Moisture and Water Activity - Routledge Handbooks

This article was downloaded by: 10.3.98.104On: 30 Dec 2021Access details: subscription numberPublisher: CRC PressInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place, London SW1P 1WG, UK

Handbook of Processed Meats and Poultry Analysis

Leo M.L. Nollet, Fidel Toldrá

Moisture and Water Activity

Publication detailshttps://www.routledgehandbooks.com/doi/10.1201/9781420045338.ch3

Young W. ParkPublished online on: 12 Nov 2008

How to cite :- Young W. Park. 12 Nov 2008, Moisture and Water Activity from: Handbook ofProcessed Meats and Poultry Analysis CRC PressAccessed on: 30 Dec 2021https://www.routledgehandbooks.com/doi/10.1201/9781420045338.ch3

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35

Chapter 3

Moisture and Water Activity

Young W. Park

Contents3.1 Introduction ..................................................................................................................... 363.2 Properties of Water in a Food System ............................................................................... 373.3 Water in Meat and Other Foods ...................................................................................... 383.4 Water Activity .................................................................................................................. 383.5 Mechanism of Drying Related to Moisture Determination ...............................................413.6 Sampling Methods for Moisture Determination .............................................................. 423.7 Methodologies of Moisture Determination ...................................................................... 43

3.7.1 Direct Methods .....................................................................................................453.7.1.1 Air-Oven Drying ....................................................................................453.7.1.2 Vacuum-Oven Drying ............................................................................513.7.1.3 Freeze-Drying .........................................................................................533.7.1.4 Distillation Methods ...............................................................................533.7.1.5 Karl Fischer Titration Method ............................................................... 563.7.1.6 Chemical Desiccation ............................................................................ 583.7.1.7 Th ermogravimetric Method ................................................................... 583.7.1.8 Gas Chromatography ............................................................................. 583.7.1.9 Application of Direct Methods in Moisture Determination ....................59

3.7.2 Indirect Methods ...................................................................................................593.7.2.1 Refractometry .........................................................................................593.7.2.2 Infrared Absorption Spectroscopy ...........................................................613.7.2.3 Near Infrared–Refl ectance Spectroscopy ................................................613.7.2.4 Microwave Absorption Method ............................................................. 633.7.2.5 Dielectric Capacitance ........................................................................... 633.7.2.6 Conductivity Method ............................................................................ 63

CRC_45318_Ch003.indd 35CRC_45318_Ch003.indd 35 9/8/2008 2:46:02 PM9/8/2008 2:46:02 PM

© 2009 by Taylor & Francis Group, LLC

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36 � Handbook of Processed Meats and Poultry Analysis

3.7.2.7 Sonic and Ultrasonic Absorption ........................................................... 643.7.2.8 Cryoscopic Methods .............................................................................. 643.7.2.9 Other Indirect Instrumental Methods ................................................... 643.7.2.10 Summary of Indirect Methods ................................................................65

References ..................................................................................................................................65

3.1 IntroductionMoisture content is one of the most important and widely used indices in processing and testing foods [1]. Th e terms “water content” and “moisture content” have been used interchangeably in lit-erature to designate the amount of water present in foodstuff s and other substances [2,3]. Because dry matter content in food is inversely related to its moisture content, moisture content has great economic importance to the food processor and consumer. In meat, water or moisture is quantita-tively the most important component of the product, constituting up to 75% by weight.

Th e amount of moisture is a measure of yield and quantity of food solids, and can be a direct index of economic value, stability, and quality of food products [1,2]. Th e abundance and chemi-cal reactivity of moisture, and the determination of its quantity, are of great concern to many industries such as food, paper, and plastics, where acceptable levels of moisture vary between materials and in some cases, very small quantities of moisture can adversely aff ect the quality of product [4].

Th e amount of water in food is also directly related to its water-holding capacity (WHC) as well as water-binding capacity. WHC is an important quality parameter for the economic value of meat. It is the ability of meat to retain the tissue water present in its structure [5]. Since meat is sold by weight, drip is unsightly to the consumer, and excessive drip is a negative determinant of meat quality. WHC is also important with respect to the manufacturing properties of the meat together with water-binding capacity, which is the ability of meat to bind added water [5]. Th e desirability of meat with low water-holding and water-binding capacity is dependent on the purposes of both retail consumption and manufacturing.

Although the determination of moisture content in foods is highly important, the accurate analysis of moisture is frequently one of the most diffi cult tasks encountered by the food chem-ist. Th is is largely attributable to the diffi culty of complete separation of all the water from a food sample without causing simultaneous decomposition of the product [2]. Th e production of water by decomposition and loss in weight would aff ect the accuracy of the determination [2,6]. Th e loss of volatile constituents from the food is another diffi culty involved in moisture determination. Th e complexity of moisture assay will be dependent on the conditions of the food and the nature of other substances present [6,7].

Accurate, rapid, and simple methods of moisture assay applicable to all types of foods are continuously sought, although it may be doubtful that such a goal will ever be achieved [5]. However, an ideal method for moisture assay has been suggested [8]. Th e requirements are (a) to be rapid, (b) to be applicable to the broadest range of materials, (c) to be performable preferably even by nontechnical persons with brief training, (d) to use a readily available apparatus of low initial investment and low cost per test, (e) to have reasonable accuracy and good precision, and (f) to present no operational hazards. Analytical methods of moisture determination are usually selected for either rapidity or accuracy, even if both goals are simultaneously sought, especially in industry applications.



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Moisture and Water Activity � 37

3.2 Properties of Water in a Food SystemWater is a ubiquitous substance in nature and is unusually reactive due to its high polarity [9]. Physically, water can be present in three diff erent forms as gas, liquid, and solid state. It exists in the gaseous state as monomolecular water vapor, in liquid state largely as dihydrol, in which two mole-cules of water are bound by hydrogen bond forces, and in several solid forms as ice varying in degree of association [10]. However, for moisture analysis, it is generally more important to recognize the diff erent types of interactions of water within a food rather than the physical state of water [11].

Historically, water in a foodstuff exists in two forms, known as “free” and “bound” [6]. How-ever, water can be classifi ed in at least three forms [2,11–14]. Th e fi rst form of water in foods exists as free water in the intergranular spaces and within the pores of the material. Such water serves as a dispersing medium for hydrophilic macromolecules such as proteins, gums, and phenolics to form molecular or colloidal solutions, and as a solvent for the crystalline compounds. Th e second form of water is adsorbed as a very thin, mono- or polymolecular layer on the internal or external sur-faces of the solid components (i.e., starches, pectins, cellulose, and proteins) by molecular forces or capillary condensation. Th is water is closely associated with absorbing macromolecules by van der Waals forces or hydrogen bond formation. Th e third form of water is in chemical combination as water of hydration, so-called “bound water.” Carbohydrates such as dextrose, maltose, and lactose form stable monohydrates, and salts such as potassium tartrate also form hydrates. Water of hydra-tion can be clearly observed from gels of proteins or polysaccharides in which the bound water is fi rmly held by hydrogen bonds [2,13–14].

Because of a variety of defi nitions, the concept of bound water is quite controversial, and ter-minating its use as a term has been suggested [3,11,15–17]. One alternative classifi cation scheme involves three broad types of water—free, adsorbed, and chemically bonded [1,14,18]. Th is is why the state of water in colloidal systems and the nature of bound water are still not clear. It has been suggested that water found in biological material may exist as (a) occluded water, (b) capillary water, (c) osmotic water, (d) colloidal water bound by physical forces, and (e) chemically bound water [19].

Bound water has been defi ned by the majority of researchers as the form of water that remains unchanged when the food is subjected to a particular heat treatment [2]. A certain proportion of the total water present in the biocolloids, for example, is not separated readily by freezing (even at −230°C) or drying [2]. At −125°C, all the free water is usually frozen, and the remaining bound water is not frozen at considerably lower temperature [14,19]. Such bound-water concentration varies from one food to another. Th e ultimate accuracy of an analytical method for moisture determination is related to the bound water and not the free water. Part of the water in a sample during drying is retained for longer times at higher temperatures than the remainder. Th e range of bound water in foods is less than 0.5 to over 30% of the total water present, corresponding to 0.1–2.2 g/g total solids [14,19]. All water may be considered as bound water to a variable extent, except for surface water. Monolayer values are most commonly calculated using the Brunauer–Emmett–Teller (BET) or Guggenheim–Anderson–de Boer (GAB) equations for modeling moisture sorption isotherm data [20]. Th e most tightly bound water is the BET monolayer water [6]. For most foods, the BET monolayer values range from a few percent to approximately 12% (wet basis) of the food or food component [21].

