are reg-ularly exposed to sunlight are likely to have asufficient supply of vitamin D. Excessiveintakes are toxic.
Vitamin D is extremely stable, and little orno loss is experienced in processing and stor-age. Vitamin D in milk is not affected by pas-teurization, boiling, or sterilization (Hartmanand Dryden 1965). Frozen storage of milk orbutter also has little or no effect on vitamin Dlevels, and the same result is obtained duringstorage of dry milk.
The vitamin D potency of milk can beincreased in several ways: by feeding cowssubstances that are high in vitamin D activ-ity, such as irradiated yeast; by irradiatingmilk; and by adding vitamin D concentrates.The latter method is now the only commonlyused procedure. The practice of irradiatingmilk to increase the vitamin D potency hasbeen discontinued, undoubtedly because ofthe deteriorative action of the radiation onother milk components. Vitamin D is addedto milk to provide a concentration of 400 IU
per quart. Addition of vitamin D to marga-rine is at a level of 550IU per 100 g.
Tocopherols (Vitamin E)
The tocopherols are derivatives of tocol,and the occurrence of a number of relatedsubstances in animal and vegetable productshas been demonstrated. Cottonseed oil wasfound to contain a-, p-, and y-tocopherol,and a fourth, 5-tocopherol, was isolated fromsoybean oil. Several other tocopherols havebeen found in other products, and Morton(1967) suggests that there are four to-copherols and four tocotrienols. The toco-trienols have three unsaturated isoprenoidgroups in the side chain. The structure oftocol is given in Figure 9-5 and the struc-tures of the tocopherols and tocotrienols inFigure 9-6. The four tocopherols are charac-terized by a saturated side chain consisting ofthree isoprenoid units. The tocotrienols havethree double bonds at the 3', 7', and 1Y car-bons of the isoprenoid side chain (Figure
A
B
Figure 9-5 Structural Formula of (A) Tocol and (B) a-Tocopherol
9-6). The carbons at locations 4' and 8' inthe side chains of the tocopherols are asym-metric, as is the number 2 carbon in the chro-man ring. The resulting possible isomers aredescribed as having R or S rotation. The nat-ural tocopherols and tocotrienols are pre-dominantly RRR isomers. Morton (1967)has summarized the chemistry of the to-copherols as shown in Figure 9-7.
On oxidation, oc-tocopherol can form ameta-stable epoxide that can be irreversiblyconverted to oc-tocopherolquinone. Reduc-
tion of the quinone yields a quinol. To-copherolquinones occur naturally. Oxidationwith nitric acid yields the o-quinone or to-copherol red, which is not found in nature.Alpha-tocopheronic acid and a-tocopher-onolactone are some of the products ofmetabolism of tocopherol. Much of the bio-logical activity of the tocopherols is relatedto their antioxidant activity. Because a-to-copherol is the most abundant of the differ-ent tocopherols, and because it appears tohave the greatest biological activity, the oc-
Figure 9-6 Chemical Structure of the Tocopherols and Tocotrienols
Tocotrienol
Tocotrlenol
OfPT5
Tocotrienol
5,7,8 - Trimethyl5,8 - Dimethyl7,8 - Dimethyl8 - Methyl
R,
CH,CH,HH
R2
CH,HCH,H
R9
CH,CH,CH,CH1
Tocopherol
Tocopherol
aftTa
Tocopherol
5,7,8 - Trimethyl5,8 • Dimethyl7,8 - Dimethyl8 - Methyl
R,
CH,CH,HH
R,
CH,HCH,H
R,
CH,CH,CH,CH,
tocopherol content of foods is usually con-sidered to be most important.
The biological activity of the tocopherolsand tocotrienols varies with the number andposition of the methyl groups on the chro-man ring and by the configuration of theasymmetric carbons in the side chain. The Rconfiguration at each chiral center has thehighest biological activity. Because the dif-ferent isomers have different activities, it isnecessary to measure each homolog and con-vert these to RRR-oc-tocopherol equivalents(Ct-TE). One oc-TE is the activity of 1 mg ofRRR-oc-tocopherol (Eitenmiller 1997). Thevitamin E activity of oc-tocopherol isomersand synthetic tocopherols is listed in Table9-6.
Tocopherols are important as antioxidantsin foods, especially in vegetable oils. With
few exceptions, animal and vegetable prod-ucts contain from about 0.5 to 1.5 mg/100 g;vegetable oils from 10 to 60 mg/100 g; andcereal germ oils, which are a very goodsource, from 150 to 500 mg/100 g. Vegetableoils have the highest proportion of oc-toco-pherol, which amounts to about 60 percent ofthe total tocopherols. Refining of vegetableoils, carried out under normal precautions(such as excluding air), appears to result inlittle destruction of tocopherol. The toco-pherol and tocotrienol content of selectedfats and oils and their primary homologs arelisted in Table 9-7. The seed oils containonly tocopherol. Tree oils, palm, palm ker-nel, coconut oil, and rice bran oil also con-tain major amounts of tocotrienols. Theprocessing of vegetable oils by deodorizationor physical refining removes a considerable
Figure 9-7 Chemistry of the Tocopherols. Source: From R.A. Morton, The Chemistry of Tocopherols,in Tocopherole, K. Lang, ed., 1967, Steinkopff Verlag, Darmstadt, Germany.
a-focopforomc acid a-focop/ierono/acfon*
o-qufnonf
«e~ toccphtrolquinon*
quinol
#.-tocoph*rolmetastable
tpoxid*
irnvtrsibly onstanding
Table 9-6 Vitamin E Activity of cc-Tocopherollsomers and Synthetic Tocopherols
Name ILJ/mg
d-cc-tocopherol (2R4/R8/R) RRR-a- 1̂ 49tocopherol
1 -a-tocopherol (2S4'R8'R) 0.46dl-a-tocopherol 1.10
all-rac-a-tocopherol2R4/R8/S-a-tocopherol 1.342S4'R8'S-oc-tocopherol 0.552S4'S8'S-a-tocopherol 1.092S4'S8'R-a-tocopherol 0.312R4'S8'R-a-tocopherol 0.852S4'S8'S-a-tocopherol 1.10d-a-tocopheryl acetate RRR-a- 1.36
tocopheryl acetatedl-a-tocopherol 1.00
all-rac-a-tocopherol acetate
Source: Reprinted with permission from R.R. Eiten-miller, Vitamin E Content of Fats and Oils: NutritionalImplications, Food Technol., Vol. 51, no. 5, p. 79, ©1997, Institute of Food Technologists.
portion of the tocopherols, and these steam-volatile compounds accumulate in the fattyacid distillate (Ong 1993). This product is animportant source of natural vitamin E prepa-rations. Baltes (1967) carried out tests inwhich two easily oxidizable fats, lard andpartially hydrogenated whale oil, were stabi-lized with a-tocopherol and ascorbylpalmi-tate and citric acid as synergists. Withoutantioxidants, these fats cannot be used in thecommercial food chain. Amounts of a-toco-pherol ranging from 0.5 to 10 mg/100 g wereeffective in prolonging the storage life ofsome samples up to two years.
The tocopherol content of some animaland vegetable products as reported by Thaler(1967) is listed in Table 9-8. Cereals and
cereal products are good sources of to-copherol (Table 9-9). The distribution oftocopherol throughout the kernels is not uni-form, and flour of different degrees ofextraction can have different tocopherol lev-els. This was shown by Menger (1957) in astudy of wheat flour (Table 9-10).
Processing and storage of foods can resultin substantial tocopherol losses. An exampleis given in Table 9-11, where the loss oftocopherol during frying of potato chips isreported. After only two weeks' storage ofthe chips at room temperature, nearly half ofthe tocopherol was lost. The losses were onlyslightly smaller during storage at freezertemperature. Boiling of vegetables in waterfor up to 30 minutes results in only minorlosses of tocopherol. Baking of white breadresults in a loss of about 5 percent of thetocopherol in the crumb.
The human daily requirement of vitamin Eis estimated at 30 IU. Increased intake ofpolyunsaturated fatty acids increases theneed for this vitamin.
Vitamin K
This vitamin occurs in a series of differentforms, and these can be divided into twogroups. The first is vitamin K1 (Figure 9-8),characterized by one double bond in the sidechain. The vitamins K2 have a side chainconsisting of a number of regular units of thetype
CH3
R-[CH2—CH=C—CH2]n—H
where n can equal 4, 5, 6, 7, and so forth.Vitamin K1 is slowly decomposed by at-
mospheric oxygen but is readily destroyedby light. It is stable against heat, but unstableagainst alkali.
The human adult requirement is estimatedat about 4 mg per day. Menadione (2-methyl1,4-naphtoquinone) is a synthetic productand has about twice the activity of naturallyoccurring vitamin K.
Vitamin K occurs widely in foods and isalso synthesized by the intestinal flora. Goodsources of vitamin K are dark green vegeta-bles such as spinach and cabbage leaves, andalso cauliflower, peas, and cereals. Animalproducts contain little vitamin K1, except forpork liver, which is a good source.
The Vitamin K levels in some foods,expressed in menadione units, are given inTable 9-12.
WATER-SOLUBLE VITAMINS
Vitamin C (L-Ascorbic Acid)
This vitamin occurs in all living tissues,where it influences oxidation-reduction reac-tions. The major source of L-ascorbic acid infoods is vegetables and fruits (Table 9-13).
Source: Reprinted with permission from R.R. Eitenmiller, Vitamin E Content of Fats and Oils: Nutritional Implica-tions, Food Technol., Vol. 51, no. 5, p. 80, © 1997, Institute of Food Technologists.
Table 9-7 Tocopherol (T) and Tocotrienol (T3) Content of Vegetable Oils and Their Primary Homologs
Fats and Oils
SunflowerCottonseedSafflowerSafflower — high
linolenicSafflower — high oleicPalm
Canola
CornSoybeanRice branPeanutOliveCocoa butterPalm kernelButterLardCoconut
Total T+T3(mg/100g)
46-6778
49-8041
3289-117
65
78-10996-1159-160
375.1203.4
1.1-2.30.6
1.0-3.6
OC-7E/10Og
35-6343
41-4641
3121-34
25
20-3417-20
0.9-4116
5.13.01.9
1.1-2.30.6
0.3-0.7
%T
100100100100
10017-55
100
95100
19-491001009938
10010031
%T3
OOOO
O45-83
O
5O
51-81OO1
62OO
69
Primary Homologs
oc-T, y-Toc-T, y-Toc-T, 5-T, Y-T p-TCC-T1 P-T
(X-T1 P-T, Y-Toc-T, oc-TS, 6-T3, oc-T, 6-
T3Y-T, (X-T, 8-T,Ct-TS(Tr)1 P-T(Tr)Y-T, oc-T, 8-T, Y-T3, 6-T3Y-T, 8-T, oc-TY-T3, ccT, (X-T3, p-T, p-T3Y-T, oc-T, 8-Toc-TY-T, 8-T, oc-T, cc-T3cc-T3, CC-TCC-Tcc-TY-T3, cc-T3, 8-T, cc-T, P-
T3
Table 9-8 Tocopherol Content of Some Animaland Vegetable Food Products
Total Tocopherol asProduct a-Tocopherol (mg/100 g)Beef liver 0.9-1.6Veal, lean 0.9Herring 1.8Mackerel 1.6Crab, frozen 5.9Milk 0.02-0.15Cheese 0.4Egg 0.5-1.5Egg yolk 3.0Cabbage 2-3Spinach 0.2-6.0Beans 1-4Lettuce 0.2-0.8 (0.06)Peas 4-6Tomato 0.9 (0.4)Carrots 0.2(0.11)Onion 0.3 (0.22)Potato ?(0.12)Mushrooms 0.08
Source: From H. Thaler, Concentration and Stabil-ity of Tocopherols in Foods, in Tocopherols, K. Lang,ed., 1967, Steinkopff Verlag, Darmstadt, Germany.
L-ascorbic acid (Figure 9-9) is a lactone(internal ester of a hydroxycarboxylic acid)and is characterized by the enediol group,which makes it a strongly reducing com-pound. The D form has no biological activ-ity. One of the isomers, D-isoascorbic acid,or erythorbic acid, is produced commerciallyfor use as a food additive. L-ascorbic acid isreadily and reversibly oxidized to dehydro-L-ascorbic acid (Figure 9-10), which retainsvitamin C activity. This compound can befurther oxidized to diketo-L-gulonic acid, in a
Table 9-9 Tocopherol Content of Cereals andCereal Products
Total Tocopherol as oc-Product Tocopherol (mg/1 OO g)Wheat 7-10Rye 2.2-5.7Oats 1.8-4.9Rice (with hulls) 2.9Rice (polished) 0.4Corn 9.5Whole wheat meal 3.7Wheat flour 2.3-5.4Whole rye meal 2.0-4.5Oat flakes 3.85Corn grits 1.17Corn flakes 0.43White bread 2.15Whole rye bread 1.3Crisp bread 4.0
Source: From H. Thaler, Concentration and Stabil-ity of Tocopherols in Foods, in Tocopherols, K. Lang,ed., 1967, Steinkopff Verlag, Darmstadt, Germany.
nonreversible reaction. Diketo-L-gulonic acidhas no biological activity, is unstable, and isfurther oxidized to several possible com-pounds, including 1-threonic acid. Dehydra-tion and decarboxylation can lead to theformation of furfural, which can polymerizeto form brown pigments or combine withamino acids in the Strecker degradation.
Humans and guinea pigs are the only pri-mates unable to synthesize vitamin C. Thehuman requirement of vitamin C is not welldefined. Figures ranging from 45 to 75 mg/day have been listed as daily needs. Contin-ued stress and drug therapy may increase theneed for this vitamin.
Vitamin C is widely distributed in nature,mostly in plant products such as fruits (espe-
Table 9-10 Tocopherol Content of Wheat and ItsMilling Products
Tocopherolmg/100g
Product Ash (%) (Dry Basis)
Whole wheat 2X35 5̂ 04Flour 1 (fine) 1.68 5.90Flour 2 1.14 4.27Flour 3 0.84 3.48Flour 4 0.59 2.55Flour 5 0.47 2.35Flour 6 (coarse) 0.48 2.13Germ 4.10 25.0
Source: From A. Menger, Investigation of the Stabil-ity of Vitamin E in Cereal Milling Products and BakedGoods, Brot. Geback, Vol. 11, pp. 167-173,1957 (Ger-man).
cially citrus fruits), green vegetables, toma-toes, potatoes, and berries. The only animalsources of this vitamin are milk and liver.Although widely distributed, very high levelsof the vitamin occur only in a few products,such as rose hips and West Indian cherries.The concentration varies widely in differenttissues of fruits; for example, in apples, theconcentration of vitamin C is two to threetimes as great in the peel as in the pulp.
Vitamin C is the least stable of all vitaminsand is easily destroyed during processing andstorage. The rate of destruction is increasedby the action of metals, especially copper
Table 9-11 Tocopherol Losses DuringProcessing and Storage of Potato Chips
Tocopherol(mg/100 g) Loss (%)
Oil before use 82 —Oil after use 73 11Oil from fresh chips 75 —After two weeks at 39 48
room temperatureAfter one month at 22 71
room temperatureAfter two months at 17 77
room temperatureAfter one month at 28 63
-120CAfter two months at 24 68
-12.0C
and iron, and by the action of enzymes.Exposure to oxygen, prolonged heating inthe presence of oxygen, and exposure to lightare all harmful to the vitamin C content offoods. Enzymes containing copper or iron intheir prosthetic groups are efficient catalystsof ascorbic acid decomposition. The mostimportant enzymes of this group are ascorbicacid oxidase, phenolase, cytochrome oxi-dase, and peroxidase. Only ascorbic acid oxi-dase involves a direct reaction among en-zyme, substrate, and molecular oxygen. Theother enzymes oxidize the vitamin indirectly.Phenolase catalyzes the oxidation of mono-
Figure 9-8 Structural Formula of Vitamin K1
Table 9-12 Vitamin K in Some Foods (Expressedas Menadione Units per 100 g of Edible Portion)
Product Units/1 OO g
Cabbage, white 70Cabbage, red 18Cauliflower 23Carrots 5Honey 25Liver (chicken) 13Liver (pork) 111Milk 8Peas 50Potatoes 10Spinach 161Tomatoes (green) 24Tomatoes (ripe) 12Wheat 17Wheat bran 36Wheat germ 18
Table 9-13 Vitamin C Content of Some Foods
Ascorbic AcidProduct (mg/1 OO g)
Black currants 200Brussels sprouts 10OCauliflower 70Cabbage 60Spinach 60Orange 50Orange juice 40-50Lemon 50Peas 25Tomato 20Apple 5Lettuce 15Carrots 6Milk 2.1-2.7Potatoes 30
L-ASCORBIC ACID D-ASCORBIC ACID
L-ARABOASCORBIC ACIDD-ISOASCORBIC ACID(ERYTHORBIC ACID)
Figure 9-9 Structural Formulas of L-Ascorbic Acid and Its Stereoisomers
and dihydroxy phenols to quinones. Thequinones react directly with the ascorbicacid. Cytochrome oxidase oxidizes cyto-chrome to the oxidized form and this reactswith L-ascorbic acid.
Peroxidase, in combination with phenoliccompounds, utilizes hydrogen peroxide tobring about oxidation. The enzymes do notact in intact fruits because of the physicalseparation of enzyme and substrate. Mechan-ical damage, rot, or senescence lead to cellu-lar disorganization and initiate decomposition.Inhibition of the enzymes in vegetables isachieved by blanching with steam or by
Table 9-14 Effect of Blanching Method onAscorbic Acid Levels of Broccoli
Ascorbic Acid (mg/100 g)
Factor Effect Reduced Dehydro Total
"Raw 94̂ 0 4̂ 0 98.2Water blanch 45.3 5.7 51.0Steam blanch 48.8 7.4 56.2
Source: From D. Odland and M.S. Eheart, AscorbicAcid, Mineral and Quality Retention in Frozen BroccoliBlanched in Water, Steam, and Ammonia-Steam, J.Food ScL, Vol. 40, pp. 1004-1007, 1975.
electronic heating. Blanching is necessarybefore vegetables are dried or frozen. In fruitjuices, the enzymes can be inhibited by pas-teurization, deaeration, or holding at lowtemperature for a short period. The effect ofblanching methods on the ascorbic acid con-tent of broccoli was reported by Odland andEheart (1975). Steam blanching was foundto result in significantly smaller losses ofascorbic acid (Table 9-14). The retention ofascorbic acid in frozen spinach depends onstorage temperature. At a very low tempera-ture (-290C), only 10 percent of the initiallypresent ascorbic acid was lost after one year.At -12°, the loss after one year was muchhigher, 55 percent. The presence of metalchelating compounds stabilizes vitamin C.These compounds include anthocyanins andflavonols, polybasic or polyhydroxy acidssuch as malic and citric acids, and polyphos-phates.
Ascorbic acid is oxidized in the presenceof air under neutral and alkaline conditions.At acid pH (for example, in citrus juice), thevitamin is more stable. Because oxygen isrequired for the breakdown, removal of oxy-gen should have a stabilizing effect. For theproduction of fruit drinks, the water shouldbe deaerated to minimize vitamin C loss. Thetype of container may also affect the extent
Figure 9-10 Oxidation of L-Ascorbic Acid
L-ASCORBICACID
DEHYDRO-L-ASCORBICACID
DIKETO-L-GULONICACID
of ascorbic acid destruction. Use of tin cansfor fruit juices results in rapid depletion ofoxygen by the electrochemical process ofcorrosion. In bottles, all of the residual oxy-gen is available for ascorbic acid oxidation.To account for processing and storage losses,it is common to allow for a loss of 7 to 14 mgof ascorbic acid per 100 mL of fruit juice.Light results in rapid destruction of ascorbicacid in milk. It has been shown (Sattar anddeMan 1973) that transparent packagingmaterials permit rapid destruction of vitaminC (Figure 9-11). The extent of ascorbic aciddestruction is closely parallel to the develop-ment of off-flavors. The destruction of ascor-bic acid in milk by light occurs under theinfluence of riboflavin as a sensitizer. Thereaction occurs in the presence of light andoxygen, and the riboflavin is converted tolumichrome.
Factors that affect vitamin C destructionduring processing include heat treatment andleaching. The severity of processing condi-tions can often be judged by the percentageof ascorbic acid that has been lost. Theextent of loss depends on the amount ofwater used. During blanching, vegetablesthat are covered with water may lose 80 per-cent; half covered, 40 percent; and quartercovered, 40 percent of the ascorbic acid. Par-ticle size affects the size of the loss; forexample, in blanching small pieces of car-rots, losses may range from 32 to 50 percent,and in blanching large pieces, only 22 to 33percent. Blanching of cabbage may result ina 20 percent loss of ascorbic acid, and subse-quent dehydration may increase this to a totalof 50 percent. In the processing of milk,losses may occur at various stages. From aninitial level of about 22 mg/L in raw milk,
Figure 9-11 Effect of Exposure Time at Light Intensity of 200 Ft-C on the Loss of Ascorbic Acid inMilk. Packaging materials: (1) clear plastic pouch, (2) laminated nontransparent pouch, (3) carton, (4)plastic 3-quart jug. Source: From A. Sattar and J.M. deMan, Effect of Packaging Material on Light-Induced Quality Deterioration of Milk, Can. lnst. Food ScL Technol J., Vol. 6, pp. 170-174, 1973.
EXPOSURE TIME hours
L-A
SC
OR
BIC
A
CID
%
lo
ss
the content in the product reaching the con-sumer may be well below 10 mg/L. Furtherlosses may occur in the household duringstorage of the opened container.
The processing of milk into various dairyproducts may result in vitamin C losses. Icecream contains no vitamin C, nor doescheese. The production of powdered milkinvolves a 20 to 30 percent loss, evaporatedmilk a 50 to 90 percent loss. Bullock et al.(1968) studied the stability of added vitaminC in evaporated milk and found that adding266 mg of sodium ascorbate per kg was suf-ficient to ensure the presence of at least 140mg/L of ascorbic acid during 12 months ofstorage at 210C. Data on the stability of vita-min C in fortified foods have been assembledby deRitter (1976) (Table 9-15).
There are many technical uses of ascorbicacid in food processing. It is used to preventbrowning and discoloration in vegetablesand fruit products; as an antioxidant in fats,fish products, and dairy products; as a stabi-lizer of color in meat; as an improver offlour; as an oxygen acceptor in beer process-ing; as a reducing agent in wine, partiallyreplacing sulfur dioxide; and as an addednutrient. The vitamin is protected by sulfurdioxide, presumably by inhibiting polyphe-nolase.
Vitamin B1 (Thiamin)
This vitamin acts as a coenzyme in themetabolism of carbohydrates and is presentin all living tissues. It acts in the form of thi-amin diphosphate in the decarboxylation ofoc-keto acids and is referred to as cocarboxy-lase. Thiamin is available in the form of itschloride or nitrate, and its structural formulais shown in Figure 9-12. The molecule con-tains two basic nitrogen atoms; one is in theprimary amino group, the other in the quater-
Table 9-15 Vitamin C Stability in Fortified Foodsand Beverages after Storage at 230C for 12Months, Except as Noted
Retention
No. ofSam- Mean Range
Product pies (%) (%)
Ready-to-eat cereal 4 71 60-87Dry fruit drink mix 3 94 91-97Cocoa powder 3 97 80-100Dry whole milk, air 2 75 65-84
packDry whole milk, gas 1 93 —
packDry soy powder 1 81 —Potato flakes1 3 85 73-92Frozen peaches 1 80 —Frozen apricots2 1 80 —Apple juice 5 68 58-76Cranberry juice 2 81 78-83Grapefruit juice 5 81 73-86Pineapple juice 2 78 74-82Tomato juice 4 80 64-93Vegetable juice 2 68 66-69Grape drink 3 76 65-94Orange drink 5 80 75-83Carbonated beverage 3 60 54-64Evaporated milk 4 75 70-82
1 Stored for 6 months at 230C.2Thawed after storage in freezer for 5 months.
Source: From E. deRitter, Stability Characteristicsof Vitamins in Processed Foods, Food Technol., Vol.30, pp. 48-51,54, 1976.
nary ammonium group. It forms salts withinorganic and organic acids. The vitamincontains a primary alcohol group, which isusually present in the naturally occurringvitamin in esterified form with ortho-, di-, or
triphosphoric acid. In aqueous solution, thecompound may occur in different forms,depending on pH. In acid solution, the equi-librium favors the formation of positive ions(Figure 9-13). The thiol- form is favored inalkaline medium. This form can react withcompounds containing sulfhydryl groups toform disulfide bridges. It has been suggestedthat thiamin occurs in some foods linked toprotein by disulfide bridges.
Small quantities of thiamin are present inalmost all foods of plant and animal origin.Good sources are whole cereal grains; organmeats such as liver, heart, and kidney; leanpork; eggs; nuts; and potatoes (Table 9-16).Although thiamin content is usually mea-
sured in mg per 100 g of a food, another unithas been used occasionally, the IU corre-sponding to 3 jug of thiamin-hydrochloride.The human daily requirement is related tothe carbohydrate level of the diet. A mini-mum intake of 1 mg per 2,000 kcal is consid-ered essential. Increased metabolic activity,such as that which results from heavy work,pregnancy, or disease, requires higher intake.
Thiamin is one of the more unstable vita-mins. Various food processing operationsmay considerably reduce thiamin levels.Heat, oxygen, sulfur dioxide, leaching, andneutral or alkaline pH may all result indestruction of thiamin. Light has no effect.The enzyme is stable under acid conditions;
Figure 9-12 Structural Formula of Thiamin. Hydrochloride: X = Cl , HCl; Mononitrate: X = NO3 .
Figure 9-13 Behavior of Thiamin in Aqueous Solutions. Source: Reprinted with permission from J.Schormuller, The Composition of Foods, © 1965, Springer.
alkaline
Pseudobasepositive ions
acid
Thiol form
Table 9-16 Thiamin Content of Some Foods
Thiamin(mg/100g)
Product Edible Portion
Almonds 0.24Corn 0.37Egg 0.11Filberts 0.46Beef heart 0.53Beef liver 0.25Macaroni (enriched) 0.88Macaroni (not enriched) 0.09Milk 0.03Peas 0.28Pork, lean 0.87Potatoes 0.10Wheat (hard red spring) 0.57Wheat flour (enriched) 0.44Wheat flour (not enriched) 0.08
at pH values of 3.5 or below, foods can beautoclaved at 12O0C with little or no loss ofthiamin. At neutral or alkaline pH, the vita-min is destroyed by boiling or even by stor-age at room temperature. Even the slightalkalinity of water used for processing mayhave an important effect. Bender (1971)reports that cooking rice in distilled waterreduced thiamin content negligibly, whereascooking in tap water caused an 8 to 10 per-cent loss, and cooking in well water caused aloss of up to 36 percent.
Some fish species contain an enzyme thatcan destroy thiamin. Sulfur dioxide rapidlydestroys thiamin. For this reason, sulfurdioxide is not permitted as an additive infoods that contain appreciable amounts ofthiamin.
Baking of white bread may result in thia-min loss of 20 percent. Thiamin loss in milk
processing is as follows: pasteurization, 3 to20 percent; sterilization, 30 to 50 percent;spray drying, 10 percent; and roller drying,20 to 30 percent. Cooking of meat causeslosses that are related to size of cut, fat con-tent, and so on. Boiling loss is 15 to 40 per-cent; frying, 40 to 50 percent; roasting, 30 to60 percent; and canning, 50 to 75 percent.Similar losses apply to fish. Because thiaminand other vitamins are located near the branof cereal grains, there is a great loss duringmilling. White flour, therefore, has a greatlyreduced content of B vitamins and vitamin E(Figure 9-14). Not only is thiamin contentlowered by milling, but also storage of wholegrain may result in losses. This depends onmoisture content. At normal moisture levelof 12 percent, five months' storage results ina 12 percent loss; at 17 percent moisture, a30 percent loss; and at 6 percent moisture, noloss at all. Because of the losses that arelikely to occur in cereal grain processing andin the processing of other foods, a programof fortification of flour is an important factorin preventing vitamin deficiencies. Table9-17 lists the nutrients and recommendedlevels for grain products fortification (NationalAcademy of Sciences 1974b). A summary ofdata relating processing treatment to thiaminstability has been given by deRitter (1976)(Table 9-18).
Vitamin B2 (Riboflavin)
The molecule consists of a d-ribitol unitattached to an isoalloxazine ring (Figure9-15). Anything more than a minor changein the molecule results in a loss of vitaminactivity. Aqueous solutions of riboflavin areyellow with a yellowish-green fluorescence.The vitamin is a constituent of two coen-zymes, flavin mononucleotide (FMN) andflavin adenine dinucleotide (FAD). FMN is
riboflavin-5'-phosphate and forms part ofseveral enzymes, including cytochrome creductase. The flavoproteins serve as elec-tron carriers and are involved in the oxida-tion of glucose, fatty acids, amino acids, andpurines.
Very good sources of riboflavin are milkand milk products; other sources are beefmuscle, liver, kidney, poultry, tomatoes, eggs,green vegetables, and yeast (Table 9-19).
Riboflavin is stable to oxygen and acid pHbut is unstable in alkaline medium and is
Figure 9-14 Relation Between Extraction Rate and Proportion of Total Vitamins of the Grains Retainedin Flour. Source: Reprinted with permission from W.R. Aykroyd and J. Doughty, Wheat in HumanNutrition, © 1970, Food and Agriculture Organization of the United Nations.
Extraction rate (%)
Propo
rtion o
f vita
min r
etaine
d (%)
Table 9-17 Nutrients and Levels Recommendedfor Inclusion in Fortification of Cereal-GrainProducts1
Level
Nutrient (mg/lb) (mg/100g)
Vitamin A2 22 O48Thiamin 2.9 0.64Riboflavin 1.8 0.40Niacin 24.0 5.29Vitamin B6 2.0 0.44Folic acid 0.3 0.07Iron 40 8.81Calcium 900 198.2Magnesium 200 44.1Zinc 10 2.2
1Wheat flour, corn grits, cornmeal, rice. Othercereal-grain products in proportion to their cereal-graincontent.
2Retinol equivalent.
Source: Reprinted with permission from NationalAcademy of Sciences, Recommended Dietary Allow-ances, 8th rev. ed., © 1974, National Academy of Sci-ences.
very sensitive to light. When exposed tolight, the rate of destruction increases as pHand temperature increase. Heating underneutral or acidic conditions does not destroythe vitamin.
The human requirement for riboflavin var-ies with metabolic activity and body weightand ranges from 1 to 3 mg per day. Normaladult requirement is 1.1 to 1.6 mg per day. Inmost cases, the riboflavin of foods is presentin the form of the dinucleotide, the phospho-ric acid ester, or is bound to protein. Only inmilk does riboflavin occur mostly in the freeform.
Under the influence of light and alkalinepH, riboflavin is transformed into lumiflavin,
Table 9-18 Thiamin Stability in Foods
Reten-Product Treatment tion (%)
Nine canned Processing 31-89vegetables
Four canned Storage, 2-3 mo 73-94vegetables @ room
temperatureCereals Extrusion cooking 48-90Fortified ready-to- Storage, 12 mo @ 10O
eat cereal 230CBread (white, Commercial 74-79
whole wheat) bakingDevil's food cake Baking 0-7
(PH9)
Source: From E. deRitter, Stability Characteristics ofVitamins in Processed Foods, Food Technol., Vol. 30,pp. 48-51,54, 1976.
an inactive compound with a yellowish-green fluorescence. Under acid conditions,riboflavin is transformed into another inac-tive derivative, lumichrome, and ribitol. Thiscompound has a blue fluorescence. The trans-
Figure 9-15 Structural Formula of Riboflavin.Riboflavin: R = OH; Riboflavin phosphate: R =PO3NaOH.
Table 9-19 Riboflavin Content of Some Foods
Riboflavin (mg/100 g)Product Edible Portion
Beef oTeCabbage 0.05Eggs 0.30Chicken 0.19Beef liver 3.26Chicken liver 2.49Beef kidney 2.55Peas 0.29Spinach 0.20Tomato 0.04Yeast (dry) 5.41Milk 0.17Nonfat dry milk 1.78
formation into lumiflavin in milk results inthe destruction of ascorbic acid.
The light sensitivity of riboflavin results inlosses of up to 50 percent when milk isexposed to sunlight for two hours. The natureof the packaging material significantlyaffects the extent of riboflavin destruction. Itappears that the wavelengths of light respon-sible for the riboflavin destruction are in thevisible spectrum below 500 to 520 nm. Ultra-violet light has been reported to have nodestructive effect on riboflavin (Hartman andDry den 1965). Riboflavin is stable in drymilk for storage periods of up to 16 months.Pasteurization of milk causes only minorlosses of riboflavin.
Vitamin B6 (Pyridoxine)
There are three compounds with vitaminB6 activity. The structural formula of pyri-
doxine is presented in Figure 9-16. Theother two forms of this vitamin are differentfrom pyridoxine—they have another substit-uent on carbon 4 of the benzene ring. Pyri-doxal has a -CHO group in this position andpyridoxamine has a -CH2NH2 group. Allthree compounds can occur as salts. VitaminB6 plays an important role in the metabolismof amino acids, where it is active in the coen-zyme form pyridoxal-5-phosphate. The threeforms of vitamin B6 are equally active inrats; although it can be expected that thesame applies for humans, this has not beendefinitely established.
Vitamin B6 is widely distributed in manyfoods (Table 9-20), and deficiencies of thisvitamin are uncommon. The recommendedallowance for adults has been established at2 mg per day. The requirement appears toincrease with the consumption of high-pro-tein diets.
Vitamin B6 occurs in animal tissues in theform of pyridoxal and pyridoxamine or astheir phosphates. Pyridoxine occurs in plantproducts.
Pyridoxine is stable to heat and strongalkali or acid; it is sensitive to light, espe-cially ultraviolet light and when present inalkaline solutions. Pyridoxal and pyridoxam-
Figure 9-16 Structural Formula of Pyridoxine
Table 9-20 Vitamin B6 Content of Some Foods
Product Vitamin B6 (\ig/g)
Wheat 3.2-6.1Whole wheat bread 4.2White bread 1.0Orange juice 0.52-0.60Apple juice 0.35Tomatoes 1.51Beans, canned 0.42-0.81Peas, canned 0.44-0.53Beef muscle 0.8-4.0Pork muscle 1.23-6.8Milk, pasteurized 0.5-0.6Yeast 50
ine are rapidly destroyed when exposed toair, heat, or light. Pyridoxamine is readilydestroyed in food processing operations.
Because it is difficult to determine thisvitamin in foods, there is a scarcity of infor-mation on its occurrence. Recent data estab-lish the level in milk as 0.54 mg per liter.Other sources are meats, liver, vegetables,whole grain cereals, and egg yolk.
The effects of processing on pyridoxinelevels in milk and milk products have beenreviewed by Hartman and Dryden (1965).No significant losses have been reported to
result from pasteurization, homogenization,or production of dried milk. Heat steriliza-tion of milk, however, has been reported toresult in losses ranging from 36 to 49 per-cent. Losses occur not only during the heattreatment but also during subsequent storageof milk. These storage losses have beenattributed to a conversion of pyridoxal topyridoxamine and then to a different form ofthe vitamin. Wendt and Bernhart (1960) haveidentified this compound as fe/s^-pyridoxaldisulfide (Figure 9-17). This compound isformed by reaction of pyridoxal and activesulfhydryl groups. The latter are formed dur-ing heat treatment of milk proteins. Exposureof milk to daylight in clear glass bottles foreight hours resulted in a vitamin B6 loss of21 percent.
Food canning results in losses of vitaminB6 of 20 to 30 percent. Milling of wheat mayresult in losses of up to 80 to 90 percent.Baking of bread may result in losses of up to17 percent.
A review of some stability data of vitaminB6 as prepared by deRitter (1976) is given inTable 9-21.
Niacin
The term niacin is used in a generic sensefor both nicotinic acid and nicotinamide
Figure 9-17 Structural Formula of fcw-4-Pyridoxal Disulfide
(Figure 9-18). Nicotinamide acts as a com-ponent of two important enzymes, NAD andNADP, which are involved in glycolysis, fatsynthesis, and tissue respiration. Niacin isalso known as the pellagra preventive factor.The incidence of pellagra has declined but isstill a serious problem in parts of the NearEast, Africa, southeastern Europe, and inNorth American populations that subsist on
corn diets. When corn is treated with alkalior lime, as for the tortilla preparation in Cen-tral America, the amount of available niacincan be greatly increased. Tryptophan can beconverted by the body into niacin. Manydiets causing pellagra are low in good-qual-ity protein as well as in vitamins. Corn pro-tein is low in tryptophan. The niacin of cornand other cereals may occur in a bound form,
Figure 9-18 Structural Formulas of (A) Nicotinic Acid and (B) Nicotinamide
Source: From E. deRitter, Stability Characteristics of Vitamins in Processed Foods, Food Technol., Vol. 30, pp.48-51,54,1976.
Table 9-21 Vitamin B6 Stability in Foods
Product
Bread (added B6)Enriched corn meal
Enriched macaroni
Whole milk
Infant formula, liquid
Infant formula, dryBoned chicken
Treatment
Baking12 mo @ 380C + 50% relative
humidity12 mo @ 380C + 50% relative
humidity
Evaporation and sterilizationEvaporation and sterilization
+ 6 mo @ room temperatureProcessing and sterilization
Spray dryingCanningIrradiation (2.79 megarads)
Retention (%)
100
90-95
100
SaccharomycesCarlsbergensis
3018
Chick5544
Rat6541
33-50 (natural)84 (added)
69-835768
called niacytin, that can be converted intoniacin by alkali treatment.
The human requirement of niacin is re-lated to the intake of tryptophan. Animalproteins contain approximately 1.4 percentof tryptophan, vegetable proteins about 1percent. A dietary intake of 60 mg of tryp-tophan is considered equivalent to 1 mg ofniacin. When this is taken into account,average diets in the United States supply500 to 1,000 mg tryptophan per day and 8 to17 mg niacin for a total niacin equivalent of16 to 33 mg. The RDA for adults, expressedas niacin, is 6.6 mg per 1,000 kcal, and notless than 13 mg when caloric intake is lessthan 2,000 kcal.