Small changes in water content can exert a large infl uence on storage stability of low-moisture foods. Irreversible changes in texture of foods also occur during freezing and freezing storage. Th us, removal of free water rather than bound water from dried foods has been known to improve storage stability [22]. Likewise, decrease in free-water content of foods to be preserved by freezing, concentration, partial dehydration, or addition of sugar is believed to improve the storage stability of frozen foods and food products [14].



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Researchers have shown that water displays abnormally high values of certain physical con-stants, including specifi c heat, specifi c gravity, heat of infusion, heat of evaporation, surface ten-sion, and viscosity. Th ese special constants may be derived from its remarkable and variable solvent power, high dielectric constant, dissolving and ionization ability, and its own molecular aggrega-tion tendency [23]. Th e characteristics of water in chemical reactivity, volatility, solvent power, electrical properties (high dielectric constant, conductivity, and magnetic resonance absorption), thermal conductivity, and light scattering and absorption have been used in the determination of moisture content in foods and other materials [2,13,23].

3.3 Water in Meat and Other FoodsWater in meat is associated with muscle tissue, and proteins have a central role in the mechanism of water binding. Th e water content of meat is inversely related to fat content, whereas it is unaf-fected by protein content, except in young animals [5]. Muscle proteins impart a gel structure to the tissue in the living animal, and very little loss of water occurs from tissue cut immediately after slaughter. Th is is attributed to the water molecule behaving as a dipole and binding strongly to surfaces by a number of noncovalent forces [5].

Previously, up to 60% of water was thought to be bound by the myofi brils, but this fi gure was an overestimate, and about 10% would be more realistic. Approximately 85% of water is bound between the thick and thin myofi brils [5]. Because this binding is looser than in the living ani-mal, some loss as drip from freshly cut surfaces is inevitable. Th e amount of drip loss is mainly a function of postmortem changes, which aff ect the pH of the meat and the changes in myofi brillar volume [5].

Drastic changes in WHC and tenderness in meat occur during heating of the meat prod-ucts, including shrinkage and hardening of tissue and the release of juice, which are caused by changes in the meat proteins. Th is considerable decrease of WHC during heating is attributable to a tightening of the myofi brillar network by heat denaturation of the proteins [24]. Changes in WHC during heating are closely related to alterations in the tenderness and rigidity of tissue, and most decrease in WHC occurs at temperature between 30 and 50°C. Th e sarcomere length of the muscle fi bers of raw meat is closely connected with the tenderness of cooked meat [24].

Th ere are wide variations in moisture content of natural and processed foods of animal and plant origins (Table 3.1) [17]. First, with respect to the moisture content of meat and fi sh products, their moisture content depends primarily on the fat content and varies to a lesser degree with the age, source, and growth season of the animal. Th e range of moisture content in meat and fi sh is from 50 to 70%, whereas some organs may contain up to 80% water. Sausages have wide varia-tions of water content. Poultry meats contain from 50% in geese to 75% in chicken, whereas fresh and dried eggs have approximately 74 and 5% water, respectively [1].

3.4 Water ActivityAlthough it is not perfect, a relationship exists between the water content of food and its perish-ability. Dehydration is performed for the purpose of reducing the water content of a food, which in turn increases the concentration of solutes and decreases the perishability of the food. However, various types of food with the same level of water content exhibit signifi cant diff erences in stability and perishability [17].



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Water content alone is not a reliable indicator of perishability of food products, which is par-tially due to the diff erences in intensity of association of water with nonaqueous constituents. Th e term “water activity” (aw) was developed to indicate the intensity with which water associates with various nonaqueous constituents [17]. Water engaged in strong association has less ability to support degradative activities than that in weaker association. Th e degradative activities of water include the growth of microorganisms and hydrolytic chemical reactions. aw can predict food sta-bility, safety, and other properties more reliably than water content can. Although aw is not perfect, it correlates suffi ciently well with rates of microbial growth and many degradative reactions as to be a useful indicator of product stability and microbial safety [17].

aw can be defi ned as aw = f/fo = p/po, where f is the fugacity of the solvent (fugacity means the escaping tendency of a solvent from solution), and fo is the fugacity of the pure solvent. p/po is the term measured, and sometimes does not equal aw [17]. Water activity is a property of solutions, and is the ratio of vapor pressure of the solution to the vapor pressure of pure water at the same temperature [25]. Water activity is related to relative humidity, and under equilibrium conditions, water activity equals RH/100. Quantitatively, aw is a measure of unbound, free water in a system available to support biological and chemical reactions. aw, not absolute water content, is what bacteria, enzymes, and chemical reactants encounter, aff ecting food materials at the microenvi-ronmental level.

Th e eff ect of aw on microorganism growth is very important in intermediate-moisture foods. At the usual temperatures permitting microbial growth, most bacteria require a water activity in the range of 0.9–1.00. Th e minimum aw below which most important food bacteria will not grow is about 0.90, depending on the specifi c bacteria [25]. Some halophilic bacteria may grow at an aw of 0.75, and certain osmophilic yeasts can grow even at lower aw, but these microorganisms seldom cause food spoilage. Compared to most bacteria, molds are more resistant to dryness. Molds can grow well on foods having an aw of about 0.80, and can show slow growth at room temperature

Table 3.1 Water Contents of Various Foods

Food Water Content (%)

Meat

Pork, raw, composite of lean cuts 53–60 Beef, raw, retail cuts 50–70 Chicken, all classes, raw meat without skin 74 Fish, muscle proteins 65–81

Fruit

Berries, cherries, pears 80–85 Apples, peaches, oranges, grapefruit 90–90 Rhubarb, strawberries, tomatoes 90–95

Vegetables

Avocado, bananas, peas (green) 74–80 Beets, broccoli, carrots, potatoes 85–90 Asparagus, beans (green), cabbage, caulifl ower, lettuce

90–95

Source: Fennema, O.R. in Food Chemistry, Marcel Dekker, New York, 1996, 17–94.



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for several months on some foods with an aw as low as 0.70. Mold growth is completely inhibited at aw below 0.65. However, such low aw generally is not applicable in the fabrication of intermedi-ate-moisture foods, many of which have below 20% moisture [25].

Th e humidity at which the product neither loses nor gains moisture is the equilibrium relative humidity (ERH) [2,25]. Diagrammatic plots of such data yield water sorption isotherms as shown in Figure 3.1 [17,26]. Th e ERH diff er between foods. Below the atmospheric humidity level, food can be dried further, whereas above this humidity, it may pick up moisture from the atmosphere [2]. Th e ERH at diff erent temperature can be measured by exposing the dried food sample to diff erent levels of humidity in bell jars and weighing the sample after several hours of exposure [25]. Figure 3.1 illustrates that the product comes into equilibrium at 4% moisture at 100°C and 40% RH. Similarly, if the food product further dries to 2% moisture, the equilibrium is attained at 15% RH and 100°C [2,25]. Similar water sorption isotherms have been established for a wide variety of food products.

Figure 3.1 Moisture desorption isotherms for potatoes at various temperatures. (Görling, P., in Fundamental Aspects of the Dehydration of Foodstuffs, Society of Chemical Industry, London, 1958.)

0°20°

60°

100°

40°

80°

0.5

0.4

0.3

0.2

0.1

Moi

stur

e co

nten

t (g/

g D

.M.)

0 0.2 0.4 0.6 0.8 1.0p/po



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3.5 Mechanism of Drying Related to Moisture DeterminationRemoval of moisture from a meat sample is a critical aspect of the moisture determination. Th e rate of drying aff ects the quantifi cation of moisture in the food. It is desirable to understand the mecha-nism of drying or moisture removal in the moisture determination of a sample. During the drying process, moisture loss from the sample occurs in two distinct stages or periods, which is detailed in the following.

In the initial stage, the drying rate remains constant, and equals that of evaporation from a free liquid surface. Th is stage of drying is controlled by surface evaporation, which is known as the constant-rate drying period [11,25,27,28]. Th is phase of constant rate continues as long as water reaches the surface of the material as fast as evaporation takes place [2,11,25,27]. After this stage, there is a sudden drop in the drying rate at the end of the constant-rate period, where the drying rate decreases dramatically, due to moisture diff usion being reduced by physical or chemical interactions within the food [11,25]. Th is sudden fall of drying rate is caused by the physicochemically bound water. Th is infl ection point is frequently referred to as the critical moisture content [2,11,25].

Th e second stage of drying period, known as the falling-rate drying period, begins at the infl ection point and extends to the fi nal moisture content [25,27,28]. An example of a drying curve for carrots under unspecifi ed drying conditions is shown in Figure 3.2 [25]. As shown, zero percent water is usually never reached under the typical operating conditions required to yield a high-quality dehydrated food product [25]. During moisture determination, however, quality is not the ultimate goal, and harsher operating conditions can be employed [11].

More extreme time–temperature combinations may be used for the analysis of moisture. However, care must be taken to avoid using higher temperatures causing product decomposition. Although decomposition of sugars can be most commonly illustrated [1,11,14,18], other products can also decompose when exposed to temperatures that are too high. For example, Figure 3.3 indicates decomposition of fl our or other sugar-containing food products at temperatures above

Figure 3.2 The phases of moisture removal in a food-drying process. (Charm, S.E, The Funda-mentals of Food Engineering, AVI Publishing, Westport, CT, 1971.)

100

50

0Drying time

Falling-rate drying period

Critical moisture content

Constant-rate drying period



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180°C, as depicted by the discontinuity in the straight line [29]. Instead of using such high tem-peratures, the rate of moisture removal can be increased by drying under reduced pressure [29]. Researchers have shown that vacuum drying is particularly useful for foods that decompose at relatively low temperatures.

3.6 Sampling Methods for Moisture DeterminationTh e accuracy of moisture determination of a meat sample is largely dependent on the method of selection and the handling of representative samples from larger batches [1,6,18,29]. Handling a sample during moisture determination is important because moisture is easily gained from or lost to the atmosphere due to water-activity gradients between the food and atmosphere [11]. Th erefore, exposure to the atmosphere should be minimized for the bulk food, or samples taken from a bulk food, to avoid moisture exchange [1,11,18]. Once the samples are taken, they should be quickly placed in dry rigid plastic or glass containers with tight closures and clear labels, followed by storage at an appropriate temperature before chemical analysis [11]. When refrigerated samples are tested, the sample container should be allowed to warm up to room temperature before opening to prevent moisture from condensing on the cold food. In addition to avoiding sample exposure to atmo-sphere, it is also essential that a representative sample of the food must be prepared before moisture analysis. Unless the samples are representative of the sample population, no method of moisture determination is of any value. A single absolutely representative sample is diffi cult to obtain for laboratory analysis, since foodstuff s and food ingredients are relatively heterogeneous materials. Th us, sampling errors can sometimes be greater than the experimental error of analysis [6,7].

Figure 3.3 Effect of temperature on the moisture content. Determination of fl our. (Park, Y.W. and L.N. Bell, Handbook of Food Analysis, Marcel Dekker, New York, 2002.)

Theoricalextrapolatedmoisture content

17

16

15

14

13

12

11

10

100 150 200 250

Drying temperature/°C

Moi

stur

e lo

st b

y dr

ying

(%

)



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Finding a general sampling method applicable to all food types is diffi cult, but random sam-pling is the most recommended fundamental concept [6,7]. Random sampling is most appropri-ate for relatively homogenous food samples, whereas stratifi ed random sampling is employed for heterogeneous food samples [7]. For the stratifi ed random sampling, the sample population is subdivided into small groups that may be treated as homogenous [6,29]. Food samples should be as homogenous as possible to have precise analytical results for moisture content. Th e homogeni-zation depends on the type of food sample. Reduction in the size of food particles and thorough mixing of samples can be effi ciently accomplished using a number of mechanical devices. Blend-ers, mincers, graters, homogenizers, powder mills, and grinders are essential pieces of equipment for the homogenization of dry, moist, and wet samples [11].

Depending on the moisture determination technique, ground samples may be passed through a sieve of suitable mesh size (18–40 mesh) to obtain a uniform particle size distribution [1]. Varia-tions in particle size can infl uence moisture values if too small a sample is analyzed [7]. Th e aliquot size of a powdered bulk sample can be reduced using the process known as quartering [7,29]. In quartering, the bulk food is formed into a uniform pile on a large sheet of glazed paper, glass, or the surface of a clean, laminated bench top. Th e pile is divided into four equal parts by separating quarter segments. Two quarters are rejected, and the other two quarters are thoroughly mixed. Th e process is repeated until a suitable sample size is obtained. Th e quartering method is depicted in Figure 3.4.

Meat, fi sh products, and some vegetables having high moisture content are best homogenized using a modern domestic food processor or blender followed by mixing. Th e minimum weight of the sample should be carefully considered, depending on the food type. Th e Offi cial Methods of Analysis of AOAC International describes minimum weight considerations for specifi c food prod-ucts [30]. Food-product conditions to be considered in sampling for moisture measurement are summarized in Table 3.1.

3.7 Methodologies of Moisture DeterminationMany reviews on methodologies of moisture determination have been published [1,3,14,18,29]. Pande published one of the more extensive reviews, Handbook of Moisture Determination and Control, in four volumes [7]. Detailed methodologies for specifi c food products can be found in the Offi cial Methods of Analysis of AOAC International [30].

Generally, analytical methods of moisture determination can be classifi ed in two ways, as shown in Table 3.2. One way is by the four major analytical principles—drying, distillation, chemical, and physical methods [1]. Th e other is by direct and indirect procedures based on

Figure 3.4 Quartering method of sample size reduction for homogeneous sampling. (Park, Y.W. and L.N. Bell, Handbook of Food Analysis, Marcel Dekker, New York, 2002.)

Accept

Accept

Reject

Reject

Accept

Accept

Reject

Reject

Mix acceptedquadrants and

re divide

Mix acceptedquadrants and

re divide

if needed



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the underlying scientifi c theory shown in Table 3.3 [6]. In direct methods, moisture analysis normally involves removing water from the solid-food samples by drying, distillation, extrac-tion, or another method, and its quantity is measured by weighing, titration, and so forth [6]. However, for the indirect methods, moisture is not removed from the sample and quantifi ed directly; instead, the properties of the food that depend on either the amount of water or number of hydrogen atoms are measured [6]. Th is indicates that indirect methods must be calibrated against standard moisture values that have been precisely determined using one or more of the direct methods. Th erefore, the accuracy of indirect methods is dependent on the analytical values of direct measurements against which they are calibrated.

It is known that direct methods usually give accurate and even absolute values for moisture determination, although they are mostly tedious, manual, and time-consuming [6]. However, indirect methods are rapid, nondestructive, and off er the possibility of automation for continuous determination [6]. For the purpose of comparison, methodologies of moisture determination are classifi ed into two categories, namely direct and indirect methods. In addition, Tables 3.4 and 3.5 show the advantages and disadvantages of each individual method under the two classifi cations.

Table 3.2 Factors Affecting Sample Preparation for Moisture Determination

1 Characteristics of food sampleSolid versus liquid Homogeneous versus heterogeneous Type of water interactions within food

2 Particle size and shape of sample3 Sample preparation

Homogenization, blending, mixing, grating, milling, sieve size, heat from mechanical sampling device

4 Representative samplingRandom sampling Stratifi ed random sampling Quartering

5 Sample contamination Chemical Microbial (yeast, molds, bacteria) Atmospheric (moisture, dust)

6 Aging of sample Oxidation Decomposition Environmental relative humidity State of hysteresis Adsorption Desorption Equilibrium moisture content

7 Sample storage Storage period Storage temperature Time to be analyzed

Source: Park, Y.W. in Handbook of Food Analysis, Marcel Dekker, New York, 1996, 59–92; Park, Y.W. and L.N. Bell in Handbook of Food Analysis, Marcel Dekker, New York, 2002, 55–82.



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3.7.1 Direct Methods

3.7.1.1 Air-Oven Drying

Air-oven drying is very convenient, and is one of the most widely and commonly used meth-ods for routine moisture determination in laboratories around the world [1,7]. Drying can be accomplished using either convection-type ovens or forced-draft ovens [1]. Th e ovens should be thermally regulated to ±0.5°C and have minimal temperature variations (less than ±3°C) within the oven [1]. Forced-draft ovens off er a more consistent temperature throughout the oven than convection ovens [18]. Modern drying ovens are usually heated by electricity or infrared (IR) heaters and can be equipped with built-in balances for routine and fast analysis,

Table 3.3 Classifi cation of Analytical Methods for Moisture Determination

Classifi cation by Four Major Principles

Classifi cation by Direct/Indirect Procedures

Drying methods Direct methods Oven drying Gravimetric methods Vacuum drying Oven drying Freeze drying (lyophilization) Air oven Chemical desiccation Vacuum oven Thermogravimetric analysis Freeze dryingDistillation methods Thermogravimetric analysis Direct distillation Chemical desiccation Refl ux distillation Distillation methodsChemical methods Direct distillation Karl Fischer titration Refl ux distillation Generation of acetylene Chemical titration methodPhysical methods Karl Fischer IR absorption Extraction method NIR refl ectance GC GC Indirect methods NMR Spectroscopic methods Refractometry IR absorption Neutron scattering NIR refl ectance Electrical NMR Microwave absorption Mass spectrometry Dielectric capacitance Electrical methods Conductivity Microwave absorption Cryoscopic methods Conductivity

Dielectric capacitanceSonic and ultrasonic

methods Neutron scattering Refractometry Cryoscopic methods




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Table 3.4 Advantages and Disadvantages of Direct Methods for Moisture Determination

Method Advantages Disadvantages

Oven drying Standard conventional method Variations of temperature due to particle size, sample weight, position in the oven, etc.