Table 9-22 Niacin Content of Some Foods
Niacin (mg/100 g)Product Edible Portion
Barley (pearled) 3.1Beans (green, snap) 0.5Beans (white) 2.4Beef (total edible) 4.4Beef kidney 6.4Beef liver 13.6Chicken (dark meat) 5.2Chicken (light meat) 10.7Corn (field) 2.2Haddock 3.0Milk 0.1Mushrooms 4.2Peanuts 17.2Peas 2.9Potatoes 1.5Spinach 0.6Wheat 4.3Yeast (dry) 36.7
Good dietary sources of this vitamin areliver, kidney, lean meat, chicken, fish, wheat,barley, rye, green peas, yeast, peanuts, andleafy vegetables. In animal tissues, the pre-dominant form of niacin is the amide. Niacincontent of some foods are listed in Table9-22.
Niacin is probably the most stable of the Bvitamins. It is unaffected by heat, light, oxy-gen, acid, or alkali. The main loss resultingfrom processing involves leaching into theprocess water. Blanching of vegetables maycause a loss of about 15 percent. Processes inwhich brines are used may cause losses of upto 30 percent. Processing of milk, such aspasteurization, sterilization, evaporation,and drying have little or no effect on nico-tinic acid level. Virtually all the niacin inmilk occurs in the form of nicotmamide. Inmany foods, application of heat, such asroasting or baking, increases the amount ofavailable niacin. This results from the changeof bound niacin to the free form.
Vitamin B12 (Cyanocobalamine)
This vitamin possesses the most complexstructure of any of the vitamins and is uniquein that it has a metallic element, cobalt, in themolecule (Figure 9-19). The molecule is acoordination complex built around a centraltervalent cobalt atom and consists of twomajor parts—a complex cyclic structure thatclosely resembles the porphyrins and a nu-cleotide-like portion, 5,6-dimethyl-l-(oc-D-ribofuranosyl) benzimidazole-3'-phosphate.The phosphate of the nucleotide is esterifiedwith l-amino-2-propanol; this, in turn, isjoined by means of an amide bond with thepropionic acid side chain of the large cyclicstructure. A second linkage with the largestructure is through the coordinate bondbetween the cobalt atom and one of the nitro-
gen atoms of the benzimidazole. The cyanidegroup can be split off relatively easily, forexample, by daylight. This reaction can bereversed by removing the light source. Thecyano group can also be replaced by othergroups such as hydroxo, aquo, and nitroto.Treatment with cyanide will convert thesegroups back to the cyano form. The differentforms all have biological activity.
Cyanocobalamine is a component of sev-eral coenzymes and has an effect on nucleicacid formation through its action in cycling5-methyl-tetrahydrofolate back into the folatepool. The most important dietary sources ofthe vitamin are animal products. Vitamin B12
is also produced by many microorganisms. Itis not surprising that vitamin B12 deficiencyof dietary origin only occurs in vegetarians.
The average diet in the United States isconsidered to supply between 5 and 15 |Hg/day. In foods, the vitamin is bound to pro-teins via peptide linkages but can be readilyabsorbed in the intestinal tract. The RDA is 3|ig for adults and adolescents.
Few natural sources are rich in vitaminB12. However, only very small amounts arerequired in the diet. Good sources are leanmeat, liver, kidney, fish, shellfish, and milk(Table 9-23). In milk, the vitamin occurs ascobalamine bound to protein.
Vitamin B12 is not destroyed to a greatextent by cooking, unless the food is boiled inalkaline solution. When liver is boiled inwater for 5 minutes, only 8 percent of thevitamin B12 is lost. Broiling of meat mayresult in higher losses. Pasteurization causes
Figure 9-19 Structural Formula of Cyanocobalamine
Table 9-23 Vitamin B12 Content of Some Foods
Product Vitamin B12
Beef muscle 0.25-3.4 jig/1 OO g
Beef liver 14-152 (ig/1 OO gMilk 3.2-12.4 |ig/LShellfish 600-970 p.g/1 OO g (dry wt)Egg yolk 0.28-1.556 jig/1 OO g
only a slight destruction of vitamin B12 inmilk; losses range from 7 to 10 percentdepending on pasteurization method. Moredrastic heat treatment results in higher losses.Boiling milk for two to five minutes causes a30 percent loss, evaporation about 50 percent,and sterilization up to 87 percent. The loss indrying of milk is smaller; in the production ofdried skim milk, the vitamin B12 loss is about30 percent. Ultra-high-temperature steriliza-tion of milk does not cause more vitamin B12
destruction than does pasteurization.
Folic Acid (Folacin)
Folic acid is the main representative of aseries of related compounds that containthree moieties: pterin, /7-aminobenzoic acid,and glutamic acid (Figure 9-20). The com-mercially available form contains one glutamic
acid residue and is named pteroylglutamicacid (PGA). The naturally occurring formsare either PGA or conjugates with varyingnumbers of glutamic acid residues, such astri- and heptaglutamates. It has been sug-gested that folic acid deficiency is the mostcommon vitamin deficiency in North Amer-ica and Europe. Deficiency is especiallylikely to occur in pregnant women.
The vitamin occurs in a variety of foods,especially in liver, fruit, leafy vegetables, andyeast (Table 9-24) (Hurdle et al. 1968; Stre-iff 1971). The usual form of the vitamin inthese products is a polyglutamate. The actionof an enzyme (conjugase) is required to lib-erate the folic acid for metabolic activity;this takes place in the intestinal mucosa. Thefolacin of foods can be divided into two maingroups on the basis of its availability to L.casei: (1) the so-called free folate, which isavailable to L. casei without conjugase treat-ment; and (2) the total folate, which alsoincludes the conjugates that are not normallyavailable to L. casei. About 25 percent of thedietary folacin occurs in free form. Thefolate in vegetables occurs mainly in the con-jugated form; the folate in liver occurs in thefree form.
The RDA for folacin is 400 (Ig for adults.There is an additional requirement of 400|iig/day during pregnancy and 200 |iig/dayduring breastfeeding.
Figure 9-20 Structural Formula of Folic Acid
Table 9-24 Folate Content of Some Foods
FolateProduct fag/g)
Beef, boiled 0.03Chicken, roasted 0.07Cod, fried 0.16Eggs, boiled 0.30Brussels sprouts, boiled 0.20Cabbage, boiled 0.11Lettuce 2.00Potato, boiled 0.12Spinach, boiled 0.29Tomato 0.18Orange 0.45Milk 0.0028Bread, white 0.17Bread, brown 0.38Orange juice, frozen reconstituted 0.50Tomato juice, canned 0.10
Many of the naturally occurring folates areextremely labile and easily destroyed bycooking. Folic acid itself is stable to heat inan acid medium but is rapidly destroyedunder neutral and alkaline conditions. Insolution, the vitamin is easily destroyed bylight. Folate may occur in a form more active
than PGA; this is called folinic acid or citro-vorum factor, which is N5-formyl-5, 6, 7, 8-tetrahydro PGA (Figure 9-21). The folate ofmilk consists of up to 20 percent of folinicacid. It has been reported that pasteurizationand sterilization of milk involve only smalllosses or no loss. Hurdle et al. (1968) re-ported that boiling of milk causes no loss infolate; however, boiling of potato results in a90 percent loss and boiling of cabbage a 98percent loss. Reconstitution of dried milkfollowed by sterilization as can occur withbaby formulas may lead to significant folacinlosses. Fermentation of milk and milk prod-ucts may result in greatly increased folatelevels. Blanching of vegetables and cookingof meat do not appear to cause folic acidlosses. Table 9-25 contains a summary offolate stability data prepared by deRitter(1976). Citrus fruit and juices are relativelygood sources of folic acid, which is presentmostly as the reduced 5-methyl tetrahydro-folate (monoglutamate form). There are alsopolyglutamate derivatives present (White etal. 1991).
Pantothenic Acid
The free acid (Figure 9-22) is very unsta-ble and has the appearance of a hygroscopic
Figure 9-21 Structural Formula of Folinic Acid
Source: From E. deRitter, Stability Characteristics ofVitamins in Processed Foods, Food Technol., Vol. 30,pp. 48-51,54, 1976.
oil. The calcium and sodium salts are morestable. The alcohol (panthenol) has the samebiological activity as the acid. Only the dex-trorotatory or D form of these compoundshas biological activity. Pantothenic acidplays an important role as a component ofcoenzyme A, and this is the form in which itoccurs in most foods.
Pantothenic acid occurs in all living cellsand tissues and is, therefore, found in mostfood products. Good dietary sources includemeats, liver, kidney, fruits, vegetables, milk,egg yolk, yeast, whole cereal grains, and nuts(Table 9-26). In animal products, most of thepantothenic acid is present in the bound
Table 9-26 Pantothenic Acid Content of SomeFoods
Pantothenic AcidProduct fag/g)
Beef, lean 10Wheat 11Potatoes 6.5Split peas 20-22Tomatoes 1Orange 0.7Walnuts 8Milk 1.3-4.2Beef liver 25-60Eggs 8-48Broccoli 46
form, but in milk only about one-fourth ofthe vitamin is bound.
There is no recommended dietary allow-ance for this vitamin because of insufficientevidence to base one on. It is estimated thatadult dietary intake in the United Statesranges from 5 to 20 mg/day, and 5 to 10 mg/day probably represents an adequate intake.
The vitamin is stable to air, and labile todry heat. It is stable in solution in the pHrange of 5 to 7 and less stable outside thisrange. Pasteurization and sterilization ofmilk result in very little or no loss. The pro-duction and storage of dried milk involveslittle or no loss of pantothenic acid. Manu-
Table 9-25 Folic Acid Stability in Foods
Product
CabbagePotatoesRiceBeef, pork,
and chickenVarious
foods
Treatment
Boiled 5 minBoiled 5 minBoiled 15 minBoiled 15 min
Cooked
Retention ofFolic Add
Free (%)
3250
<50
27
Total(%)549210
<50
55
Figure 9-22 Structural Formula of Pantothenic Acid. Pantothenic acid: R = COOH; Panthenol: R =CH2OH.
Figure 9-23 Structural Formula of Biotin
facture of cheese involves large losses duringprocessing, but during ripening the pan-tothenic acid content increases, due to syn-thesis by microorganisms. Blanching of veg-etables may involve losses of up to 30 per-cent. Boiling in water involves losses thatdepend on the amount of water used.
Biotin
The structural formula (Figure 9-23) con-tains three asymmetric carbon atoms, andeight different stereoisomers are possible.Only the dextrorotatory D-biotin occurs innature and has biological activity. Biotinoccurs in some products in free form (vege-tables, milk, and fruits) and in other productsis bound to protein (organ meats, seeds, andyeast). Good sources of the vitamin are meat,liver, kidney, milk, egg yolk, yeast, vegeta-bles, and mushrooms (Table 9-27).
Biotin is important in a number of meta-bolic reactions, especially in fatty acid syn-thesis. The biotin supply of the human or-ganism is only partly derived from the diet.
An important factor in biotin's availabilityis that some of the vitamin is derived fromsynthesis by intestinal microorganisms; thisis demonstrated by the fact that three to six
Table 9-27 Biotin Content of Some Foods
Product Biotin fog/1 OO g)
"Milk 1.1-3.7Tomatoes 1Broad beans 3Cheese 1.1-7.6Wheat 5.2Beef 2.6Beef liver 96Lettuce 3.1Mushrooms 16Potatoes 0.6Spinach 6.9Apples 0.9Oranges 1.9Peanuts 34
times more biotin is excreted in the urinethan is ingested with the food. The dailyintake of biotin is between 100 and 300 (Ig.No recommended dietary allowance hasbeen established. Biotin is deactivated byraw egg white. This is caused by the glyco-protein avidin. Heating of avidin will destroythe inactivator capacity for biotin.
Data on the stability of biotin are limited.The vitamin appears to be quite stable. Heattreatment results in relatively small losses.The vitamin is stable to air and is stable atneutral and acid pH. Pasteurization and ster-ilization of milk result in losses of less than10 percent. In the production of evaporatedand dried milk, losses do not exceed 15 per-cent.
VITAMINS AS FOOD INGREDIENTS
In addition to their role as essential micro-nutrients, vitamins may serve as food ingre-
Figure 9-24 Prevention of Lipid Free RadicalFormation by Ascorbyl Palmitate. Source: FromM.L. Liao and P.A. Seib, Selected Reactions ofL-Ascorbic Acid Related to Foods, Food Tech-noL, Vol. 41, no. 11, pp. 104-107, 1987.
dients for their varied functional properties(Institute of Food Technologists 1987). Vita-min C and vitamin E have found widespreaduse as antioxidants. In lipid systems, vitamin
E may be used as an antioxidant in fats thathave little or no natural tocopherol content.Ascorbic acid in the form of its palmitic acidester, ascorbyl palmitate, is an effective anti-oxidant in lipid systems. Ascorbyl palmitateprevents the formation of lipid free radicals(Figure 9-24) and thereby delays the initia-tion of the chain reaction that leads to thedeterioration of the fat (Liao and Seib 1987).Ascorbyl palmitate is used in vegetable oilsbecause it acts synergistically with naturallyoccurring tocopherols. The tocopherols arefat-soluble antioxidants that are used in ani-mal fats. Ascorbic acid reduces nitrous acidto nitric oxide and prevents the formation ofN-nitrosamine. The reaction of nitrous acidand ascorbic acid is given in Figure 9-25(Liao and Seib 1987). Ascorbic acid is alsowidely used to prevent enzymic browning infruit products. Phenolic compounds are oxi-dized by polyphenoloxidase to quinones.The quinones rapidly polymerize to formbrown pigments. This reaction is easily re-versed by ascorbic acid (Figure 9-26).
LIPID RADICALASCORBYL PALMITATERADICAL
ASCORBYLPALMITATE
FREERADICAL
PARTIAL STRUCTUREOF LIPID
2-NITRITE ESTEROF L-ASCORBIC ACID
NITRICOXIDE
L-ASCORBICACID
L-ASCORBATE RADICAL
Figure 9-25 Reaction Between Nitrous Acid and Ascorbic Acid. Source: From M.L. Liao and P.A.Seib, Selected Reactions of L-Ascorbic Acid Related to Foods, Food Technol, Vol. 41, no. 11, pp.104-107, 1987.
The carotenoids p-carotene and p-apo-8-carotenal are used as colorants in fat-basedas well as water-based foods.
Other functions of ascorbic acid are inhibi-tion of can corrosion in canned soft drinks,
REFERENCES
Baltes, J. 1967. Tocopherols as fat stabilizers. In To-copherol, K. Lang, ed. Darmstadt, Germany: Stein-kopff Verlag.
Bender, A.E. 1971. The fate of vitamins in food pro-cessing operations. In Vitamins, ed. M. Stein. Lon-don: Churchill Livingstone.
Bullock, D.H., et al. 1968. Stability of vitamin C inenriched commercial evaporated milk. J. Dairy ScL51:921-923.
Combs, G.F. 1992. The vitamins: Fundamental aspectsin nutrition and health. San Diego: Academic Press.
deMan, J.M. 1981. Light-induced destruction of vita-min A in milk. J. Dairy ScL 64: 2031-2032.
deMan, J.M., et al. 1986. Stability of vitamin A bead-lets in nonfat dry milk. Milchwissenschaft 41:468^69.
protection of flavor and color of wine, pre-vention of black spot formation in shrimp,stabilization of cured meat color, and doughimprovement in baked goods (Institute ofFood Technologists 1987).
deRitter, E. 1976. Stability characteristics of vitaminsin processed foods. Food TechnoL 30: 48-51, 54.
Eitenmiller, R.R. 1997. Vitamin E content of fats andoils: Nutritional implications. Food TechnoL 51, no.5:78-81.
Gross, J. 1987. Pigments in fruits. London: HarcourtBrace Jovanovich.
Hartman, A.M., and Dryden, L.P. 1965. Vitamins inmilk and milk products. Am. Dairy Sci. Assoc.Champaign, IL.
Hurdle, A.D.F., et al. 1968. A method for measuringfolate in food and its application to a hospital diet.Am. J. Clin. Nutr. 21: 1202-1207.
Institute of Food Technologists. 1987. Use of vitaminsas additives in processed foods. Food TechnoL 41,no. 9: 163-168.
BROWNPIGMENTS
PHENOLICCQMPOUMD
0-QUINONE
L-ASCORBICACID
DEHYDROASCOR3ICACID
Figure 9-26 Reduction of Ortho-Quinone by Ascorbic Acid During Enzymic Browning. Source: FromM.L. Liao and RA. Seib, Selected Reactions of L-Ascorbic Acid Related to Foods, Food TechnoL, Vol.41, no. 11, pp. 104-107,1987.
Liao, M.-L., and P.A. Seib. 1987. Selected reactions ofL-ascorbic acid related to foods. Food Technol 41,no. 11: 104-107, 111.
Menger, A. 1957. Investigation of the stability of vita-min E in cereal milling products and baked goods.Brot. Gebdck 11: 167-173 (German).
Morton, R.A. 1967. The chemistry of tocopherols. InTocopherol, ed. K. Lang. Darmstadt, Germany:Steinkopff Verlag.
National Academy of Sciences. 1974a. Recommendeddietary allowances, 8th rev. ed. Washington, DC.
National Academy of Sciences. 1974b. Proposed forti-fication policy for cereal-grain products. Washing-ton, DC.
Odland, D., and M.S. Eheart. 1975. Ascorbic acid,mineral and quality retention in frozen broccoliblanched in water, steam and ammonia-steam. J.Food ScL 40: 1004-1007.
Ong, A.S.H. 1993. Natural sources of tocotrienols. InVitamin E in health and disease, ed. L. Packer and J.Fuchs. New York: Marcel Dekker.
Rouseff, R.L., and S. Nagy. 1994. Health and nutri-tional benefits of citrus fruit components. FoodTechnol. 48, no. 11: 126-132.
Sattar, A., and J.M. deMan. 1973. Effect of packagingmaterial on light-induced quality deterioration ofmilk. Can. Inst. Food ScL Technol. J. 6: 170-174.
Streiff, R.R. 1971. Folate levels in citrus and otherjuices. Am. /. Clin. Nutr. 24: 1390-1392.
Thaler, H. 1967. Concentration and stability of toco-pherols in foods. In Tocopherol, ed. K. Lang. Darms-tadt, Germany: Steinkopff Verlag
Wendt, G., and E.W. Bernhart. 1960. The structure of asulfur-containing compound with vitamin B6 activ-ity. Arch. Biochem. Biophys. 88: 270-272.
White, D.R., et al. 1991. Reverse phase HPLC/ECdetermination of folate in citrus juice by direct injec-tion with column switching. J. Agr. Food Chem. 39:714-717.
INTRODUCTION
Enzymes, although minor constituents ofmany foods, play a major and manifold rolein foods. Enzymes that are naturally presentin foods may change the composition ofthose foods; in some cases, such changes aredesirable but in most instances are undesir-able, so the enzymes must be deactivated.The blanching of vegetables is an example ofan undesirable change that is deactivated.Some enzymes are used as indicators in ana-lytical methods; phosphatase, for instance, isused in the phosphatase test of pasteurizationof milk. Enzymes are also used as processingaids in food manufacturing. For example,rennin, contained in extract of calves' stom-achs, is used as a coagulant for milk in theproduction of cheese.
Food science's emphasis in the study ofenzymes differs from that in biochemistry.The former deals mostly with decompositionreactions, hydrolysis, and oxidation; the lat-ter is more concerned with synthetic mecha-nisms. Whitaker (1972) has prepared anextensive listing of the uses of enzymes infood processing (Table 10-1) and this gives agood summary of the many and varied possi-ble applications of enzymes.
NATURE AND FUNCTION
Enzymes are proteins with catalytic prop-erties. The catalytic properties are quite spe-cific, which makes enzymes useful in ana-lytical studies. Some enzymes consist onlyof protein, but most enzymes contain addi-tional nonprotein components such as carbo-hydrates, lipids, metals, phosphates, or someother organic moiety. The complete enzymeis called holoenzyme; the protein part,apoenzyme\ and the nonprotein part, coj'ac-tor. The compound that is being converted inan enzymic reaction is called substrate. In anenzyme reaction, the substrate combineswith the holoenzyme and is released in amodified form, as indicated in Figure 10-1.An enzyme reaction, therefore, involves thefollowing equations:
* iEnzyme + substrate ^ ^* complex
*2
*3
^ - enzyme + products
The equilibrium for the formation of thecomplex is given by
K = [E][S]m [ES]
Enzymes
CHAPTER 10
Enzyme
Amylases
Cellulase
Dextran-sucrase
Invertase
Lactase
TannasePentosanaseNaringinase
Pectic enzymes (use-ful)
Food
Baked goodsBrewing
Cereals
Chocolate-cocoaConfectioneryFruit juicesJelliesPectinSyrups and sugars
VegetablesBrewingCoffeeFruits
Sugar syrupsIce creamArtificial honeyCandy
Ice Cream
FeedsMilk
BrewingMillingCitrus
Chocolate-cocoaCoffee
FruitsFruit juices
OlivesWines
Purpose or Action
Increase sugar content for yeast fermentationConversion of starch to maltose for fermentation;
removal of starch turbiditiesConversion of starch to dextrins, sugar; increase
water absorptionLiquidification of starches for free flowRecovery of sugar from candy scrapsRemove starches to increase sparkling propertiesRemove starches to increase sparkling propertiesAn aid in preparation of pectin from apple pomaceConversion of starches to low molecular weight dex-
trins (corn syrup)Hydrolysis of starch as in tenderization of peasHydrolysis of complex carbohydrate cell wallsHydrolysis of cellulose during drying of beansRemoval of graininess of pears; peeling of apricots,
tomatoesThickening of syrupThickening agent, bodyConversion of sucrose to glucose and fructoseManufacture of chocolate-coated, soft, cream can-
diesPrevent crystallization of lactose, which results in
grainy, sandy textureConversion of lactose to galactose and glucoseStabilization of milk proteins in frozen milk by
removal of lactoseRemoval of polyphenolic compoundsRecovery of starch from wheat flourDebittering citrus pectin juice by hydrolysis of the
glucoside, naringinHydrolytic activity during fermentation of cocoaHydrolysis of gelatinous coating during fermentation
of beansSofteningImprove yield of press juices, prevent cloudiness,
improve concentration processesExtraction of oilClarification
continues
Table 10-1 Uses and Suggested Uses of Enzymes in Food Processing
Enzyme
Pectic enzymes(deteriorative)
Proteases(useful)
Proteases(deteriorative)
Lipase (useful)
Lipase(deteriorative)
Phosphatases
NucleasesPeroxidases
(useful)
Food
Citrus juice
FruitsBaked goods
Brewing
Cereals
Cheese
Chocolate-cocoaEggs, egg productsFeeds
Meats and fish
MilkProtein hydrolysates
WinesEggsCrab, lobsterFlourCheeseOilsMilk
Cereals
Milk and dairy productsOilsBaby foodsBrewingMilkFlavor enhancersVegetablesGlucose determinations
Purpose or Action
Destruction and separation of pectic substances ofjuices
Excessive softening actionSoftening action in doughs; cut mixing time, increase
extensibility of doughs; improvement in grain, tex-ture, loaf volume; liberate p-amylase
Body, flavor and nutrients development during fer-mentation; aid in filtration and clarification, chill-proofing
Modify proteins to increase drying rate, improveproduct handling characteristics; manufacture ofmiso and tofu
Casein coagulation; characteristic flavors duringaging
Action on beans during fermentationImprove drying propertiesUse in treatment of waste products for conversion to
feedsTenderization; recovery of protein from bones, trash
fish; liberation of oilsIn preparation of soybean milkCondiments such as soy sauce and tamar sauce;
specific diets; bouillon, dehydrated soups, gravypowders, processed meats
ClarificationShelf life of fresh and dried whole eggsOvertenderization if not inactivated rapidlyInfluence on loaf volume, texture if too activeAging, ripening, and general flavor characteristicsConversion of lipids to glycerol and fatty acidsProduction of milk with slightly cured flavor for use in
milk chocolateOverbrowning of oat cakes; brown discoloration of
wheat branHydrolytic rancidityHydrolytic rancidityIncrease available phosphateHydrolysis of phosphate compoundsDetection of effectiveness of pasteurizationProduction of nucleotides and nucleosidesDetection of effectiveness of blanchingIn combination with glucose oxidase
continues
Table 10-1 continued
Enzyme
Peroxidases(deteriorative)
Catalase
Glucose oxidase
Polyphenol oxidase(useful)
Polyphenol oxidase(deteriorative)
Lipoxygenase
Ascorbic acid oxidaseThiaminase
Food
VegetablesFruitsMilkVariety of products
Variety of products
Glucose determination
Tea, coffee, tobacco
Fruits, vegetables
Vegetables
Vegetables, fruitsMeats, fish
Purpose or Action
Off-flavorsContribution to browning actionDestruction of H2O2 in cold pasteurization
To remove glucose and/or oxygen to prevent brown-ing and/or oxidation; used in conjunction with glu-cose oxidase
Removal of oxygen and/or glucose from productssuch as beer, cheese, carbonated beverages,dried eggs, fruit juices, meat and fish, milk powder,wine to prevent oxidation and/or browning; used inconjunction with catalase
Specific determination of glucose; used in conjunc-tion with peroxidase
Development of browning during ripening, fermenta-tion, and/or aging process
Browning, off-flavor development, loss of vitamins
Destruction of essential fatty acids and vitamin A;development of off-flavors
Destruction of vitamin C (ascorbic acid)Destruction of thiamine
Source: Reprinted with permission from J. R. Whitaker, Principles of Enzymology for the Food Sciences, 1 972, bycourtesy of Marcel Dekker, Inc.
Table 10-1 continued
whereE, S, and ES are the enzyme, substrate, and
complex, respectivelyKm is the equilibrium constant
This can be expressed in the form of theMichaelis-Menten equation, as follows:
v = v is][S] + Kn
wherev is the initial short-time velocity of the
reaction at substrate concentration [S]
V is the maximum velocity that can beattained at a high concentration of thesubstrate where all of the enzyme is inthe form of the complex
This equation indicates that when v is equal toone-half of K the equilibrium constant Km isnumerically equal to S. A plot of the reactionrates at different substrate concentrations canbe used to determine Km. Because it is notalways possible to attain the maximum reac-tion rate at varying substrate concentrations,the Michaelis-Menten equation has beenmodified by using reciprocals and in this
form is known as the Lineweaver-Burkeequation,
i - I K™v " V+ V[S]
Plots of 1/v as a function of 1/[S] result instraight lines; the intercept on the Y-axis rep-resents 1/V; the slope equals KnJV', and fromthe latter, Km can be calculated.
Enzyme reactions follow either zero-orderor first-order kinetics. When the substrateconcentration is relatively high, the concen-tration of the enzyme-substrate complex willbe maintained at a constant level and the
amount of product formed is a linear func-tion of the time interval. Zero-order reactionkinetics are characteristic of catalyzed reac-tions and can be described as follows:
d[S]_k.dt
whereS is substrate and k° is the zero-order reac-
tion constant
First-order reaction kinetics are character-ized by a graduated slowdown of the forma-tion of product. This is because the rate of itsformation is a function of the concentration
Products
Figure 10-1 The Nature of Enzymes—Substrate Reactions
Holoenzyme - product complex
Holoenzyme-substrote complex
Holoenzyme Apoenzyme - substrote complex
SubstroteCofoctorApoenzyme
of unreacted substrate, which decreases asthe concentration of product increases. First-order reaction kinetics follow the equation,
^ = ft1 ( [S l - [ F l )
whereP is product and k{ is the first-order reac-
tion constant
For relatively short reaction times, theamount of substrate converted is proportionalto the enzyme concentration.
Each enzyme has one—and some enzymeshave more—optimum pH values. For mostenzymes this is in the range of 4.5 to 8.0.Examples of pH optima are amylase, 4.8;invertase, 5.0; and pancreatic a-amylase, 6.9.The pH optimum is usually quite narrow,although some enzymes have a broader opti-mum range; for example, pectin methyl-esterase has a range of 6.5 to 8.0. Someenzymes have a pH optimum at very high orvery low values, such as pepsin at 1.8 andarginase at 10.0.
Temperature has two counteracting effectson the activity of enzymes. At lower temper-atures, there is a g10 of about 2, but at tem-peratures over 4O0C, the activity quicklydecreases because of denaturation of the pro-tein part of the enzymes. The result of thesefactors is a bell-shaped activity curve with adistinct temperature optimum.
Enzymes are proteins that are synthesizedin the cells of plants, animals, or microorgan-isms. Most enzymes used in industrial appli-cations are now obtained from microor-ganisms. Cofactors or coenzymes are small,heat-stable, organic molecules that mayreadily dissociate from the protein and canoften be removed by dialysis. These coen-zymes frequently contain one of the B vita-mins; examples are tetrahydrofolic acid andthiamine pyrophosphate.
Specificity
The nature of the enzyme-substrate reac-tion as explained in Figure 10-1 requires thateach enzyme reaction is highly specific. Theshape and size of the active site of theenzyme, as well as the substrate, are impor-tant. But this complementarity may be evenfurther expanded to cover amino acid resi-dues in the vicinity of the active site, hydro-phobic areas near the active site, or thepresence of a positive electrical charge nearthe active site (Parkin 1993). Types of speci-ficity may include group, bond, stereo, andabsolute specificity, or some combination ofthese. An example of the specificity of en-zymes is given in Figure 10-2, which illus-trates the specificity of proline-specific pep-tidases (Habibi-Najafi and Lee 1996). Theamino acid composition of casein is high inproline, and the location of this amino acid inthe protein chain is inaccessible to commonaminopeptidases and the di- and tripepti-dases with broad specificity. Hydrolysis ofthe proline bonds requires proline-specificpeptidases, including several exopeptidasesand an endopeptidase. Figure 10-2 illustratesthat this type of specificity is related to thetype of amino acid in a protein as well as itslocation in the chain. Neighboring aminoacids also determine the type of peptidaserequired to hydrolyze a particular peptidebond.
Classification
Enzymes are classified by the Commissionon Enzymes of the International Union ofBiochemistry. The basis for the classificationis the division of enzymes into groupsaccording to the type of reaction catalyzed.This, together with the name or names ofsubstrate(s), is used to name individualenzymes. Each well-defined enzyme can be
described in three ways—by a systematicname, by a trivial name, and by a number ofthe Enzyme Commission (EC). Thus, theenzyme oc-amylase (trivial name) has thesystematic name a-l,4-glucan-4-glucanohy-drolase, and the number EC 3.2.1.1. The sys-tem of nomenclature has been described byWhitaker (1972; 1974) and Parkin (1993).
Enzyme Production
Some of the traditionally used industrialenzymes (e.g., rennet and papain) are pre-pared from animal and plant sources. Recentdevelopments in industrial enzyme produc-tion have emphasized the microbial enzymes(Frost 1986). Microbial enzymes are veryheat stable and have a broader pH optimum.Most of these enzymes are made by sub-merged cultivation of highly developedstrains of microorganisms. Developments in
biotechnology will make it possible to trans-fer genes for the elaboration of specificenzymes to different organisms. The majorindustrial enzyme processes are listed inTable 10-2.
HYDROLASES
The hydrolases as a group include allenzymes that involve water in the formationof their products. For a substrate AB, thereaction can be represented as follows:
AB + HOH -4 HA + BOH
The hydrolases are classified on the basis ofthe type of bond hydrolyzed. The mostimportant are those that act on ester bonds,glycosyl bonds, peptide bonds, and C-Nbonds other than peptides.
Figure 10-2 Mode of Action of Prolme-Specific Peptidases. Source: Reprinted with permission fromM.B. Habibi-Najafi and B.H. Lee, Bitterness in Cheese: A Review, Crit. Rev. Food ScL Nutr., Vol. 36,No. 5, p. 408. Copyright CRC Press, Boca Raton, Florida.
ProteasesRennetTrypsinPapainFungalFungal (rennins)Bacterial
GlycosidasesBacterial oc-amylase
Fungal cc-amy-lase
(3-amylaseAmyloglucosi-
dasePectinaseCellulaseYeast lactaseMold lactase
OthersGlucose
isomeraseGlucose oxidaseMold catalaseAnimal catalaseLipase
Calf stomachAnimal pancreasCarica papaya fruitAspergillus oryzaeMucorspp.Bacillus spp.
Bacillus spp.Aspergillus oryzae
BarleyAspergillus niger
Aspergillus nigerMoldsKluyveromyces spp.Aspergillus spp.
Various microbialsources
Aspergillus nigerAspergillus nigerLiverMolds
Source: From G.M. Frost, Commercial Production of Enzymes, in Developments in Food Proteins, BJ.F. Hud-son, ed., 1986, Elsevier Applied Science Publishers Ltd.
Enzyme Source
Table 10-2 Major Industrial Enzymes and the Process Used for Their Production
Sub
mer
ged
Ferm
enta
tion
Surfa
ce F
erm
enta
tion
lntra
cellu
lar
Ext
race
llula
r
Con
cent
ratio
n
Pre
cipi
tatio
n
Dry
ing
Pel
letin
g
Furth
er P
urifi
catio
n
Sol
id P
rodu
ct
Sol
utio
n P
rodu
ct
Imm
obili
zed
Pro
duct
Esterases
The esterases are involved in the hydrolysisof ester linkages of various types. The prod-ucts formed are acid and alcohol. Theseenzymes may hydrolyze triglycerides andinclude several Upases; for instance, phos-pholipids are hydrolyzed by phospholipases,and cholesterol esters are hydrolyzed by cho-lesterol esterase. The carboxylesterases areenzymes that hydrolyze triglycerides such astributyrin. They can be distinguished fromUpases because they hydrolyze soluble sub-strates, whereas Upases only act at the water-lipid interfaces of emulsions. Therefore, anycondition that results in increased surfacearea of the water-lipid interface will increasethe activity of the enzyme. This is the reasonthat lipase activity is much greater in homog-enized (not pasteurized) milk than in the non-homogenized product. Most of the lipolyticenzymes are specific for either the acid or thealcohol moiety of the substrate, and, in thecase of esters of polyhydric alcohols, theremay also be a positional specificity.
Lipases are produced by microorganismssuch as bacteria and molds; are produced byplants; are present in animals, especially inthe pancreas; and are present in milk. Li-pases may cause spoilage of food becausethe free fatty acids formed cause rancidity. Inother cases, the action of Upases is desirableand is produced intentionally. The boundarybetween flavor and off-flavor is often a verynarrow range. For instance, hydrolysis ofmilk fat in milk leads to very unpleasant off-flavors at very low free fatty acid concentra-tion. The hydrolysis of milk fat in cheesecontributes to the desirable flavor. These dif-ferences are probably related to the back-ground upon which these fatty acids aresuperimposed and to the specificity for par-ticular groups of fatty acids of each enzyme.
In seeds, Upases may cause fat hydrolysisunless the enzymes are destroyed by heat.Palm oil produced by primitive methods inAfrica used to consist of more than 10 per-cent of free fatty acids. Such spoilage prob-lems are also encountered in grains and flour.The activity of lipase in wheat and othergrains is highly dependent on water content.In wheat, for example, the activity of lipaseis five times higher at 15.1 percent than at 8.8percent moisture. The lipolytic activity ofoats is higher than that of most other grains.
Lipases can be divided into those that havea positional specificity and those that do not.The former preferentially hydrolyze the esterbonds of the primary ester positions. Thisresults in the formation of mono- and diglyc-erides, as represented by the following reac-tion:
During the progress of the reaction, the con-centration of diglycerides and monoglycer-ides increases, as is shown in Figure 10-3.
Lipase
Lipase
The (3-monoglycerides formed are resistant tofurther hydrolysis. This pattern is characteris-tic of pancreatic lipase and has been used tostudy the triglyceride structure of many fatsand oils.
The hydrolysis of triglycerides in cheese isan example of a desirable flavor-producingprocess. The extent of free fatty acid forma-tion is much higher in blue cheese than inCheddar cheese, as is shown in Table 10-3.This is most likely the result of Upases elabo-
rated by organisms growing in the bluecheese, such as P. roqueforti, P. camemberti,and others. The extent of lipolysis increaseswith age, as is demonstrated by the increas-ing content of partial glycerides during theaging of cheese (Table 10-4). In many cases,lipolysis is induced by the addition of lipoly-tic enzymes. In the North American choco-late industry, it is customary to induce somelipolysis in chocolate by means of lipase. Inthe production of Italian cheeses, lipolysis is
Figure 10-3 The Course of Pancreatic Lipase Hydrolysis of Tricaprylin. MG = monoglycerides, DG =diglycerides, TG = triglycerides. Source: From A. Boudreau and J.M. deMan, The Mode of Action ofPancreatic Lipase on Milkfat Glycerides, Can. J. Biochem., Vol. 43, pp. 1799-1805, 1965.
X HYDROLYSIS
MO
LE V
. G
LY C
ER
IDE
S
Table 10-3 Free Fatty Acids in Some DairyProducts
Product Free Fatty Acids (mg/kg)
Fresh milk 415Moderately rancid 1,027
creamButter 2,733Cheddar cheese 1,793 (avg of 12 sam-
ples)Blue cheese 23,500 to 66,700 (range
3 samples)
Source: From E.A. Day, Role of Milk Lipids in Flavorsof Dairy Products, in Flavor Chemistry, R.F. Gould, ed.,1966, American Chemical Society.
induced by the use of pregastric esterases.These are lipolytic enzymes obtained fromthe oral glands located at the base of thetongue in calves, lambs, or kids.
Specificity for certain fatty acids by somelipolytic enzymes has been demonstrated.Pancreatic lipase and milk lipase are broad-spectrum enzymes and show no specificityfor any of the fatty acids found in fats.Instead, the fatty acids that are released from
Table 10-4 Formation of Partial Glycerides inCheddar Cheese
Mono-Diglycerides glycerides
Product Type (wt %) (wt %)
Mild 7.4-7.6 1.0-2.0Medium 7.6-9.7 0.5-1.4Old 11.9-15.6 1.1-3.2
the glycerides occur in about the same ratioas they are present in the original fat. Speci-ficity was shown by Nelson (1972) in calfesterase and in a mixed pancreatin-esterasepreparation (Table 10-5). Pregastric ester-ases and lipase from Aspergillus species pri-marily hydrolyze shorter chain-length fattyacids (Arnold et al. 1975).