Convenient Diffi cult to remove all waterRelative speed and precision Loss of volatile substances during

dryingAccommodates large number of samples

Decomposition of sample (i.e., sugar)

Attain the desired temperature more rapidly

Vacuum-oven drying

Lower heating temperatures possible

Possible volatile loss

Prevents sample decomposition Lower number of samples than drying oven

Uniform heating and constant evaporation

Drying effi ciency reduced for high-moisture foods

Freeze-drying Excellent for sensitive, high-value liquid foods

Expensive

Preserves texture and appearance Long drying timeNo foaming Sample must be initially frozenNo case-hardening Most applicable to high moisture

foodsNo oxidationNo bacterial changes during drying

Distillation methods

Determines water directly rather than weight loss

Low precision of measuring device

Apparatus is simple to handle Organic solvents such as toluene pose a fi re hazard

Accuracy may be greater than oven-drying method

Organic solvents may be toxic

Takes relatively short time (30 min to 1 h) to determine

Can have higher results due to distillation of water-soluble components (e.g., glycerol and alcohol)

Prevents oxidation of sample Water droplets may adhere to internal surface of the apparatus, causing erroneous results

Not affected by environmental humidity

Emulsions may form

Suitable for samples containing volatile substances

Karl Fischer method

A standard method for moisture analysis

Chemicals of the highest purity must be used for preparing the reagent

The accuracy and precision are higher than with other methods

Titration endpoint may be diffi cult to determine visually



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assuming the food is stable [6]. Because the principle of oven drying is based on weight loss, the sample needs to be thermally stable and should not contain signifi cant amount of volatile compounds [6].

Operational procedures for the conventional method of moisture determination using a dry-ing oven and analytical balance generally involve the following steps: sample preparation, weigh-ing, drying, cooling, and reweighing. Th e general principles of the procedures are described as follows. Th ese procedures have been basically adopted from offi cially accepted AOAC procedures [30] as well as other similar references [2,11].

3.7.1.1.1 Required Apparatus

1. Weighing dishes—nickel, stainless steel, aluminum, or porcelain. Metal dishes should not be used when the sample may be corrosive.

2. Analytical balance with 0.1 mg sensitivity.

Table 3.4 (Continued)


Useful for determining water in fats and oils by preventing oxidation

The reagent is unstable and needs standardization before use

Once the apparatus is set up, determination takes a few minutes

Titration apparatus must be protected from atmospheric moisture due to extreme sensitivity of reagent to moisture

Automated equipment available Ascorbic acid and other carbonyls can react with reagents, causing over-estimation of the moisture content

Chemical desiccation

Can serve as a reference standard for other methods

Requires a long time to achieve constant dry weight

Can be done at room temperature Moisture equilibrium depends on strength of desiccantGood for measuring moisture in

substances containing volatile compounds

Thermogravimetric analysis

More automated method than standard oven drying

Excellent for research, but not practical

Weighing error is minimal because sample is not removed from oven

Small sample may not be representative

Sample size is small Sample may decompose or oxidize

GC Analysis is rapid (takes 5–10 min per sample)

Unit cost per sample may be higher than drying oven

Results similar to conventional methods

Sample extraction requiredRequires expensive equipment




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Table 3.5 Advantages and Disadvantages of Indirect Methods for Moisture Determination


Refractometry Determination takes only 5–10 min (rapid)

Temperature sensitive

Does not require complex or expensive instrumentation

Requires uniformity of fl uid samples

Simple method Solid samples (e.g., meat) require homogenization in an anhydrous solvent

Reasonable accuracyExcellent method for high-sugar products

IR Absorption Can perform multicomponent analysis

Accuracy depends on calibration against reference standard

Most versatile and selective Temperature-dependentNondestructive analysis Dependent on homogenization

effi ciency of sampleAbsorption band of water is not specifi c

NIR refl ectance spectroscopy

Rapid Refl ectance data are affected by sample particle size, shape, packing density, and homogeneity

Precise Interference between chemical groups (e.g., hydroxyl and amine)

Nondestructive Temperature-dependentNo extraction required Accuracy depends on calibration of

standard samplesMinimum sample preparation Equipment is expensive

Microwave absorption

Nondestructive Possible leakage of microwave energy during measurement

No extraction required Has relatively low sensitivity and limited range for moisture determinations

More accurate than low-frequency resistance or capacitance meters

Depends on the fl uctuation of the material density in the volume measured

Results affected by factors such as particle size, temperature, soluble salt contents, polarization, and frequency of sample

Dielectric Capacitance

Has high sensitivity due to large dielectric constant of water

Affected by texture of sample, packing, electrolytes, temperature, and moisture distribution

Convenient to industrial operations with the continuous measurement system

Potential calibration diffi culty beyond pH 2.7–6.7

System can be modifi ed to have universal applicability

Diffi cult to measure bound water at high frequencies

Conductivity Measurement is instantaneous Measures only free waterNondestructive Conversion charts are needed to

obtain total moisture values



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3. Desiccator—containing an effi cient desiccant such as phosphorus pentoxide, calcium sulfate, or calcium chloride.

4. Atmospheric oven. 5. Blender—Oster, Waring, or equivalent for high-moisture samples. 6. Grinder and mill for low-moisture samples. 7. Spatula or plastic spoon. 8. Steam bath—it is used for predrying high-moisture samples such as dairy products. 9. Crucible tongs. 10. Th ermometer (0–130°C).

Table 3.5 (Continued)


Precise Accuracy and precision are affected by temperature, electrolyte content, and contact between electrode and samples

Diffi cult to maintain calibration of the equipment

Sonic and ultrasonic absorption

Bound water can be determined in aqueous solution of electrolytes and nonelectrolytes

Dependent on the type of medium for sound passes

Nondestructive Appropriate standards required to get total moisture content

Mass spectroscopy Can analyze simultaneously a large number of components from a complex matrix

High variation between theoretical moisture values and hydrated substances

No electrical leakage problem due to low potentials applied to the beam tube

Major instrumental problem is memory effect from the preceding sample

NMR Spectroscopy Very rapid analysis Cost of equipment is highAccurate Separate calibration curves are

required for different substancesNondestructive Constant and correct sample weight

requiredApplicable to many types of foods

Not applicable for foods having variable lipid contents

Can differentiate between free and bound water

Particle size and packing of granular samples have no effect on signal absorption

Neutron scattering method

Density and moisture measured simultaneously

Applicable only to substances that are relatively proton-free

The absolute error is claimed to be less than ±0.5%

Expensive

Suitable for soil moisture assay




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3.7.1.1.2 Procedure for Oven Drying

1. Wash the empty dishes thoroughly, rinse, and dry in an oven for several hours at 100°C. Store in a clean desiccator at room temperature before use.

2. Mix the prepared sample thoroughly, and quickly weigh a 2–5-g sample into a preweighed dish using an analytical balance to the nearest 0.1 mg; the sample should be spread evenly across the bottom of the dish.

3. Place the dish without its cover on the metal shelf in the atmospheric oven, avoiding contact between the dish and the walls. Refer to Table 3.6 for steam-bath requirements, oven tem-peratures, and drying times for selected food products.

4. After a specifi ed time in the oven, use tongs to place the cover onto the dish, remove the dish from the oven, and place it into the desiccator for at least 30 min to cool to room temperature.

5. Weigh the dish on the analytical balance and calculate moisture loss.

3.7.1.1.3 Calculations of Moisture Content

Moisture (%)

(loss of weight 100)

(sample weight)�

�

Solids (%) 100 moisture (%)� �

Table 3.6 Atmospheric-Oven Temperatures and Time Settings for Oven Drying of Milk and Other Foods

ProductDry on

Steam BathOven Temperature

(°C ± 2)Time in

Oven (h)

Buttermilk (liquid) X 100 3Cheese (natural-type only) 105 16–18Chocolate and cocoa 100 3Cottage cheese 100 3Cream (liquid and frozen) X 100 3Egg albumin (liquid) X 130 0.75Egg albumin (dried) X 130 0.75Ice cream and frozen desserts X 100 3.5Milk (whole, low fat, and skim) X 100 3 Condensed skim 100 3 Evaporated milk 100 3Nuts (almonds, peanuts, walnuts, etc.) 130 3

Note: X indicates that samples must be partially dried on steam bath before placing in oven.