Specificity of Upases may be expressed ina number of different ways—substrate spe-cific, regiospecific, nonspecific, fatty acylspecific, and stereospecific. Examples ofthese specificities have been presented byVilleneuve and Foglia (1997) (Table 10-6).
Substrate specificity is the ability to hydro-lyze a particular glycerol ester, such as when
Table 10-5 Free Fatty Acids Released from Milkfat by Several Lipolytic Enzymes
Source: From J.H. Nelson, Enzymatically Produced Flavors for Fatty Systems, J. Am. Oil Chem. Soc., Vol. 49,pp. 559-562, 1972.
Fatty Acid
4:06:08:010:012:014:016:018:1 and 18:218:0
Milk Lipase
13.92.11.83.02.77.721.629.210.5
Steapsin
10.72.91.53.74.010.721.624.313.4
PancreaticLipase
14.42.11.43.33.810.124.025.59.7
Calf Esterase
35.002.51.33.15.113.215.914.23.2
EsterasePancreatin
15.853.63.05.54.48.519.321.110.1
Source: Reprinted with permission from R ViIIe-neuve and T.A. Foglia, Lipase Specificities: PotentialApplication in Lipid Bioconversions, J. Am. Oil Chem.Soc., Vol. 8, p. 641, © 1997, AOCS Press.
a lipase can rapidly hydrolyze a triacylglyc-erol, but acts on a monoacylglycerol onlyslowly. Regiospecificity involves a specificaction on either the sn-1 and sn-3 positionsor reaction with only the sn-2 position. The
1,3-specific enzymes have been researchedextensively, because it is now recognized thatUpases in addition to hydrolysis can catalyzethe reverse reaction, esterification or transes-terification. This has opened up the possibil-ity of tailor-making triacylglycerols with aspecific structure, and this is especiallyimportant for producing high-value fats suchas cocoa butter equivalents. The catalyticactivity of lipases is reversible and dependson the water content of the reaction mixture.At high water levels, the hydrolytic reactionprevails, whereas at low water levels the syn-thetic reaction is favored. A number of lipasecatalyzed reactions are possible, and thesehave been summarized in Figure 10-4 (Ville-neuve and Foglia 1997). Most of the lipasesused for industrial processes have beendeveloped from microbes because these usu-ally exhibit high temperature tolerance.Lipases from Mucor miehei and Candidaantarctica have been cloned and expressed inindustry-friendly organisms. Lipases fromgenetically engineered strains will likely beof major industrial importance in the future(Godtfredsen 1993). Fatty acid-specific li-pases react with either short-chain fatty acids(Penicillium roqueforti) or some long-chainfatty acids such as a's-9-unsaturated fattyacids (Geotrichum candidum). Stereospecificlipases react with only fatty acids at the sn-1or sn-3 position.
The applications of microbial lipases in thefood industry involve the hydrolytic as wellas the synthetic capabilities of these enzymesand have been summarized by Godtfredsen(1993) in Table 10-7.
The lipase-catalyzed interesterificationprocess can be used for the production of tri-acylglycerols with specific physical proper-ties, and it also opens up possibilities formaking so-called structured lipids. An exam-ple is a triacylglycerol that carries an essen-
Table 10-6 Examples of Lipase Specificities
Specificity
Substrate specificMonoacylglyercolsMono- and diacylglyc-
erolsTriacylglycerols
Regiospecific1,3-regioselective
sn-2-regioselectiveNonspecific
Fatty acylspecificShort-chain fatty acid
(FA)
cis-9 unsaturated FA
Long-chain unsatur-ated FA
Stereospecificsn-1 Stereospecific
sn-3 Stereospecific
Lipase
Rat adipose tissuePenicillium camem-
bertiiPenicillium sp.
Aspergilllus nigerRhizopus arrhizusMucor mieheiCandida antarctica APenicillium expan-
sumAspergillus sp.Pseudomonas
cepacia
Penicillium roqueforti
Premature infantgastric
Geotrichum candi-dum
Botrytis cinerea
Humicola lanuginosaPseudomonas
aeruginosaFusarium solani
cutinaseRabbit gastric
tial fatty acid (e.g., DHA-docosahexaenoicacid) in the sn-2 position and short-chainfatty acids in the sn-1 and sn-3 positions.Such a structural triacylglycerol would rap-idly be hydrolyzed in the digestive tract andprovide an easily absorbed monoacylglyc-erol that carries the essential fatty acid(Godtfredsen 1993).
The lipases that have received attention fortheir ability to synthesize ester bonds havebeen obtained from yeasts, bacteria, andfungi. Lipases can be classified into three
groups according to their specificity (Macrae1983). The first group contains nonspecificlipases. These show no specificity regardingthe position of the ester bond in the glycerolmolecule, or the nature of the fatty acid.Examples of enzymes in this group arelipases of Candida cylindracae, Corynebac-terium acnes, and Staphylococcus aureus.The second group contains lipases with posi-tion specificity for the 1- and 3-positions ofthe glycerides. This is common among mi-crobial lipases and is the result of the steri-
Figure 10-4 Lipase Catalyzed Reactions Used in Oil and Fat Modification. Source: Reprinted with per-mission from R Villeneuve and T.A. Foglia, Lipase Specificities: Potential Application in Lipid Biocon-versions, /. Am. Oil Chem. Soc., Vol. 8, p. 642, © 1997, AOCS Press.
Acidolysis
Alcoholysis
Transesterification
lnteresterification
Esterification
Hydrolysis
cally hindered ester bond of the 2-position'sinability to enter the active site of theenzyme. Lipases in this group are obtainedfrom Aspergillus niger, Mucor javanicus,and Rhizopus arrhizus. The third group oflipases show specificity for particular fattyacids. An example is the lipase from Geotri-chum candidum, which has a marked speci-ficity for long-chain fatty acids that contain acis double bond in the 2-position. The knowl-edge of the synthetic ability of lipases hasopened a whole new area of study in the mod-ification of fats. The possibility of modifyingfats and oils by immobilized lipase technol-ogy may result in the production of food fatsthat have a higher essential fatty acid contentand lower trans levels than is possible withcurrent methods of hydrogenation.
Amylases
The amylases are the most importantenzymes of the group of glycoside hydro-
lases. These starch-degrading enzymes canbe divided into two groups, the so-calleddebranching enzymes that specifically hy-drolyze the 1,6-linkages between chains,and the enzymes that split the 1,4-linkagesbetween glucose units of the straight chains.The latter group consists of endoenzymesthat cleave the bonds at random points alongthe chains and exoenzymes that cleave atspecific points near the chain ends. Thisbehavior has been represented by Marshall(1975) as a diagram of the structure of amy-lopectin (Figure 10-5). In this molecule, the1,4-oc-glucan chains are interlinked by 1,6-ct-glucosidic linkages resulting in a highlybranched molecule. The molecule is composed of three types of chains; the A chainscarry no substituent, the B chains carry otherchains linked to a primary hydroxyl group,and the molecule contains only one C chainwith a free reducing glucose unit. Thechains are 25 to 30 units in length in starchand only 10 units in glycogen.
Source: Reprinted with permission from S.E. Godtfredsen, Lipases, Enzymes in Food Processing, T.Nagodawithana and G. Reed, eds., p. 210, © 1993, Academic Press.
Table 10-7 Application of Microbial Lipases in the Food Industry
Industry
Dairy
Bakery
BeverageFood dressing
Health foodMeat and fishFat and oil
Effect
Hydrolysis of milk fatCheese ripeningModification of butter fatFlavor improvement and shelf-life
prolongationImproved aromaQuality improvement
TransesterificationFlavor development and fat removalTransesterificationHydrolysis
Product
Flavor agentsCheeseButterBakery products
BeveragesMayonnaise, dressing, and whipped
toppingsHealth foodsMeat and fish productsCocoa butter, margarineFatty acids, glycerol, mono- and
diglycerides
Alpha-amylase (a-l,4-Glucan 4-Glucanohydrolase)
This enzyme is distributed widely in theanimal and plant kingdoms. The enzymecontains 1 gram-atom of calcium per mole.Alpha-amylase (a-1,4-glucan-4-glucanohy-drolase) is an endoenzyme that hydrolyzesthe oc-l,4-glucosidic bonds in a random fash-ion along the chain. It hydrolyzes amylopec-tin to oligosaccharides that contain two to sixglucose units. This action, therefore, leads toa rapid decrease in viscosity, but little mono-saccharide formation. A mixture of amyloseand amylopectin will be hydrolyzed into amixture of dextrins, maltose, glucose, and
oligosaccharides. Amylose is completelyhydrolyzed to maltose, although there usu-ally is some maltotriose formed, which hy-drolyzes only slowly.
Beta-amylase (a-l,4-GlucanMaltohydrolase)
This is an exoenzyme and removes suc-cessive maltose units from the nonreducingend of the glucosidic chains. The action isstopped at the branch point where the a-1,6glucosidic linkage cannot be broken by oc-amylase. The resulting compound is namedlimit dextrin. Beta-amylase is found only in
Figure 10-5 Diagrammatic Representation of Amylopectin Structure. Lines represent oc-D-glucanchains linked by 1,4-bonds. The branch points are 1,6-oc glucosidic bonds. Source: From JJ. Marshall,Starch Degrading Enzymes, Old and New, Starke, Vol. 27, pp. 377-383, 1975.
higher plants. Barley malt, wheat, sweetpotatoes, and soybeans are good sources.Beta-amylase is technologically important inthe baking, brewing, and distilling industries,where starch is converted into the ferment-able sugar maltose. Yeast ferments maltose,sucrose, invert sugar, and glucose but doesnot ferment dextrins or oligosaccharides con-taining more than two hexose units.
Glucoamylase (a-l,4-GlucanGlucohydrolase)
This is an exoenzyme that removes glucoseunits in a consecutive manner from the non-reducing end of the substrate chain. Theproduct formed is glucose only, and this dif-ferentiates this enzyme from a- and p-amy-lase. In addition to hydrolyzing the a-1,4linkages, this enzyme can also attack the a-1,6 linkages at the branch point, albeit at aslower rate. This means that starch can becompletely degraded to glucose. The enzymeis present in bacteria and molds and is usedindustrially in the production of corn syrupand glucose.
A problem in the enzymic conversion ofcorn starch to glucose is the presence oftransglucosidase enzyme in preparations ofa-amylase and glucoamylase. The transglu-cosidase catalyzes the formation of oligosac-charides from glucose, thus reducing theyield of glucose.
Nondamaged grains such as wheat andbarley contain very little a-amylase but rela-tively high levels of (3-amylase. When thesegrains germinate, the (3-amylase level hardlychanges, but the a-amylase content mayincrease by a factor of 1,000. The combinedaction of a- and (3-amylase in the germinatedgrain greatly increases the production of fer-mentable sugars. The development of a-amylase activity during malting of barley isshown in Table 10-8. In wheat flour, high a-
amylase activity is undesirable, because toomuch carbon dioxide is formed during bak-ing.
Raw, nondamaged, and ungelatinizedstarch is not susceptible to (3-amylase activ-ity. In contrast, a-amylase can slowly attackintact starch granules. This differs with thetype of starch; for example, waxy corn starchis more easily attacked than potato starch. Ingeneral, extensive hydrolysis of starch re-quires gelatinization. Damaged starch gran-ules are more easily attacked by amylases,which is important in bread making Alpha-amylase can be obtained from malt, fromfungi (Aspergillus oryzae), or from bacteria(B. subtilis). The bacterial amylases have ahigher temperature tolerance than the maltamylases.
Beta-galactosidase ($-D-GalactosideGalactohydrolase)
This enzyme catalyzes the hydrolysis of p-D-galactosides and a-L-arabinosides. It isbest known for its action in hydrolyzing lac-tose and is, therefore, also known as lactase.The enzyme is widely distributed and occursin higher animals, bacteria, yeasts, and
Table 10-8 Development of a-Amylase DuringMalting of Barley at 2O0C
Days of Steeping and a-Amylase (2CPGermination Dextrose Units)
~6 O3 555 1107 1308 135
Source: From S.R. Green, New Use of Enzymes inthe Brewing Industry, MBAA Tech. Quar., Vol. 6, pp.33-39, 1969.
plants. Beta-galactosidase or lactase is foundin humans in the cells of the intestinalmucous membrane. A condition that is wide-spread in non-Caucasian adults is character-ized by an absence of lactase. Suchindividuals are said to have lactose intoler-ance, which is an inability to digest milkproperly.
The presence of galactose inhibits lactosehydrolysis by lactase. Glucose does not havethis effect.
Pectic Enzymes
The pectic enzymes are capable of degrad-ing pectic substances and occur in higherplants and in microorganisms. They are notfound in higher animals, with the exceptionof the snail. These enzymes are commer-cially important for the treatment of fruitjuices and beverages to aid in filtration andclarification and increasing yields. Theenzymes can also be used for the productionof low methoxyl pectins and galacturonicacids. The presence of pectic enzymes infruits and vegetables can result in excessivesoftening. In tomato and fruit juices, pecticenzymes may cause "cloud" separation.
There are several groups of pecticenzymes, including pectinesterase, theenzyme that hydrolyzes methoxyl groups,and the depolymerizing enzymes polygalac-turonase and pectate lyase.
Pectinesterase (Pectin Pectyl-Hydrolase)
This enzyme removes methoxyl groupsfrom pectin. The enzyme is referred to byseveral other names, including pectase, pec-tin methoxylase, pectin methyl esterase, andpectin demethylase. Pectinesterases are foundin bacteria, fungi, and higher plants, withvery large amounts occurring in citrus fruitsand tomatoes. The enzyme is specific forgalacturonide esters and will not attack non-galacturonide methyl esters to any largeextent. The reaction catalyzed by pectinesterase is presented in Figure 10-6. It hasbeen suggested that the distribution of meth-oxyl groups along the chain affects the reac-tion velocity of the enzyme (MacMillan andSheiman 1974). Apparently, pectinesteraserequires a free carboxyl group next to anesterified group on the galacturonide chain toact, with the pectinesterase moving down thechain linearly until an obstruction is reached.
PiCtinttUrott
Figure 10-6 Reaction Catalyzed by Pectinesterase
To maintain cloud stability in fruit juices,high-temperature-short-time (HTST) pas-teurization is used to deactivate pectolyticenzymes. Pectin is a protective colloid thathelps to keep insoluble particles in suspen-sion. Cloudiness is required in commercialproducts to provide a desirable appearance.The destruction of the high levels of pectin-esterase during the production of tomatojuice and puree is of vital importance. Thepectinesterase will act quite rapidly once thetomato is broken. In the so-called hot-breakmethod, the tomatoes are broken up at hightemperature so that the pectic enzymes aredestroyed instantaneously.
Polygalacturonase (Poly-a~l,4-Galacturonide Glycanohydrolase)
This enzyme is also known as pectinase,and it hydrolyzes the glycosidic linkages inpectic substances according to the reactionpattern shown in Figure 10-7. The polyga-lacturonases can be divided into endoen-zymes that act within the molecule on a-1,4linkages and exoenzymes that catalyze thestepwise hydrolysis of galacturonic acid
molecules from the nonreducing end of thechain. A further division can be made by thefact that some polygalacturonases act princi-pally on methylated substrates (pectins),whereas others act on substrates with freecarboxylic acid groups (pectic acids). Theseenzymes are named polymethyl galactur-onases and polygalacturonases, respectively.The preferential mode of hydrolysis and thepreferred substrates are listed in Table 10-9.Endopolygalacturonases occur in fruits andin filamentous fungi, but not in yeast or bac-teria. Exopolygalacturonases occur in plants(for example, in carrots and peaches), fungi,and bacteria.
Pectate Lyase (Poly-a-l,4-D-GalacturonideLyase)
This enzyme is also known as trans-elimi-nase; it splits the glycosidic bonds of a glu-curonide chain by trans elimination ofhydrogen from the 4- and 5-positions of theglucuronide moiety. The reaction pattern ispresented in Figure 10-8. The glycosidicbonds in pectin are highly susceptible to thisreaction. The pectin lyases are of the endo-type and are obtained exclusively from fila-
Figure 10-7 Reaction Catalyzed by Polygalacturonase
PolygolocturonaM
mentous fungi, such as Aspergillus niger.The purified enzyme has an optimum pH of5.1 to 5.2 and isoelectric point between 3 and4 (Albersheim and Kilias 1962).
Commercial Use
Pectic enzymes are used commercially inthe clarification of fruit juices and wines andfor aiding the disintegration of fruit pulps.By reducing the large pectin molecules intosmaller units and eventually into galactur-onic acid, the compounds become water sol-uble and lose their suspending power; also,their viscosity is reduced and the insolublepulp particles rapidly settle out.
Most microorganisms produce at least onebut usually several pectic enzymes. Almostall fungi and many bacteria produce theseenzymes, which readily degrade the pectinlayers holding plant cells together. This leadsto separation and degradation of the cells,and the plant tissue becomes soft. Bacterialdegradation of pectin in plant tissues isresponsible for the spoilage known as "softrot" in fruits and vegetables. Commercialfood grade pectic enzyme preparations maycontain several different pectic enzymes.Usually, one type predominates; this dependson the intended use of the enzyme prepara-tion.
Figure 10-8 Reaction Catalyzed by Pectin Lyase
Ptctin LyQM
Table 10-9 Action of Polygalacturonases
Type of Attack
RandomRandomTerminalTerminal
Enzyme
Endo-polymethylgalacturonaseEndo-polygalacturonaseExo-polymethylgalacturonaseExo-polygalacturonase
Preferred Substrate
PectinPectic acidPectinPectic acid
Proteases
Proteolytic enzymes are important in manyindustrial food processing procedures. Thereaction catalyzed by proteolytic enzymes isthe hydrolysis of peptide bonds of proteins;this reaction is shown in Figure 10-9. Whi-taker (1972) has listed the specificity require-ments for the hydrolysis of peptide bonds byproteolytic enzymes. These include thenature of R1 and R2 groups, configuration ofthe amino acid, size of substrate molecule,and the nature of the X and Y groups. Amajor distinguishing factor of proteolyticenzymes is the effect of R1 and R2 groups.The enzyme oc-chymotrypsin hydrolyzespeptide bonds readily only when R1 is part ofa tyrosyl, phenylalanyl, or tryptophanyl resi-due. Trypsin requires R1 to belong to an argi-nyl or lysyl residue. Specific requirement forthe R2 groups is exhibited by pepsin and thecarboxypeptidases; both require R2 to belongto a phenylalanyl residue. The enzymes re-quire the amino acids of proteins to be in theL-configuration but frequently do not have astrict requirement for molecular size. Thenature of X and Y permits the division of pro-teases into endopeptidases and exopepti-dases. The former split peptide bonds in arandom way in the interior of the substratemolecule and show maximum activity whenX and Y are derived. The carboxypeptidasesrequire that Y be a hydroxyl group, the ami-nopeptidases require that X be a hydrogen,
and the dipeptidases require that X and Yboth be underived.
Proteolytic enzymes can be divided intothe following four groups: the acid proteases,the serine proteases, the sulfhydryl proteases,and the metal-containing proteases.
Acid Proteases
This is a group of enzymes with pH optimaat low values. Included in this group are pep-sin, rennin (chymosin), and a large numberof microbial and fungal proteases. Rennin,the pure enzyme contained in rennet, is anextract of calves' stomachs that has beenused for thousands of years as a coagulatingagent in cheese making. Because of the scar-city of calves' stomachs, rennet substitutesare now widely used, and the coagulantsused in cheese making usually contain mix-tures of rennin and pepsin and/or microbialproteases. Some of the microbial proteaseshave been used for centuries in the Far Eastin the production of fermented foods such assoy sauce.
Rennin is present in the fourth stomach ofthe suckling calf. It is secreted in an inactiveform, a zymogen, named prorennin. Thecrude extract obtained from the dried stom-achs (veils) contains both rennin and proren-nin. The conversion of prorennin to rennincan be speeded up by addition of acid. Thisconversion involves an autocatalytic process,in which a limited proteolysis of the proren-
Figure 10-9 Reaction Catalyzed by Proteases
nin occurs, thus reducing the molecularweight about 14 percent. The conversion canalso be catalyzed by pepsin. The processinvolves the release of peptides from the N-terminal end of prorennin, which reduces themolecular weight from about 36,000 to about31,000. The molecule of prorennin consistsof a single peptide chain joined internally bythree disulfide bridges. After conversion torennin, the disulfide bridges remain intact.As the calves grow older and start to eatother feeds as well as milk, the stomachstarts to produce pepsin instead of rennin.The optimum activity of rennin is at pH 3.5,but it is most stable at pH 5; the clotting ofcheese milk is carried out at pH values of 5.5to 6.5.
The coagulation or clotting of milk by ren-nin occurs in two stages. In the first, theenzymic stage, the enzyme acts on K-caseinso that it can no longer stabilize the caseinmicelle. The second, or nonenzymic stage,involves the clotting of the modified caseinmicelles by calcium ions. The enzymic stageinvolves a limited and specific action on theK-casein, resulting in the formation of insol-uble para-K-casein and a soluble macropep-tide. The latter has a molecular weight of6,000 to 8,000, is extremely hydrophilic, andcontains about 30 percent carbohydrate. Theglycomacropeptide contains galactosamine,galactose, and N-acetyl neuraminic acid(sialic acid). The splitting of the glycomac-ropeptide from K-casein involves the break-ing of a phenylalanine-methionine bond inthe peptide chain. Other clotting enzymes—including pepsin, chymotrypsin, and micro-bial proteases—break the same bond andproduce the same glycomacropeptide.
Pepsin is elaborated in the mucosa of thestomach lining in the form of pepsinogen.The high acidity of the stomach aids in theautocatalytic conversion into pepsin. This
conversion involves splitting several peptidefragments from the N-terminal end of pepsin-ogen. The fragments consist of one largepeptide and several small ones. The largepeptide remains associated with pepsinogenby noncovalent bonds and acts as an inhibi-tor. The inhibitor dissociates from pepsin at apH of 1 to 2. In the initial stages of the con-version of pepsinogen to pepsin, six peptidebonds are broken, and continued action onthe large peptide (Figure 10-10) results inthree more bonds being hydrolyzed. In thisprocess, the molecular weight changes from43,000 to 35,000 and the isoelectric pointchanges from 3.7 to less than 1. The pepsinmolecule consists of a single polypeptidechain that contains 321 amino acids. The ter-tiary structure is stabilized by three disulfidebridges and a phosphate linkage. The phos-phate group is attached to a seryl residue andis not essential for enzyme activity. The pHoptimum of pepsin is pH 2 and the enzyme isstable from pH 2 to 5. At higher pH values,the enzyme is rapidly denatured and loses itsactivity. The primary specificity of pepsin istoward the R2 group (see the equation shownin Figure 10-9), and it prefers this to be aphenylalanyl, tyrosyl, or tryptophanyl group.
The use of other acid proteases as substi-tutes for rennin in cheese making is deter-mined by whether bitter peptides are formedduring ripening of the cheese and by whetherinitial rapid hydrolysis causes excessive pro-tein losses in the whey. Some of the acid pro-teases used in cheese making include prep-arations obtained from the organisms Endo-thia parasitica, Mucor miehei, and Mucorpusillus. Rennin contains the enzyme chy-mosin, and the scarcity of this naturalenzyme preparation for cheese makingresulted in the use of pepsin for this purpose.Pepsin and chymosin have primary struc-tures that have about 50 percent homology
and quite similar tertiary structures. Themolecular mass of the two enzymes is simi-lar, 35 kDa, but chymosin has a higher pi.Much of the chymosin used in cheese mak-ing is now obtained by genetic engineeringprocesses. In the production of soy sauce andother eastern food products, such as miso (anoriental fermented food) and ketjap (Indone-sian type soy sauce), the acid proteases ofAspergillus oryzae are used. Other productsinvolve the use of the fungus Rhizopus oli-gosporus. Acid proteases also play a role inthe ripening process of a variety of softcheeses. This includes the Penicillia used inthe blue cheeses, such as Roquefort, Stilton,and Danish blue, and in Camembert andBrie. The molds producing the acid proteases
may grow either on the surface of the cheeseor throughout the body of the cheese.
Serine Proteases
This group includes the chymotrypsins,trypsin, elastase, thrombin, and subtilisin.The name of this group of enzymes refers tothe seryl residue that is involved in the activesite. As a consequence, all of these enzymesare inhibited by diisopropylphosphorofluori-date, which reacts with the hydroxyl groupof the seryl residue. They also have an imida-zole group as part of the active site and theyare all endopeptides. The chymotrypsins,trypsin and elastase, are pancreatic enzymesthat carry out their function in the intestinal
Figure 10-10 Structure of Pepsinogen and Its Conversion to Pepsin. Source: From RA. Bovey andS.S. Yanari, Pepsin, in The Enzymes, Vol. 4, RD. Boyer et al., eds., 1960, Academic Press.
tract. They are produced as inactive zymo-gens and are converted into the active formby limited proteolysis.
Sulfhydryl Proteases
These enzymes obtain their name from thefact that a sulfhydryl group in the molecule isessential for their activity. Most of theseenzymes are of plant origin and have foundwidespread use in the food industry. Theonly sulfhydryl proteases of animal originare two of the cathepsins, which are presentin the tissues as intracellular enzymes. Themost important enzymes of this group arepapain, ficin, and bromelain. Papain is anenzyme present in the fruit, leaves, and trunkof the papaya tree (Carica papaya). Thecommercial enzyme is obtained by purifica-tion of the exudate of full-grown but unripepapaya fruits. The purification involves useof affinity chromatography on a column con-taining an inhibitor (Liener 1974). This pro-cess leads to the full activation of theenzyme, which then contains 1 mole of sulf-hydryl per mole of protein. The crude papainis not fully active and contains only 0.5 moleof sulfhydryl per mole of protein. Bromelainis obtained from the fruit or stems of thepineapple plant (Ananas comosus). Thestems are pressed and the enzyme precipi-tated from the juice by acetone. Ficin isobtained from the latex of tropical fig trees(Ficus glabrata). The enzyme is not homo-geneous and contains at least three differentproteolytic components.
The active sites of these plant enzymescontain a cysteine and a histidine group thatare essential for enzyme activity. The pHoptimum is fairly broad and ranges from 6 to7.5. The enzymes are heat stable up to tem-peratures in the range of 60 to 8O0C. Thepapain molecule consists of a single polypep-
tide chain of 212 amino acids. The molecularweight is 23,900. Ficin and bromelain con-tain carbohydrate in the molecule; papaindoes not. The molecular weights of theenzymes are quite similar; that of ficin is25,500 and that of bromelain, 20,000 to33,200. These enzymes catalyze the hydroly-sis of many different compounds, includingpeptide, ester, and amide bonds. The varietyof peptide bonds split by papain appears toindicate a low specificity. This has beenattributed (Liener 1974) to the fact thatpapain has an active site consisting of sevensubsites that can accommodate a variety ofamino acid sequences in the substrate. Thespecificity in this case is not determined bythe nature of the side chain of the amino acidinvolved in the susceptible bond but rather bythe nature of the adjacent amino acids.
Commercial use of the sulfhydryl pro-teases includes stabilizing and chill proofingof beer. Relatively large protein fragmentsremaining after the malting of barley maycause haze in beer when the product is storedat low temperatures. Controlled proteolysissufficiently decreases the molecular weightof these compounds so that they will remainin solution. Another important use is in thetenderizing of meat. This can be achieved byinjecting an enzyme solution into the carcassor by applying the enzyme to smaller cuts ofmeat. The former method suffers from thedifficulty of uneven proteolysis in differentparts of the carcass with the risk of overten-derizing some parts of the carcass.
Metal-Containing Proteases
These enzymes require a metal for activityand are inhibited by metal-chelating com-pounds. They are exopeptidases and includecarboxypeptidase A (peptidyl-L-amino-acidhydrolase) and B (peptidyl-L-lysine hydro-
lase), which remove amino acids from theend of peptide chains that carry a free oc-car-boxyl group. The aminopeptidases removeamino acids from the free oc-amino end ofthe peptide chain. The metalloexopeptidasesrequire a divalent metal as a cofactor; thecarboxypeptidases contain zinc. Theseenzymes are quite specific in the action; forexample, carboxypeptidase B requires theC-terminal amino acid to be either arginineor Iysine; the requirement for carboxypepti-dase A is phenylalanine, tryptophan, or iso-leucine. These specificities are comparedwith those of some other proteolytic enzymesin Figure 10-11. The carboxypeptidases arerelatively small molecules; molecularweight of carboxypeptidase A is 34,600.The amino peptidases have molecularweights around 300,000. Although many ofthe aminopeptidases are found in animal tis-sues, several are present in microorganisms(Riordan 1974).
Protein Hydrolysates
Protein hydrolysates is the name given to afamily of protein breakdown products ob-tained by the action of enzymes. It is alsopossible to hydrolyze proteins by chemicalmeans, acids, or alkali, but the enzymatic
method is preferred. Many food productssuch as cheese and soy sauce are obtained byenzymatic hydrolysis. The purpose of theproduction of protein hydrolysates is toimprove nutritional value, cost, taste, antige-nicity, solubility, and functionality. The pro-teins most commonly selected for producinghydrolysates are casein, whey protein, andsoy protein (Lahl and Braun 1994). Proteinscan be hydrolyzed in steps to yield a series ofproteoses, peptones, peptides, and finallyamino acids (Table 10-10). These productsshould not be confused with hydrolyzed veg-etable proteins, which are intended as flavor-ing substances.
The extent of hydrolysis of protein hy-drolysates is measured by the ratio of theamount of amino nitrogen to the total amountof nitrogen present in the raw material (AN/TN ratio). Highly hydrolyzed materials haveAN/TN ratios of 0.50 to 0.60. To obtain thedesired level of hydrolysis in a protein, acombination of proteases is selected. Serineprotease prepared from Bacillus lichenifor-mis has broad specificity and some prefer-ence for terminal hydrophobic amino acids.Peptides containing terminal hydrophobicamino acids cause bitterness. Usually a mix-ture of different proteases is employed. Thehydrolysis reaction is terminated by adjust-
Figure 10-11 Specificity of Some Proteolytic Enzymes
Carboxypeptidase B
Trypsin Pepsin, Chymotrypsin
Carboxypeptidase A
ing the pH and increasing the temperature toinactivate the enzymes. The process for pro-ducing hydrolysates is shown in Figure 10-12(Lahl and Braun 1994). Protein hydrolysatescan be used as food ingredients with specificfunctional properties or for physiological ormedical reasons. For example, hydrolyzed pro-teins may lose allergenic properties by suit-ably arranged patterns of hydrolysis (Cordle1994).
OXIDOREDUCTASES
Phenolases
The enzymes involved in enzymic brown-ing are known by the name polyphenoloxi-dase and are also called polyphenolase orphenolase. It is generally agreed (Mathew
and Parpia 1971) that these terms include allenzymes that have the capacity to oxidizephenolic compounds to oquinones. This canbe represented by the conversion of odihy-droxyphenol to oquinone,
Table 10-10 Protein Hydrolysate Products Produced from Casein and Whey Protein Concentrate(WPC)
a Commercial hydrolysates produced by Deltown Specialties, Fraser, NY.b Determined by reverse-phase HPLC.c Ratio of amino nitrogen present in the hydrolysate to the total amount of nitrogen present in the substrate.
Source: Reprinted with permission from WJ. Lahl and S.D. Braun, Enzymatic Production of Protein Hydrolysatesfor Food Use, Food Technology, Vol. 48, No. 10, p. 69, © 1994, Institute of Food Technologists.
Hydrolysatea
Intact protein
Proteose
Peptone
Peptides
Peptides and free amino acids
Protein Source
CaseinWPC
CaseinWPC
CaseinWPC
CaseinWPC
CaseinWPC
Average MolecularWeight*
28,50025,0006,0006,8002,0001,400
400375260275
AN/TNC
0.070.060.130.110.240.240.480.430.550.58
enzyme
The action of polyphenolases is detrimen-tal when it leads to browning in bruised andbroken plant tissue but is beneficial in theprocessing of tea and coffee. The enzymeoccurs in almost all plants, but relativelyhigh levels are found in potatoes, mush-
rooms, apples, peaches, bananas, avocados,tea leaves, and coffee beans.
In addition to changing odiphenols into o-quinones, the enzymes also catalyze the con-version of monophenols into o-diphenols, asfollows:
Figure 10-12 Process for the Production of Protein Hydrolysates. Source'. Reprinted with permissionfrom WJ. Lahl and S JD. Braun, Enzymatic Production of Protein Hydrolysates for Food Use, FoodTechnology, Vol. 48, No. 10, p. 70, © 1994, Institute of Food Technologists.
*IF NECESSARY
DRY HYDROLYZEDPROTEIN/PEPTONE
POWDER
1) EVAPORATE2) PASTEURIZE3) SPRAY DRY
BACTERIOSTAT*FILTERED, CLARIFIEDHYDROLYZED
PROTEIN/PEPTONE
REPEATPROCESSIFAPPLICATIONDICTATES
I)FILTER2) PASTEURIZE ORGANIC SOLIDS AND
FILTER AID
UNFILTERED HYDROLYSATEHYDROLYZED PROTEIN/PEPTONEAS
AQUEOUS SOLUTIONS & SOLIDS
1) DIGEST 3 HR-1 WEEK2) DENATURE ENZYME
BACTERIOSTAT*
PURCHASED ENZYME AND/ORPREPARATION
STERILIZED PROTEIN+
ENZYME SYSTEM
STERILIZED PROTEINSUSPENSION OR SOLUTION
STERILIZE
PROTEIN SUSPENSION ORSOLUTION
H2O DISSOLVE OR SUSPEND
PROTEIN SOURCE
where BH2 stands for an 0-diphenolic com-pound.
To distinguish this type of activity from theone mentioned earlier, it is described ascresolase activity, whereas the other is re-ferred to as catecholase activity. For bothtypes of activity, the involvement of copperis essential. Copper has been found as a com-ponent of all polyphenolases. The activity ofcresolase involves three steps, which can berepresented by the following overall equation(Mason 1956):
Protein-Cu2+-O2 + monophenol —»
Protein-Cu2+ + o-quinone + H2O
The protein copper-oxygen complex isformed by combining one molecule of oxy-gen with the protein to which two adjacentcuprous atoms are attached.
Catecholase activity involves oxidizingtwo molecules of odiphenols to two mole-cules of 0-quinones, resulting in the reduc-tion of one molecule of oxygen to twomolecules of water. The action sequence aspresented in Figure 10-13 has been proposedby Mason (1957). The enzyme-oxygen com-plex serves as the hydroxylating or dehy-droxylating intermediate, and (Cu)n repre-sents the actual charge designation of thecopper at the active site. In preparations highin cresolase activity, n = 2, and in prepara-tions high in catecholase activity, n = 1. Theoverall reaction involves the use of one mole-cule of oxygen, one atom of which goes intothe formation of the diphenol, and the other,which is reduced to water. This can beexpressed in the following equation given byMathew and Parpia (1971):
enzymeMonophenol + O2 + o-diphenol *
0-diphenol + quinone + H2O
The substrates of the polyphenol oxidaseenzymes are phenolic compounds present inplant tissues, mainly flavonoids. These in-clude catechins, anthocyanidins, leucoantho-cyanidins, flavonols, and cinnamic acidderivatives. Polyphenol oxidases from differ-ent sources show distinct differences in theiractivity for different substrates. Some spe-cific examples of polyphenolase substratesare chlorogenic acid, caffeic acid, dicatechol,protocatechuic acid, tyrosine, catechol, di-hydroxyphenylalanine, pyrogallol, and cat-echins.
To prevent or minimize enzymic browningof damaged plant tissue, several approachesare possible. The first and obvious one,although rarely practical, involves the exclu-sion of molecular oxygen. Another approachis the addition of reducing agents that canprevent the accumulation of 0-quinones.
4-methyl-catechol
p-cresol
Heat treatment is effective in deactivating theenzymes. Metal complexing agents may deac-tivate the enzyme by making the copperunavailable.
One of the most useful methods involvesthe use of L-ascorbic acid as a reducingagent. This is practiced extensively in thecommercial production of fruit juices andpurees. The ascorbic acid reacts with the o-quinones and changes them back into o-diphenols (Figure 10-14).
Glucose Oxidase (p-D-Glucose: OxygenOxidoreductase)
This enzyme catalyzes the oxidation of D-glucose to 5-D-gluconolactone and hydrogenperoxide in the presence of molecular oxy-gen, as follows:
C6H12O6 CnZyme ' C6H10O6 + H2O2
The enzyme is present in many fungi andis highly specific for p-D-glucopyranose. Ithas been established that the enzyme does
not oxidize glucose by direct combinationwith molecular oxygen. The mechanism asdescribed by Whitaker (1972) involves theoxidized form of the enzyme, flavin adeninedinucleotide (FAD), which serves as a dehy-drogenase. Two hydrogen atoms are removedfrom the glucose to form the reduced state ofthe enzyme, FADH2, and 5-D-gluconolac-tone. The enzyme is then reoxidized bymolecular oxygen. The gluconolactone ishydrolyzed in the presence of water to formD-gluconic acid.
In food processing, glucose oxidase is usedto remove residual oxygen in the head spaceof bottled or canned products or to removeglucose. Light has a deteriorative effect oncitrus beverages. Through the catalytic actionof light, peroxides are formed that lead tooxidation of other components, resulting invery unpleasant off-flavors. Removing theoxygen by the use of a mixture of glucoseoxidase and catalase will prevent these per-oxides from forming. The glucose oxidasepromotes the formation of gluconic acid withuptake of one molecule of oxygen. The cata-
Figure 10-13 Phenolase Catalyzed Reactions. (A) activation of phenolase. (B,C,D) Two-step four-elec-tron reduction of oxycuprophenolase, and the associated hydroxylation of monophenols. Source: FromH.S. Mason, Mechanisms of Oxygen Metabolism, Adv. Enzymol, Vol. 19, pp. 79-233, © 1957.
lase decomposes the hydrogen peroxideformed into water and one half-molecule ofoxygen. The net result is the uptake of onehalf-molecule of oxygen. The overall reac-tion can be written as follows:
, glucose oxidaseglucose + 72 O7 * glucomc
catalase . ,acid
Recent information suggests that this applica-tion is not effective because of reversibleinhibition of the glucose oxidase by the dyesused in soft drinks below pH 3 (Hammer1993).