Source: AOAC, Offi cial Methods of Analysis of AOAC International, AOAC Interna-tional, Arlington, VA, 1995.



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It is important to carefully consider the length of time required in the oven when the drying method is used for moisture determination. Diff erent oven temperatures and drying times for many food products have been determined and can be found in the Offi cial Methods of Analysis of AOAC International [30]; some of these are summarized in Table 3.6. For products without an offi cial method, samples are periodically weighed during the drying process until the change in mass is negligible. Typically, two successive weighings an hour apart should show a mass change of less than 2 mg/5 g sample [1]. Because many of these fl uid products contain large amounts of sugar, the predrying temperatures of the steam bath should be kept below 70°C to prevent sample decomposition [14,18].

Th e accuracy of any particular drying procedure for the determination of moisture can be infl uenced by a number of factors. Erroneous results in moisture determination by oven drying may result from variations in sample weighings, oven conditions, drying conditions, and post-drying treatments. Sample weighing is infl uenced by adsorption of atmospheric vapor, length of weighing time, spillage, and balance accuracy. Oven conditions that infl uence the accuracy of moisture determination include temperature, air velocity, pressure, and relative humidity. Factors associated with the drying conditions are size and shape of the sample container, type and location of the heating element, drying time, scorching, loss of volatile compounds, and decomposition. Postdrying factors such as fi nal temperature at weighing, desiccator effi ciency, loss of dried sample, and balance buoyancy eff ect may also contribute to erroneous data. Advantages and disadvantages of air-oven drying methods, as well as those of other direct methods, are listed in Table 3.4.

3.7.1.2 Vacuum-Oven Drying

Since many drawbacks associated with air-oven drying can be overcome by vacuum-oven dry-ing [6,7], this method is generally considered as the standard and most accurate drying method for moisture analysis in foods. Vacuum drying usually can heat foods up to 98–102°C, with low pressure of 25–100 mm Hg [1,14]. Lower temperatures (60–70°C) are used for high-sugar food products to prevent decomposition [1,7,14,18]. Moisture can be evaporated more quickly at the reduced pressure, and drying times can be dramatically reduced [1]. It may be impossible to obtain an absolute moisture content of the sample by drying methods, but vacuum drying can yield a close and reproducible estimate of the true moisture content of a food [1].

Th ere are several types of vacuum ovens available. Laboratory type vacuum ovens can be con-nected to a vacuum line and electrically heated. Th ese vacuum ovens are typically equipped with airtight front doors using vacuum grease on a rubber gasket. Although a vacuum of 100–600 mm Hg can be maintained inside the sample chamber [7], it is usually desirable to have pressures below 50 mm Hg, because the reduced pressure will increase the rate of drying [1]. Dry air is introduced into the vacuum oven during drying; without purging dry air into the oven, the vapor pressure of water inside the oven would reduce the usefulness of the vacuum oven, especially for high-mois-ture foods [1,18]. According to the AOAC procedures, moisture contents are usually determined by heating in a vacuum oven at 100°C for 2–6 h at a pressure of 25–100 mm Hg [30]; Table 3.7 lists some drying conditions for selected food products by vacuum oven and other direct methods for moisture determination of foods as recommended by AOAC. Th e advantages and disadvan-tages of the vacuum drying method are also described in Table 3.4.

Th e required apparatus and general procedure for vacuum-drying method are delineated as follows [2,11]:



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3.7.1.2.1 Required Apparatus

1. Vacuum oven—thermostatically controlled and connected with a vacuum pump capable of maintaining the pressure in the oven below 25 mm Hg. Th e oven should have a dry air inlet that passes through an indicating desiccant and a trap for releasing the vacuum.

2. Dishes—metal dishes with close-fi tting lids and fl at bottoms to provide maximum area of contact with the heating plate.

3. Other apparatus and equipment are the same as those for air-oven drying.

3.7.1.2.2 Procedure for Vacuum Drying

1. Metal sample dishes must be washed and dried in a laboratory oven. After cooling in a desiccator, weigh the dish to 0.1 mg.

2. Weigh the sample (3.0–5.0 g) into the preweighed dish using an analytical balance. Distribute the sample evenly over the bottom of the dish. Some samples require predrying as described for air-oven drying to prevent decomposition and splattering.

3. Put the sample dishes in the vacuum oven, partially uncover the dish, evacuate the oven, and dry the sample at an appropriate temperature and vacuum pressure. During drying, admit a slow current of air dried by passage through the indicating desiccant into the oven.

4. Turn off the vacuum pump after 5 h and slowly readmit dry air into the oven. Press the cover lightly onto the dish using tongs, transfer the dish to a desiccator to cool, and reweigh.

5. Dry for another hour to ensure that constant weight has been achieved.

Table 3.7 Moisture Content of Meat Samples Obtained by Conventional and GC Methods

Moisture (%)a

Samples GC Conventional

Fat 6.9 6.6b

Pork jowl 27.2 27.1b

Pork trim 36.6 37.2b

Navels 38.2 38.0b

Salami 40.5 39.3b

Emulsion, frankfurterc 51.1 51.0b

Cow meat 55.4 54.9d

Bull meat 69.4 70.6d

Cheek meat 71.4 69.9d

Turkey 75.9 76.3d

a Mean of fi ve determinations.b Toluene distillation.c Emulsion obtained immediately before extrusion.d Oven drying at 105°C for 24 h.

Source: Reineccius, G.A. and P.B. Addis, J. Food Sci., 38, 355, 1973.



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3.7.1.2.3 Calculations of Moisture

Moisture ( )water volume

sample weight%

( )

( )�

�100

Solids (%) 100 moisture (%)� �

3.7.1.3 Freeze-Drying

Th ere is no better method than freeze-drying for preserving freshness and textural quality of dried foods. Freeze-drying or lyophilization is especially suited for drying high-value liquid foods such as coff ee and juices, as well as high-value solid foods such as strawberries, shrimp, diced chicken, sliced mushrooms, and even steaks and chops [25]. In recent years, the lyophilization process has evolved into a highly advanced drying technique. Th is drying method has the limitation that its cost may be two to fi ve times greater per weight of water removed than other drying methods [25]. Much of the development work, therefore, has focused on optimizing both the lyophilization process and equipment to lower drying costs [25]. However, freeze-drying has many advantages, some of which are listed in Table 3.4.

Th e main principle of lyophilization is facilitating the sublimation of water from the sample under reduced pressure and temperature conditions. Sublimation is the direct conversion of ice to water vapor without melting into liquid water; thus, lyophilization preserves the physical structure of the food. Water evaporation from ice (sublimation) occurs at temperatures below 0°C and pres-sures below 4.6 mm Hg [31]. Under these conditions, water in the food remains frozen and water vapor leaves the food faster than water in the surrounding atmosphere reentering onto the food, which causes a net reduction in the moisture content of the frozen sample [25]. Heat is frequently applied to the frozen food to enhance the sublimation rate within the vacuum chamber of the dryer. Th e maximum drying rate occurs when the vacuum is maintained at 0.1–2 mm Hg and heat is added just short of melting the ice [25]. As freeze-drying progresses, moisture is initially removed from the surface and continues to recede toward the center of the frozen food until the fi nal ice sublimes, leaving a moisture content of less than 5% [25]. Completion of drying times for freeze-drying may be 8 h or longer. Th e high cost of the equipment may limit the availability of this freeze-dry method for ordinary moisture analysis of food samples. However, this lyophilization method is desirable as a component of a standard reference method for moisture determination [1].

3.7.1.4 Distillation Methods

Th e property of “azeotropy” in water is utilized for this method, where water is simultaneously distilled with an immiscible liquid at a constant ratio. Th ere are two main types of distillation methods for moisture determination—direct distillation and refl ux distillation [1,14,18]. First, for the direct distillation method, a food is heated in a liquid (e.g., mineral oil), which is immiscible with water and has a high boiling point [1,14,18]. Th e water in the food distills directly from this liquid, condenses, and is collected in a graduated tube; the volume of the water removed is then measured.



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Second, the refl ux distillation procedure is more commonly used than the direct distillation method [1,6–7,14,18]. Th is procedure makes use of the azeotropic properties of solvent mixtures. Water and an immiscible solvent, such as toluene or xylene, distill off together during heating at a constant ratio and frequently at a temperature lower than the boiling point of either component [31]. As an example, the respective boiling points of water and toluene are 100 and 110.6°C, but the boiling point of the binary mixture is 85°C; the distillation ratio of the mixture is approxi-mately 20% water and 80% toluene [31]. If water is denser than its co-distillate, as in the case with toluene, the water is again collected in a suitable measuring apparatus, where the water is separated and then the water volume is measured.