This enzyme mixture can also remove glu-cose from eggs before drying to preventMaillard type browning reactions in the driedproduct.
Catalase (Hydrogen Peroxide: HydrogenPeroxide Oxidoreductase)
Catalase catalyzes the conversion of twomolecules of hydrogen peroxide into waterand molecular oxygen as follows:
catalase2H2O2 + 2 H2O+ O2
This enzyme occurs in plants, animals, andmicroorganisms. The molecule has four sub-units; each of these contains a protohemingroup, which forms part of four indepen-dent active sites. The molecular weight is240,000. Catalase is less stable to heat than isperoxidase. At neutral pH, catalase will rap-idly lose activity at 350C. In addition to cata-lyzing the reaction shown above (catalaticactivity), catalase can also have peroxidatic
Figure 10-14 Reaction of L-Ascorbic Acid with o-Quinone in the Prevention of Enzymic Browning
L-Ascorbic acid L-Dehydro-ascorbie acid
A-Methyl 0-benzoquinone4-Methylcatechol
activity. This occurs at low concentrations ofhydrogen peroxide and in the presence ofhydrogen donors (e.g., alcohols).
In plants, catalase appears to have twofunctions. First is the ability to dispose of theexcess H2O2 produced in oxidative metabo-lism, and second is the ability to use H2O2 inthe oxidation of phenols, alcohols, and otherhydrogen donors. The difference in heat sta-bility of catalase and peroxidase was demon-strated by Lopez et al. (1959). They foundthat blanching of southern peas for oneminute in boiling water destroys 70 to 90percent of the peroxidase activity and 80 to100 percent of the catalase activity.
The combination of glucose oxidase andcatalase is used in a number of food process-ing applications, including the removal oftrace glucose or oxygen from foods and inthe production of gluconic acid from glu-cose. Greenfield and Lawrence (1975) havestudied the use of these enzymes in theirimmobilized form on an inorganic support.
Peroxidase (Donor: Hydrogen PeroxideOxidoreductase)
The reaction type catalyzed by peroxidaseinvolves hydrogen peroxide as an acceptor,and a compound AH2 as a donor of hydrogenatoms, as shown:
peroxidaseH2O2+ AH2 * 2 H2O+ A
In contrast to the action of catalase, no molec-ular oxygen is formed.
The peroxidases can be classified into thetwo groups, iron-containing peroxidases andflavoprotein peroxidases. The former can befurther subdivided into ferriprotoporphyrinperoxidases and verdoperoxidases. The firstgroup contains ferriprotoporphyrin III (pro-tohemin) as the prosthetic group (Figure
10-15). The common plant peroxidases(horseradish, fig, and turnip) are in thisgroup and the enzymes are brown whenhighly purified. The second group includesthe peroxidases of animal tissue and milk(lactoperoxidase). In these enzymes, theprosthetic group is an iron porphyrin nucleusbut not protohemin. When highly purified,these enzymes are green in color. Flavopro-tein peroxidases occur in microorganismsand animal tissues. The prosthetic group isFAD.
The linkage between the iron-containingprosthetic group and the protein can be stabi-lized by bisulfite (Embs and Markakis 1969).It is suggested that the bisulfite forms a com-plex with the peroxidase iron, which stabi-lizes the enzyme.
Because of the widespread occurrence ofperoxidase in plant tissues, Nagle and Haard(1975) have suggested that it plays an impor-tant role in the development and senescenceof plant tissues. It plays a role in biogenesisof ethylene; in regulating ripening and senes-cence; in the degradation of chlorophyll; andin the oxidation of indole-3-acetic acid.
The enzyme can occur in a variety of mul-tiple molecular forms, named isoenzymes orisozymes. Such isoenzymes have the sameenzymatic activity but can be separated byelectrophoresis. Nagle and Haard (1975)separated the isoperoxidases of bananas intosix anionic and one cationic component bygel electrophoresis. By using other methodsof separation, an even greater number ofisoenzymes was demonstrated.
Peroxidase has been implicated in the for-mation of the "grit cells" or "stone cells" ofpears (Ranadive and Haard 1972). Boundperoxidase but not total peroxidase activitywas higher in the fruit that contained exces-sive stone cells. The stone cells or sclereidsare lignocellulosic in nature. The presence
of calcium ions causes the release of wallbound peroxidase and a consequent decreasein the deposition of lignin.
The peroxidase test is used as an indicatorof satisfactory blanching of fruits and vegeta-bles. However, it has been found that theenzymes causing off-flavors during frozenstorage can, under some conditions, be regen-erated. Regeneration of enzymes is a rela-tively common phenomenon and is morelikely to occur the faster the temperature israised to a given point in the blanching pro-cess. The deactivation and reactivation of per-oxidase by heat was studied by Lu andWhitaker (1974). The rate of reactivation wasat a maximum at pH 9 and the extent of reac-tivation was increased by addition of hematin.
The deactivation of peroxidase is a func-tion of heating time and temperature. Lac-toperoxidase is completely deactivated byheating at 850C for 13 seconds. The effect ofheating time at 760C on the deactivation of
lactoperoxidase is represented in Figure10-16, and the effect of heating temperatureon the deactivation constant is shown in Fig-ure 10-17. Lactoperoxidase can be regener-ated under conditions of high temperatureshort time (HTST) pasteurization. Figure10-18 shows the regeneration of lactoperoxi-dase activity in milk that is pasteurized for10 seconds at 850C. The regeneration effectdepends greatly on storage temperature; thelower the storage temperature, the smallerthe regeneration effect.
Lactoperoxidase is associated with theserum proteins of milk. It has an optimumpH of 6.8 and a molecular weight of 82,000.
Lipoxygenase (Linoleate: OxygenOxidoreductase)
This enzyme, formerly named lipoxidase,is present in plants and catalyzes the oxida-tion of unsaturated fats. The major source of
Figure 10-15 Structural Formula of Ferriprotoporphyrin III (Protohemin). Source: From J.R. Whi-taker, Food Related Enzymes, 1974, American Chemical Society.
lipoxygenase is legumes, soybeans, andother beans and peas. Smaller amounts arepresent in peanuts, wheat, potatoes, and rad-ishes. Lipoxygenase is a metallo-proteinwith an iron atom in its active center. Inplants two types of lipoxygenase exist: type Ilipoxygenase peroxidizes only free fattyacids with a high stereo- and regioselectivity;type II lipoxygenase is less specific for freelinoleic acid and acts as a general catalyst forautoxidation. Type I reacts with fats in a foodonly after free fatty acids have been formedby lipase action; type II acts directly on tri-acylglycerols.
Lipoxygenase is highly specific and at-tacks the c/5--c/5--l,4-pentadiene group con-tained in the fatty acids linoleic, linolenic,and arachidonic, as follows:
-CH=CH-CH2-CH=CH-
The specificity of this enzyme requires thatboth double bonds are in the cis configura-
tion; in addition, there is a requirement thatthe central methylene group of the 1,4-penta-diene group occupies the co-8 position on thefatty acid chain and also that the hydrogen tobe removed from the central methylene groupbe in the L-position. Although the exactmechanism of the reaction is still in somedoubt, there is agreement that the essentialsteps are as represented in Figure 10-19. Ini-tially, a hydrogen atom is abstracted from theco-8 methylene group to produce a free radi-cal. The free radical isomerizes, causing con-jugation of the double bond and isomerizationto the trans configuration. The free radicalthen reacts to form the co-6 hydroperoxide.
Lipoxygenase is reported to have a pHoptimum of about 9. However, these valuesare determined with linoleic acid as sub-strate, and in natural systems the substrate isusually present in the form of triglycerides.The enzyme has a molecular weight of
HEATING TIME
Figure 10-16 Deactivation of Lactoperoxidaseas a Function of Heating Time. Source: From F.Kiermeier and C. Kayser, Heat Inactivation ofLactoperoxidase (in German), Z. Lebensm.Untersuch. Forsch., Vol. 113, 1960.
Figure 10-17 Dependence of the Rate Constantof Heat Deactivation of Lactoperoxidase on theHeating Temperature. Source: From F Kier-meier and C. Kayser, Heat Inactivation of Lac-toperoxidase (in German), Z Lebensm. Untersuch.Forsch.,Vo\. 113, 1960.
TEMPERATURE
INA
CTI
VA
TIO
N
CO
NS
TAN
T
PE
RO
XID
AS
E A
CTI
VIT
Y
102,000 and an isoelectric point of 5.4. Theperoxide formation by lipoxygenase is inhib-ited by the common lipid antioxidants. Theantioxidants are thought to react with the freeradicals and interrupt the oxidation mecha-nism.
Most manifestations of lipoxygenase infoods are undesirable. However, it is used inbaking to bring about desirable changes.Addition of soybean flour to wheat flourdough results in a bleaching effect, becauseof oxidation of the xanthophyll pigments. Inaddition, there is an effect on the rheologicaland baking properties of the dough. It hasbeen suggested that lipoxygenase acts indi-rectly in the oxidation of sulfhydryl groupsin the gluten proteins to produce disulfidebonds. When raw soybeans are ground withwater to produce soy milk, a strong andunpleasant flavor develops that is called
Figure 10-19 Essential Steps in the Mechanismof the Lipoxygenase-Catalyzed Oxidation of the1,4-Pentadiene Group
Figure 10-18 Regeneration in Ultra High Temperature Treated Milk as a Function of Storage Temper-ature. (A) 20C; (B) 2O0C; (C) 370C. Source: From F. Kiermeier and C. Kayser, Heat Inactivation ofLactoperoxidase (in German), Z. Lebensm. Untersuch. Forsch., Vol. 113, 1960.
STORAGE TIME - HOURS
PE
RO
XID
AS
E A
CT
IVIT
Y -
%
painty, green, or beany. Carrying out thegrinding in boiling water instantly deacti-vates the enzyme, and no off-flavor isformed. Blanching of peas and beans isessential in preventing the lipoxygenase-cat-alyzed development of off-flavor. In additionto the development of off-flavors, the enzymemay be responsible for destruction of caro-tene and vitamin A, chlorophyll, bixin, andother pigments.
In some cases the action of lipoxygenaseleads to development of a characteristicaroma. Galliard et al. (1976) found that themain aroma compounds of cucumber, 2-trans hexenol and 2-trans, 6-a's-nonadienal,are produced by reaction of linolenic acidand lipoxygenase to form hydroperoxide
(Figure 10-20); these are changed into cisunsaturated aldehydes by hydroperoxidelyase. The cis unsaturated aldehydes are trans-formed by isomerase into the correspondingtrans isomers. These same substances inanother matrix would be experienced as off-flavors. The use of lipoxygenase as a versatilebiocatalyst has been described by Gardner(1996).
Xanthine Oxidase (Xanthine: OxygenOxidoreductase)
This enzyme catalyzes the conversion ofxanthine and hypoxanthine to uric acid. Thereaction equation is given in Figure 10-21;heavy arrows indicate the reactions catalyzed
Lipoxygenase
Hydroperoxidelyase
3-c/s hexenol 3-c/s, 6-c/s nonadienal
(Isomerase) (Isomerase)
2-trans hexenol 2-frans, 6-c/s nonadienal
Figure 10-20 Lipoxygenase Catalyzed Formation of Aroma Compounds in Cucumber. Source:Reprinted from Biochim. Biophys. Acta., Vol. 441, T. Galliard, D.R. Phillips, and J. Reynolds, TheFormation of cw-3-nonenal, trans-2-nonenal and Hexanol from Linoleic Acid Hydroperoxide Isomersby a Hydroperoxide Cleavage Enzyme System in Cucumber (Cucumis Sativus) Fruits, p. 184, Copy-right 1976, with permission from Elsevier Science.
by the enzyme and the dashed arrows repre-sent the net result of the catalytic process(Whitaker 1972). Although xanthine oxi-dase is a nonspecific enzyme and many sub-stances can serve as substrate, the rate ofoxidation of xanthine and hypoxanthine ismany times greater than that of other sub-strates.
Xanthine oxidase has been isolated frommilk and obtained in the crystalline state.The molecular weight is 275,000. One moleof the protein contains 2 moles of FAD, 2gram-atoms of molybdenum, 8 gram-atomsof nonheme iron, and 8 labile sulfide groups.The 8 labile sulfide groups are liberated inthe form of H2S upon acidification or boilingat pH 7. The optimum pH for activity is 8.3.The xanthine oxidase in milk is associatedwith the fat globules and, therefore, followsthe fat into the cream when milk is separated.It seems to be located in small particles(microsomes) that are attached to the fat
globules. The microsomes also contain theenzyme alkaline phosphatase. The micro-somes can be dislodged from the fat globulesby mechanical treatment such as pumpingand agitation and by heating and cooling.The enzyme is moderately stable to heat butno less so than peroxidase.
IMMOBILIZED ENZYMES
One of the most important recent develop-ments in the use of enzymes for industrialfood processing is the fixing of enzymes onwater-insoluble inert supports. The fixedenzymes retain their activity and can be eas-ily added to or removed from the reactionmixture. The use of immobilized enzymespermits continuous processing and greatlyincreased use of the enzyme. Various possi-ble methods of immobilizing enzymes havebeen listed by Weetall (1975) and Hultin(1983). A schematic representation of the
Figure 10-21 Oxidation of Hypoxanthine and Xanthine to Uric Acid by Xanthine Oxidase. Source:From J.R. Whitaker, Principles of Enzymology for the Food Sciences, 1972, Marcel Dekker, Inc.
Xanthine(enol form)
Uric acid(enol form)
Hypoxanthine Xanthine(koto form)
Uric acid(Ketoform)
available methods is given in Figure 10-22.The immobilizing methods include adsorp-tion on organic polymers, glass, metal oxides,and siliceous materials such as bentonite andsilica; entrapment in natural or syntheticpolymers, usually polyacrylamide; microen-capsulation in polymer membranes; ionexchange; cross-linking; adsorption and
cross-linking combined; copolymerization;and covalent attachment to organic polymers.The chemistry of immobilizing enzymes hasbeen covered in detail by Stanley and Olson(1974).
A summary of immobilization methodshas been provided by Adlercreutz (1993) andis presented in Exhibit 10-1. In membrane
ENTRAPMENTADSORPTION
carr ier enzyme
polymer
ION EXCHANGE
enzyme
MICROENCAPSULATION
enzyme
membrane carrier
ADSORPTION a CROSS-LINKING
enzymeCROSS-LINKING
enzyme
c a r r i e r
COPOLYMERIZATION
enzyme
COVALENT ATTACHMENT
enzyme
carr ie rpoly me r
Figure 10-22 Methods of Immobilizing Enzymes. Source: From H.H. Weetall, Immobilized Enzymesand Their Application in the Food and Beverage Industry, Process Biochem., Vol. 10, pp. 3-6, 1975.
enzyme
Exhibit 10-1 Summary of EnzymeImmobilization Methods
1. chemical methods• covalent binding• cross-linking
2. physical methods• adsorption• physical deposition• entrapment
-in polymer gels-in microcapsules
• membranes3. two-phase systems
• organic-aqueous• aqueous-aqueous
reactors, the reaction product is separatedfrom the reaction mixture by a semiperme-able membrane. In two-phase systems, ahydrophobic reaction product can be sepa-rated from the aqueous reaction mixture bytransfer to the organic solvent phase. Inaqueous-aqueous systems, two incompatiblepolymers in aqueous solution form a two-phase system.
Immobilizing enzymes is likely to changetheir stability, and the method of attachmentto the carrier also affects the degree of stabil-ity. When a high molecular weight substrateis used, the immobilizing should not be doneby entrapment, microencapsulation, or copo-lymerization, because enzyme and substratecannot easily get in contact. One of thepromising methods appears to be covalentcoupling of enzymes to inorganic carrierssuch as porous silica glass particles. Not allof the immobilized enzyme is active, due toeither inactivation or steric hindrance. Usu-ally, only about 30 to 50 percent of the boundenzyme is active.
Immobilized enzymes can be used in oneof two basic types of reactor systems. Thefirst is the stirred tank reactor where theimmobilized enzyme is stirred with the sub-strate solution. This is a batch system and,after the reaction is complete, the immobi-lized enzyme is separated from the product.The other system employs continuous flowcolumns in which the substrate flowsthrough the immobilized enzyme containedin a column or similar device. A simplifiedflow diagram of such a system is given inFigure 10-23.
IMMOBILIZED ENZYME SYSTEM
FEED PRETREATMENTIMMOBtLIZED ENZYME
COLUMN ORSTIRRID TANK REACTOR PRODUCT
TEMPERATURE ANDpH ADJUSTMENT
DEAfRATlONFlOW CONTROL
MONITORING DtVICi
Figure 10-23 Row Diagram of an Immobilized Enzyme System (Column Operation of Lactase Immo-bilized on Phenol-Formaldehyde Resin with Glutaraldehyde). Source: From W.L. Stanley and A.C.Olson, The Chemistry of Immobilizing Enzymes, J. Food ScL, Vol. 39, pp. 660-666, 1974.
The characteristics of immobilized enzymesare likely to be somewhat different thanthose of the original enzyme. The pH opti-mum can be shifted; this depends on the sur-face charge of the carrier (Figure 10-24).Another property that can be changed is theMichaelis constant, Km. This value can be-come either larger or smaller. Immobilizingmay result in increased thermal stability(Figure 10-25), but in some cases the ther-mal stability is actually decreased.
Many examples of the use of immobilizedenzymes in food processing have beenreported. One of the most important of theseis the use of immobilized glucose isomeraseobtained from Streptomyces for the produc-tion of high-fructose corn syrup (Mermel-stein 1975). In this process, the enzyme isbound to an insoluble carrier such as diethylamino ethyl cellulose or a slurry of the fixed
enzyme coated onto a pressure-leaf filter.The filter then serves as the continuous reac-tor through which the corn syrup flows. Theproduct obtained by this process is a syrupwith 71 percent solids that contains about 42percent fructose and 50 percent glucose; ithas high sweetening power, high ferment-ability, high humectancy, reduced tendencyto crystallize, low viscosity, and good flavor.
Examples of the use of immobilized en-zymes in food processing and analysis havebeen listed by Olson and Richardson (1974)and Hultin (1983). L-aspartic acid and L-malic acid are produced by using enzymescontained in whole microorganisms that areimmobilized in a polyacrylamide gel. Theenzyme aspartase from Escherichia coli isused for the production of aspartic acid.Fumarase from Brevibacterium ammoni-agenes is used for L-malic acid production.
Figure 10-24 Effect of Immobilizing on the pH Optimum of Papain. Source: From H.H. Weetall,Immobilized Enzymes and Their Application in the Food and Beverage Industry, Process Biochem.,Vol. 10, pp. 3-6, 1975.
PH
PERC
ENT
OF
MA
XIM
UM
AC
TIVI
TY
IMMOBILIZED PAPAIN(NEGATIVE CHARGED SURFACE)
IMMOBILIZED PAPAIN(POSITIVE CHARGED SURFACE)
SOLUBLEPAPAIN
The most widely used immobilized en-zyme process involves the use of the enzymeglucose isomerase for the conversion of glu-cose to fructose in corn syrup (Carasik andCarroll 1983). The organism Bacillus coagu-lans has been selected for the production ofglucose isomerase. The development of theimmobilized cell slurry has not proceeded tothe point where half-lives of the enzyme aremore than 75 days. A half-life is defined asthe time taken for a 50 percent decrease inactivity. Such immobilized enzyme columnscan be operated for periods of over threehalf-lives.
REFERENCES
Adlercreutz, P. 1993. Immobilized enzymes. InEnzymes in food processing, ed. T. Nagodaurithanaand G. Reed. San Diego, CA: Academic Press, Inc.
Albersheim, P., and U. Kilias. 1962. Studies relating tothe purification and properties of pectin transelimi-nase. Arch. Biochem. Biophys. 97: 107-115.
The second important application of im-mobilized enzymes is the hydrolysis of lac-tose to glucose and galactose in milk andmilk products by lactase (Sprossler andPlainer 1983). Several lactase sources areavailable; from yeast, Saccharomyces lactisand S. fragilis, or from fungi, Aspergillusoryzae or A. niger. The enzymes vary in theiroptimum pH and optimum temperature, aswell as other conditions.
It is to be expected that the use of immobi-lized enzymes in food processing will con-tinue to grow rapidly in the near future.
Arnold, R.G., et al. 1975. Application of lipolyticenzymes to flavor development in dairy products. J.Dairy ScL 58: 1127-1143.
Carasik, W., and J.O. Carroll. 1983. Development ofimmobilized enzymes for production of high-fruc-tose corn syrup. Food Technol 37, no. 10: 85-91.
Figure 10-25 Effect of Immobilizing of the Thermal Stability of Papain. Source: From H.H. Weetall,Immobilized Enzymes and Their Application in the Food and Beverage Industry, Process Biochem.,Vol. 10, pp. 3-6, 1975.
TEMPERATURE (°c)
IMMOBILIZED PAPAIN
SOLUBLEPAPAIN
°/o
OF
IN
ITIA
L A
CT
IVIT
Y
Cordle, C.T. 1994. Control of food allergies using pro-tein hydrolysates. Food Technol 48, no. 10: 72-76.
Embs, RJ., and P. Markakis. 1969. Bisulfite effect onthe chemistry and activity of horseradish peroxidase.J. Food Sd. 34:500-501.
Frost, G.M. 1986. Commercial production of enzymes.In Developments in food proteins, Vol. 4, ed. B.J.F.Hudson. New York: Elsevier Applied Science Pub-lishers.
Galliard, T., et al. 1976. The formation of ds-3-none-nal, frans-2-nonenal and hexanal from linoleic acidhydroperoxide isomers by a hydroperoxide cleavageenzyme system in cucumber (Cucumis sativus)fruits. Biochim. Biophys. Acta 441: 181-192.
Gardner, H.W. 1996. Lipoxygenase as a versatile bio-catalyst. J. Am. Oil Chem. Soc. 73: 1347-1357.
Godtfredsen, S.E. 1993. Lipases. In Enzymes in foodprocessing, ed. T. Nagodawithana and G. Reed. SanDiego, CA: Academic Press, Inc.
Greenfield, P.P., and R.L. Lawrence. 1975. Character-ization of glucose oxidase and catalase on inorganicsupports. / Food Sd. 40: 906-910.
Habibi-Najafi, M.B., and B.H. Lee. 1996. Bitterness incheese: A review. Crit. Rev. Food Sd. Nutr. 36:397-411.
Hammer, F.E. 1993. Oxidoreductases. In Enzymes infood processing, ed. T. Nagodawithana and G. Reed.San Diego, CA: Academic Press, Inc.
Hultin, H.O. 1983. Current and potential uses of immo-bilized enzymes. Food Technol. 37, no. 10: 66-82,176.
Lahl, W.J., and S.D. Braun. 1994. Enzymatic produc-tion of protein hydrolysates for food use. Food Tech-nol. 48, no. 10:68-71.
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INTRODUCTION
The possibility of harmful or toxic sub-stances becoming part of the food supplyconcerns the public, the food industry, andregulatory agencies. Toxic chemicals may beintroduced into foods unintentionally throughdirect contamination, through environmen-tal pollution, and as a result of processing.Many naturally occurring food compoundsmay be toxic. A summary of the varioustoxic chemicals in foods (Exhibit 11-1) waspresented in a scientific status summary ofthe Institute of Food Technologists (1975).Many toxic substances present below certainlevels pose no hazard to health. Some sub-stances are toxic and at the same time essen-tial for good health (such as vitamin A andselenium). An understanding of the proper-ties of additives and contaminants and howthese materials are regulated by governmen-tal agencies is important to the food scientist.Regulatory controls are dealt with in Chapter12.
Food additives can be divided into twomajor groups, intentional additives and inci-dental additives. Intentional additives arechemical substances that are added to foodfor specific purposes. Although we have lit-tle control over unintentional or incidentaladditives, intentional additives are regulated
by strict governmental controls. The U.S.law governing additives in foods is the FoodAdditives Amendment to the Federal Food,Drug and Cosmetic Act of 1958. Accordingto this act, a food additive is defined as fol-lows:
The term food additive means any sub-stance the intended use of which results,or may reasonably be expected to result,directly or indirectly in its becoming acomponent or otherwise affecting thecharacteristics of any food (includingany substance intended for use in pro-ducing, manufacturing, packing, pro-cessing, preparing, treating, packaging,transporting, or holding food; and in-cluding any source of radiation intendedfor any such use), if such a substance isnot generally recognized, among expertsqualified by scientific training and expe-rience to evaluate its safety, as havingbeen adequately shown through scien-tific procedures (or, in the case of a sub-stance used in food prior to January 1,1958, through either scientific proce-dures or experience based on commonuse in food) to be safe under the condi-tion of its intended use; except that sucha term does not include pesticides, color
Additives and Contaminants
CHAPTER 11
additives and substances for which priorsanction or approval was granted.
The law of 1958 thus recognizes the fol-lowing three classes of intentional additives:
1. additives generally recognized as safe(GRAS)
2. additives with prior approval3. food additives
Coloring materials and pesticides on rawagricultural products are covered by otherlaws. The GRAS list contains several hun-dred compounds, and the concept of such alist has been the subject of controversy (Hall1975).
Before the enactment of the 1958 law, U.S.laws regarding food additives required that afood additive be nondeceptive and that anadded substance be either safe and thereforepermitted, or poisonous and deleterious and
therefore prohibited. This type of legislationsuffered from two main shortcomings: (1) itequated poisonous with harmful, and (2) theonus was on the government to demonstratethat any chemical used by the food industrywas poisonous. The 1958 act distinguishesbetween toxicity and hazard. Toxicity is thecapacity of a substance to produce injury.Hazard is the probability that injury willresult from the intended use of the substance.It is now well recognized that many compo-nents of our foods, whether natural or added,are toxic at certain levels but harmless oreven nutritionally essential at lower levels.The ratio between effective dose and toxicdose of many compounds, including suchcommon nutrients as amino acids and salts,is of the order of 1 to 100. It is now manda-tory that any user of an additive must petitionthe government for permission to use thematerial and must supply evidence that thecompound is safe.
Exhibit 11-1 Toxic Chemicals in Foods
NATURAL
• normal components of natural foodproducts
• natural contaminants of natural foodproducts-microbiological origin: toxins-nonmicrobiological origin: toxicants
(e.g., Hg, Se) consumed in feeds byanimals used as food sources
MAN-MADE
• agricultural chemicals (e.g., pesticides,fertilizers)
• food additives• chemicals derived from food packag-
ing materials• chemicals produced in processing of
foods (e.g., by heat, ionizing radiation,smoking)
• inadvertent or accidental contaminants-food preparation accidents or mis-
takes-contamination from food utensils-environmental pollution-contamination during storage or
transport
An important aspect of the act is the so-called Delaney clause, which specifies thatno additive shall be deemed safe if it is foundto induce cancer in man or animal. Such spe-cial consideration in the case of cancer-pro-ducing compounds is not incorporated in thefood laws of many other countries.
INTENTIONAL ADDITIVES
Chemicals that are intentionally introducedinto foods to aid in processing, to act as pre-servatives, or to improve the quality of thefood are called intentional additives. Theiruse is strictly regulated by national and inter-national laws. The National Academy of Sci-ences (1973) has listed the purposes of foodadditives as follows:
• to improve or maintain nutritional value• to enhance quality• to reduce wastage• to enhance consumer acceptability• to improve keeping quality• to make the food more readily available• to facilitate preparation of the food
The use of food additives is in effect a foodprocessing method, because both have thesame objective—to preserve the food and/ormake it more attractive. In many food pro-cessing techniques, the use of additives is anintegral part of the method, as is smoking,heating, and fermenting. The National Acad-emy of Sciences (1973) has listed the follow-ing situations in which additives should notbe used:
• to disguise faulty or inferior processes• to conceal damage, spoilage, or other
inferiority• to deceive the consumer
• if use entails substantial reduction inimportant nutrients
• if the desired effect can be obtained byeconomical, good manufacturing prac-tices
• in amounts greater than the minimumnecessary to achieve the desired effects
There are several ways of classifying in-tentional food additives. One such methodlists the following three main types of addi-tives:
1. complex substances such as proteins orstarches that are extracted from otherfoods (for example, the use of casein-ate in sausages and prepared meats)
2. naturally occurring, well-defined chem-ical compounds such as salt, phos-phates, acetic acid, and ascorbic acid
3. substances produced by synthesis,which may or may not occur in nature,such as coal tar dyes, synthetic (3-caro-tene, antioxidants, preservatives, andemulsifiers
Some of the more important groups ofintentional food additives are described inthe following sections.
Preservatives
Preservatives or antimicrobial agents playan important role in today's supply of safeand stable foods. Increasing demand for con-venience foods and reasonably long shelf lifeof processed foods make the use of chemicalfood preservatives imperative. Some of thecommonly used preservatives—such as sul-fites, nitrate, and salt—have been used forcenturies in processed meats and wine. Thechoice of an antimicrobial agent has to bebased on a knowledge of the antimicrobial
spectrum of the preservative, the chemicaland physical properties of both food and pre-servative, the conditions of storage and han-dling, and the assurance of a high initialquality of the food to be preserved (Davidsonand Juneja 1990).
Benzoic Acid
Benzoic acid occurs naturally in manytypes of berries, plums, prunes, and somespices. As an additive, it is used as benzoicacid or as benzoate. The latter is used moreoften because benzoic acid is sparsely solu-ble in water (0.27 percent at 180C) andsodium benzoate is more soluble (66.0 g/100mL at 2O0C). The undissociated form of ben-zoic acid is the most effective antimicrobialagent. With a pKa of 4.2, the optimum pHrange is from 2.5 to 4.0. This makes it aneffective antimicrobial agent in high-acidfoods, fruit drinks, cider, carbonated bever-ages, and pickles. It is also used in marga-rines, salad dressings, soy sauce, and jams.
Parabens
Parabens are alkyl esters of /?-hydroxyben-zoic acid. The alkyl groups may be one ofthe following: methyl, ethyl, propyl, butyl, orheptyl. Parabens are colorless, tasteless, andodorless (except the methyl paraben). Theyare nonvolatile and nonhygroscopic. Theirsolubility in water depends on the nature ofthe alkyl group; the longer the alkyl chainlength, the lower the solubility. They differfrom benzoic acid in that they have antimi-crobial activity in both acid and alkaline pHregions.
The antimicrobial activity of parabens isproportional to the chain length of the alkylgroup. Parabens are more active againstmolds and yeasts than against bacteria, and
more active against gram-positive than gram-negative bacteria. They are used in fruit-cakes, pastries, and fruit fillings. Methyl andpropyl parabens can be used in soft drinks.Combinations of several parabens are oftenused in applications such as fish products,flavor extracts, and salad dressings.
Sorbic Acid
Sorbic acid is a straight-chain, trans-transunsaturated fatty acid, 2,4-hexadienoic acid.As an acid, it has low solubility (0.15 g/100mL) in water at room temperature. The salts,sodium, or potassium are more soluble inwater. Sorbates are stable in the dry form; theyare unstable in aqueous solutions because theydecompose through oxidation. The rate ofoxidation is increased at low pH, by increasedtemperature, and by light exposure.
Sorbic acid and sorbates are effectiveagainst yeasts and molds. Sorbates inhibityeast growth in a variety of foods includingwine, fruit juice, dried fruit, cottage cheese,meat, and fish products. Sorbates are mosteffective in products of low pH includingsalad dressings, tomato products, carbonatedbeverages, and a variety of other foods.
The effective level of sorbates in foods isin the range of 0.5 to 0.30 percent. Some ofthe common applications are shown in Table11-1. Sorbates are generally used in sweet-ened wines or wines that contain residualsugars to prevent refermentation. At the lev-els generally used, sorbates do not affectfood flavor. However, when used at higherlevels, they may be detected by some peopleas an unpleasant flavor. Sorbate can bedegraded by certain microorganisms to pro-duce off-flavors. Molds can metabolize sor-bate to produce 1,3 pentadiene, a volatilecompound with an odor like kerosene. Highlevels of microorganisms can result in the
degradation of sorbate in wine and result inthe off-flavor known as geranium off-odor(Edinger and Splittstoesser 1986). The com-pounds responsible for the flavor defect areethyl sorbate, 4-hexenoic acid, 1-ethoxy-hexa-2,4-diene, and 2-ethoxyhexa-3,5-diene.The same problem may occur in fermentedvegetables treated with sorbate.
Sulfites
Sulfur dioxide and sulfites have long beenused as preservatives, serving both as antimi-crobial substance and as antioxidant. Theiruse as preservatives in wine dates back toRoman times. Sulfur dioxide is a gas that canbe used in compressed form in cylinders. It isliquid under pressure of 3.4 atm and can beinjected directly in liquids. It can also beused to prepare solutions in ice cold water. Itdissolves to form sulfurous acid. Instead ofsulfur dioxide solutions, a number of sulfitescan be used (Table 11-2) because, when dis-solved in water, they all yield active SO2.
The most widely used of these sulfites ispotassium metabisulfite. In practice, a valueof 50 percent of active SO2 is used. Whensulfur dioxide is dissolved in water, the fol-lowing ions are formed:
SO2 (gas) -> SO2 (aq)SO2 (aq)+ -» H2O -> H2SO3
H2SO3 -» H+ + HSO3- (K1 = 1.7 x 1(T2)HSO3
1 -> H+ + SO321 (K2 = 5 x IO"6)
2HSO3- -> S2O52- + H2O
All of these forms of sulfur are known as freesulfur dioxide. The bisulfite ion (HSO3") canreact with aldehydes, dextrins, pectic sub-stances, proteins, ketones, and certain sugarsto form addition compounds.
Table 11-1 Applications of Sorbates as Antimicrobial Agents
Products
Dairy products: aged cheeses, processed cheeses, cottage cheese, cheesespreads, cheese dips, sour cream, yogurt
Bakery products: cakes, cake mixes, pies, fillings, mixes, icings, fudges, toppings,doughnuts
Vegetable products: fermented vegetables, pickles, olives, relishes, fresh saladsFruit products: dried fruit, jams, jellies, juices, fruit salads, syrups, purees, concen-
tratesBeverages: still wines, carbonated and noncarbonated beverages, fruit drinks, low-
calorie drinksFood emulsions: mayonnaise, margarine, salad dressingsMeat and fish products: smoked and salted fish, dry sausagesMiscellaneous: dry sausage casings, semimoist pet foods, confectionery
Levels (%)
0.05-0.30
0.03-0.30
0.02-0.200.02-0.25
0.02-0.10
0.05-0.100.05-0.300.05-0.30
Source: Reprinted with permission from J.N. Sofos and RF. Busta, Sorbic Acid and Sorbates, in Antimicrobials inFoods, P.M. Davidson and A.L. Branen, eds., p. 62,1993, by courtesy of Marcel Dekker, Inc.
The addition compounds are known asbound sulfur dioxide. Sulfur dioxide is usedextensively in wine making, and in wine acet-aldehyde reacts preferentially with bisulfite.Excess bisulfite reacts with sugars. It is pos-sible to classify bound SO2 into three forms:aldehyde sulfurous acid, glucose sulfurousacid, and rest sulfurous acid. The latter holdsthe SO2 in a less tightly bound form. Sulfitesin wines serve a dual purpose: (1) antisepticor bacteriostatic and (2) antioxidant. Theseactivities are dependent on the form of SO2
present. The various forms of SO2 in wineare represented schematically in Figure 11-1.The free SO2 includes the water-soluble SO2
and the undissociated H2SO3 and constitutesabout 2.8 percent of the total. The bisulfiteform constitutes 96.3 percent and the suifiteform 0.9 percent (all at pH 3.3 and 2O0C).The bound SO2 is mostly (80 percent)present as acetaldehyde SO2, 1 percent asglucose SO2, and 10 to 20 percent as restSO2. The various forms of suifite have differ-ent activities. The two free forms are the onlyones with antiseptic activity. The antioxidantactivity is limited to the SO3
2" ion (Figure11-1). The antiseptic activity of SO2 ishighly dependent on the pH, as indicated inTable 11-3. The lower the pH the greater the
antiseptic action of SO2. The effect of pH onthe various forms of sulfur dioxide is shownin Figure 11-2.
Sulfurous acid inhibits molds and bacteriaand to a lesser extent yeasts. For this reason,SO2 can be used to control undesirable bac-teria and wild yeast in fermentations withoutaffecting the SO2-tolerant cultured yeasts.According to Chichester and Tanner (1968),the undissociated acid is 1,000 times moreactive than HSO3~ for Escherichia coli, 100to 500 times for Saccharomyces cerevisiae,and 100 times for Aspergillus niger.
The amount of SO2 added to foods is self-limiting because at levels from 200 to 500ppm the product may develop an unpleasantoff-flavor. The acceptable daily intake (ADI)is set at 1.5 mg/kg body weight. Becauselarge intakes can result from consumption ofwine, there have been many studies on re-ducing the use of SO2 in wine making.Although some other compounds (such assorbic acid and ascorbic acid) may partiallyreplace SO2, there is no satisfactory replace-ment for SO2 in wine making.
The use of SO2 is not permitted in foodsthat contain significant quantities of thia-mine, because this vitamin is destroyed bySO2. In the United States, the maximum per-
Chemical
Sulfur dioxideSodium suifite, anhydrousSodium suifite, heptahydrateSodium hydrogen suifiteSodium metabisulfitePotassium metabisuifiteCalcium suifite
Formula
SO2
Na2SO3
Na2SO3-? H2ONaHSO3
Na2S2O5
K2S2O5
CaSO3
Content of Active SO2
100.00%50.82%25.41%61.56%67.39%57.63%64.00%
Table 11-2 Sources of SO2 and Their Content of Active SO2
mitted level of SO2 in wine is 350 ppm.Modern practices have resulted in muchlower levels of SO2. In some countries SO2 isused in meat products; such use is not per-mitted in North America on the grounds thatthis would result in consumer deception. SO2
is also widely used in dried fruits, where lev-els may be up to 2,000 ppm. Other applica-tions are in dried vegetables and dried potato
Table 11-3 Effect of pH on the Proportion ofActive Antiseptic SO2 of Wine Containing 100mg/L Free SO2
pH Active SO2 (mg/L)
~22 3?!o2.8 8.03.0 5.03.3 3.03.5 1.83.7 1.24.0 0.8
products. Because SO2 is volatile and easilylost to the atmosphere, the residual levelsmay be much lower than the amounts origi-nally applied.
Nitrates and Nitrites
Curing salts, which produce the character-istic color and flavor of products such asbacon and ham, have been used throughouthistory. Curing salts have traditionally con-tained nitrate and nitrite; the discovery thatnitrite was the active compound was made inabout 1890. Currently, nitrate is not consid-ered to be an essential component in curingmixtures; it is sometimes suggested thatnitrate may be transformed into nitrite, thusforming a reservoir for the production ofnitrite. Both nitrates and nitrites are thoughtto have antimicrobial action. Nitrate is usedin the production of Gouda cheese to preventgas formation by butyric acid-forming bac-teria. The action of nitrite in meat curing is
Figure 11—1 The Various Forms of SO2 in Wine and Their Activity. Source: Reprinted with permis-sion from J.M. deMan, 500 Years of Sulfite Use in Winemaking, Am. Wine Soc. /., Vol. 20, pp. 44-46,© 1988, American Wine Society.
I antjoxidont
activeantiseptic
acetaldehyde SO2
rest SO2
bound SO2
TOTAL SO 2
free SO2
HSOj
giucose su?
considered to involve inhibition of toxin for-mation by Clostridium botulinum, an impor-tant factor in establishing safety of curedmeat products. Major concern about the useof nitrite was generated by the realizationthat secondary amines in foods may react toform nitrosamines, as follows:
analytical procedures are difficult, there is asyet no clear picture of the occurrence of nitro-samines. The nitrosamines may be either vol-atile or nonvolatile, and only the latter areusually included in analysis of foods. Nitro-samines, especially dimethyl-nitrosamine, havebeen found in a number of cases when curedmeats were surveyed at concentrations of afew |Ltg/kg (ppb). Nitrosamines are usuallypresent in foods as the result of processingmethods that promote their formation (Hav-ery and Fazio 1985). An example is the spraydrying of milk. Suitable modifications ofthese process conditions can drasticallyreduce the nitrosamine levels. Considerablefurther research is necessary to establish whynitrosamines are present only in some sam-ples and what the toxicological importanceof nitrosamines is at these levels. Thereappears to be no suitable replacement fornitrite in the production of cured meats such
Figure 11-2 Effect of pH on the lonization of Sulfurous Acid in Water
pH
X OF
TOT
AL S
ULPH
UROU
S AC
ID
The nitrosamines are powerful carcino-gens, and they may be mutagenic and terato-genic as well. It appears that very smallamounts of nitrosamines can be formed incertain cured meat products. These levels arein the ppm or the ppb range and, because
as ham and bacon. The ADI of nitrite hasbeen set at 60 mg per person per day. It isestimated that the daily intake per person inCanada is about 10 mg.
Cassens (1997) has reported a dramaticdecline in the residual nitrite levels in curedmeat products in the United States. The cur-rent residual nitrite content of cured meatproducts is about 10 ppm. In 1975 an averageresidual nitrite content in cured meats wasreported as 52.5 ppm. This reduction ofnitrite levels by about 80 percent has beenattributed to lower ingoing nitrite, increaseduse of ascorbates, improved process control,and altered formulations.
The nitrate-nitrite intake from naturalsources is much higher than that from pro-cessed foods. Fassett (1977) estimated thatthe nitrate intake from 100 g of processedmeat might be 50 mg and from 100 g ofhigh-nitrate spinach, 200 mg. Wagner andTannenbaum (1985) reported that nitrate incured meats is insignificant compared tonitrite produced endogenously. Nitrate isproduced in the body and recirculated to theoral cavity, where it is reduced to nitrite bybacterial action.
Hydrogen Peroxide
Hydrogen peroxide is a strong oxidizingagent and is also useful as a bleaching agent.It is used for the bleaching of crude soya lec-ithin. The antimicrobial action of hydrogenperoxide is used for the preservation ofcheese milk. Hydrogen peroxide decom-poses slowly into water and oxygen; this pro-cess is accelerated by increased temperatureand the presence of catalysts such as cata-lase, lacto-peroxidase and heavy metals. Itsantimicrobial action increases with tempera-ture. When hydrogen peroxide is used forcheese making, the milk is treated with 0.02
percent hydrogen peroxide followed by cata-lase to remove the hydrogen peroxide. Hy-drogen peroxide can be used for sterilizingfood processing equipment and for steriliz-ing packaging material used in aseptic foodpackaging systems.
Sodium Chloride
Sodium chloride has been used for centu-ries to prevent spoilage of foods. Fish, meats,and vegetables have been preserved with salt.Today, salt is used mainly in combinationwith other processing methods. The antimi-crobial activity of salt is related to its abilityto reduce the water activity (aw), therebyinfluencing microbial growth. Salt has thefollowing characteristics: it produces anosmotic effect, it limits oxygen solubility, itchanges pH, sodium and chloride ions aretoxic, and salt contributes to loss of magne-sium ions (Banwart 1979). The use of sodiumchloride is self-limiting because of its effecton taste.
Bacteriocins
Nisin is an antibacterial polypeptide pro-duced by some strains of Lactococcus lactis.Nisin-like substances are widely producedby lactic acid bacteria. These inhibitory sub-stances are known as bacteriocins. Nisin hasbeen called an antibiotic, but this term isavoided because nisin is not used for thera-peutic purposes in humans or animals. Nisin-producing organisms occur naturally in milk.Nisin can be used as a processing aid againstgram-positive organisms. Because its effec-tiveness decreases as the bacterial load in-creases, it is unlikely to be used to cover upunhygienic practices.
Nisin is a polypeptide with a molecularweight of 3,500, which is present as a dimer
of molecular weight 7,000. It contains someunusual sulfur amino acids, lanthionine andp-methyl lanthionine. It contains no aromaticamino acids and is stable to heat.
The use of nisin as a food preservative hasbeen approved in many countries. It has beenused effectively in preservation of processedcheese. It is also used in the heat treatment ofnonacid foods and in extending the shelf lifeof sterilized milk.
A related antibacterial substance is nata-mycin, identical to pimaricin. Natamycin iseffective in controlling the growth of fungibut has no effect on bacteria or viruses. Infermentation industries, natamycin can beused to control mold or yeast growth. It has alow solubility and therefore can be used as asurface treatment on foods. Natamycin isused in the production of many varieties ofcheese.
Acids
Acids as food additives serve a dual pur-pose, as acidulants and as preservatives.Phosphoric acid is used in cola soft drinks toreduce the pH. Acetic acid is used to providetartness in mayonnaise and salad dressings.A similar function in a variety of other foodsis served by organic acids such as citric, tar-taric, malic, lactic, succinic, adipic, and fu-maric acid. The properties of some of thecommon food acids are listed in Table 11-4(Peterson and Johnson 1978). Members ofthe straight-chain carboxylic acids, propionicand sorbic acids, are used for their antimicro-bial properties. Propionic acid is mainly usedfor its antifungal properties. Propionic acidapplied as a 10 percent solution to the sur-face of cheese and butter retards the growthof molds. The fungistatic effect is higher atpH 4 than at pH 5. A 5 percent solution ofcalcium propionate acidified with lactic acid
to pH 5.5 is as effective as a 10 percent una-cidified solution of propionic acid. The sodiumsalts of propionic acid also have antimicro-bial properties.
Antioxidants
Food antioxidants in the broadest sense areall of the substances that have some effect onpreventing or retarding oxidative deteriora-tion in foods. They can be classified into anumber of groups (Kochhar and Rossell1990).
Primary antioxidants terminate free radicalchains and function as electron donors. Theyinclude the phenolic antioxidants, butylatedhydroxyanisole (BHA), butylated hydroxy-toluene (BHT), tertiary butyl hydroquinone(TBHQ), alkylgalates, usually propylgallate(PG), and natural and synthetic tocopherolsand tocotrienols.
Oxygen scavengers can remove oxygen ina closed system. The most widely used com-pounds are vitamin C and related substances,ascorbyl palmitate, and erythorbic acid (theD-isomer of ascorbic acid).
Chelating agents or sequestrants removemetallic ions, especially copper and iron, thatare powerful prooxidants. Citric acid is widelyused for this purpose. Amino acids and eth-ylene diamine tetraacetic acid (EDTA) areother examples of chelating agents.
Enzymic antioxidants can remove dissolvedor head space oxygen, such as glucose oxi-dase. Superoxide dismutase can be used toremove highly oxidative compounds fromfood systems.
Natural antioxidants are present in manyspices and herbs (Lacroix et al. 1997; Six1994). Rosemary and sage are the mostpotent antioxidant spices (Schuler 1990).The active principles in rosemary are car-nosic acid and carnosol (Figure 11-3). Anti-
Table 11-4 Properties of Some Common Food Acids
C4H6O6
Crystalline
150.09
75.05
147.0
1.04x1 0~3
5.55x1 0~5
H3PO4
85% WaterSolution82.00
27.33
OO
7.52 x 10~3
6.23 x 10~8
3x10-13
C4H6O5
Crystalline
134.09
67.05
144.0
4 x 1Q-4
9 xlO"6
C3H6O3
85% WaterSolution90.08
90.08
OO
1.37 x 1Q-4
C6H10O6
Crystalline
178.14
178.14
59.0
2.5 x 10"4
(gluconicacid)
C4H4O4
Crystalline
116.07
58.04
0.63
1 x10~3
3x10~5
C6H8O7
Crystalline
192.12
64.04
181.00
8.2 xlO"4
1.77 x 10"5
3.9 XlO"6
C6H10O4
Crystalline
146.14
73.07
1.4
3.7x10~5
2.4x10-®
C2H4O2
Oily Liquid
60.05
60.05
OO
8x10~5
Empiricalformula
Physical form
Molecularweight
Equivalentweight
Sol. in water(g/100ml_solv.)
lonizationconstants
KI
K2
K3
Tartaric AcidPhosphoric
AcidMalic AcidLactic Acid
Glucono-Delta-
LactoneFumaric
AcidCitric AcidAdipic AcidAcetic AcidProperty
Structure
oxidants from spices can be obtained asextracts or in powdered form by a processdescribed by Bracco et al. (1981).
The level of phenolic antioxidants permit-ted for use in foods is limited. U.S. regula-tions allow maximum levels of 0.02 percentbased on the fat content of the food.
Sometimes the antioxidants are incorpo-rated in the packaging materials rather thanin the food itself. In this case, a larger num-ber of antioxidants is permitted, providedthat no more than 50 ppm of the antioxidantsbecome a component of the food.
Emulsifiers
With the exception of lecithin, all emulsifi-ers used in foods are synthetic. They arecharacterized as ionic or nonionic and bytheir hydrophile/lipophile balance (HLB).All of the synthetic emulsifiers are deriva-tives of fatty acids.
Lecithin is the commercial name of a mix-ture of phospholipids obtained as a byprod-uct of the refining of soybean oil. Phos-phatidylcholine is also known as lecithin, butthe commercial product of that name con-tains several phospholipids including phos-
phatidylcholine. Crude soybean lecithin isdark in color and can be bleached withhydrogen peroxide or benzoyl peroxide. Lec-ithin can be hydroxylated by treatment withhydrogen peroxide and lactic or acetic acid.Hydroxylated lecithin is more hydrophilic,and this makes for a better oil-in-water emul-sifier. The phospholipids contained in leci-thin are insoluble in acetone.
Monoglycerides are produced by transes-terification of glycerol with triglycerides.The reaction proceeds at high temperature,under vacuum and in the presence of an alka-line catalyst. The reaction mixture, afterremoval of excess glycerol, is known as com-mercial monoglyceride, a mixture of about40 percent monoglyceride and di- and tri-glycerides. The di- and triglycerides have noemulsifying properties. Molecular distilla-tion can increase the monoglyceride contentto well over 90 percent. The emulsifyingproperties, especially HLB, are determinedby the chain length and unsaturation of thefatty acid chain.
Hydroxycarboxylic and fatty acid estersare produced by esterifying organic acids tomonoglycerides. This increases their hydro-philic properties. Organic acids used are ace-
carnosolcarnosic acid
Figure 11-3 Chemical Structure of the Active Antioxidant Principles in Rosemary
tic, citric, fumaric, lactic, succinic, or tartaricacid. Succinylated monoglycerides are syn-thesized from distilled monoglycerides andsuccinic anhydride. They are used as doughconditioners and crumb softeners (Krog1981). Acetic acid esters can be producedfrom mono- and diglycerides by reactionwith acetic anhydride or by transesterifica-tion. They are used to improve aeration infoods high in fat content and to control fatcrystallization. Other esters may be pre-pared: citric, diacetyl tartaric, and lactic acid.A product containing two molecules of lacticacid per emulsifier molecule, known asstearoyl-2-lactylate, is available as the sodiumor calcium salt. It is used in bakery products.
Polyglycerol esters of fatty acids are pro-duced by reacting polymerized glycerol withedible fats. The degree of polymerization ofthe glycerol and the nature of the fat providea wide range of emulsifiers with differentHLB values.
Polyethylene or propylene glycol esters offatty acids are more hydrophilic than mono-glycerides. They can be produced in a rangeof compositions.
Sorbitan fatty acid esters are produced bypolymerization of ethylene oxide to sorbitanfatty acid esters. The resulting polyoxyethyl-ene sorbitan esters are nonionic hydrophilicemulsifiers. They are used in bakery prod-ucts as antistaling agents. They are known aspolysorbates with a number as indication ofthe type of fatty acid used (e.g., lauric,stearic, or oleic acid).
Sucrose fatty acid esters can be producedby esterification of fatty acids with sucrose,usually in a solvent system. The HLB varies,depending on the number of fatty acids ester-ified to a sucrose molecule. Monoesters havean HLB value greater than 16, triesters lessthan 1. When the level of esterification in-creases to over five molecules of fatty acid,
the emulsifying property is lost. At high lev-els of esterification the material can be usedas a fat replacer because it is not absorbed ordigested and therefore yields no calories.
Bread Improvers
To speed up the aging process of wheatflour, bleaching and maturing agents areused. Benzoyl peroxide is a bleaching agentthat is frequently used; other compounds—including the oxides of nitrogen, chlorinedioxide, nitrosyl chloride, and chlorine—areboth bleaching and improving (or maturing)agents. Improvers used to ensure that doughwill ferment uniformly and vigorously in-clude oxidizing agents such as potassiumbromate, potassium iodate, and calcium per-oxide. In addition to these agents, there maybe small amounts of other inorganic com-pounds in bread improvers, including ammo-nium chloride, ammonium sulfate, calciumsulfate, and ammonium and calcium phos-phates. Most of these bread improvers canonly be used in small quantities, becauseexcessive amounts reduce quality. Severalcompounds used as bread improvers areactually emulsifiers and are covered underthat heading.
Flavors
Included in this group is a wide variety ofspices, oleoresins, essential oils, and naturalextractives. A variety of synthetic flavors con-tain mostly the same chemicals as those foundin the natural flavors, although the natural fla-vors are usually more complex in composi-tion. For legislative purposes, three categoriesof flavor compounds have been proposed.
1. Natural flavors and flavoring sub-stances are preparations or single sub-stances obtained exclusively by phys-
ical processes from raw materials intheir natural state or processed forhuman consumption.
2. Nature-identical flavors are producedby chemical synthesis or from aro-matic raw materials; they are chemi-cally identical to natural products usedfor human consumption.
3. Artificial flavors are substances thatare not present in natural products.
The first two categories require consider-ably less regulatory control than the latterone (Vodoz 1977). The use of food flavorscovers soft drinks, beverages, baked goods,confectionery products, ice cream, desserts,and so on. The amounts of flavor compoundsused in foods are usually small and generallydo not exceed 300 ppm. Spices and oleores-ins are used extensively in sausages and pre-pared meats. In recent years, because ofpublic perception, the proportion of naturalflavors has greatly increased at the expenseof synthetics (Sinki and Schlegel 1990).Numerous flavoring substances are on thegenerally recognized as safe (GRAS) list.Smith et al. (1996) have described some ofthe recent developments in the safety evalua-tion of flavors. They mention a significantrecent development in the flavor industry—the production of flavor ingredients usingbiotechnology—and describe their safetyassessment.
Flavor Enhancers
Flavor enhancers are substances that carrythe property of umami (see Chapter 7) andcomprise glutamates and nucleotides. GIu-tamic acid is a component amino acid of pro-teins but also occurs in many protein-con-taining foods as free glutamic acid. In spiteof their low protein content, many vegetables
have high levels of free glutamate, includingmushrooms, peas, and tomatoes. Sugita(1990) has listed the level of bound and freeglutamate in a variety of foods. Glutamate isan element of the natural ripening processthat results in fullness of taste, and it hasbeen suggested as the reason for the popular-ity of foods such as tomatoes, cheese, andmushrooms (Sugita 1990).
The nucleotides include disodium 5'-ino-sinate (IMP), adenosine monophosphate(AMP), disodium 5'-guanylate (GMP), anddisodium xan thy late (XMP). IMP is foundpredominantly in meat, poultry, and fish;AMP is found in vegetables, crustaceans,and mollusks; GMP is found in mushrooms,especially shiitake mushrooms.
Monosodium glutamate (MSG) is the sod-ium salt of glutamic acid. The flavor-enhanc-ing property is not limited to MSG. Similartaste properties are found in the L-forms ofoc-amino dicarboxylates with four to sevencarbon atoms. The intensity of flavor isrelated to the chemical structure of thesecompounds. Other amino acids that havesimilar taste properties are the salts ofibotenic acid, tricholomic acid, and L-thean-ine.
The chemical structure of the nucleotidesis shown in Figure 7-21. They are purineribonucleotides with a hydroxyl group oncarbon 6 of the purine ring and a phosphateester group on the 5'-carbon of the ribose.Nucleotides with the ester group at the 2' or3' position are tasteless. When the estergroup is removed by the action of phos-phomonoesterases, the taste activity is lost. Itis important to inactivate such enzymes infoods before adding 5'-nucleotide flavor en-hancers.
The taste intensity of MSG and its concen-tration are directly related. The detectionthreshold for MSG is 0.012 g/100 mL; for
sodium chloride it is 0.0037 g/100 mL; andfor sucrose it is 0.086 g/100 mL. There is astrong synergistic effect between MSG andIMP. The mixture of the two has a tasteintensity that is 16 times stronger than thesame amount of MSG. MSG contains 12.3percent sodium; common table salt containsthree times as much sodium. By using flavorenhancers in a food, it is possible to reducethe salt level without affecting the palatabil-ity or food acceptance. The mode of actionof flavor enhancers has been described byNagodawithana (1994).
Sweeteners
Sweeteners can be divided into twogroups, nonnutritive and nutritive sweeten-ers. The nonnutritive sweeteners include sac-charin, cyclamate, aspartame, acesulfame K,and sucralose. There are also others, mainlyplant extracts, which are of limited impor-tance. The nutritive sweeteners are sucrose;glucose; fructose; invert sugar; and a varietyof polyols including sorbitol, mannitol, malt-itol, lactitol, xylitol, and hydrogenated glu-cose syrups.
The chemical structure of the most impor-tant nonnutritive sweeteners is shown in Fig-ure 11-4. Saccharin is available as the sod-ium or calcium salt of orthobenzosulfimide.The cyclamates are the sodium or calciumsalts of cyclohexane sulfamic acid or the aciditself. Cyclamate is 30 to 40 times sweeterthan sucrose, and about 300 times sweeterthan saccharin. Organoleptic comparison ofsweetness indicates that the medium inwhich the sweetener is tasted may affect theresults. There is also a concentration effect.At higher concentrations, the sweetnessintensity of the synthetic sweeteners increasesat a lower rate than that which occurs withsugars. This has been ascribed to the bitter-
ness and strong aftertaste that appears atthese relatively high concentrations.
Cyclamates were first synthesized in 1939and were approved for use in foods in theUnited States in 1950. Continued tests on thesafety of these compounds resulted in the1967 finding that cyclamate can be convertedby intestinal flora into cyclohexylamine,which is a carcinogen. Apparently, only cer-tain individuals have the ability to convertcyclamate to cyclohexylamine (Collings 1971).In a given population, a portion are noncon-verters, some convert only small amounts,and others convert large amounts.
Aspartame is a dipeptide derivative, L-aspartyl-L-phenylalanine methyl ester, whichwas approved in the United States in 1981for use as a tabletop sweetener, in dry bever-age mixes, and in foods that are not heat pro-cessed. This substance is metabolized in thebody to phenylalanine, aspartic acid, andmethanol. Only people with phenylketonuriacannot break down phenylalanine. Anothercompound, diketopiperazine, may also beformed. However, no harmful effects fromthis compound have been demonstrated. Themain limiting factor in the use of aspartameis its lack of heat stability (Homier 1984).
A new sweetener, approved in 1988, isacesulfame K. This is the potassium salt of6-methyl-1,2,3-oxathiozine-4(3H)-one-2, 2-dioxide (Figure 11-4). It is a crystallinepowder that is about 200 times sweeter thansugar. The sweetening power depends to acertain degree on the acidity of the food it isused in. Acesulfame K is reportedly morestable than other sweeteners. The sweet tasteis clean and does not linger. Sucralose is atrichloroderivative of the C-4 epimer galac-tosucrose. It is about 600 times sweeter thansucrose and has a similar taste profile. Oneof its main advantages is heat stability, so itcan be used in baking.
Blending of nonnutritive sweeteners maylead to improved taste, longer shelf life,lower production cost, and reduced con-sumer exposure to any single sweetener(Verdi and Hood 1993). The dihydrochal-cone sweeteners are obtained from phenolicglycosides present in citrus peel. Such com-pounds can be obtained from naringin ofgrapefruit or from the flavonoid neohesperi-din. The compound neohesperidin dihydro-chalcone is rated 1,000 times sweeter thansucrose (Inglett 1971). Horowitz and Gentili(1971) investigated the relationship betweenchemical structure and sweetness, bitterness,and tastelessness. Several other natural com-pounds having intense sweetness have beendescribed by Inglett (1971); these includeglycyrrhizin (from licorice root) and a taste-modifying glycoprotein named miraculinthat is obtained from a tropical fruit knownas miracle berry. Stevioside is an extractfrom the leaves of a South American plantthat is 300 times sweeter than sugar. Thau-matin, a protein mixture from a West African
fruit, is 2,000 times sweeter than sugar, butits licorice-like aftertaste limits its useful-ness.
It has been suggested that sugars from theL series could be used as low-calorie sweet-eners. These sugars cannot be metabolized inthe normal way, as D sugars would, andtherefore pass through the digestive systemunaltered. Their effect on the body has notbeen sufficiently explored.
Possible new sweeteners have been de-scribed by Gelardi (1987).
Phosphates
These compounds are widely used as foodadditives, in the form of phosphoric acid asacidulant, and as monophosphates and poly-phosphates in a large number of foods andfor a variety of purposes. Phosphates serve asbuffering agents in dairy, meat, and fishproducts; anticaking agents in salts; firmingagents in fruits and vegetables; yeast food inbakery products and alcoholic beverages;
Figure 11-4 Chemical Structure of Sodium Saccharin, Sodium Cyclamate, Cyclohexylamine, andAcesulfame K
Na-Saccharin Na-cyclamate cyclohexylamine
Acesulfame K
and melting salts in cheese processing. Phos-phorus oxychloride is used as a starch-modi-fying agent.
The largest group of phosphates and themost important in the food industry is theorthophosphates (Figure 11-5). The phos-phate group has three replaceable hydrogens,giving three possible sodium orthophos-phates—monosodium, disodium, and triso-dium phosphate. The phosphates can bedivided into othophosphates, polyphos-phates, and metaphosphates, the latter havinglittle practical importance. Polyphosphateshave two or more phosphorus atoms joinedby an oxygen bridge in a chain structure. Thefirst members of this series are the pyrophos-phates, which have one P-O-P linkage. Thecondensed phosphate with two linkages istripolyphosphate. Alkali metal phosphateswith chain lengths greater than three are usu-
ally mixtures of polyphosphates with variedchain lengths. The best known is sodiumhexametaphosphate. The longer chain lengthsalts are glasses. Hexametaphosphate is not areal metaphosphate, since these are ring struc-tures and hexametaphosphate is a straight-chain polyphosphate. Sodium hexametaphos-phate has an average chain length of 10 to 15phosphate units.
Phosphates are important because theyaffect the absorption of calcium and otherelements. The absorption of inorganic phos-phorus depends on the amount of calcium,iron, strontium, and aluminum present in thediet. Chapman and Pugsley (1971) have sug-gested that a diet containing more phospho-rus than calcium is as detrimental as a simplecalcium deficiency. The ratio of calcium tophosphorus in bone is 2 to 1. It has been rec-ommended that in early infancy, the ratioshould be 1.5 to 1; in older infants, 1.2 to 1;and for adults, 1 to 1. The estimated annualper capita intake in the United States is 1 gCa and 2.9 g P, thus giving a ratio of 0.35.The danger in raising phosphorus levels isthat calcium may become unavailable.
Coloring Agents
In the United States two classes of coloradditives are recognized: colorants exemptfrom certification and colorants subject tocertification. The former are obtained fromvegetable, animal, or mineral sources or aresynthetic forms of naturally occurring com-pounds. The latter group of synthetic dyesand pigments is covered by the Color Addi-tives Amendment of the U.S. Food, Drug andCosmetic Act. In the United States thesecolor compounds are not known by theircommon names but as FD&C colors (Food,Drug and Cosmetic colors) with a color anda number (Noonan 1968). As an example,
QRTHO
PYRO
IBJ
LONG CHAIN
Figure 11-5 Structure of Ortho- and Poly-phosphate Salts
FD&C red dye no. 2 is known as amaranthoutside the United States. Over the years theoriginally permitted fat-soluble dyes havebeen removed from the list of approved dyes,and only water-soluble colors remain on theapproved list.
According to Newsome (1990) only ninesynthetic colors are currently approved forfood use and 21 nature-identical colors areexempt from certification. The approvedFD&C colors are listed in Exhibit 11-2. Cit-rus red no. 2 is only permitted for externaluse on oranges, with a maximum level of 2ppm on the weight of the whole orange. Itsuse is not permitted on oranges destined forprocessing.
Lakes are insoluble forms of the dyes andare obtained by combining the color withaluminum or calcium hydroxide. The dyesprovide color in solution, and the lakes serveas insoluble pigments.
Exhibit 11-2 Color Additives Permitted forFood Use in the United States and Their Com-mon Names
• FD&C red no. 3 (erythrosine)• FD&C red no. 40 (allura red)• FD&C orange B• FD&C yellow no. 6 (sunset yellow)• FD&C yellow no. 5 (tartrazine)• FD&C green no. 3 (fast green)• FD&C blue no. 1 (brillian blue)• FD&C blue no. 2 (indigotine)• Citrus red no. 2
Source: Reprinted with permission from R.L.Newsome, Natural and Synthetic ColoringAgents, in Food Additives, A.L. Branen, P.M.Davidson, and S. Salminen, eds., p. 344, 1990,by courtesy of Marcel Dekker, Inc.
The average per capita consumption offood colors is about 50 mg per day. Food col-ors have been suspect as additives for manyyears, resulting in many deletions from theapproved list. An example is the removal ofFD&C red no. 2 or amaranth in 1976. In theUnited States, it was replaced by FD&C redno. 40. The removal from the approved listwas based on the observation of reproductiveproblems in test animals that consumed ama-ranth at levels close to the ADI. As a conse-quence, the Food and Agriculture Organ-ization (FAO)AVorld Health Organization(WHO) reduced the ADI to 0.75 mg/kg bodyweight from 1.5 mg/kg. Other countries,including Canada, have not delisted ama-ranth.
The natural or nature-identical colors areless stable than the synthetic ones, more vari-able, and more likely to introduce undesir-able flavors. The major categories of naturalfood colors and their sources are listed inTable 11-5.
Food Irradiation
Food irradiation is the treatment of foodsby ionizing radiation in the form of beta,gamma, or X-rays. The purpose of food irra-diation is to preserve food and to prolongshelf life, as other processing techniquessuch as heating or drying have done. For reg-ulatory purposes irradiation is considered aprocess, but in many countries it is consid-ered to be an additive. This inconsistency inthe interpretation of food irradiation resultsin great obstacles to the use of this processand has slowed down its application consid-erably. Several countries are now in the pro-cess of reconsidering their legislationregarding irradiation. Depending on the radi-ation dose, several applications can be distin-guished. The unit of radiation is the Gray
(Gy), which is a measure of the energyabsorbed by the food. It replaced the olderunitrad(l Gy = 100 rad).
Radiation sterilization produces foodsthat are stable at room temperature andrequires a dose of 20 to 70 kGy. At lowerdoses, longer shelf life may be obtained,especially with perishable foods such asfruits, fish, and shellfish. The destruction ofSalmonella in poultry is an application forradiation treatment. This requires doses of 1to 10 kGy. Radiation disinfestation of spicesand cereals may replace chemical fumi-gants, which have come under increasingscrutiny in recent years. Dose levels of 8 to30 kGy would be required. Other possibleapplications of irradiation processing areinhibition of sprouting in potatoes andonions and delaying of the ripening of trop-ical fruits.
Nutrition Supplements
There are two fundamental reasons forthe addition of nutrients to foods consumedby the public: (1) to correct a recognizeddeficiency of one or more nutrients in thediets of a significant number of peoplewhen the deficit actually or potentiallyadversely affects health; and (2) to maintainthe nutritional quality of the food supply ata level deemed by modern nutrition scienceto be appropriate to ensure good nutritionalhealth, assuming only that a reasonablevariety of foods are consumed (Augustinand Scarbrough 1990).
A variety of compounds are added tofoods to improve the nutritional value of aproduct, to replace nutrients lost during pro-cessing, or to prevent deficiency diseases.Most of the additives in this category are
Source: Reprinted with permission from R.L. Newsome, Natural and Synthetic Coloring Agents, in Food Addi-tives, A.L Branen, P.M. Davidson, and S. Salminen, eds., p. 333,1990, by courtesy of Marcel Dekker, Inc.
Table 11-5 Major Categories of Natural Food Colors and Their Sources
Colorant
AnthocyaninsBetalains
CaramelCarotenoids
Annatto (bixin)Canthaxanthinp-apocarotena!
ChlorophyllsRiboflavinOthers
Carmine (cochineal extract)Turmeric (curcuma)Crocetin, crocin
Sources
Grape skins, elderberriesRed beets, chard, cactus fruits, pokeberries, bougainvillea, ama-
ranthusModified sugar
Seeds of Blxa orellanaMushrooms, crustaceans, fish, seaweedOranges, green vegetablesGreen vegetablesMilk
Coccus cati insectCurcuma longaSaffron
vitamins or minerals. Enrichment of flourand related products is now a well-recog-nized practice. The U.S. Food and DrugAdministration (FDA) has established defini-tions and standards of identity for the enrich-ment of wheat flour, farina, corn meal, corngrits, macaroni, pasta products, and rice.These standards define minimum and maxi-mum levels of addition of thiamin, ribofla-vin, niacin, and iron. In some cases, optionaladdition of calcium and vitamin D isallowed. Margarine contains added vitaminsA and D, and vitamin D is added to fluid andevaporated milk. The addition of the fat-sol-uble vitamins is strictly controlled, becauseof the possible toxicity of overdoses of thesevitamins. The vitamin D enrichment of foodshas been an important measure in the elimi-nation of rickets. Another example of thebeneficial effect of enrichment programs isthe addition of iodine to table salt. This mea-sure has virtually eliminated goiter.
One of the main potential deficiencies inthe diet is calcium. Lack of calcium is associ-ated with osteoporosis and possibly severalother diseases. The recommended daily allow-ance for adolescents/young adults and theelderly has increased from the previous rec-ommendation of 800 to 1,200 mg/day to1,500 mg/day. This level is difficult toachieve, and the use of calcium citrate in for-tified foods has been recommended by Labin-Goldscher and Edelstein (1996). Sloan andStiedemann (1996) highlighted the relation-ship between consumer demand for fortifiedproducts and complex regulatory issues.
Migration from Packaging Materials
When food packaging materials weremostly glass or metal cans, the transfer ofpackaging components to the food consistedpredominantly of metal (iron, tin, and lead)
uptake. With the advent of extensive use ofplastics, new problems of transfer of toxi-cants and flavor and odor substances becameapparent. In addition to polymers, plasticsmay contain a variety of other chemicals,catalysts, antioxidants, plasticizers, colo-rants, and light absorbers. Depending on thenature of the food, especially its fat content,any or all of these compounds may beextracted to some degree into the food (Bie-beretal. 1985).
Awareness of the problem developed in themid 1970s when it was found that mineralwaters sold in polyvinyl chloride (PVC) bot-tles contained measurable amounts of vinylchloride monomer. Vinyl chloride is a knowncarcinogen. The Codex Alimentarius Com-mittee on Food Additives and Contaminantshas set a guideline of 1 ppm for vinyl chlo-ride monomer in PVC packaging and 0.01ppm of the monomer in food (Institute ofFood Technologists 1988). Another additivefound in some PVC plastics is octyl tin mer-captoacetate or octyl tin maleate. Specificregulations for these chemicals exist in theCanadian Food and Drugs Act.
The use of plastic netting to hold and shapemeat during curing resulted in the finding ofN-nitrosodiethylamine and N-nitrosodibu-tylamine in hams up to levels of 19 ppb(parts per billion) (Sen et al. 1987). Laterresearch established that the levels of nitro-samines present were not close to violativelevels (Marsden and Pesselman 1993).
Plasticizers, antioxidants, and colorants areall potential contaminants of foods that arecontained in plastics made with these chemi-cals. Control of potential migration of plasticcomponents requires testing the containerswith food simulants selected to yield infor-mation relevant to the intended type of foodto be packaged (DeKruyf et al. 1983; Bieberetal. 1984).
Other Additives
In addition to the aforementioned majorgroups of additives, there are many others in-cluding clarifying agents, humectants, glazes,polishes, anticaking agents, firming agents,propellants, melting agents, and enzymes.These intentional additives present consider-able scientific and technological problems aswell as legal, health, and public relationschallenges. Future introduction of new addi-tives will probably become increasingly dif-ficult, and some existing additives may bedisallowed as further toxicological studiesare carried out and the safety requirementsbecome more stringent.
INCIDENTAL ADDITIVES ORCONTAMINANTS
Radionuclides
Natural radionuclides contaminate air,food, and water. The annual per capita intakeof natural radionuclides has been estimatedto range from 2 Becquerels (Bq) for 232Th toabout 130 Bq for 40K (Sinclair 1988). TheBq is the International System of Units (SI)unit of radioactivity; 1 Bq = 1 radioactivedisintegration per second. The previouslyused unit of radioactivity is the Curie (Ci); 1Ci = 3.7 x 1010 disintegrations per second,and 1 Bq = 27 x 10~12 Ci. The quantity ofradiation or energy absorbed is expressed inSievert (Sv), which is the SI unit of doseequivalent. The absorbed dose (in Gy) ismultiplied by a quality factor for the particu-lar type of radiation. Rem is the previouslyused unit for dose equivalent; 100 rem = 1Sv.
The effective dose of Th and K radionu-clides is about 400 |nSv per capita per year,with half of it resulting from 40K. The total
exposure of the U.S. population to naturalradiation has been estimated at about 3 mSv.In addition, 0.6 mSv is caused by man-maderadiation (Sinclair 1988).
Radioactive Fallout
Major concern about rapidly increasinglevels of radioactive fallout in the environ-ment and in foods developed as a result ofthe extensive testing of nuclear weapons bythe United States and the Soviet Union in the1950s. Nuclear fission generates more than200 radioisotopes of some 60 different ele-ments. Many of these radioisotopes are harm-ful to humans because they may be incor-porated into body tissues. Several of theseradioactive isotopes are absorbed efficientlyby the organism because they are relatedchemically to important nutrients; for exam-ple, strontium-90 is related to calcium andcesium-137 to potassium. These radioactiveelements are produced by the followingnuclear reactions, in which the half-life isgiven in parentheses:
p- p-90Kr(BBsCC) ^ 9 0Rb(IJmIn) ^ 9 0 S r (28 y)
P- p-137I (22 sec) ** 137Xe (3.8 min) *• 137Cs (29 y)
The long half-life of the two end productsmakes them especially dangerous. In anatmospheric nuclear explosion, the tertiaryfission products are formed in the strato-sphere and gradually come down to earth.Every spring about one-half to two-thirds ofthe fission products in the stratosphere comedown and are eventually deposited by precip-itation. Figure 11-6 gives a schematic out-line of the pathways through which thefallout may reach us.
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HISTORICAL OVERVIEW
Attempts at regulating the composition offoods go back to the Middle Ages. Primarilyrestricted to certain food items such as breador beer, these ancient regulations were in-tended to protect the consumer from fraudu-lent practices. The original Bavarian beerpurity law dating from the Middle Ages isstill quoted today to indicate that nothing butwater, malt, yeast, and hops have been usedin the production of beer. The foundationsfor many of our modern food laws were laidin the last quarter of the 19th century.Increasing urbanization and industrializationmeant that many people had less control overthe food that had to be brought into the urbancenters. Foodstuffs were deliberately con-taminated to increase bulk or improve ap-pearance. Chalk was mixed with flour, andvarious metal salts were added to improvecolor (Reilly 1991). Some of these addedsubstances were highly toxic. One practiceleading to disastrous results was the distilla-tion of rum in stills constructed of lead.