A rapid distillation can be achieved by distillation with a boiling liquid, which transfers heat eff ectively to the sample [1]. Th e lowered boiling point of the distillation mixture causes less decomposition of the food during heating [1,18]. Th is procedure also minimizes oxidative reactions [6–7,31]. Th is distillation method is especially suitable for samples having a high concentration of volatile compounds. Th e research data collected from azeotropic distillation have shown consistently the theoretical moisture content to within 0.1% [6]. A moisture value comparison between refl ux distillation using toluene and oven-drying methods showed similar levels for a variety of products [31]. Th ere are some potential diffi culties for the refl ux method, such as emulsion formation and suspended water droplets. Using clean glassware and allowing the apparatus to cool before reading the volume of collected water help with these two prob-lems [18]. Table 3.4 delineates the advantages and disadvantages associated with the distillation procedures.

An apparatus of the refl ux distillation system is shown in Figure 3.5. Th is system consists of a heating source under a round-bottom boiling fl ask, and the fl ask contains the food sample and the solvent (e.g., toluene). Th e round-bottom fl ask is connected to a Bidwell–Sterling receiver, which will collect and measure the distilled water in a side arm. A condenser is positioned directly above the side arm of the Bidwell–Sterling receiver. Th e apparatus, reagents, and procedure of the refl ux distillation are outlined as follows [2,11].

3.7.1.4.1 Apparatus

1. Refl ux distillation apparatus (see Figure 3.5) 2. Heating mantle

3.7.1.4.2 Reagents

1. Xylene or toluene

3.7.1.4.3 Procedure

1. Weigh a food sample containing 2–5 g water and place the sample in an appropriately sized round-bottom fl ask. Th is could be 10–15 g cheese or 40 g spice.

2. Add enough suitable solvent (e.g., toluene) to cover the food, usually 60–100 mL. 3. Assemble the refl ux apparatus as shown in Figure 3.5. 4. Run cold water through the condenser and gradually heat the fl ask until refl uxing starts.



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5. Adjust the heating to produce two drops of condensate per second. When the rate of water accumulation decreases, increase heat to yield four drops per second.

6. When no additional moisture is collected in the side arm, rinse the condenser with the solvent and continue heating a few more minutes. Total heat time is typically 1–1.5 h.

7. Turn off the heat and allow the apparatus to cool, especially the side arm. 8. Record the volume of water in the side arm.

3.7.1.4.4 Calculations

Moisture (%)

(loss of weight 100)

(sample weight)�

�

Figure 3.5 Apparatus for azeotropic distillation method with Bidwell-Sterling receiver. (Park, Y.W. and L.N. Bell, Handbook of Food Analysis, Marcel Dekker, New York, 2002.)

Condenser

Bidwell−SterlingReceiver

Boilingflask

Heater

10 ml



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Detailed experimental protocols for specifi c food products can be found in the Offi cial Methods of Analysis of AOAC International [30] as well as other sources [2,11].

3.7.1.5 Karl Fischer Titration Method

Th e Karl Fischer method for moisture analysis is based on the reduction of iodine by sulfur diox-ide in the presence of water, which was proposed by Bunsen in 1853. Th e main chemical principle of the method is

2H2O + SO2 + I2 → H2SO4 + 2HI

Fischer modifi ed the conditions of the reaction, enabling quantifi cation of moisture [32]. Th e Karl Fischer method has become a standard method for moisture determination of liquids and solids due to its selectivity, high precision, and speed [9]. It is especially applicable for measuring moisture in foods for which heating methods give erratic results [1,18]. Moisture assay using this chemical technique has been approved for dried vegetables, oils and fats, cacao products, liquid molasses, and sugar-rich foods [1,30]. Th is method has superior sensitivity compared to other methods, being able to quantify the amount of water to a few parts per million [6–7,18]. Th e accuracy and precision of the Karl Fischer method have been found to be higher than those of other methods [6]. A more detailed listing of the advantages and disadvantages of the Karl Fischer method is found in Table 3.4.

Th e titration reagent for the Karl Fischer method consists of a mixture of iodine, pyridine, sulfur dioxide, and methanol. Th e titration of water with this reagent follows the two-step reaction shown as follows [1,18,33].

O

N

I

N

H

O

N

SO2

SO2

CH3OH+

H

N

SO4CH3

N

I2 + SO2 + H2O + 3 + 2

Th e preceding reactions illustrate that titration of 1 mol water requires 1 mol iodine, 1 mol sulfur dioxide, 3 mol pyridine, and 1 mol methanol. Th e titration is performed either by volumet-ric titration, where the endpoint is indicated by the appearance of brown color from free iodine (detected either visually or by photometric determination), or by coulometric titration, where the endpoint is determined by a potentiometer [18]. Several diagrammatic representations of the Karl Fischer apparatus are shown by Pande [7].

Th e Offi cial Methods of Analysis of AOAC International [30] shows the offi cial Karl Fischer method for specifi c food products and typical use of automated equipment. Th e titration is automatically performed by these Karl Fischer instruments until the endpoint is reached. Th e moisture content is calculated from the amount of titrant consumed, which is often expressed in



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milligrams. Milligrams of water can be converted into percent moisture using the initial sample mass. Liquid or solid food samples may be directly introduced into the reaction vessel if the water is easily accessible to the reagent. Th e water is frequently extracted into anhydrous methanol in solid foods where the water is not accessible. A known amount of the methanol/water solution is then injected into the reaction vessel. A methanol blank should also be prepared in a similar man-ner. Th e amount of water in this methanol blank would indicate whether any moisture from the atmosphere was introduced during sample preparation. Th e moisture in the blank sample should be subtracted from the moisture value of the food. Th e apparatus, reagents, and procedure of man-ual Karl Fischer method for a solid containing inaccessible water are described in the following.

3.7.1.5.1 Apparatus

1. Burette–automatic fi lling type, all glass, fully protected against moisture ingress 2. Titration vessel—having an agitation device such as magnetic stirrer slightly pressurized

with dry inert gas (N2 or CO2) to exclude air 3. Electrometric apparatus and galvanometer—suitable for “dead stop” endpoint technique

3.7.1.5.2 Reagents

1. Methanol (anhydrous). 2. Karl Fischer reagent—to minimize loss of active reagent from side reactions, many labora-

tory suppliers provide the Karl Fischer reagent as two solutions: iodine in methanol and sulfur dioxide in pyridine. Th e solutions are mixed shortly before use.

3.7.1.5.3 Procedure

1. Weigh an amount of sample containing approximately 100 mg water into a predried 50-mL round-bottom fl ask.

2. Add 40 mL methanol into the fl ask, quickly place it on the heating range, and connect the refl ux condenser

3. Boil the contents of the fl ask gently under refl ux for 15 min. 4. Stop heating with the condenser attached, and let it drain for 15 min. 5. Remove and stopper the fl ask. 6. Pipette a 10-mL aliquot of the extract into the titration vessel, titrate with the Karl Fischer

reagent to the “dead stop” endpoint, and record the volume of titrant used. 7. Run a blank fl ask without a sample following the same procedures described earlier.

3.7.1.5.4 Calculation

Using the preceding procedure, the following equation is used to calculate moisture content. If a known sample mass was introduced directly into the titration vessel rather than extracting with methanol, the dilution factor (i.e., 4) may be removed from the equation.

Moisture ( )reagent used for sample [mL] reagent u

%([ . ]

�4 0 1� � � �F ssed for blank [mL]

sample weight

)



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Th e standardization factor of the reagent, F, can be determined from titrating samples containing known amounts of water. It is measured in milligrams of water per milliliters of reagent [18].

Pyridine-free Karl Fischer reagents are available due to health concerns, and these reagents do not compromise the moisture determination. Th e replacement solvent systems actually can speed up the analysis and improve its precision [1]. As described earlier, the titration can be performed manually or using semiautomated equipment, but completely automated equipment is recom-mended, especially when Karl Fischer analyses are to be performed on a routine basis.

3.7.1.6 Chemical Desiccation

Th e chemical desiccation method is carried out by desiccation in an evacuated desiccator containing a substance that strongly absorbs moisture, usually for dried foods. Th e amount of water removed from the food depends on the strength of the desiccant employed [30]. Relative effi ciencies of various desiccating agents were compared in several studies, as shown in Table 3.6 [31,34,35]. Th e most eff ective desiccating agents are phosphorus pentoxide, barium oxide, and magnesium per-chlorate [30]. However, phosphorous pentoxide becomes explosive if it absorbs too much moisture [20]. Calcium sulfate (Drierite™) is a commonly used desiccant despite not being as eff ective.