The first food laws in the United Kingdomwere enacted in 1860 and 1875, and the firstCanadian food law was passed in 1875. Inthe United Sates the first comprehensive fed-eral food law came into effect in 1906. This
law prohibited the use of certain harmfulchemicals in foods and the interstate com-merce of misbranded or adulterated foods.Public concern about adulteration and falsehealth claims during the 1930s led to the fed-eral Food, Drug and Cosmetic Act (FDCA)in 1938. A major weakness of this law wasthat the burden of proof of the toxicity of achemical was entirely upon the government.Any substance could be used until such timewhen it was proven in a court of law that thesubstance was harmful to health. A selectcommittee of the U.S. House of Representa-tives, the Delaney committee, studied the lawand recommended its revision. The revisedlaw, which went into effect in 1958, is knownas the Food Additives Amendment of thefederal Food, Drug and Cosmetic Act. Underthis act, no chemical can be used in fooduntil the manufacturer can demonstrate itssafety. The U.S. Food and Drug Administra-tion (FDA) is responsible only for evaluatingthe safety evidence submitted by the appli-cant. The principle of establishing the safetyof chemicals before they can be used is nowbecoming widely accepted in U.S. and inter-national food laws.
A peculiar aspect of the federal act of 1958is the so-called Delaney clause, which stipu-lates that any substance that is found to cause
Regulatory Control of FoodComposition, Quality, and Safety
CHAPTER 12
cancer in humans or animals is banned fromuse in food at any level. This controversialclause has been the subject of much discus-sion over the years. Suspected carcinogenscan be dealt with in other food law systemsunder the general provisions of safety.
The establishment of the safety of a chemi-cal has become more and more difficult overthe years. There are several reasons for this.First, analytical instrumentation can detectever smaller levels of a substance. Where itwas once common to have levels of detectionof parts per million, now levels of detectioncan be as low as parts per billion or parts pertrillion. At these levels, chemicals becometoxicologically insignificant. Second, therequirements for safety have become morecomplex. Initially, the safety of a chemicalwas determined by its acute toxicity mea-sured on animals and expressed as LD50, thedose level that results in a 50 percent mortal-ity in a given test population. As the scienceof toxicology has matured, safety require-ments have increased; safety testing now fol-lows a standard pattern as exemplified by theproposed system for food safety assessmentshown in Figure 12-1. Third, new process-
ing techniques and novel foods have beendeveloped. Many years of research wererequired to demonstrate the safety of radia-tion pasteurization of foods, and even nowonly limited use is made of radiation treat-ment of food and food ingredients. The issueof the safety of novel foods has gained newimportance since the introduction of geneti-cally modified crops. In addition to therequirements of the safety decision tree ofFigure 12-1, the issue of allergenicity hasarisen. Toxicity is assumed to affect every-one in a similar way, but allergic reactionsaffect only certain individuals. Allergic reac-tions can be of different degrees of severity.A major allergic reaction can result in ana-phylactic shock and even death. Regulationsare now being developed in several countriesrelated to placing warning labels on foodscontaining certain allergens. One example ofpossible transfer of allergenicity to anotherfood occurred when a company explored thegenetic modification of soybeans to improveprotein content. A Brazil nut storage proteingene was selected for transfer into the soy-bean genetic makeup. When it was foundthat people who were allergic to nuts also
Figure 12-1 Proposed System for Food Safety Assessment. From Food Safety Council, 1982.
+ = presents socially unacceptable risk- = does not present a socially unacceptable riskS = metabolites known and safeU = metabolites unknown or of doubtful safety? = decision requires more evidence
AcceptAcceptReject
RejectReject
ChronicToxicity
Reject
Accept
SubchronicToxicity &Reproduction
Reject
GeneticToxicology
Metabolism &Pharmacokinetics
AcuteToxicityExposure
AssessmentDefined
Test Material
became allergic to the genetically alteredsoybean, the commercial development ofthis type of genetically modified soybeanwas abandoned. A fourth difficulty in regula-tory control of food composition and qualityis the often overlapping authority of differentagencies. In many countries, the basic foodlaw is the responsibility of the health depart-ment. However, control of meat products,animal health, and veterinary drug residuesmay reside in agriculture departments. Somecountries such as Canada have a separatedepartment dealing with fish and fisheries.Environmental issues sometimes come underthe jurisdiction of industry departments. Inaddition, countries may have a federal struc-ture where individual states or provincesexercise complete or partial control. Beforethe enactment of the FDCA in the UnitedStates, it was argued that food safety shouldbe under the control of individual states.Canada is a federation, but the CanadianFood and Drugs Act is federal legislationthat applies to all provinces and territories. Incontrast, the situation in Australia, also a fed-eration, makes each state responsible for itsown food laws. Recent efforts there havetried to harmonize state food laws by intro-duction in each state of a "model food act"(Norris and Black 1989).
Usually, food laws are relatively short andsimple documents that set out the generalprinciples of food control. They are accom-panied by regulations that provide specificdetails of how the principles set out in thefood law should be achieved. In the UnitedStates the law deals with food, drugs, andcosmetics; in Canada the regulations dealwith food and drugs. The tendency today isto provide laws that specifically deal withfood. The separation of food laws and regu-lations makes sense because the regulationscan be constantly updated without going
through the difficult process of changing thelaw.
Food and drugs have traditionally beenconsidered separate categories in the legisla-tive process. Until relatively recently, healthclaims on foods were prohibited in manycountries. However, in recent years consum-ers have been deluged with health informa-tion relating to their foods. Some of thisinformation has been negative, such as infor-mation about the effect of fat on the inci-dence of heart disease; other information hasbeen positive as for instance the beneficialeffect of dietary fiber.
There is increasing interest in a group ofsubstances known as nutraceuticals or func-tional foods and food supplements. A nutra-ceutical can be defined as any food or foodingredient that provides medical or healthbenefits, including the prevention and treat-ment of disease. These materials cover a grayarea between foods and drugs and presentdifficulties in developing proper regulatorycontrols. It has been stated (Camire 1996)that dietary supplements in the United Statesof America enjoy a favored status. They donot require proof of either efficacy or safety.Dietary supplements include a large varietyof substances such as vitamins, minerals,phytochemicals, and herbal or botanical ex-tracts (Pszczola 1998).
SAFETY
The safety of foods—including food addi-tives, food contaminants, and even some ofthe major natural components of foods—isbecoming an increasingly complex issue.Prior to the enactment of the Food AdditivesAmendment to the FDCA, food additivecontrol required that a food additive be non-deceptive and that an added substance be
either safe and therefore permitted, or poi-sonous and deleterious and therefore prohib-ited. This type of legislation suffered fromtwo main shortcomings: (1) it equated poi-sonous with harmful and (2) the onus was onthe government to demonstrate that anychemical used by the food industry was poi-sonous. The 1958 act distinguishes betweentoxicity and hazard: Toxicity is the capacityof a substance to produce injury, and hazardis the probability that injury will result fromthe intended use of a substance. It is nowwell recognized that many components ofour foods, whether natural or added, aretoxic at certain levels but harmless or evennutritionally essential at lower levels. Someof the fat-soluble vitamins are in this cate-gory. The ratio between effective dose andtoxic dose of many compounds, includingsuch common nutrients as amino acids andsalts, is of the order of 1 to 100. Today anyuser of an additive must petition the govern-ment for permission to use the material andsupply evidence that the compound is safe.
The public demand for absolute safety isincompatible with modern scientific under-standing of the issues. Safety is not absolutebut rather a point on a continuum; the exactposition involves judgments based on scien-tific evidence and other important factorsincluding societal, political, legal, and eco-nomic issues. Modern legislation movesaway as much as possible from the non-science factors. Several recent issues havedemonstrated how difficult this can be. Insome cases scientific knowledge is unavail-able, and decision making is difficult. Inaddition, we now know that food safetyrelates to all parts of the food chain, notmerely the industrial processing of foods.What happens on the farm in terms of use ofparticular animal feeds or use of agriculturalchemicals up to the handling of foods in food
service establishments are all part of the foodsafety problem.
Scheuplein and Flamm (1989) stated thatthe assurance of safety by the FDA hasmoved away from a comfortable assuranceof absolute safety to an assurance of somevery small yet distinctly uncomfortable levelof risk. It appears that the public is lessinclined to accept even a very low level ofrisk related to the food supply than the oftenmuch greater risks of many of our dailyactivities.
In the United States, safety is oftenexpressed as the principle of "reasonable cer-tainty of no harm." This principle hasreplaced the earlier idea of "zero tolerance"for toxic substances. The idea of zero toler-ance is incorporated in the Delaney clause ofthe Food Additives Amendment.
As the science of toxicology developed,the requirements for establishing safety be-came more demanding. At one time the LD50
was sufficient to establish safety. The effectof dose level is very important in toxicology.The effects, which vary from no effect dose(NED) levels to fatal effect, have been sum-marized in Figure 12-2 (Concon 1988). Twotypes of substances exist; type I shows nobeneficial effects and type II shows nutri-tional and/or therapeutic beneficial effects.
LD50 is a measure of acute toxicity. Overtime, many other test requirements havebeen added to establish safety as shown inthe safety decision tree developed by theFood Safety Council (1982). In this systeman organized sequence of tests is prescribed(see Figure 12-1). Other tests in this systeminvolve genetic toxicity, metabolism, phar-macokinetics (the pathways of chemicals inthe system and their possible accumulationin organs), subchronic toxicity, teratogenic-ity (birth defects), and chronic toxicity. To allthis are added tests for carcinogenicity and
allergenicity. Most of these tests are per-formed on animals. The no-effect levelascertained with animals is then divided by asafety factor of 100 to arrive at a safe levelfor humans. The idea of establishing a safetymargin for chronic toxicity was accepted bythe FDA in 1949.
The sequence of events leading from toxi-cological investigations to the formulation ofregulations is shown in Figure 12-3 (Vettorazi1989). The important part of this procedure isthe interpretation. This is carried out by quali-fied experts who develop recommendationsbased on the scientific data produced. It issometimes possible for different groups ofexperts (such as groups in different countries)to come up with differing recommendationsbased essentially on the same data.
U.S. FOOD LAWS
The basic U.S. law dealing with foodsafety and consumer protection is the Food,Drug and Cosmetic Act (FDCA) of 1938 as
amended by the Food Additives Amendmentof 1958. The FDCA applies to all foods dis-tributed in the United States, including foodsimported from other countries. A number ofother acts are important for the productionand handling of foods. Some of the moreimportant ones include the following:
• The Meat Inspection Act of 1906. Theresponsibility for the safety and whole-someness of meat and meat productsfalling under the provisions of this act isdelegated to the U.S. Department ofAgriculture (USDA). The USDA's re-sponsibilities include inspection of meat-processing facilities and animals beforeand after slaughter, inspection of meatproducts and meat-processing labora-tories, and premarket clearance of meatproduct labels. When a food productcontains less than 3 percent meat, theproduct comes under the jurisdiction ofthe FDA. Similar laws are the PoultryProducts Inspection Act and the Egg
Figure 12-2 Relationship Between Dose Level and Toxic Effects. Source: Reprinted with permissionfrom J.M. Concon, Food Toxicology. Part A—Principles and Concepts. Part B—Contaminants andAdditives, p. 16, 1988, by courtesy of Marcel Dekker, Inc.
Type II:
No Effect Beneficial Effects(nutritional and/ortherapeutic)
Toxic Effects Fatal Effects
Increasing Dose
Type I:
No Effect(harmless)
Toxic Effects Fatal Effects
Product Inspection Act. Both of these arethe responsibility of USDA.
• The Safe Drinking Water Act. Passed in1974, this law gives the FDA authorityto regulate bottled drinking water andthe Environmental Protection Agencyauthority to set standards for drinkingwater supplies.
• The Nutrition Labeling and EducationAct of 1990 (NLEA). This is an extensionof the FDCA and requires that all foodsintended for retail sales are provided withnutrition labeling. Mandatory nutritionlabeling is not required in most othercountries unless a health claim is made.
• Alcoholic beverages come under theauthority of the Bureau of Alcohol,Tobacco and Firearms (BATF), an orga-nization unique to the United States. It isnoteworthy that some of the labelingrequirements for other foods do notapply to alcoholic beverages.
The various U.S. agencies involved in foodcontrol and their responsibilities are summa-rized in Table 12-1. The FDA is the agencyprimarily responsible for the control of food,and its authority derives from the U.S.Department of Health and Human Services.The USDA is responsible for meat, poultry,and egg products. These activities are carriedout by a number of organizations withinUSDA. The Food Safety and Inspection Ser-vice (FSIS), the Food and Nutrition Service(FNS), and the Agricultural Marketing Ser-vice (AMS) are all part of this activity.
The Food Additives Amendment to theFDCA (see Chapter 11) recognizes the fol-lowing three classes of intentional additives:
1. those generally recognized as safe(GRAS)
2. those with prior approval3. food additives
Figure 12-3 Critical Points and Objectives of Toxicological Evaluation of Food Additives. Source:Reprinted with permission from G. Vettorazi, Role of International Scientific Bodies, in InternationalFood Regulation Handbook, R.D. Middlekauff and P. Shubik, eds., p. 489, 1989, by courtesy of Mar-cel Dekker, Inc.
TOXICOLOGICALMETHODOLOGY1
2APPROPRIATE
INVESTIGATIONS
3ADEQUATE
INFORMATION
REGULATIONS
TOXICOLOGICALEVALUATION
TOXICOLOGICALRECOMMENDATIONS
INTERPRETATION
5
4
6
Coloring materials and pesticides on rawagricultural products are covered by otherlaws. The GRAS list contains several hun-dred compounds, and the concept of such alist has been the subject of a good deal ofcontroversy (Hall 1975). The concept of aGRAS list is unique to the U.S. regulatorysystem; there is no equivalent in the legisla-tion of other countries.
An important aspect of U.S. food laws ismandatory nutritional labeling. Nutritionallabeling in Canada and Europe is voluntaryand only becomes mandatory if a healthclaim is made.
Another trend in food legislation is thechange from prescriptive regulations to therequirement of total quality assurance sys-
tems. This means that food industries will berequired to adopt HACCP systems (hazardanalysis critical control points).
CANADIAN FOOD LAWS
In May 1997 a completely reorganizedsystem of food control in Canada went intoeffect with the creation of the Canadian FoodInspection Agency (CFIA). The CFIA com-bines into a single organization food controlfunctions of at least four federal depart-ments. This major change was intended tosimplify a complex and fragmented system.
Prior to the formation of CFIA, food con-trol responsibilities were shared by the fol-
Table 12-1 Food Safety Responsibilities of 12 U.S. Agencies
a GIPSA replaced USDA's Grain Inspection Service.
Agency
Food and Drug Administration (FDA)
Food Safety and Inspection Service (FSIS)Animal and Plant Health Inspection Service
(APHIS)Grain Inspection, Packers and Stockyard
Administration (GIPSA)3
Agricultural Marketing Service (AMS)
Agricultural Research Service (ARS)National Marine Fisheries Service (NMFS)Environmental Protection Agency (EPA)Centers for Disease Control and Prevention
(CDC)Federal Trade Commission (FTC)U.S. Customs Service (Customs)Bureau of Alcohol, Tobacco and Firearms
(ATF)
Responsibilities
Ensures safety of all foods except meat, poultry, and eggproducts. Also, ensures safety of animal drugs andfeeds.
Ensures safety of meat, poultry, and egg products.Protects animals and plants from disease and pests or
when human health may be affected.Inspects grain, rice, and related products for quality and
aflatoxin contamination.
Grades quality of egg, dairy, fruit, vegetable, meat, andpoultry products.
Performs food safety research.Conducts voluntary seafood inspection program.Establishes pesticide tolerance levels.Investigates foodborne disease problems.
Regulates advertising of food products.Examines/collects food import samples.Regulates alcoholic beverages.
lowing federal departments: Health Canada(HC), Agriculture and Agri-food Canada(AAFC), Fisheries and Oceans Canada(FOC), and Industry Canada (IC).
The major law relating to food safety isthe Food and Drugs Act and regulations.Until May 1997 HC was responsible forfood, health, safety, and nutrition as well asfor administering the Food and Drugs Actand regulations (Smith and Jukes 1997).Food labeling regulations are part of Foodand Drugs Act and regulations, but enforce-ment was shared with AAFC. AAFC admin-istered the Meat Inspection Act and theCanadian Agricultural Products Act. FOCadministered the Fish Inspection Act. TheConsumer Packaging and Labeling Act stan-dardizes the form and manner of essentialinformation on the label of all prepackagedconsumer products including foods. Therequired information includes the commonname of the product, the net quantity, andname and address of the company or personresponsible for the product. Canadian regu-lations require this information to be pro-vided in both official languages, English andFrench.
Because the Food and Drugs Act is crimi-nal law, it applies to all foods sold in Canada.The laws administered by AAFC and FOCare not criminal law and, therefore, do notapply to foods produced and sold within thesame province. This is similar to the situa-tion in the United States.
Provinces and municipalities have a cer-tain level of involvement with food control.Provincial regulations are mainly concernedwith health issues and the control of certaincommodities such as dairy products.
The establishment of the CFIA in 1997significantly changed the system. CFIA isresponsible for the enforcement and/oradministration of 11 statutes regulating food,
animal and plant health, and related prod-ucts. This involves a consolidation of theinspection and animal and plant health ser-vices of HC, AAFC, and FOC. A singlebody, the CFIA, is now responsible for thefederal control of all food products.
The establishment of the CFIA is only thefirst step in a complete overhaul of the Cana-dian food control system. One of the imme-diate goals is the development of a CanadianFood Act, and the harmonization of federaland provincial acts. Approximately 77 differ-ent federal, provincial, and territorial actsregulate food in Canada. Through the Cana-dian Food Inspection System (CFIS), a com-mon regulatory base will be developed, asdepicted in Figure 12-4. An important aspectof future food regulations will be the relianceon HACCP for safety assurance.
EUROPEAN UNION (EU) FOOD LAWS
The EU at this time involves 15 indepen-dent states, and one of the aims of the unionis to facilitate trade among member states. Toachieve the harmonization of food laws, aprogram was instituted to develop a commonset of food laws. The EU food laws apply inall of the 15 member nations, but theenforcement remains with the individualmember states. The EU is governed by threebodies, the European Council (the Council),which consists of ministers from the membercountries; the European Parliament, which isformed from members elected in the membercountries; and the European Commission(the Commission). The Commission is theworking organization that develops laws.The Council approves the laws, and the Par-liament has an advisory function. The EUlaws, adopted by the Council, may take thefollowing forms:
• Regulations. These are directly appliedwithout the need for national measuresto implement them.
• Directives. These bind member states asto the objectives to be achieved whileleaving the national authorities the powerto choose the form and means to beused.
• Decisions. These are binding in all theiraspects upon those to whom they areaddressed. A decision may be addressedto any or all member states, to undertak-ings, or to individuals.
• Recommendations and opinions. Theseare not binding.
The Commission began preparing a com-prehensive directive on food additives in1988. The comprehensive directive on foodadditives will have two major parts: (1) a listof all the additives and their conditions ofuse, and (2) the purity criteria of these addi-
tives, together with other specifications suchas sampling methods and methods of analy-sis.
An interesting development in EU foodlaws is the decision of the Commission todiscontinue issuing vertical directives (verti-cal relates to commodity-specific issues) andto concern itself with horizontal regulations(horizontal relates to general issues acrosscommodities).
An important recent issue concerns theNovel Food Regulation, which is a system offormal, mandatory, premarket evaluation andapproval for most innovative foods and foodproduction processes (Huggett and Conzel-mann 1997). Novel foods are all foods andfood ingredients that have not hitherto beenused for human consumption to a significantdegree in the EU. The Novel Food Regula-tion requires additional specific labeling ofany characteristic, food property (such ascomposition, nutritional value, or nutritional
Figure 12-4 Common Regulatory Base Suggested for the Canadian Food System
INTERPRETATIVE GUIDELINES
INTERPRETATIVE GUIDELINES
"CORE" REGULATIONS
COMMON REGULATORY BASE (CRB)
COMMON LEGISLATIVE BASE (CLB)E.G. FOOD ACT
HARMONIZED COMMODITY/SECTOR-SPECIFIC REGULATIONS
effects), or intended use that renders the foodno longer equivalent to its conventionalcounterpart. This regulation, therefore, re-quires specific labeling for foods producedthrough genetic engineering. U.S. regula-tions do not require labeling to describe theuse of genetic engineering in developing anew variety of food.
A food safety crisis developed in Europebeginning in the late 1980s and early 1990s.The disease in cattle known as bovinespongiform encephalopathy (BSE), popu-larly know as mad cow disease, assumed epi-demic proportions in England, and more thana million head of cattle had to be destroyed.The problem with BSE is twofold: the patho-genic agent(s) has not been identified, andthe transmission to humans is suspected butnot proven. There is a human spongiformencephalopathy, Creutzfeldt-Jakob disease(CJD), which is rare and usually affects olderpeople; a new variant (vCJD) affectsyounger persons (Digulio et al. 1997). Manyunanswered questions about the disease andits possible effect on humans as well asincompetent handling of the issue by politi-cians created a great deal of unease by thepublic in Europe. The possibility of transferof the pathogenic agent via rendered meatand bone meal (MBM) has been suggested.
The BSE scare reinforced the importanceof involving consumers and other groups inthe consultative process in the developmentof EU legislation (Figure 12-5).
The EU passed a directive in 1993 requiringall food companies in the EU to implement aneffective HACCP system by December 1995.The directive covers not only large andmedium-sized businesses but also small com-panies and even small bakery shops and cater-ing establishments. This directive makes thefood manufacturer liable for damages sufferedas a result of product defects.
INTERNATIONAL FOOD LAW:CODEX ALIMENTARIUS
The Codex Alimentarius Commission is ajoint effort by two organizations of theUnited Nations—the Food and AgricultureOrganization (FAO), headquartered in Rome,and the World Health Organization (WHO),headquartered in Geneva. The Codex Ali-mentarius Commission is responsible fordeveloping a set of rules known as the CodexAlimentarius (CA). The CA has no legal sta-tus, and its adoption is voluntary. Its purposeis to serve as a reference for food safety andstandardization on a worldwide basis and toserve as a model for adoption by nations thatdo not have the resources to develop theirown standards. Working under the commis-sion are worldwide general subject commit-tees, a series of worldwide commodity com-mittees, and regional coordinating commit-tees (Figure 12-6).
The fact that CA is a joint effort of FAOand WHO is fortunate and meaningful. Eventoday in the United States, the FDA is con-stantly searching to serve both the consum-ing public and the food industry withoutcreating an impression of being partial to oneside or the other.
Since its inception, the CA Commissionhas produced a large volume of standards,codes of practice, and guidelines. It hasdeveloped more than 220 commodity stan-dards, more than 40 codes of practice, amodel food law, a code of ethics, and limitsfor more than 500 food additives. In addi-tion, the commission scrutinized 2,000 pesti-cides and established limits on 200 of them(Mendez 1993). The work on pesticide resi-dues has resulted in establishing maximumresidue limits (MRLs) for a wide range ofpesticides in many food commodities. The
commission has studied the safety of a largevariety of food additives, considering bothtoxicology and efficacy. The commission hasalso been active in the area of the safe use ofveterinary drugs and has set maximum resi-due levels for these compounds. The codesof hygienic/technological practice have beendeveloped for a wide range of food commod-ities.
An important recent development in thework of the CA is its change in emphasis. Itis gradually moving away from the vertical
approach to laws (that is, laws relating to asingle commodity) to horizontal laws (morebroadly based laws that apply across allfoods and food commodities). The CA pro-cedure for the elaboration of standards is acomplex process involving eight steps. Re-cently, the CA Commission decided to dis-continue work on a standard for mayonnaise.This trend of moving away from verticalstandards is not confined to CA. It is alsotaking place in EU legislation and in manynational systems.
Figure 12-5 The Consultative Process Used in the Development of EU Food Legislation. Source:Reprinted with permission from R. Haigh and R Deboyser, Food Additives and the European Eco-nomic Community, in International Food Regulation Handbook, R.D. Middlekauff and R Shubik, eds.,1989, by courtesy of Marcel Dekker, Inc.
GovernmentAdministrations
(advice)Scientific Committeefor Food
(advice) (advice)
Agriculture
Industry
Food Manufacturers
Retailers
Consumers
Other InterestsCOMMISSION(17 members)
(proposal)(proposal)
Economic and SocialCommittee
Standing Committeeon Foodstuffs
COUNCIL(12 Ministers)(opinion)
(opinion)
Legislation
European Parliament
The importance of CA standards for inter-national trade increased significantly as aresult of the formation in 1995 of the WorldTrade Organization (WTO), headquartered inGeneva. The WTO is the successor to theGeneral Agreement on Tariffs and Trade(GATT), and most trading nations of theworld are members of WTO. One of the mainpurposes of WTO is to promote trade throughthe elimination of nontariff trade barriers. Inthe area of food trade, "health requirements"often were used as a trade barrier. To improve
the rules that were in effect during the GATTperiod, WTO established the Agreement onSanitary and Phytosanitary Measures, knownas the S&P Agreement. This agreement dealswith trade in agricultural products of animaland plant origin. Under this agreement, mem-ber states of WTO agree to settle trade dis-putes on the basis of scientific facts and useof the CA standards. A recent case that wasbrought before the WTO panel involved therefusal by the EU to allow importation ofbeef originating in the United States that is
Figure 12-6 Structure of the Codex Alimentarius Commission
Food hygieneAnalysisand sampling
Pesticideresidues
Foodlabeling
Generalprinciples
Food additivesand contaminants
e.g., Fats and oils
Import/exportinspection
and certification
Residues of veterinarydrugs in food
Worldwidegeneral subject
committees
ExecutiveCommittee
CodexAiimentariusCommission
FAO/WHOSecretariat
Worldwidecommoditycommittees
Regionalcoordinatingcommittees
produced using growth hormones. TheUnited States argued on the basis of scientificevidence that this practice did not result inany detectable residue of the hormones in thebeef. The WTO panel has ruled in favor ofthe U.S. position.
The labeling of food causing severe aller-gic reaction in some people has resulted inthe draft list of foods in May 1996. Severeallergic reactions may cause anaphylaxis andpossible death in sensitive persons. The listincludes the following foods:
• cereals containing gluten (wheat, rye,barley, oats, spelt, or their hybridizedstrains and products of these)
• Crustacea and products of these• eggs and egg products
• fish and fish products• peanuts, soybeans, and products of these• milk and milk products (lactose included)• tree nuts and nut products• sulfite in concentrations of 10 mg/kg or
more
This CA proposal is likely to be adopted forinclusion in the food laws of many countries.
The possibility of transfer of allergenicityfrom an existing food to a new geneticallyengineered variety is one of the major con-cerns relating to novel foods produced bygenetic engineering. Assessment of the aller-genic potential is a critical component of thesafety assessment of crops developed byusing plant biotechnology (Fuchs and Ast-wood 1996).
Table 12-2 Comparison of Flour Enrichment Requirements in Canada and the United States
Source: Reprinted from Health Canada, Health Protection Branch consultative document on draft proposals-sub-jects: (1) fortification of flour and pasta with folic acid, (2) harmonization of flour enrichment with the United States ofAmerica, (3) optional enrichment of flour.
Nutrient
MandatoryThiamineRiboflavinNiacinFolic acidIron
OptionalVitamin B6
Folic acidPantothenic
acidMagnesiumCalcium
Canada(Flour, White Flour, Enriched Flour, or
Enriched White Flour)
Minimum per 100 g
0.44 mg0.27 mg3.5 mg
2.9 mg
0.25 mg0.04 mg1.0mg
150mg110mg
Maximum per 10Og
0.77 mg0.48 mg6.4 mg
4.3 mg
0.31 mg0.05 mg1.3mg
190mg140mg
United States(Enriched Flour)
Amount per 100 g
0.64 mg0.40 mg5.29 mg0.15 mg4.40 mg
211 mg
HARMONIZATION
Harmonization of food laws betweennations and trading blocks is important forthe promotion of international trade. Harmo-nization does not necessarily mean that foodlaws have to become identical in differentjurisdictions. It may rather be a case of estab-lishing the principle of equivalency. It can beassumed that if the basic principles of thedifferent laws are essentially the same (theassurance of a safe and wholesome food sup-ply) and their enforcement is satisfactory,then products produced in one country canbe accepted as complying with the law inanother country. Harmonization of food lawsbetween trading partners in free trade groupsis important in promoting free trade. Thebest example is the efforts of harmonizingfood laws in the countries of the EU. Theestablishment of the WTO has increased theimportance of the CA and will have an effectin establishing CA as the worldwide refer-
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Camire, M.E. 1996. Blurring the distinction betweendietary supplements and foods. Food Technol. 50,no. 6: 160.
Concon, J.M. 1988. Food toxicology. Part A: Princi-ples and concepts. Part B: Contaminants and addi-tives. New York: Marcel Dekker.
Digulio, K., et al. 1997. International symposium onspongiform encephalopathies: Generating rationalpolicy in the face of public fears. Trends Food Sd.Technol. 8: 204-206.
Food Safety Council. 1982. A proposed food safetyevaluation process: Final report of board of trustees.Washington, DC.
Fuchs, R.L., and J.D. Astwood. 1996. Allergenicityassessment of foods derived from genetically modi-fied plants. Food Technol. 50, no. 2: 83-88.
ence for settling disputes about nontarifftrade barriers.
Other efforts at harmonizing food lawsoccur between partners of the North Ameri-can Free Trade Agreement (NAFTA) involv-ing the United States, Canada, and Mexico,and between Australia and New Zealand. Asuggested approach to legislative harmoniza-tion is depicted in Figure 12-4. The exist-ence of a model food act developed by CAshould be an incentive in bringing about har-monization within and between countries.
Food laws developed in various countriesreflect the way governments are organizedand the state of development of the foodindustry. Different interpretations of scientificand nutritional information can result in estab-lishment of different standards. This is dem-onstrated by the comparison of Canadian andU.S. rules on flour enrichment (Table 12-2).
The increasing efforts of harmonization offood laws around the world will continue asinternational trade in food products contin-ues to grow.
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Scheuplein, RJ., and W.G. Flamm. 1989. A historicalperspective on FDA's use of risk assessment. InInternational food regulation handbook, ed. R.D.Middlekauff and P. Shubik. New York: Marcel Dek-ker.
Smith, T.M., and DJ. Jukes. 1997. Food control sys-tems in Canada. Crit. Rev. Food Sd. Nutr. 37: 299-251.
Vettorazi, G. 1989. Role of international scientific bod-ies. In International food regulation handbook, ed.R.D. Middlekauff and P. Shubik. New York: MarcelDekker.
The International System (SI) of the Unitsrests upon seven base units and two supple-mentary units as shown in Table A-I. Fromthe base units, derived units can be obtainedto express various quantities such as area,power, force, etc. Some of these have specialnames as listed in Table A—2. Multiples andsubmultiples are obtained by using prefixesas shown in Table A—3.
Older units in the metric system and theavoirdupois system are still widely used inthe literature, and the information supplied inthis appendix is given for convenience inconverting these units, Table A—4.
TEMPERATURE
O 0C = 273 0KCelsius was formerly called Centigrade100 0C = (100 x 1.8) + 32 0F = 212 0FO 0C = 32 0F0F = (0C x 1.8)+ 320C = (0F-32)-*- 1.8
Table A-1 Base Units and Supplementary Units
Quantity Unit Symbol
Base UnitsLength meter mMass kilogram kgTime second sElectric current ampere ATemperature kelvin KLuminous candela cd
intensityAmount of mole mol
substance
SupplementaryUnitsPlane angle radian radSolid angle steradian sr
Units and Conversion Factors
APPENDIX A
Quality
ForceEnergyPowerPressureElectrical potentialElectrical resistanceElectrica conductanceElectrical chargeElectrical capacitanceMagnetic fluxMagnetic flux densityInductanceFrequencyIlluminationLuminous flux
Unit
newtonjoulewattpascalvoltohmSiemenscoulombfaradweberteslahenryhertzluxlumen
Symbol
NJWPaVQSCFWbTHHzIxIm
Formula
kg.m/s2
N.mJ/sN/m2
W/AV/A1/0A.sC/VV.sWb/m2
Wb/A
27C/S
cd.sr/m2
cd.sr
Multiplier
1 000 000 000 000
1 000 000 000
1 000 000
1 000
1 OO
10
0.1
0.01
0.001
0.000 001
0.000 000 001
0.000 000 000 001
Exponent Form
1012
109
106
103
102
io-1
io-1
to-2
io-3
10'6
io-9
io-12
Prefix
tera
gigamega
kilo
hecto
deca
deci
centi
milli
micro
nano
pico
Sl Symbol
T
G
M
k
h
da
d
C
m
in
n
P
Table A-3 Multiples and Submultiples
Table A-2 Derived Units with Special Names
0.304800.092900.0283228.316853.785414.5460935.23830.061.699010.227120.272770.101970.453590.907181.016050.016020.068950.0010.09807
1.3333133.771254.18681 .055043.61 03
0.859851 03
1.16300 1Q-3
0.29307 10~3
0.251990.7460.735500.093511
meters (m)m2
m3
dm3 - liters (L)liter (=1000 cc)liter (=1 00OmL)liter (L)m3/hm3/hm3/hm3/hkg(=1000g)
kgMetric ton (MT)M ton (=1000 kg)kg/dm3
bar(=10N/cm2)barbarmbarmbarkJ (kiloJoule)kJkJkcalkWkW (kJ/sec)kcal/hkWkWfoot-candle (ft-c)centipoise (cp)centisokes (cSt)
feet (ft) (=12 in)
ft2
CU ft (ft3)
ft3
US gal (=128 USf I. oz)Imp gal (=160 IfI. oz)US bushelL/mincu ft/minUSGPmIGPMNewton (N)lb(av)(=16oz)Short ton (= 2000 lbs)Long ton (= 2240 lbs)Ib/ft3
psimbar(= 100 Pascals)mH2O
mmHg (torr)inHg (6O0F)kcalBTUkWhkWhkcal/hBTU/hBTU/hHP (electr.)Metric hpluxmPa.smma/s
3.2808410.7639135.314670.035310.264170.219970.0283816.666670.588584.402873.666159.806652.204621.102310.9642162.4278914.50377
1.0103
10.19716
0.750010.029610.238850.947830.27778 1Q-3
1.1 6300 10~3
0.859851 03
3.41219 103
3.968381.340481.3596210.7611
To Convert Into Multiply By
Table A-4 Conversion Factors
Multiply By Into To Convert
GreekCharacter
A aBPTyA6EE, €
ZCHTI
09,0I iK K, KAXMJI
Nv
H^OoHTC, 03Pp£ a, qT T
YD
0 cp, <|)
X X1FiJ/Qco
Gree/c A/ame
alphabetagammadeltaepsilonzetaetathetaiotakappalambdamunuxiomicron
Pirhosigmatauupsilonphichipsiomega
RomanEquivalent
AaBb
GgDdE iZzEeTh thIiKkLlM mNnXx0 6
PpRrSsTtYyPh phChchPs psUo
Greek Alphabet
APPENDIX B
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Index
Index terms Links
A A bands 145
Acesulfame-κ 182
Acetaldehyde 297 303
Acetals 167
Acetic acid 271
Acetophenoses 284
Acetylated sugars 167
Acetylformoin 176
Acetyl glucosamine 172
Achromatic colors 234
Acid detergent fiber 204
Acids as acidulants 438 as preservatives 438
Acids, structure and physical properties 438 439
Acidulants, acids acetic 438 adipic 438 citric 438 fumaric 438 lactic 438 malic 438 phosphoric 438 444 succinic 438 tartaric 438
Actin 114 144
Active methylene group 59
Active oxygen method 61 67
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Actomyosin 28 146
Acute toxicity 476
Acyclic aldehyde 165
Acylglycerols 34 See also Glycerides monoacyl 81 103 104 triacyl. See Triglycerides
Additives 429 incidental 449 intentional 429
Additives and contaminants 429
Additives, incidental (contaminants) 449 antibiotics 460 asbestos 459 bacterial and fungal toxins 464 dioxin 455 from industry 455 natural toxins 467 pesticides 451 polychlorinated biphenyls 458 polycylclic aromatic hydrocarbons 463 radioactive fallout 449 radionuclides 449 trace metals 461
Additives, intentional, classification 431 See also Food additives
Additives, intentional, major classes acidulants 438 antimicrobial agents 431 antioxidants 438 bleaching agents 438 440 441 bread improvers 441 coloring agents 445 emulsifiers 440 flavor enhancers 442
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Additives, intentional, major classes (Continued) flavors 441 irradiation 446 nutritional supplements 447 phosphates 444 preservatives 431 spices 442 sweeteners 443
Additives, intentional, minor classes 449 anticaking 449 clarifying agents 449 firming 449 glazes 449 humectants 449 polishes 450 propellants 449
Adenosine triphosphate 29 298
Adhesiveness 313 334
ADI (acceptable daily intake) 434 437
Adipic acid 271
Adipic anhydride 187
Adsorption, heat of 11
Adsorption isotherm 6 28
Aeration 89
Aftertaste 294
Agar 200
Agaricaceae 451
Agaropectin 201
Agarose 201
Agent orange 458
Aggregated gels 138
Aggregation, protein 117
Aglycone 167
Agreement on Sanitary and Phytosanitary Measures 486
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Agricultural Marketing Service 480
Agriculture and Agri-food Canada 482
AH, B theory 269
Albumins 111 113
Alcoholic beverages 303
Alcohol, sugars. See Sugar alcohols
Aldehyde oxidase 221
Aldehydes 57
Aldehyde sulfurous acid 434
Aldehydo-glycerides 59
Aldoseamine 120
Aldoses 167
Aldosylamine 128
Aldotriose 163
Aleurone grains 156
Algin 201
Alginate 199
Aliphatic alcohol 51
Alkali cellulose 203
Alkaline taste 263 276
Alkaloids 273 305
Alkanones 300
Allergenicity 476
Allergenic properties 413
Allergic reactions 487
Alliinase 469
Allium 469
Alpha amylase 181
Alpha helix 115 146
Aluminum 223
Aluminum phosphate 223
Alzheimer’s disease 223
Amadori rearrangement 120 122 132
Amaranth 446
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Amino acid, composition beef 113 casein 121 corn 113 egg 113 gelatin 121 149 milk 113 139 millet 113 peas 113 rice 113 serum albumin 121 sorghum 113 wheat 113
Amino acids 111
Amino acids, general acidic 112 amides 112 aromatic 112 basic 112 cyclic 112 essential 111 139 hydrocarbon 112 hydroxyl 112 polar 115 structure 112 sulfur 112
Amino acids, individual alanine 112 arginine 112 131 asparagine 112 aspartic 112 132 cysteine 112 121 131 152 cystine 112 121 132 152 glutamic acid 112 132 glutamine 112 glycine 112
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Amino acids, individua (Continued) histidine 112 132 hydroxy proline 120 isoleucine 112 leucine 112 lysine 112 120 123 132
152 methionine 112 131 132 152
157 ornithine 130 phenylalanine 112 proline 112 115 120 122
141 serine 112 threonine 112 132 tryptophane 112 131 132 152
157 valine 112
Amino acids, sulfur 111
Amino acrylic acid 132
Amino-nitro-propoxybenzene 267
Amino-propanol 380
Amino sugars 167
Ammonium chloride 441
Ammonium phosphate 441
Ammonium sulfate 441
Amorphous foods 20
Amphiphilic 134
Amygdalin 167
Amylase(s) 139 258 402
Amyl esters 301
Amyloglucosidase 259
Amylopectin 184
Amylose 184
Amy lose complexing 105
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Anaphylactic shock 476
Angle of tilt 88
Anhydride formation 174
Anhydro base 255
Anhydro sugars 167
Animal depot fat 36
Animal proteins 138
Anisaldehyde oxime 269
Anisidine value 60
Anisotropic particles 345
Annatto 252
Anomers 163
Anthocyanidins 252 cyanidin 253 254 delphinidin 253 254 destruction 253 255 lakes 254 malvidin 253 pelargonidin 253 peonidin 253 254 petunidin 253 spectrum UV 253
Anthocyanins 252 295
Antibiotics 460
Antibiotics as contaminants causing allergies 460 causing resistant organisms 460 used in agriculture 460
Anticaking agents 449
Antifirming effect 105
Antimicrobial agents. See preservatives
Antioxidants 62 386 438 440 ascorbyl palmitate 64 BHA 62 63 BHT 62 63
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Antioxidants (Continued) chelating agents 438 enzymic 438 natural 438 oxygen scavengers 438 PG 62 63 phenolic 438 spices and herbs 438 TBHQ 62 63 tocopherols 62
Antiparallel hydrogen bonding 270
Antiparallel sheets 142
AN/TN ratio 412
Apo carotenal 251 358 387
Apo carotenoic ethyl ester 358
Apoenzyme 389
Apolar molecules 4
Apparent viscosity 319
Applesauce 225
Apricot 246
Araboxylan 192
Arachidonic acid 41
Aroclor 458
Aroma character 289
Asbestos 459
Asbestos in filtration beverages 460 in water supply 460
Ascorbic acid oxidase 368
Ascorbic acid 366
Ascorbic L, acid. See Vitamin C
Ascorbyl palmitate 365 386 438
Ascorbyl palmitate as synergist 365
Ashing 209
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Asparagus 226
Aspartame 182
Aspergillus flavus 465
Aspergillus niger 172 402 407 427 434
Aspergillus oryzae 404 410 427
Astaxanthin 251
Astringency 276 294
Asymmetric carbon 163 363
ATP 29 298
Autoxidation 54
Avidin 150 385
Avogadro’s number 8
B β-amino alanine 133
β-elimination 133
β-fructofuranosidase 172
β-furanoside 165
Bacillus coagulans 427
Bacillus licheniformis 412
Bacillus subtilis 404
Back extrusion 332
Bacterial growth, aw 22
Bacterial toxins 464
Bacteriocins 437 as preservative 438 in milk 438 natamycin 438 Nisin 438 pimaricin 438
Barbecuing 463
Barium 140 450
Bavarian beer purity law 475
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Beans 226
Beef fat. See Fats and oils, individual
Beer haze 411
Bell peppers 306
Benzidine value 57
Benzoic acid 432
Benzoic acid as preservative 432
Benzopyrene nucleus 256
Benzoyl peroxide 440 441
Benzpyrene 463
Benzyl alcohol 269
Beryllium salt 266
Betacyanins 258
Beta-ionone 251
Betalains 258
Betanin 258
Betaxanthins 258
BET isotherm 8 9 19
Bifidus factor 172
Bile acids 51
Bimolecular lipid layers 104
Binding energy 3
Bingham body 324
Biopolymers 19
Biotechnology 487
Biotin 385 avidin 385 sources 355 385 stability 356 385 structure 385
Birefringence 185
Bisulfite 418
Bitter peptides 300
Bitter taste 263 273
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Bixa orellana 246
Bixin 244
Black body 230
Blanching 360 422
Bleaching 50
Block copolymer 195
Blood serum albumin 143
Bloom 99
Bond angles 16
Bonito 280
Botulinum toxins 464
Bovine spongiform encephalopathy 484
Branched chain fatty acids 36
Branched dextrins 180
Brassica species 44
Brassicasterol 51 55
Brazil nut 476
Bread flavor 297
Bread improvers 441 bleaching agents 441 inorganic compounds 441
Bread, specific heat 14
Brevibacterium ammoniagenes 426
Brittleness 311 334
Brookfield viscometer 329
Brothy taste 300
Brownian movement 342 344
Browning, nonenzymatic. See nonenzymatic browning
Buccal proteins 295
Buffering capacity 272
Bulking agent 194
Bulk sweeteners 183
Bureau of Alcohol Tobacco and Firearms 480
Burgers model 323
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Butter 338
Butter flavor 284 297
Butter, thixotropic hardness 328
Butylated hydroxy anisole 62
Butylated hydroxy toluene 62
Butyric acid 284
C Cadmium 463
Caffeine 273 468 469
Calcium 220
Calcium caseinate phosphate complex 139
Calcium channels 264
Calcium-chelate 214
Calcium saccharate 170
Calcium sensitive casein 141
Calcium sulfate 441
Campesterol 55
Camphoraceous 287
Canadian Agricultural Products Act 482
Canadian food control system 482
Canadian Food Inspection Agency 481
Cancer 223
Candida antarctica 400
Candida cylindracae 401
Canned foods, metal uptake 223
Canola oil 37 106 See Fats and oils, individual
Cantaxanthin 251
Capillary condensation 12
Capillary viscometers 330
Capillary water 7 11
Capsaicin 276 306
Capsicum 276
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Caramel 259
Caramel color production 259
Caramel flavor 176
Caramelization 127 175
Carbanion 134
Carbinol base 255
Carbohydrates 163
Carbohydrates, classification 163 cellulose 191 206 cyclodextrins 194 dextrins 194 disaccharides 171 182 fiber 203 glycogen 163 gums 197 206 hemicelluloses 191 206 hexoses 165 hydrocolloids 199 lignin 193 206 monosaccharides 163 182 oligosaccarides 169 182 pectins 194 206 pentosans 191 polyols 181 polysaccharrides 182 184 starch 179
Carbohydrates, content in foods 164
Carbohydrates, physical properties gum gels 197 202 lactose 177 178 pectin gels 206 starch gels 187 starch crystallinity 184 starch paste 187 sucrose 179
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Carbonates 212
Carbonic anhydrase 220
Carbonium ion 256
Carbon number, glycerides 35 51
Carbonyls 57
Carmelan 175
Carnosic acid 438
Carnosol 438
Carotene 245 357
Carotenoids 244 annatto 252 astaxanthin 250 canthaxanthin 250 capsanthin 250 capsorubin 250 carotene, α, β, γ 245 cryptoxanthin 250 in corn 248 in eggs 246 251 in milk fat 248 in oranges 247 in palm oil 248 252 in peaches 247 in salmon 246 in tomatoes 246 isozeaxanthin 250 lutein 250 lycopene 245 249 251 physalien 250 stability 251 synthetic 251 torularhodin 250 zeaxanthin 249 250
Carrageenan 199 201
Carrots 244
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Carry-through properties 62
Carvacrol 305
Carvone 268 305
Casein 119 139 413 See also Proteins, casein
Casein, α, α3, αS2, κ 141 409
Casein, genetic variants 141
Caseinate agglomeration 202
Caseinate particles, stability tests 213
Caseinates 143
Casein hydrolysates 413 414
Casein micelle 140 141
Casson model 319
Catalase heat stability 418
Catalyst 71
Catalysts, hydrogenation. See Hydrogenation, fats
Catalysts, interesterification 80
Catecholase activity 415
Celery 284
Cellobiose 172 191
Cellular structure 340
Cellulose 191 crystallinity 191 modified 199 203 molecular weight 191 molecule 191
Cephalin 52
Ceramides 52
Cereal germ oils 364
Cereal proteins 112
Cesium 137 450
Chain elongation 35
Chain fission 59
Chain reaction 56
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Chain shortening, fatty acids 35
Chalcones 255
Charcoal broiling 463
Charge frequency 136
Cheese flavor 300
Chelates 210
Chemisorption 73
Chemoreceptor 287
Chernobyl 451
Chewiness 313 334
Chicken fat 39
Chilies 306
Chiral 45
Chiral center 363 364
Chlorine 441
Chlorine bleaching 458
Chlorine dioxide 441
Chlorophyll 26 242
Chlorophyllase 243
Chlorophyllin 243
Chloroplastids 242
Chocolate 48 95 343
Chocolate aroma 285
Chocolate microstructure 343
Chocolate milk 202
Cholecalciferol 361
Cholesterol 51 53 54
Cholesterol acetate 53
Cholesterol esters 53
Chondrus crispus 201
Chroma 236
Chroman ring 364
Chromaticity coordinates 232
Chromatogram 39 41
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Chromen sulfonic acid 255
Chromium 219
Chromium intake 223
Chromoproteins 114
Chronic toxicity 476
Chrysotile asbestos 460
CIE system 229
Cinnamaldehyde 305
cis-cis-1,4 diene system 68
Citral 301
Citrates 212
Citric acid 271 365 370
Citric acid as synergist 365
Citronellyl acetate 301
Citrus flavor 300
Citrus fruit 246
Clarifying agents 449
Clathrates 4
Clausius Clapeyron law 10
Clostridium botulinum 436 464
Cloud separation 405
Clupein 114
Coating fats 100
Cobalt 219 220
Cocarboxylase 372
Cocoa butter 45 81 95 isosolids blends 100 101 polymorphism 99 solid fat profiles 102
Cocoa butter equivalents 99
Cocoa butter improvers 99
Cocoa butter substitutes 99
Coconut. See Fats and oils, individual
Coconut oil 34
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Codex Alimentarius 484
Codmium contamination in crustaceans 463
Coenzyme 355
Coenzyme A 384
Cofactor 389
Coffee 301
Coffee flavor 301
Cognac 303
Cohesion 311
Cohesiveness 333
Collagen 114 147 148
Colloidal dispersions 342
Colloidal particles 212
Colloids 342 anisotropic particles 345 definition 342 dimensions 344 particle size 342 345
Color 229 beets 258 Cabernet Sauvignon grapes 253 corn 248 crude palm oil 238 eggs 246 251 fruits 254 honey 235 maple syrup 235 meat 240 242 243 milk fat 248 oranges 247 paprika 252 peaches 247 raspberries 253 red fish 251 salmon 246
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Color (Continued) tomato 246 251 vegetables 254
Colorants 239
Colorants not requiring certification 260
Colorants, synthetic 260
Color difference 238
Coloring agents 445
Coloring agents, additives 446 FD&C colors 446 lakes 446 nature-identical 446 447 permitted 446 synthetic dyes 446
Color measurements chromaticity coordinates 232 CIE system 229 233 235 complementary 234 gloss 239 Hunter 229 232 237 238 Lovibond 229 238 Munsell 229 236 primary colors 231 spectrophotometry 229 230 tristimulus values 232 wavelength 234
Color, pigments anthocyanins 252 benzopyran 239 betalains 258 caramel 259 carotenoids 244 chlorophylls 241 244 245 cholemyoglobin 242 dyes 260
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Color, pigments (Continued) flavonoids 252 from artifacts 239 hemoglobin 240 241 isoprenoid 239 myoglobin 240 241 nitrosohemochrome 242 nitrosomyoglobin 242 not requiring certification 260 oxymyoglobulin 240 241 pheophytins 243 sulfmyoglobin 242 synthetic 260 tannins 257 tetrapyrrole 239 240 titanium dioxide 260 xanthophylls 244 251
Color purity 234
Color rendition 230
Color triangle 231
Complementary colors 234
Complex ions 213
Complex light types 230
Component fatty acids 36
Composite foods, aw 27
Compression cell 333
Compression force 316
Conalbumin 150
Confectionery fats 94 95
Configurations, sugars aldoses 173 Fischer formula 163 Haworth formula 163 tautomeric forms 166
Congeners 458
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Conglycinin 159
Coniferaldehyde 304
Coniferyl alcohol 193
Conjugase 382
Conjugated dienes 59 74
Conjugated double bonds 244
Conjugation 58
Connective tissue 147
Consistency 41 76 311
Consumer Packaging and Labeling Act 482
Contaminants 429 See also Additives, incidental
Continuous phase 101
Contractile elements 145
Contraction 147
Contributory flavor compounds 289 297
Cooked flavor 142
Cooling effect 96
Coolness 84 263 276
Coordination bonds 240
Copolymerization 424
Copper 219 220
Coprecipitates 143
Corn oil. See Fats and oils, individual
Corn starch 183
Corn sweeteners 179
Corn syrup 179
Corrosion of tin cans 371
Corynebacterium acnes 401
Cottonseed oil. See Fats and oils, individual
Cottonseed protein 113
Coumaryl alcohol 193
Covalent bonds 346
Creep curve 320
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Cresolase activity 415
Creutzfeldt-Jakob disease 484
Critical nucleation temperature 17
Crocetin 244
Cross bonding of starch 187 340
Crosshead, texture measurement 332
Cross linking 424
Cross striation 144
Cruciferae 268
Crucifera oils 43
Crustacea 50
Cryoprotection 20
Cryptoxanthin 247 358
Crystal growth water 16 19
Crystallization, fats 84 crystal polymorph 86 87 88 crystal formation 88 crystal size 85 latent heat 82 nucleation 84 super cooling 84
Crystallization, sugars 176 lactose 177 sorbitol 183 sucrose 176 177
Crystallization velocity, water 17
Crystal size of fats 85
Crystal structure, fats 86 cocoa butter 99 double chain 92 hydrogenated palm oil 94 hydrogenated sunflower oil 90 long spacings 89 melting points, polymorphs 89 palm oil triglycerides 93
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Crystal structure, fats (Continued) PEP 92 93 polymorphism 86
hexagonal (α) 88 90 orthorombic (β’} 88 90 transition 89 92 triclinic (β) 88 90
PSP 92 93 short spacings 88 triple chain 92
Crystal structure, water 15 17
Cubic phase 104
Cubic structure, water 16
Cucumbers 297
Cuminaldehyde 305
Cutin 203
Cyanidin 253
Cyanocobalamine 380 See also Vitamin B12
Cyanogenic glycosides 468
Cyclic diketones 302
Cyclic monomeric glycerides 68
Cyclic monomers 69
Cyclodextrins 194
Cyclohexylamine 443
Cyclopentenyl ring 69
Cysteine sulfoxides 469
Cytochrome C reductase 375
Cytochrome oxidase 368
D Dashpot in texture 322
Decadienal-2,4 59
Deep frying fats 66 See also Fat, deep frying
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Deformation in texture 311 316
Dehydrated foods 21 27
Dehydro-ascorbic acid 367
Dehydrocholesterol 361
Dehydro retinol 359
Delaney clause 431 475
Delphinidin 253
Denaturation 117
Dental health 221
Denture tenderometer 333
Deodorization, oils 50 66
Deoxynivalenol 467
Deoxyosulose 126
Deoxyribonucleic acid 167
Deoxyribose-2 167
Deoxy sugars 167
Derived lipids 33
Desaturation, fatty acids 35
Desorption, hydrogenation fats 73
Desorption isotherm 6
Detinning 463
Dextrinization 189
Dextrose equivalent 179
DHA 401
Diabetic foods 168
Diacetyl 303
Diamino sugar 120
Diels-Alder reaction 68
Dienal-2,4 297
Dietary fiber 203 206 definition 206 methods 206
Differential scanning calorimetry 88 311
Diffused reflection 239
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Dihydroperoxides 59
Dihydropyrazines 286
Diisopropylphosphorofluoridate 410
Diketo gulonic acid 367
Dilatancy 318
Dilatometry 83
Dillapiol 305
Dimethyl mercury 461
Dimethyl nitrosamine 436
Dimethyl sulfide 297 299
Dioxin 455
Dioxin, contaminants 455 458 from agent orange 458 from combustion 458 from herbicides 458 from packaging 458 from pulp and paper 458
Dioxygenase 358
Diphenols 414
Directed interesterification 77
Disaccharides cellobiose 171 172 lactose 170 171 177 178 maltose 170 171 melibiose 170 sucrose 170 171 177 178
Disperse phase 101
Disperse systems 311
Distarch adipate 187
Distarch phosphate 187
Distilled monoglyceride 81
Distribution coefficients 232
Disulfide bonds 111 154 373
Docosahexaenoic acid 37
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Dominant wavelength, color 234
Domoic acid 469
Double bond migration in fatty acids 73
Double chain length, X-ray diffraction 89
Double helix 146
Double helix in starch 185
Dough 154 193 335
Drip loss, meat 18 214
Dulcin 267
Dynamic testing, texture 326
Dynamic viscosity 318
E EDTA 438
Effective dose, in toxicity 478
Egg protein 112 150
Egg white lysozyme 142
Egg yolk 42 251
Egg yolk fat. See Fats and oils, individual
Eicosapentaenoic acid 37
Einstein equation 344
Elaidic acid 34 92
Elastic body 320
Elasticity, texture 313
Elastin 114
Electron microscopy 156 311
Electrophoresis 113
Electroplating 220
Ellagic acid 257
Ellagitannins 257
Emulsification 134
Emulsifiers 440 fatty acid esters 441 HLB 441
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Emulsifiers (Continued) hydroxylated lecithin 440 monoglycerides and esters 441 polyglycerol esters 441
Emulsifiers, nonionic 104 atmul 104 myrj 104 span 104 tween 104
Emulsion destabilization 18
Emulsion flocculation 346
Emulsions 101
Emulsion stability 137
Enal-2-trans 297
Enantiomer 45 47
Endo-α-amylase 179
Endothia parasitica 409
Enediol 174
Enolization 123 174
Enrichment Fe 220
Entanglement 339
Entrainment 95
Entropy 3
Enzymatic activity, aw 22
Enzymatic antioxidant superoxide dismutase 438
Enzymatic browning, flavonoids 256
Enzyme activity 21 23 24
Enzyme classification 394
Enzyme deactivation 389
Enzyme immobilization methods 425
Enzyme kinetics 393
Enzyme positional specificity 397
Enzyme reactors 425
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Enzymes 389 acid protease 408 aldehyde oxidase 221 alkaline phosphatase 423 alliinase 469 amino peptidase 412 amylase 139 390 396 402 amylase α (endo) 179 181 259 amyloglucosidase 181 259 ascorbic acid oxidase 368 392 carbonic anhydrase 220 carboxypeptidase 408 412 catalase 392 396 417 437 cellulase 390 396 chlorophyllase 243 chymosin 409 chymotrypsin 408 410 cocarboxylase 372 coenzyme A 384 coenzyme FAD 374 coenzyme FMN 374 cytochrome C reductase 375 cytochrome oxidase 368 dextran, sucrase 390 elastase 410 esterase 397 ficin 411 fumarase 426 galactosidase 171 404 glucoamylase 404 glucose isomerase 180 396 426 427 glucose oxidase 392 396 416 glucosidase, β 191 glycosidase 396 glycosyl transferase 194 hydrolases 395
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Enzymes (Continued) invertase 390 lactase 171 390 396 404
427 lactoperoxidase 419 437 lipase 391 396 400 401 lipoxydase 243 lipoxygenase 392 419 lysozyme 115 naringinase 390 nuclease 391 oxidorecductase 413 pancreatic lipase 398 papain 396 411 pectate lyase 405 pectic 390 396 405 pectin esterase 405 pectin methyl esterase 394 pentosanase 390 pepsin 408 peroxidase 139 368 391 418
423 phenolase 368 413 phosphatase 139 389 391 polygalacturonase 405 polymethyl galacturonase 406 polyphenolase 220 371 polyphenol oxidase 392 413 pregastric esterase 399 proline specific peptidase 394 prorennin 408 protease 391 408 proteolytic 115 pullulanase 181 rennin 389 396 serin protease 410
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Enzymes (Continued) subtilisin 410 sulfhydryl protease 411 sulfite oxidase 221 superoxide dismutase 438 tannase 390 thiaminase 392 thrombin 410 trans-eliminase 406 trypsin 396 408 410 xanthine dehydrogenase 221 xanthine oxidase 139 221 422
Enzymes groups amylases 402 esterases 397 hydrolases 395 lipoxygenases 419 oxidoreductases 413 pectic 405 proteases 408
Enzymes in food processing 390 396
Enzyme sources 408
Enzyme specificity 394 399 400
Epoxide 363
Equal point 233
Equatorial hydroxyl 164
Equilibrium relative humidity 4
Equivalency 488
Ergocalciferol 361
Ergosterol 361
Erucic acid 37 43
Erythorbic acid 211 367 438
Escherichia coli 426 434
Essential amino acids 111 139
Essential oils 304 441
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Ester interchange, interesterification 77
Ester phosphate 142
Ethereal 287
Ethoxy hexadiene 433
Ethyl decadienoate 301
Ethylene 418
Ethyl sorbate 433
Eugenol 305
Euglobulin 142
European Commission 482
European Council 482
European Parliament 482
European Union decisions 483 directives 483 food laws 482 recommendations 483 regulations 483
Eutectic 99
Evaporated milk 372
Even distribution, glycerides 47
Evening primose oil 43
Excitation purity 234
Extensigraph 336
Extractives, natural 441
F Fabricated foods 33
F-actin 146
FAO 484
Farinograph 335
Fat content 33
Fat crystallization 15
Fat crystal network 339 347 348
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Fat, deep frying 65 chemical reactions 68 69 free fatty acid 66 oxidation 66 polymerization 70 stability of oils 67 69 trans-formation 66 68
Fat, hydrogenation 71 See also Hydrogenation, fats
Fat microstructure 346
Fat, oxidation 54 aldehyde formation 59 60 68 autoxidation 54 during frying 66 free radical 59 61 hydroperoxide formation 58 60 61 light induced 65 lipoxygenase 62 oxidized flavor 70 72 peroxide formation 61 photooxidation 63 products 59 68 72 sensitizers 64 singlet oxygen 63 65
Fat plasticity 339
Fat replacers olestra 107 salatrim 107
Fats and oils 33 content in foods 35
Fats and oils, classes fruit coat 34 37 kernel 34 mammal depot 34 38
529 Index terms Links
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Fats and oils, classes (Continued) marine 34 37 44 45
48 53 77 milk fat, ruminant 34 37 40 52
55 94 seed oils 34 37
Fats and oils, individual canola 37 44 46 67
69 93 106 chicken 39 49 cocoa butter 42 46 49 50
55 69 83 85 96 99 102
coconut 42 45 47 55 94
corn 53 55 56 66 69 93
cottonseed 46 55 69 73 93
egg yolk 42 45 evening primrose 43 heated 65 herring 55 human 38 lard (pig) 39 48 53 55
57 63 69 94 96
linseed 53 56 menhaden 44 mustard 42 44 46 107 novel 106 olive 42 46 53 56
69 93 palm kernel 42 45 47 69
94 102 palm oil 42 46 69 93
94 95 97
530 Index terms Links
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Fats and oils, individual (Continued) palm olein 69 97 palm stearin 69 97 102 peanut 46 56 69 73
93 rapeseed 37 42 44 46
53 55 56 77 replacer 107 rice bran 46 safflower 42 sesame 43 sheep 39 40 soybean 37 42 44 46
53 55 67 69 93
soybean, low linoleic 37 sunflower 46 67 69 90
106 tallow (beef) 39 53 55 69
84 94 teaseed 53 turkey 39 vegetable 42
Fats, texture 337
Fat texture, effect of crystallization temperature 337 temperature treatment 337
Fatty acid composition canola 46 cocoa butter 46 coconut 47 cottonseed 46 egg yolk 45 fish oils, blue whiting 45
capelin 45 dog fish 45
531 Index terms Links
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Fatty acid composition (Continued) menhaden 44 saith 45 sprat 45
milk fats, bovine 40 goat 40 sheep 40
mustard 46 olive 46 palm 46 palm kernel 47 peanut 46 rapeseed 46 rice bran 46 soybean 46 sunflower 46
Fatty acid methyl ester 79
Fatty acids 33 110 ante-iso 39 arachidonic 38 41 60 branch chain 36 conjugated 58 59 66 68
74 cyclic 71 DHA 37 38 41 elaeostearic 38 elaidic 38 Environmental Protection Agency 37 38 41 77 erucic 37 38 even carbon 36 gadoleic 38 hydrogenated 36 hydroxy 36 iso 39 isomers 36 39 40 43
75
532 Index terms Links
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Fatty acids (Continued) isomers-cis 43 75 isomers-geometric 73 isomers-positional 73 isomers-trans 43 58 66 68
73 75 keto 39 linolenic 38 58 60 65 linolenic γ 38 43 long chain 36 medium chain 36 44 multibranched 39 myristoleic 38 nomenclature 35 odd carbon 36 oleic 36 38 60 palmitic 36 38 palmitoleic 38 petroselinic 38 polyunsaturated 41 saturated 58 short chain 36 stearic 36 unconjugated 43 unsaturated 35 vaccenic 38
FD & C colors 260 446
Federal Food Drug and Cosmetic Act 475 479
Fehling solution 169
Ferrous sulfate 220
Fiber 203 analysis 203 bulking capacity 206 components 205 crude fiber 203 dietary 203
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Fiber (Continued) sources 204 205 total dietary 203 205 206
Fibrillar proteins 144
Film formation 200
Filtration of beer 460
Firming agents 449
Firmness, texture 311
Fish flavor 299 liver oil 357 protein 131 149
actin 150 actomyosin 150 concentrate 150 myosin 150 tropomyosin 150
Fisher formula 163
Fisheries and Oceans Canada 482
Fish Inspection Act 482
Flatulence 172
Flavin adenine dinucleotide 374 416
Flavin mononucleotide 374
Flavonoids 134 252 415 hesperidin 257 quercetin 256 structures 256
Flavor 263 analysis 291 astringency 276 294 bitter taste 263 273 275 295 chemical structure and taste 266 description 291 295 enhancements 278 enhancers 442
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Flavor (Continued) interrelationships of taste 275 odor 282 odor and molecular structure 283 odor description 289 off-flavor 291 296 of foods 297 potentiation 279 profile method 292 receptor mechanism 263 264 release 84 reversion 54 70 71 salty taste 263 272 273 sour taste 263 270 sweet taste 263 268 taste 263 theories of olfaction 287 threshold 59
Flavor, additives 441 artificial 442 natural 441 nature identical 442
Flavor compounds and description alcoholic beverages 303 304 bread 297 cheese 300 coffee 301 304 dairy products 296 297 fish 299 fruit 300 margarine 295 meat 298 milk 300 spices and herbs 304 tea 301 unsaturated aldehydes 294
535 Index terms Links
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Flavor compounds and description (Continued) vanilla 306 vegetables 301 wine 292 293
Flavor enhancer, additives 442 ibotenic acid 442 L-theanine 442 MSG 442 nucleotides 442 tricholomic acid 442
Flavor enhancers 278 characteristics 278 glutamic acid 278 ibotenic acid 281 282 maltol 281 MSG production 279 nucleotides 280 synergistic effect 280 tricholomic acid 281 282 Umami 278
Flavylium 252
Flocculation 18 117
Floral 287
Flour enrichment 488
Flow 311
Fluidity 187
Fluorescence 64 126 128 374 376
Fluorescent light 230
Fluorine 219
Foaming 134
Folacin. See Folic acid.