Usually at room temperature, desiccation of the sample is achieved. With few exceptions, des-iccation techniques are lengthy procedures, frequently requiring weeks and even months for the sample to achieve constant weight [1,6,20]. Th e equilibrium time depends strongly on the forces holding water in the sample relative to the desiccant. Slight heating may be used in conjunction with the desiccants to enhance moisture removal from the food. Although this method has some limitations, results obtained using chemical desiccation can serve as reference standards for cali-brating moisture contents of more rapid procedures [30].

3.7.1.7 Thermogravimetric Method

Th e moisture of a food is removed by heating in thermogravimetric analysis (TGA), which resem-bles an automated version of the standard oven-drying method. Th e TGA instrumentation is equipped with a thermobalance, which automatically measures and records the weight loss of a food sample as a function of time and temperature while the sample is being heated [36]. For the procedure of TGA, a small amount of sample is loaded into the balance, which then heats under a controlled temperature program. Analytical data are recorded in the form of thermogravimetric curves, which plot the sample mass as a function of temperature. Moisture is continuously evapo-rated from the sample and the weight loss is recorded until the sample has reached a constant weight. Errors associated with sample weighing are minimized in TGA, because multiple sample transfers are not required for the analytical processes. However, care must be taken to prevent thermal decomposition of the sample to avoid erroneous results, as in other methods using heat for moisture determination.

Moisture assay using TGA has been shown to give results similar to other methods. Th e TGA method is advantageous in that it can be used to quantify chemically bonded water of hydration, as well as to analyze total moisture content.

3.7.1.8 Gas Chromatography

Th e gas chromatography (GC) method has versatile capability in analytical chemistry, which can be applied to moisture assay of foods [1,6]. Th e principle of the GC method for moisture



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determination is as follows. A known amount of the food sample is initially homogenized, and water is extracted into an anhydrous solvent such as methanol, ethanol, or isopropanol. Th e extract is then analyzed using GC such that quantitative separation of the water–solvent mixture can occur. A Poropak Q column and a thermal conductivity detector have been used previously for moisture analysis of meat products [37]. Th e quantifi cation of moisture content is carried out by determining the peak areas of water and solvent; these areas are then compared to the areas of solutions containing known amounts of water (i.e., a standard curve). Th e mois-ture values determined by GC compared with values determined by distillation or oven-drying methods have shown that the GC values were not diff erent from those obtained by the more conventional methods [33]. Table 3.7 illustrates the moisture contents of meat samples obtained by conventional and GC methods. Th e GC analysis is rapid, but requires specialized and expen-sive equipment.

3.7.1.9 Application of Direct Methods in Moisture Determination

As mentioned earlier, various direct methods have been discussed for moisture analysis. Th e most widely used method is air-oven drying, which is usually conducted for the least temperature-sensitive foods. A complete moisture removal is assumed at some appropriate time/temperature combination. Vacuum-oven drying speeds up moisture removal and is especially useful for foods susceptible to decomposition (e.g., sugar-containing products). Since moisture can be removed more rapidly at lower pressures, the vacuum-oven method uses pressures lower than that of the atmosphere to reduce the boiling point of water while preventing product decompo-sition. Distillation methods give comparable moisture values to those from the oven methods. Distillation methods are carried out at atmospheric pressure, and often completed in a shorter time than the oven-drying methods. Th e Karl Fischer titration method is rapid, although it remains primarily a laboratory technique. Th is method also requires a considerable degree of skill for the performing analyst. Vacuum desiccation requires too much time to be considered as a method for product quality control. GC, although able to give rapid results comparable to other methods, requires a large initial capital investment. For the analysis of diff erent food samples, an offi cial method for moisture determination can be selected, depending on the types of foods analyzed [30]. Th e offi cial methods for moisture determination of various foods recognized by the Association of Offi cial Analytical Chemists International are described in Table 3.8 [30].

3.7.2 Indirect MethodsAs mentioned earlier, a variety of indirect methods for moisture determination also exist. Th ese methods measure a property of water that is dependent on its content. Th us, moisture contents can be calculated using appropriate calibration curves.

3.7.2.1 Refractometry

Refractometry is an optical method measuring the refractive index of a solution, which can be used for determining its moisture content. A schematic presentation of the refraction, or bend-ing, of light as it passes through two diff erent sucrose solutions is shown in Figure 3.6. Th e more



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Figure 3.6 Schematic moisture determination by refractometry. (Park, Y.W. and L.N. Bell, Handbook of Food Analysis, Marcel Dekker, New York, 2002.)

Light source

90% water

10% sugar

50% water50% sugar

R I = 1.348 R I = 1.420

Table 3.8 Comparison of Direct Methods for Moisture Determination of Foods as Recommended by AOAC International

Moisture-Assaying Method

Temperature (°C)

Pressure (mm Hg)

Sample Weight (g)

Time Required (h) Food Products

Air-oven drying 100–102 760 2 16 –18 Meat products100 760 5 3 Cane and beet sugar130 760 2 1–2 Flour100 760 2 Until constant

weightCacao products

Vacuum-oven drying

60–70 50–100 2–5 2–6 Dried fruits, honey, syrup

98–100 25 2–5 5 Pasta products, coffee, wheat fl our

100 100 2–3 4 –5 Dried milk, cheese, nuts, tea

75 100 5 5 Fats and oilsDistillation 129–134 760 10–15 1–2 Cheese (amyl

alcohol and xylene)85 760 40 1–2 Spices (toluene)

Karl Fischer method

— — 5–15 A few minutes (if apparatus is set up)

Cacao products2–3 Dried vegetables5–25 Fats and oils0.5–1 Molasses

Source: AOAC, Offi cial Methods of Analysis of AOAC International, AOAC International, Arlington, VA, 1995.



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refraction, the higher the concentration in the solution. Th e moisture content can be rapidly deter-mined by measuring the refractive index of a solution or slurry, using an appropriate calibration curve [1,6,18,38].

Th e sample can be homogenized with an anhydrous solvent (e.g., isopropanol) for solid or semisolid foods, and then the refractive index of the solution is measured using a refractometer. A calibration curve is produced by measuring the refractive index of solutions containing the same solvent with known amounts of added water. Th e moisture content of the sample is calculated using the calibration curve, and the mass of food is homogenized in the solvent. Because the refractive index measurement is temperature-sensitive, a uniform sample is required, and strict temperature control is necessary.

3.7.2.2 Infrared Absorption Spectroscopy

Many reports have shown that IR spectroscopy can be used for moisture determination [1,6–7,18]. Th e IR spectrum of a chemical compound has been described as one of its most characteristic physical properties [7]. Since IR has these properties, it is one of the most versatile methods for measuring the moisture content of a large variety of solid, liquid, or gas substances by employing specifi c wavelengths at which maximum absorption is expected to occur. For water, the spectral region of interest is 700–2400 nm; absorption bands occurring at 1450 and 1940 nm are frequently used [1,6–7,18]. Determination of the moisture content of a sample can be performed by compar-ing the band intensity with that of the same band for standard concentrations of water.

Th e basic concept of this methodology is that an IR beam passes through an optical fi lter, which consequently transmits energy at a specifi c wavelength through the sample cell and then to a detector [39]. Ideally, the wavelength used is that of maximum absorption for the compound being measured. Th e IR technique must be calibrated using standards of known concentration. Th en the absorption values of the sample can be compared with those of the standards for moisture determi-nation. An appropriate calibration technique [18,30] is required for each chemical compound.

3.7.2.3 Near Infrared–Refl ectance Spectroscopy

Near infrared (NIR)–refl ectance spectroscopy technology has been developed recently to use its high resolving power of refl ectance spectra in the NIR range (800–2500 nm) as an analytical tool for components analysis. Th e mid-IR range (2,500–24,000 nm) has high resolution in the absorption spectrum and can absorb IR radiation eff ectively from many compounds, but resolu-tion of the refl ectance spectrum is poor [40].

Since this methodology developed in early 1970s, NIR–refl ectance spectroscopy has assumed immense economic importance as a rapid, integrated multicomponent testing method for a wide range of products. Th e NIR technique has been widely used to predict the composition (i.e., mois-ture, oil, and protein) of grains and oil seeds as well as other foods [41]. Th e primary advantage of the method is the speed of analysis. Th e accuracy of the NIR refl ectance method depends on the calibration curve, derived from wet chemical analysis of the standard samples. If the calibration sample set does not adequately represent the range of the unknown samples, then the analysis will be error-prone. Linear calibration curves (correlation coeffi cients >0.98) have been established between the moisture values of raw pork and beef determined by the oven-drying method and by the NIR–refl ectance spectroscopic method [41].

Th e NIR spectrophotometer can generate the refl ectance spectra with a monochromator oper-ated in single- or double-beam mode, which then can be downloaded into a computer [41,42].