Foliar penetration 220
Folic acid (folacin) 382 sources 355 382
536 Index terms Links
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Folic acid (folacin) (Continued) stability 356 383 384 structure 382
Food additives 429
Food additives, regulations Delaney clause 431 definition 429 480 food additives 430 480 GRAS 430 480 nondeceptive 430 431 prior approval 430 purposes 431
Food and Nutrition Service 480
Food irradiation 446
Food Safety and Inspection Service 480
Food simulants 448
Food supplements 477
Formic acid 61
Fortification 374
Fortification of cereal grain products 376
Fractionation, fats 94 dry 94 milk fat 49 94 multistage 98 palm kernel oil 102 palm oil 98 solvent 90 94 96 tallow 94
Fracture resistance 344
Free energy of adsorption 288
Free radical 56 59
Freeze drying 15
Freezing 14 18
Freezing, sucrose solution 18
Fructooligosaccharides 172
537 Index terms Links
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Fructose 167
Fruit brandies 303 coat fats 34 flavor 300 freezing expansion 18 mineral content 219 puree 329
Frying 65
Fumaric acid 75
Functional foods 477
Functionality 87 134
Functional properties of proteins 111
Fungal toxins 464
Furanones 286
Furanose ring 163
Furans 302
Furcellaran 199
Furda method 205
Furfural 367
Fusarium 467
Fusel alcohols 303
G GAB sorption isotherm 8
G actin 146
Galactans 192
Galacto-mannoglycan 200
Galactopyranose 195
Galacturonic acid 195 405
Gallic acid 257
Gallotannins 257
Gamma linolenic acid 43
Garlic flavor 301
538 Index terms Links
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Gas liquid chromatography 41 44 285 291
GATT 486
Gelatin gels 137
Gelatinization temperature 185 340
Gelatin types, commercial 149
Gel electrophoresis 153
Gel formation 147 149 152 180 187 347
Gels 117 138
Genetic engineering 67 106
Genetic toxicology 476
Genetic variants 140
Gentiobiose 167 175
Geometrical characteristics 313
Geotrichum candidum 400 402
Geranium off-odor 433
Geranyl geraniol 54 56
Germination 404
Gibbs-Donnan equilibrium 216
Ginger 276 305
Gingerol 305
Glass transition 18 19 20 130 179
Glass transition temperature 19 20 21 179 181
Glassy lactose 177
Glazed pottery 463
Glazes 449
Gliadin 111 114 154
Globulins 111 114
Gloss 99 239
Glucitol 168
Glucono delta lactone 160
Glucopyranosides 270
539 Index terms Links
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Glucosamine 151
Glucosan 175
Glucose-6-phosphate 186
Glucose isomerase 180
Glucose sulfurous acid 434
Glucose syrup 179
Glucose units 402
Glucosinolates 44 469
Glutamates 442
Glutamic acid taste 278
Glutelin 111 114 154
Glutelin subunits 156
Gluten 153
Gluten-sorption isotherm 9
Glyceraldehyde 163
Glyceride composition canola 93 corn 93 cottonseed 93 interesterification 79 milk fat 51 monoglycerides 81 natural fats 45 olive 93 palm oil 93 98 palm olein 98 peanut 93 racemic mixture 45 random 45 47 48 soybean 93 sunflower 93 tripalmitin 91
Glycerides 34
Glyceride structure 45
Glycerol esters 34
540 Index terms Links
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Glycinin 114
Glycogen 190
Glycogen granules 145
Glycomacropeptide 409
Glycophore 270
Glycoproteins 114
Glycosides 167 273
Glycosylamine 120
Goiter 221
Goitrin 268
Goitrogens 468
Good manufacturing practice 30
Gouda cheese 435
Graininess 88
Grapefruit juice 225
GRAS list 430 480
Green pepper 285
Grit cells 418
Groundstate oxygen 63
Growth hormones 487
Guanylate 280
Guar gum 199
Gum arabic 200
Gumminess 313 334
Gums 197 algin 201 arabic 200 carrageenan 199 201 202 guar 200 locust bean 200 modified celluloses 203
Gymnemagenin 277
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H HACCP 484
Half-life 449
Hardness 311
Hardstock 81
Harmonization 488
Haworth formula 163
Hazard 430 478
Health Canada 482
Heat denaturation 117
Heated fats 65
Heat of sorption 11 12
Heavy metal salts 266
Helix formation in starch 184
Hemagglutination 150
Heme pigments 220
Hemiacetal 163
Hemicelluloses 191
Hemochrome 242
Hemoglobin 240
Henderson-Hasselbach equation 212
Herbicides 458
Herbs 304
Hershel-Bulkley model 319
Hesperidin 167 257 274
Heterocyclic compounds 163
Heteropolysaccharides 192
Hexadienoic acid 432
Hexagonal crystal structure 88
Hexahydroxydiphenic acid 257
Hexenal 71
Hexenoic acid 433
Hexenol 284 422
Hexitols 168
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Hexoses 165
Heyns rearrangement 122
High erucic rapeseed oil 77
High fructose corn syrup 180 426
High lysine corn 156
High melting glycerides 89
High oleic sunflower oil 67 106
Hilum 185
Histidine 299
Histones 114
HLB 440
Holoenzyme 389
Honey, color 235
Hooke’s law 321
Hordenin 114
Horizontal regulations 483
Hormones 51
Hot break method 406
Hotness 263 276
HTST pasteurization 406
Hudson equation 173
Hue 234 236
Human fat. See Fats and oils, individual
Humectants 449
Humin 176
Hunter system of color 237
Hydrides 3
Hydrocarbons 51 56
Hydrochloric acid 271
Hydrocolloids 199 agar 199 200 application 199 carboxy methy cellulose 199 function 199
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Hydrocolloids (Continued) gums 197 pectins 196
Hydrogenated fats, texture 337
Hydrogenated starch hydrolyzates 182
Hydrogenation 66 71
Hydrogenation, fats 71 canola oil 78 catalysts 71 76 100 CBS 100 cottonseed oil 73 fish oils 77 frying fats 67 liquid oil polymorphism 93 nonselective 72 73 77 olefinic compounds 74 palm kernel oil 102 palm oil 93 94 peanut oil 73 pressure 72 73 rapeseed oil 77 selective 72 soybean oil 76 temperature 72 73 trans-formation 73 77
Hydrogen bonds 2 3 15
Hydrogen peroxide 437
Hydrogen sulfide 299
Hydrolysis, aw 22
Hydrolytic reactions 22
Hydrophile-lipophile balance 103
Hydrophobic bonds 4
Hydrophobic interaction 115 346
Hydrophobicity 117 136 274
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Hydrostatic compression 316
Hydrotactoid 4 6
Hydroxy benzoic acid 432
Hydroxy fatty acids 36
Hydroxylated lecithin 440
Hydroxymethyl-furfural 123 126 298
Hydroxyproline 111
Hydroxypropyl starch ether 188
Hygroscopicity 6 26
Hypobromite 149
Hypochlorite 189
Hypoxanthine 299 422
Hysteresis 8 11
H zone 145
I Ibotenic acid 281 442
Ice 1 coefficient of thermal expansion 2 density 2 heat capacity 2 heat of fusion 2 heat of sublimation 2 lattice structure 14 16 specific heat 2 vapor pressure 2
Ice cream 372
Ice crystal size 17
Ice-like cluster 16
Ice, physical properties 2
Ice, structure 14
Ideal solubility 95
Illipe butter 100
Immobilized enzymes 423
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Immobilized lipase 402
Immune globulins 138 142
Indole acetic acid 418
Induced dipole effect 184
Induction period, fat oxidation 57 64
Industry Canada 482
Information theory 287
Inherent stability 67
Inhibitors 130
Initiation 56
Ink bottle theory 10 11
Inosinate 280
Inosine monophosphate 298
Inositol 218
Inositol hexaphosphoric acid 217
Insoluble dietary fiber 204
Interesterification 77 catalyst 79 directed 77 enzymatic 81 glycerol 81 palm kernel oil 81 palm oil 82 palm stearin 81 randomization 78 79 82 96
Interfacial loading 137
Interfacial tension 101
Intermediate moisture foods 21 30
International Units 358
Invert sugar 171
Invisible fat 33
Iodine 219 131 450 133 450
reaction 184 value 40 95
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Ionic equilibria 215
Ionone ring 358
Iron 219 220
Iron bioavailability 211
Irradiated yeast 362
Irradiation 446 See also Food irradiation unit of radiation 446
Irreversible deformation 316
Isobetanin 258
Isobutanol 339
Isoelectric focussing 153
Isoelectric point 138 157
Isoelectric point, meat 213 215
Isomaltol 286
Isomaltose 180
Isomenthol 276
Isomeric fatty acids 34 36 39 40 43 58 66 68 73
Isomerization 81 174
Isoprebetanin 258
Isoprenoid side chain 362
Isosacchrosan 175
Isosolids diagram 99
Isothiocyanates 77 469
Isotropic I bands 145
Isovalerate 301
J Just noticeable difference 266 289
K Keratin 114 152
Kestose 172
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Ketones 59
Ketosamine 120
Ketoses 167
Kinematic viscosity 318
L Lachrymatory factor 301 469
Lactalbumin 138 142
Lactase 171
Lactic acid 271
Lactitol 182
Lactobacillus casei 382
Lactococcus lactis 437
Lactoglobulin 114 138 142
Lactones 284 297 301 367
Lactose 17 171
Lactose β-anhydride 177
Lactose α-hydrate 177
Lactose, physical properties 177
Lakes 254 446
Lamellar units 86
Laminaria japonica 279
Langmuir equation 7
Lanthionine 438
Lard 34 48 81
Lard stearin 98
Latent heat of crystallization 82
Laurate canola 106
Laurie oils 44 94
LD50 476
Lead 220 462
Lead acetate 266
Lead and tin contamination acid food 463
548 Index terms Links
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Lead and tin contamination (Continued) detinning of cans 463 equipment 462 gasoline 462 tin cans 462
Leaf proteins 152
Leathery state 349
Lecithin 52 440
Legumelin 114
Leucosin 114
Leukotrienes 41
Levoglucosan 174 175
Levulosan 175
Lewis acid and base 210
Ligand 210
Light effect 371
Light exposure 360
Lightness 234
Light sensitive receptors 237
Light sources 229
Lignin 193 304 306
Limit dextrin 403
Limonene 301
Lineweaver-Burke equation 393
Linoleic acid oxidation 58
Linseed oil 38 53 56
Lipase 81
Lipid oxidation 22 25
Lipid oxidation, aw 22
Lipid peroxides 360
Lipids 33 definition 33 interrelationship 34 milk 43
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Lipids (Continued) nonpolar 101 phospholipids 50 103 polar 101
Lipoproteins 114
Lipovitellenin 152
Lipovitellin 152
Lipoxydase 243 419
Lipoxygenase 62
Liver necrosis 221
Local isotherm 26
Locus 233
Locust bean gum 199
Long spacing, X-ray diffraction 89
Loosely bound water 11
Loss modulus 326
Lovibond system of color 238
Low acid foods 30
Low density lipoprotein 133
Low linolenic canola oil 106
Low linolenic linseed oil 106
Low linolenic soybean oil 106
Low methoxyl pectin 405
Lumichrome 371 376
Lumiflavin 376
Luminosity curves 230
Lutein 248
Lycopene 245
Lysine 130
Lysine loss 24
Lysine loss in proteins of cottonseed 123 fish 123 maillard reaction 120
550 Index terms Links
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Lysine loss in proteins of (Continued) milk 123 peanut 123 plasma albumin 132 wheat 123
Lysinoalanine 133
Lysosomes 145
Lysozyme 150
M Mackarel 50
Macrocyclic ketones 284
Macrocystes pyrifera 201
Magness-Taylor pressure tester 330
Maillard reaction 120 128 260 See also Nonezymatic browning
Maize, proteins 154 glutenin 154 zein 154
Malic acid 370 426
Malolactic fermentation 272
Malting of barley 411
Maltodextrins 179
Maltol 281 286 302
Maltose 171
Malvidin 253
Mammal depot fat 34
Manganese 219
Mannans 192
Mannitol 182
Maple syrup, color 235
Margarine 17 17 338 crystal structure 89 90 94 no trans 95 texture 337 338 343
551 Index terms Links
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Marine oils 34
Mass spectrometry 285 291
Matrix protein 156
Matte 239
Maximum residue limits 484
Maxwell body 322 323
Mayonnaise 137 343
Meat and bone meal 484
Meat, drip loss 214
Meat flavor 298
Meat Inspection Act (Canada) 482
Meat Inspection Act (United States) 479
Meat minerals 213 220 223
Meat proteins actin 144 actomysin 146 collagen 147 myoalbumin 146 myosin 120 144 146 stroma 144 tropomyosin 144
Meat shear cell 333 334
Meat tenderizing 411
Meat texture 334
Meat-water binding 28 29
Meaty taste 276
Mechanical body 322
Mechanical harvesting 311
Mechanical properties 311
Medium chain triglycerides 107
Melanoidins 120 126
Melibiose 172
Meltdown properties 89
Melting agents 449
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Melting points, fats 82 cocoa butter 48 99 high melting glycerides 89 milk fat 48 palm oil 97 palm olein 97 palm stearin 97 sheep depot fat 48 shortening 94 triglycerides 84 89 93 vegetable fats 44
Melting range 82
Membrane effect 16
Membrane puncturing theory 288
Menadione 366
Menthanoids 305
Mercaptans 287
Mercury 220 461
Mercury contamination in foods 462 methylmercury 461 Minamata disease 461 pulp and paper 461 seed spraying 461 structure Hg, compounds 461
Meromyosin 146
Mesomorphism 104
Metabolism 476
Metal deactivator 62
Metallic taste 263 276
Metal pickup 218
Metal uptake 223
Metameric 236
Methanethiol 299
Methional 133 300
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Menthol 276 284 305
Methoxy isobutylpyrazine 285
Methoxyl content of pectin 196
Methoxypyrazines 285
Methyl alphaethyl-n-caproate 301
Methyl cellulose 203
Methyl ketones 300
Methyl lanthionine 438
Methyl mercaptan 299
Methyl mercury 461
Methyl pyrazines 286
Methyl tetrahydrofolate 381 383
Metmyoglobin 240
Michaelis constant 426
Michaelis-Menten equation 392
Microencapsulation 424
Microorganisms, growth 21 23
Microsomes 422
Microstructure 311 341 chocolate 343 colloidal dispersions 342 345 dispersed phases 343 emulsions 343 fat 346 soybean curd 347
Migration from packaging 448
Milk Ca, P content 214 clotting 409 fat 34 flavor 299 300 minerals 212 218 219 223 powder 25 protein 138 serum proteins 138
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Minamata disease 461
Minerals 209 abundance of trace 219 determination 209 enzyme reactions 220 221 essential 209 in fruit 219 in meat 213 in milk 212 214 in plant products 216 in soybeans 218 interactions 210 in wheat grain 216 217 metal chelates 211 nonnutritive, nontoxic 209 nonnutritive, toxic 209 trace 217 uptake, canned food 223
Mint flavor complex 276 284
Miracle fruit 277
Mitochondria 145
Mixed crystals 84
Mixed gels 202
Mobility 325
Model food act 477
Modified cellulose 203
Modified starch 187 340
Molasses 170
Mold and yeast, growth 21 22 23
Molecular cross-section area 288
Molecular distillation 81 104 440
Molecular packing 86
Mollusks 50
Molybdenum 219
Monochromatic light 230
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Monoglycerides 81 103 440
Monomolecular water 7 11
Monophenols 414
Monosaccharrides Fischer formula 163 165 167 fructose 166 167 172 173 galactose 167 168 173 glucose 164 167 172 173
175 Haworth representation 165 mannose 167 168 173 tautomeric form 166
Monosodium glutamate 279
Mucins 152
Mucor javanicus 402
Mucor miehei 400 409
Mucor pusillus 409
Multilayer adsorption 28
Multiple sclerosis 223
Multi-stage fractionation 95
Multivariate analysis 291
Munsell notations 237
Munsell system of color 236
Muscle fiber 146
Muscle protein 28
Musks 283
Musky 288
Mustard oil 44
Mutarotation 169 172
Mycotoxins 465
Myofibrils 28 144
Myoglobin 240
Myosin 114 144
Myrcene 301
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N Naringin 167 273
Natamycin 438
National Academy of Sciences 431
Natural toxicants 467
Nature identical colors 446
Neat phase 104
Neohesporidose 274
Neomenthol 276
Neoisomenthol 276
Nerve activity 264
Neural response 264
Neuraminic acid 172
Neuron 288
Neutraceuticals 477
Neutral detergent fiber 204
Neutralization 50
Neutron diffraction 176
Newtonian fluids 318 320
Newtonian materials 311 318
Niacin 378 from tryptophan 379 pellagra factor 379 sources 355 380 stability 356 380 structures 379
Niacytin 380
Nickel 219 catalyst 222 content in foods 222
Nicotinic acid 378
Nicotinic amide 378
Nisin 437 461
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Nitrates, nitrites ADI 437 applications 437 curing salts 435 natural sources 437 nitrosamines 436
Nitrite cured meat 211
Nitrocyclohexanes 284
Nitrogen closure 225
Nitrogen oxides 441
Nitrosamines 386 436
Nitrosated heme 211
Nitrosodiethylamine 448
Nitrosohemochrome 242
Nitrosyl chloride 441
Nitro toluidine 267
No effect dose 478
Nonadienal 71 284 422
Nonalactone 284
Noncariogenicity 182
Nonenal 71 296
Nonenzymatic browning 21 24 120 aw 22 24 Amadori rearrangement 121 122 125 at glass transition temperature 130 browning reaction 125 caramelization 127 128 enolization 123 127 glycosylamines 125 Heyns rearrangement 123 126 hydroxymethylfurfural 123 126 inhibitors 127 130 131 lysineloss 123 130 Maillard reaction 120 127 128 129 melanoidins 120 124 126 132
558 Index terms Links
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Nonenzymatic browning (Continued) pH 131 products 129 Schiff base 124 Strecker degradation 127 velocity 120 130
Nonfermentable sweeteners 182
Nonionic emulsifiers 103
Non-Newtonian fluids dilatant 320 plastic 320 pseudoplastic 320
Nonpolar oligomeric glycerides 68
Nonprotein nitrogen 138
Nonselective hydrogenation 77
Nonspectral colors 233
Nootkatone 284
Norepinephrine 264
No-trans margarine 95
Novel food regulation 483
Novel oils and fats 106
Nuclear fission 449
Nuclear magnetic resonance 12
Nuclear magnetic resonance pulsed 83
Nucleation 16 17
Nucleation, heterogeneous 15
Nucleoproteins 114
Nucleotides 442
Nucleotides as flavor enhancers 442
Nutmeg 305
Nutrition labeling 481
Nutrition Labeling and Education Act (United States) 480
Nutrition supplement 447
Nystose 172
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O Oak casks 295 304
Octenone 277
Octyl tin maleate 448
Octyl tin mercaptoacetate 448
Odd carbon number fatty acids 36
Odor 263 282 See also Flavor analyses 291 aroma description of wine 293 description 289 intensity factor 289 just noticeable difference 289 membrane puncturing theory 288 291 olfactory organ 282 287 potency 283 285 primary odors 287 290 threshold concentration 282 287
Odorivector 287
Off-flavors 291 296 416
Oil absorption 66
O-inside form 164
Olefins 72
Oleic acid oxidation 58
Olein 94
Oleoresins 304 441
Olestra 107
Olfaction 287
Olfactory organ 282
Oligosaccharides 169 cellobiose 171 175 fructo 172 gentiobiose 168 172 in legumes 172 in cow’s milk 172
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Oligosaccharides (Continued) in soy milk 172 lactose 170 172 maltose 170 171 175 manninotriose 170 raffinose 170 sophorose 175 stachyose 170 172 sucrose 170 172 trehalose 170 verbascose 170
Olive oil 34
Onion flavor 301
O-outside form 164
Opaqueness 229
Opponent colors theory 237
Optical rotation 165
Orange 247
Organic acids 271
Ornithine 130
Ornithoalanine 133
Orthorhombic crystal structure 88
Oryzenin 114
Osazones 169
Osmotic pressure measurement 200 211
Ovomucin 114 150
Ovomucoid 150
Oxalic acid 225
Oxazoles 303
Oxidation rate 58
Oxidized glycerides 68
Oxidized sterols 68
Oxonium salts 252 255
Oxygen permeability 241
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Oxygen scavengers 438
Oxymyoglobin 240
P Packaging 26 27
Packaging materials, migration to foods 448 plastic netting 448 PVC 448
PAHs (polycyclic aromatic hydrocarbons) 463
Palatinose 183
Palm kernel oil 34 81
Palm oil 34 252 337 mid fraction 95 super olein 95
Palm stearin 81
Panose 282
Panthenol 384
Pantothenic acid 383 coenzyme A 384 sources 384 stability 356 384 structure 384
Papain immobilized 426
Papillae 264
Paprika oleoresin 252
Parabens as preservative 432
Paracasein 140
Paraffins 54 56
Parallel sheet structure 115
Partial glycerides 81 300 398
Particle size analysis 311
Pastes 187
Pattern recognition 291
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Patulin 467
PCBs (polychlorinated biphenyls), contaminants from 458 aroclor 459 hydraulic fluids 458 in fish 458 transformers 458
Peach 246
Peak time, in texture 336
Peak viscosity 341
Peanut oil. See Fats and oils, individual
Peanut protein 113
Pectic substances 169 194 acid 196 content, plants 195 degree of methylation 196 gel strength 197 hairy regions 196 LM, HM pectin 197 smooth regions 196 structure 196
Pectin 26
Pelargonidin 253
Pellagra preventive factor 379
Penetrometers 330
Penicillium camemberti 398
Penicillium roqueforte 300 398 400
Pentadiene group 420 432
Pentene radical 71
Pentitols 168 182
Pentosans 191
Peonidin 253
Pepper 276 305
Pepperminty 288
Peptides 114 273
563 Index terms Links
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Peptones 114
Peroxidase 139 257 368 421
Peroxidase test 419
Peroxides 56
Peroxide value 60
Persicaxanthin 247
Pesticides 451 455
Pesticides, chlorinated hydrocarbon 451 ADI 457 Aldrin 451 classes and names 452 DDT 452 Dieldrin 452 454 Heptachlor 452 removal 454 456 stability 452 structures 453 454
Pesticides, classes 451 chlorinated hydrocarbon
insecticides 451 organo phosphorous
insecticides 453
Pesticides, organophosphorous 453 ADI 457 classes and names 455 structure 456 water solubility 453
Petunidin 253
Pharmacokinetics 476
Phase angle 326
Phase diagram 14 19 95 96
Phenolase 368
Phenolic acid 257
Phenolic antioxidants 62
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Phenoloxidase 257
Phenols 62 303
Phenylketonuria 443
Phenylthiourea 268
Pheophorbides 243
Pheophytin 26 243
Phosphates 29 212 444
Phosphates, additives 444 firming agent 444 in nutrition 445 Na-hexa 445 ortho 445 poly 445 pyro 445 structure 445
Phosphatidylcholine 52
Phosphatidylethanolamine 52
Phosphatidylserine 52
Phosphoinositides 52
Phospholipids 50 51 52
Phosphopeptone 141
Phosphoproteins 114
Phosphoric acid 271
Phosphorus oxychloride 188 445
Phosphorus poison 76
Phosphorylation 140
Photooxidation 63
Phtalides 284
Physical properties fats 82 87 hydrides 3 ice 2 volume change, water 18 water 2
565 Index terms Links
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Physical properties, fats. See also Plastic fats hardness 87 margarines 89 90 94 shortenings 89 90 94 texture 87
Physical refining 66
Phytate 160 210
Phytic acid 218
Phytosterols 51
Picric acid 266
Picrocrocin 246
Pimaricin 438
Pinking 256
Piperine 276 305
Plane of symmetry 45
Plant breeding 67
Plant products 19
Plant proteins 152
Plasmalogens 52
Plastic 320 324
Plastic fats 71 81 86 87 cocoa butter 85 margarine 71 melting 84 shortening 71 solid fat index 83 84 solid fat content 82 84 85 100 solidification 84 tallow 84 85
Plasticizers 20
Plastic netting, migration of 448
Plastic range 81 86
Plastic viscosity 325
Polarized light microscopy 89 184 191
566 Index terms Links
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Polishes 449
Polyacrylamide gel 426
Poly chlorinated biphenyls 458
Polychlorinated dibenzodioxins 455
Polychlorinated dibenzofurans 455
Polycyclic aromatic hydrocarbons (PAHs) barbecuing 463 in smoke, foods 463 roasted coffee 463 466 structures 464 unsmoked food 466
Poly dextrose 194
Polyglutamate 382
Polyglycerol esters 441
Polyhydrons 5
Polymerization 70 81
Polymerization, sulfhydryl 120
Polymorphism, fat 86 96
Poly nuclear chelates 223
Polyols 181
Polypeptides 141
Polyphenolase 372
Polyphenols 134 295
Polyphosphates 370
Polysaccharides. See also Oligosaccharides cellulose 191 complex 191 glycogen 190 gums 197 hydrocolloids 199 molecular structure 198 sources 198 starch 179 unit length 169
Polysorbates 441
567 Index terms Links
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Polyunsaturated fatty acids 365
Polyvalent ions 210
Poly vinyl chloride 448
Popcorn 347
Popcorn crispness 349
Porosity, milk powder 26
Post-hardening 81
Potassium bromide 266
Potassium iodide 266
Potassium metabisulflte 433
Potato-dehydrated-BET Plot 9
Potato protein 113
Potato starch 183 185 404
Powdered milk 372
Power law 318
Prebetanin 258
Preservatives 431 acids 438 bacteriocins 437 benzoic acid 432 hydrogen peroxide 437 nitrites 435 parabens 432 sodium chloride 437 sorbic acid 432 433 sulfites 433
Primary colors 231
Primary odors 287
Primary oxidation products 57
Processing requirements 30
Procyanidins 257
Profile functional group concept 288
Prolamins 114
Proline 111
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Proline-rich proteins 295
Propagation 56
Propellants 449
Propionic acid as preservative 438
Propylene glycol esters 441
Propyl gallate 62
Propyl methane-thiosulfonate 301
Propyl propane-thiosulfonate 301
Prostanoids 41
Prosthetic groups 368
Protamines 114
Protein biological value 112
Protein bodies 156
Protein bonds covalent 117 disulfide 117 154 155 electrostatic 117 energy 117 hydrogen 122 hydrophobic 117 159 hydrophobicity 118 136 noncovalent 117 sulfhydryl 120
Protein coagulants 160
Protein content of foods 112 153 154
Protein crosslinking 347
Protein efficiency ratio 112
Protein gels 137 138 bean curd 137 160 clear 138 gelatin 149 isoelectric point 138 mayonnaise 179 yogurt 137
Protein hydrolysates 284 412
569 Index terms Links
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Proteins, changes during processing and storage 131 alkali treatment 132 133 ammonia treatment 133 heating 131 lipid oxidation 132 Maillard reaction 132 polyphenol reaction 134 sunlight 133 UHT sterilization 132 133
Proteins, classes conjugated 114 contractile 146 derived 114 115 fibrillar 144 lipo 152 phospho 138 serum 138 simple 113 114 soluble 146 153 stroma 144
Proteins, conjugated 113
Proteins, derived 113
Protein, separation milk 140 skim milk 143 wheat 153 whey 144
Proteins, functional properties 134 137 dispersibility 134 dough making 134 137 emulsification 134 137 foaming 134 135 137 gel formation 137 solubility 135 136 137
570 Index terms Links
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Proteins, functional properties (Continued) surface activity 134 wettability 134
Proteins of foods albumin 120 casein 114 119 137 143 cereal 112 152 egg 112 121 137 150 fibrinogen 120 fish 131 137 149 150 globulin 120 legume 152 maize 154 meat 144 milk 121 123 myosin 120 146 150 ovalbumin 138 rice 156 soybean 156 wheat 153 whey 119 139 142 143
Proteins, simple 113
Protein structure 115 alpha helix 116 146 156 159 antiparallel 116 142 159 coagulation 118 120 denaturation 118 119 double helix 146 oligomeric 117 primary 111 quartenary 159 secondary 111 116 120 159 stability 119 120 tertiary 111 115 120 142
159
Proteolysis 29 115
571 Index terms Links
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Proteoses 114
Protocrocin 246
Protohemin 417
Protopectin 26
Provitamin 355
Provitamin A 246 251 358
Pseudobase 252
Pseudoglobulin 142
Pseudoplastic 318
Pullulanase 181
Pulsed nuclear magnetic resonance 84
Punch dimension, texture 332
Pungency 276 304
Pungent 287
Purples 234
Putrid 288
PVC bottles, migration of 448
Pyranose ring 163
Pyranoside, α and β 165
Pyrazines 285
Pyridoxal 377 378
Pyridoxamine 377
Pyridoxine 377
Pyroconversion 189
Pyrolysis of carbohydrates 302
Pyrroles 303
Pyruvic acid 201
Q Quaternary protein structure 159
Quenching 64
Quercetin 256
Quercetin-3-rutinoside 226
Quinine 266 273
572 Index terms Links
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Quinones 386 413
R Racemic mixture 45
Racemization 134
Radiation disinfestation 447
Radiation pasteurization 476
Radiation unit 446
Radioactive fallout 449
Radioactive isotopes 449 half-life 449 in food chain 450 natural 449 nuclear fall out 449 pathway 450
Radionuclides 449
Raffmose 172
Rancidity 54
Random coil structure 149
Random distribution, glycerides 47 48 77
Randomization 77 See also Interesterification
Raoult’s law 4
Rapeseed oil 37
Rapid detinning 223
Raspberries 253
Reaction rate and aw 21
Receptor mechanism 263
Recommended daily allowance 358
Refining 50
Refractive index 145
Regeneration enzymes 419
Regiospecificity, enzymes 400
Relative sweetness 270
573 Index terms Links
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Relaxation, muscle 147
Relaxation time, texture 320
Rennet action 139
Rennet casein 143
Rennin sources 408
Replacer fat. See Fats and oils, individual
Resistant organisms 460
Resonance hybrids in oxidation 58 61
Restricted random distribution, glycerides 48
Rest sulfurous acid 434
Retarded elasticity, texture 321
Retinal 358
Retinin 245
Retinol 355 358
Retrogradation 186
Reverse osmosis 143
Reversion disaccharides 175
Rhamnogalacturonan 195
Rhamnoglycosides 274
Rheological testing systems cone penetrometer (AOCS) 330 337 denture tenderometer 333 extensigraph 336 Farinograph 335 FIRA-NIRD extruder 337 General Foods Texturometer 333 Instron Universal Testing Machine 330 Kramer shear press 332 339 Magness-Taylor fruit presser tester 330 penetrometers 330 rotating knife tenderometer 335 viscoamylo graph 340 342 343 344 viscometers 329 Warner-Bratzler shear 334
Rheology 316
574 Index terms Links
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Rhizopus arrhizus 402
Rhizopus oligosporus 410
Ribitol 376
Riboflavin 371 374
Ribonucleic acid 281
Ribosomes 145
Rice bran 156
Rice polish 156
Rice proteins 156
Rice starch 183
Ripening 418
Risk, safety 478
Roasted peanuts 286
Rosemary 438
Rotating knife tenderometer 335
Rubbery flow 349
Rubbery state 19
Rum 303
Rutin 226
Rutinose 256 274
S Saccharification 179
Saccharin 266 273
Saccharinic acids 174
Saccharomyces cerevisiae 434
Saccharomyces fragilis 427
Saccharomyces lactis 427
Saccomere length 335
Safe Drinking Water Act (United States) 480
Safflower oil. See Fats and oils, individual
Saffronal 246
Sage 438
Sago starch 183
575 Index terms Links
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Salatrim 107
Salmonella 447
Salty taste 264 272
Sandiness 178
Sarcolemma 144
Saturation, color 234
Saxitoxin 469
Schiff base 124
Sclerids 418
Scleroproteins 114
Scombrin 114
Seafood toxins 468
Secondary oxidation products 57
Sectilometer 337
Sedimentation methods, microstructure 311
Seeding 17 179
Seed oils 34
Selectivity 72
Selenite 221
Selenium 219
Semipermeable membrane 425
Senescence 217 370 418
Sensitizer 64
Sensory panel tests 311
Sequestrants 438
Serum albumin 114 138
Sesame oil. See Fats and oils, individual
Sewage sludge 220
Shadow matching 288
Shea oil 100
Shear force 316
Shear press 332
Sheep depot fat 48
Short chain fatty acids 300
576 Index terms Links
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Shortening 81 338
Shortening, crystal structure 89 90 94
Shortening texture 338
Shortening, trans isomers 77
Shorthand description fatty acids 35
Short range order 71 86
Short spacing, X-ray diffraction 88
Shrink temperature 147
Silicon 219
Sinapaldehyde 304
Sinapic residue 254
Sinapyl alcohol 193
Singlet oxygen 63
Sinigrin 167
Sintering fat crystals 339
Sinusoidal strain, texture 326
Sitosterol, β 51 55
Slip melting point 97
Slow-set pectin 196
Smoke 463
Snap 99
Sodium benzoate. See Benzoic acid
Sodium chloride 437
Sodium methoxide 79
Sodium stearate 105
Sodium trimetaphosphate 188
Soft rot 407
Solder 463
Solid fat content 82 83
Solid fat index 84
Solid fat profile 83 96 102
Solid solutions 84 95
Solubility diagram 97
Solvent fractionation 94
577 Index terms Links
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Sophorose 175
Sorbic acid (sorbates) as preservatives applications 432 off-flavor 433 yeasts and molds 432
Sorbitol 168 181
Sorption 4
Sorption isotherm 6 7 27 347
Sorption phenomena 4
Sour taste 263 270
Southgate method 204
Soya lecithin 103
Soybean curd 137
Soybean flour in wheat flour 421
Soybean oil 37 See also Fats and oils, individual
Soybean proteins 156 158 160
Soybeans, mineral content 218
Soy milk 160
Soy protein isolate 134
Soy sauce 408 410
Spatial representation, glyceride 46
Specialty fats 94
Specific rotation 171
Specific surface area, dispersions 344
Spectral light types 230
Spectrophotometry 229
Specular reflection 239
Spherocrystals, starch 186
Spices 304 441
Spinach 437
Spoilage, water activity 23
Spring 322
578 Index terms Links
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Springer 224
Squalene 51 56
Stachyose 172
Stainless steel 219 222
Staling 187
Staphlococcus 464
Staphylococcus aureus 401
Starch 179 340 acid conversion 180 amylase 184 185 amylopectin 184 birefringence 185 branched 184 cereal (corn, wheat) 185 187 crystallinity 184 double helix 185 enzymatic conversion 180 181 gelatinization temp 185 186 gel formation 187 granules 183 hydrolyzates 179 180 linear 184 modified 187 polymorphs 185 retrogradation 186 root tuber (potato) 187 swelling 186
Starch, glass transition 20
Starch hydrolyzates 179
Starch, modified 187 acid 187 189 adipate 187 cross-bonding 187 dextrinization 189 gel formation 187 188 190
579 Index terms Links
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Starch, modified (Continued) oxidation 187 189 phosphate 187 188 properties 190 substitution 187 188
Starch sodium octenyl succinate 188
Starch, sorption isotherm 9
Starch viscosity of suspensions corn 343 tapioca 343 waxy corn cross-bonded 342 343 344
Steam flow closure 225
Stearin 94
Stearoyl lactylate 441
Stereochemical site theory, odor 287
Stereoisomers in taste 267
Stereospecific analysis, glycerides 48
Stereospecificity 400
Stereospecific numbering, glycerides 46 47
Sterilization flavor 300
Steroids 284
Sterols 51 53 68
Stickiness 20 311
Stigmasterol 51 55
Stimulus concentration, taste 266
Stone cells 418
Storage modulus, texture 326
Strain-texture 316
Strecker degradation 128 298 367
Streptomyes 426
Stress 316
Stress decay 320
Stroma proteins 144
Strontium 89 450 90 451
580 Index terms Links
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Structural triacylglycerols 401
Struvite 226
St. Venant body 324
Subcell 88
Subchronic toxicity 476
Substrate, enzymes 389
Subunit 117
Subunit association 115
Succinylated monoglycerides 441
Succulometer cell 333
Sucrose 169
Sucrose fatty acid esters 441
Sucrose polyester 107
Sugar acids 167 276
Sugar alcohols 167 arabitol 169 galactitol 169 hexitols 168 lactitol 182 maltitol 182 183 mannitol 169 182 pentitols 168 polyols 181 sorbitol (glucitol) 168 169 181 183 xylitol 168 169 182
Sugar, reactions 174 caramelization 175 dehydration 174 enolization 174 isomerization 174 mutarotation 173 polymerization 174
Sulfide staining 225
Sulfite oxidase 221
581 Index terms Links
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Sulfites as preservatives activity 433 ADI 434 antimicrobial 433 antioxidant 433 applications 434 effect on pH 435 thiamin and 434
Sulfmyoglobin 242
Sulfones 454
Sulfoxides 454
Sulfur dioxide 130 255 373 374 433
Sulfur modified catalyst 100
Sulfur poison 76
Sunflower oil 38
Sunlight flavor 133
Supercooling 15 17 19
Super olein 81
Supersaturation sucrose 176
Suprathreshold levels 289
Surface active proteins 134
Surface activity 50
Surface area, emulsions 106
Surface tension 101 103 344
Surimi 150
Sweetened wine 432
Sweeteners 443 444
Sweeteners, artificial 182 acesulfame-K 182 aspartame 182 isomalt 183 maltitol 182 relative sweetness 183 sorbitol 182
582 Index terms Links
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Sweeteners, natural 444
Sweeteners, nonnutritive 443 acesulfame K 443 444 aspartame 443 cyclamate 443 444 cyclohexylamine 440 444 saccharine 443 444 sucralose 443
Sweetness of sweeteners 443 444
Swelling temperature 185
Swordfish 462
Synaptic area 264
Synergistic effect in flavors 280
Synergists in antioxidants 62
Synsepalum dulcificum 278
T Tackiness 199
Tallow 34 84
Tannins 134 257 276 277 295
ellagic acid based 257 gallic acid based 257 hexahydroxy diphenic acid based 26 nonhydrolyzable 257
Tapioca starch 31 43 45
Taraxanthin 251
Tartaric acid 271
Taste ability to 268 269 270 alkaloids 266 306 chemical structure 266 peptides 273 salts 266 sensations 263
583 Index terms Links
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Taste (Continued) sensitivity of the tongue 264 stereoisomers 267 269 stimulas 264 265 266
Taste, bitter 273
Taste buds 264
Taste interrelationships 275 astringency 276 277 coolness 276 277 enhancers 278 hotness 276 277 meaty 276 metallic 276 pungency 276 306 suppressants 277 278 sweet-sour 275
Taste, salty 272
Taste, sour 270
Taste, sweet 268
Tautomers 165
Tea 301
Technical monoglyceride 81
Tempering, cocoa butter 99
Tension force, texture 316
Termination in autoxidation 56
Terpenic alcohols 51
Terpinyl acetate 301
Tert-butyl hydroquinone 62
Tertiary oxidation products 57
Tetracyclines 460
Tetrahedral structure 16
Tetra hydronaphtalenes 284
Tetrapyrrole pigments 239
Tetraterpenoids 244
584 Index terms Links
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Textural instruments. See Rheological testing systems
Textural integrity 18
Texture 311 apples 333 applications 328 beans 333 beets 333 bread 333 butter 332 definition 311 dough 335 336 337 dynamic behavior 326 327 fats 330 337 339 fruit purees 329 331 fruits and vegetables 339 340 hydrogenated fats 337 interrelationships 311 margarines 332 meat 334 335 mechanical properties 314 317 microstructure 341 objective measurements 311 316 320 of foods 334 palm oil 337 physical properties 311 profile 313 314 315 317 rheological parameters 314 316 318 shortening 337 338 starch suspensions 340 terminology 312 313 314 315 types of bodies 320 viscosity 318 water activity and 347
Texture characteristics geometrical 314 measurements 316
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Texture characteristics (Continued) mechanical 314 others 31 terminology 312 317
Texture measurements by compression 330 337 by extrusion 332 by flow 330 by penetration with punches 330 331 by shearing 330 333 334 by stretching 336 creep compliance 320 deformation and strain 317 force and stress 316 relaxation time 320 shearing stress, rate of shear 318 319 326 sinusoidal strain 326 327 strain time 321 322 323 stress decay 320 stress strain 324 stress time 321 322 323 yield stress 319 yield value 319 325 326
Texture profile 314 analysis 334 geometrical characteristics 313 mechanical characteristics 313 other characteristics 313
Texture, type of bodies Bingham 324 325 326 Burgers 323 elastic 320 Hookean 321 322 Maxwell 332 375 plastic 324 plasto-elastic 325
586 Index terms Links
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Texture, type of bodies (Continued) plasto-viscoelastic 325 retarded elastic 321 322 St. Venant 32 thixotropic 326 328 342 viscoelastic 321 322 326 viscous 321 Voigt-Kelvin 322 323
Theanine 442
Theobromine 273
Theocysteine 133
Thiamin 131 372 434
Thiazoles 303
Thiazolidine 132
Thin boiling starch 187
Thiol-disulfide exchange 336
Thiophenes 303
Thiophosphates 454
Thio propanal oxide 301
Thiouracil 268
Thiourea 268
Thixotropy 326 328 338 342
Thomson equation 13
Three dimensional folding, protein 142
Three dimensional network, fat 85
Three-Mile Island 451
Threonic acid 367
Threshold value 266
Thymol 305
Tin 462
Tin contamination. See Lead and tin contamination
Tin-iron couple 223
Tin plate 225
Titanium dioxide 260
587 Index terms Links
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Titratable acidity 270
Tocopherolquinone 363
Tocopherols 62 362 386
Tocopheronic acid 363
Tocopheronolactone 363
Tocotrienols 362
Tofu 137 160 346
Tomahawk crystal shape 178
Tomato 246
Tomato paste 225
Tongue 264
Totox value 60
Toughness of fish 18
Toxicants, natural caffeine 469 470 domoic acid 469 gastrogens 468 glucosinolates 469 isothiocyanates 469 470 saxitoxin 469 seafood 469 thiosulfmates 469
Toxic chemicals in food 429 man made 430 natural 430
Toxicity 430
Toxins, bacterial and fungal aflatoxins 465 botutinum 464 465 from staphylococcus 464 in peanuts and foods 465 mycotoxins 465 patulin 467 470 vomitoxin 467 468
Trace elements 209 217
588 Index terms Links
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Trace metals 62 461 cadmium 463 lead and tin 462 mercury 461
Trace minerals 219 absorption Fe 219 abundance of 219 enzyme reactions 220 221 fruit juices, in 225 vegetables, in 224
Transglucosidation 189
Transglycosylation 183
trans-isomers 34 73
Transition metals 210
Triacylglycerols 34
Tricholomic acid 281 442
Trichromatic system 230
Triclinic crystal structure 88
Tricyclic compounds 284
Trimethylamine 299
Tripalmitin 91
Triple chain length 89
Triple helix 147
Triplet oxygen 63
Trisaturates 78
Tristimulus values 232
Tropocollagen 147
Tropomyosin 144
Trypsin digest 141
Tuna 462
U UHT sterilization 132
Ultracentrifuge 113
589 Index terms Links
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Ultrafiltration 143
Ultraviolet light 361
Umami 278 299 442
Undecalactone 284
Unfreezable water 12
Uniaxial stress 316
Unit cell 176
Universal testing machine 330
Unsaponifiables 51 53
Uric acid 422
U.S. food laws 479
V Value, color 236
VanderWaals forces 115 137 328 338 346
Vanilla 306
Vegetable flavor 301
Vegetable oils 34
Vegetables, texture 339
Vertical directives, food laws 483
Vibrational theory of olfaction 288
Vinyl chloride monomer 448
Violaxanthin 247
Viscoamylograph 340
Viscoeleastic body 321
Viscometers 328 Brookfield 329 capillary 328 falling ball 328 rotational 328
Viscosity 313 318 apparent 318 coefficient 318
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Viscosity (Continued) dynamic 318 kinematic 318 of some foods 319
Viscous isotropic phase 104
Visible fat 33
Vitamer 355
Vitamin A (retinol) 245 355 fortification 360 IU 358 359 provitamin 358 359 sources 355 357 stability 356 360 361 structure 357 358
Vitamin B12 (cyanocobalamine) 220 380 coenzyme for 381 sources 355 381 stability 356 382 structure 380 381
Vitamin B6 (pyridoxine) 377 forms 377 sources 355 377 378 stability 356 379 structures 377 378
Vitamin B2 (riboflavin) 374 fluorescence 374 376 sources 355 377 stability 356 375 structure 376
Vitamin B1 (thiamin) 372 loss of 374 SO2 destruction 374 sources 355 373 374 stability 356 373 376 structures 373
591 Index terms Links
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Vitamin C (L-ascorbic acid) 366 as antioxidant 372 destruction by enzymes 368 370 destruction by light 371 losses in dairy products 372 protective compounds of 370 sources 366 stability 356 370 structure 367 369 technical uses 372
Vitamin D (D1, D2, D3) 51 361 fortification 361 362 IU 361 sources 361 362 stability 356 361 362 structures 361
Vitamin E (tocopherols) 362 activity 365 as antioxidants 364 365 sources 355 364 stability 356 368 structures 362 363 tocotrienols 363
Vitamin fortification of flour 374
Vitamin, individual A 355 B 372
B12, cyanocobalamine 380 biotin 385 B6, pyidoxine 377 B2, riboflavin 374 B1, thiamin 372 folic acid 382 niacin 378 pantothenic acid 383
C 366
592 Index terms Links
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Vitamin, individual (Continued) D 361 E 362 K 365
Vitamin K (menadione) 365 sources 366 369 stability 356 365 structure 365 368
Vitamin loss in flour after milling 375
Vitamins 355
Vitamins, classification fat soluble 355 water soluble 366
Vitamins, food ingredients 385 antioxidants 386
Vitamin stability, general 356
Vitellenin 152
Vitellin 152
Vodka 303
Voigt Kelvin body 322
Volatile compounds coffee 302 spices 205
Vomitoxin 467
Vulgaxanthins 259
W Water 1
activity (aw) 4 21 22 30 347 437
bound loosely 11 bound tightly 12 28 bound total 12 cages 4 caloric properties 1 13
593 Index terms Links
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Water (Continued) coefficient of thermal expansion 2 content 1 crystallization 15 17 density 1 2 dielectric constant 2 heat of vaporation 2 hydration of 13 38 molecular structure 2 3 phase diagram 15 physical properties 2 refractive index 2 specific heat 2 structure 3 surface tension 2 thermal conductivity 2 types 11 unfreezeable 12 vapor pressure 2 viscosity 2 volume charge 18
Water activity (aw) 4 20 21 22 30
bacterial growth 21 color change 25 enzyme activity 1 food processing 30 food spoilage 23 glass transition temperature 12 20 hygroscopic product 26 milk powder 25 mold and yeast growth 21 packaging 26 pH 30 reaction rates in foods 22
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Water activity and texture crispness of popcorn 349 freeze dried beef 349 sorption isotherm 347 348 temperature effect 350
Water cages 4
Waxy corn 340
Waxy corn starch 404
Waxy mouthfeel 84
Wetting 344
Wheat, baking quality 337
Wheat bran 193 205 216
Wheat flour, pentosan 192
Wheat flour sorption isotherm 10
Wheat, minerals in 216 221
Wheat proteins 111 139 152 albumin 153 gliadin 153 154 globulin 153 glutenin 152 154 155 prolamin 153
Wheat starch 183
Whey protein concentrate 143
Whey protein hydrolysates 415
Whey protein isolate 138
Whipping 134
Whiskey 303
Windscale 451
Wine aroma 291 293 294
Winterization, oils 94
Work softening 339
World Health Organization (WHO) 484
WTO 486
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X Xanthine 422
Xanthine dehydrogenase 221
Xanthine oxidase 139 221
Xanthophylls 244
X-ray diffraction 88 147 176 178 184 191 311
Xylans 192
Xylitol 168 182
Xyloglucan 195
Yeast fermentation 404
Yeast growth, aw 22
Yield value 325
Young’s modulus 321
Zearalenone 467
Zeaxanthin 246 248
Zein 114 154
Zero tolerance 478
Zinc 219 220
Zingerone 276
Zymogen 408 411