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Ground samples are packed into a sample holder maintaining direct contact with a concentric IR-transmitting quartz window. Th e refl ected radiation signals of the diff used spectra from the glass window are collected with four lead sulfi de detectors equally spaced around the incident beam. Figure 3.7 depicts the NIR radiation being refl ected from a food sample to surrounding detectors. Th e signals from the detectors are amplifi ed with a logarithmic-response amplifi er, digitized, and fed into a computer. Th e wavelength range from approximately 1100 to 2500 nm is scanned every 2 nm (or 0.5 nm) along the width of the refl ectance curve [41,42]. Both the IR refl ectance (R) curve and the log(1/R) curve can be recorded as the second derivative of the original curves to help evaluate overlapping absorption bands [41,42].

Since the NIR method is not a direct method for moisture determination, the refl ectance data cannot be used directly for quantitative analysis. Moisture contents of standard samples are inputted in the computer along with the NIR data, which analyzes them with a stepwise multiple linear-regression method to develop prediction equations by a regression analysis of NIR spectral data against chemical data [41,42]. For moisture assay of the raw-meat samples, this spectral data consisted of taking the ratio of the second derivative of the log(1/R) data at two diff erent wave-lengths [41], showing the complexity of the analysis.

NIR instruments are commercially manufactured on the basis of three geometries according to the method of collecting the refl ectance. Th ese are integrating sphere, large solid-angle detec-tor, and small detector. Each of these types has advantages and disadvantages, as discussed by Norris [41]. Th e large solid-angle detector was used to collect the moisture data on pork and beef, as mentioned earlier [41].

Figure 3.7 Large, solid detector of NRI equipment. (Park, Y.W., Handbook of Food Analysis, Marcel Dekker, New York, 1996; Ruan, R.R. and P.L. Chen, Foods and Biological Materials: A Nuclear Magnetic Approach, Technomic Publishing, Lancaster, PA, 1998.)

Monochromaticradiation

Photo cell

Sample

Photo cell

Window

Sample holder



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3.7.2.4 Microwave Absorption Method

One of the distinct properties of water is that it absorbs several thousand times more microwave energy than a similar amount of a dry substance [6]. For example, at frequencies between 1 and 30 GHz, the loss tangent of water is 0.15–1.2, whereas for dry materials the loss tangent is 0.001–0.05 [30]. Because of these diff erences, the absorption of microwaves can be used to determine the water content of a variety of food products. Th e absorption of microwave energy at 2450 MHz increases linearly with increasing moisture content [30]. Th is led to the development of the micro-wave moisture meter almost 40 years ago.

Th e microwave absorption method consists of a constant source of microwave radiation, a waveguide, a detector, a microwave attenuator and amplifi er, and an indicating meter [30]. Th e sample is placed between a microwave transmitter and receiver. Th e attenuation of the sample is the diff erence in attenuation readings between the transmitter and the receiver, which is dependent on moisture content and is therefore used to construct a calibration curve [30]. Various factors can aff ect the accuracy of the microwave measurement, including leakage of microwave energy, sample temperature, particle size, polarization of diff erent material, and the presence of soluble salts [30]. Th e microwave absorption device was used to determine the moisture content of cakes, and was found to give results similar to that from a drying oven [43]. Th e microwave absorption method has not evolved into a commonly used analytical technique for moisture determination, even if the positive results have been obtained.

3.7.2.5 Dielectric Capacitance

Another distinct property of water is its dielectric constant (DC). Its DC at 20°C is about 80, whereas those of fatty acids and sucrose are both about 3 [31]. Th ere is a positive correlation between moisture content and DC, whereby a 1% increase in moisture content of a substance will theoretically increase its DC by approximately 0.8 [6]. In addition, the DC of water-containing substances increases almost linearly up to approximately 30% moisture content [6,19,30]. Instru-ments utilizing dielectric measurements for moisture determination were developed based on this principle and applied most commonly to cereals [1].

Th e major component of the dielectric instrument is a capacitance cell, which consists of two metal plates spaced apart, with equal but opposite charges [1]. Th ese charges reverse at fi xed frequencies to yield an alternating current [1]. Th e principle of moisture measurement by dielec-tric instrument is as follows. A sample placed between the two plates will become polarized and change the capacitance of the plates. Th is capacitance change, aff ected by moisture content, is measured by the instrument. On calibration with standards of known moisture content, mois-ture contents of food samples can be determined. Analytical results using a capacitance meter are infl uenced by moisture distribution, presence of electrolytes, temperature, and sample density [6,18]. Table 3.5 describes the advantages and disadvantages of the DC method and other indi-rect methods.

3.7.2.6 Conductivity Method

Th e principle of the conductivity method is that conductivity and resistance are measured in an electrical circuit containing a food sample [1]. A distinct relationship exists between the mois-ture content of materials and their electrical properties [6,32]. As the moisture content of a food



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sample increases, electrical resistance of the sample decreases, and its conductivity increases [6,18]. Measuring resistance appears to be most useful [18]. Frequently, the logarithm of resistance is plotted as a function of either moisture content or humidity. Th e logarithm of resistance is basically linear as a function of humidity, although showing curvature as a function of moisture content [30].

Several conductivity instruments are widely used by the industry for rapid routine moisture determinations by measuring either resistance or conductance of foods [1]. Th e accuracy of this method being less than ±0.5% will depend on the proper calibration [30]. An offi cial AOAC method describes a schematic of the electric circuit used to measure the conductivity of raisins and prunes [32]. To determine moisture by a conductivity meter, the food is placed between two elec-trodes and the current fl owing through the sample is measured by the change of electrical resis-tance [30]. Conductance readings are converted into moisture contents using a table that corrects for sample temperature [32]. Conductivity methods measure only the free water in the sample; the amount of bound water needs to be added to more closely approximate the total moisture content [6]. Conversion charts may be required due to the variation of bound water among samples. Th e accuracy and precision of the conductivity method are aff ected by moisture distribution, tempera-ture, and electrolyte content as well as the quality of contact between the electrodes and samples, as in the conditions of the dielectric capacitance method [6,30].

3.7.2.7 Sonic and Ultrasonic Absorption

Th e degree of absorption of sound energy would depend on the type of medium through which it is transmitted [30]. Th us, as moisture content of the medium changes, so too does the amount of sonic and ultrasonic absorption [6]. Using this principle, ultrasonic velocity measurements have been developed for the determination of moisture content [30]. In addition to absorption of sound energy, the high frequency of ultrasonic waves also enables their refl ection and refraction [6].

In this ultrasonic absorption method, the food sample is positioned between an energy gen-erator and microphone for analysis [6]. Th e energy output of the sample is amplifi ed, yielding a voltmeter reading. Voltmeter readings are converted into moisture contents using an appropriate standard curve. Ultrasonic methods have been used in laboratory settings to determine the com-position, including moisture content, of chicken [44] and dry fermented sausages [45]. Th is ultra-sonic method was shown to be rapid and nondestructive, and the analytical results were similar to other standard methods [44].

3.7.2.8 Cryoscopic Methods

Th e cryoscopic method is another indirect method for analyzing moisture, which utilizes the col-ligative property known as freezing-point depression [1,18]. Th e freezing point of water decreases as the concentration of dissolved solutes increases. Th erefore, the freezing point is inversely cor-related with the amount of water present for liquids containing a constant type of solute [18]. Th e most common use of this method is to measure water added to fl uid milk [1,18], although it could be applied to other foods.

3.7.2.9 Other Indirect Instrumental Methods

Th ere are several other indirect methods that can be applied for determination of moisture in foods, but require substantial instrumentation. Th ese methods include nuclear magnetic resonance



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(NMR) [1,6,30,46], mass spectrometry [30], and neutron scattering [30]. Th ese techniques are less frequently employed methods; therefore, they are briefl y introduced in this section, and the details can be found in other sources [30].

NMR is a fast and nondestructive method of moisture determination [1,6,30,46]. Th is technique utilizes the nuclear properties of its protons, instead of utilizing properties of water [6,30,46]. Th e challenge is to diff erentiate the proton NMR signal of water from the other hydro-gen-containing substances in the food. Th e accuracy of NMR techniques is approximately 0.2% [6]. Modern NMR techniques have been described for determination of moisture contents [47]. Table 3.5 lists the advantages and disadvantages of NMR methods.

Mass spectrometry is another indirect method for moisture determination [30]. Although mass spectrometry has been widely utilized for identifi cation of unknown substances in food samples [1], the quantifi cation of water is more problematic [18,33]. Th e greatest problem lies in the “memory eff ect” or carry-over eff ect from the already analyzed sample, which infl uences the results of the next sample [9,30]. Some success has been obtained by reacting cryogenically concentrated moisture with calcium carbide to yield acetylene, which is then quantifi ed [9]. How-ever, mass spectrometry remains an uncommon method for moisture determination due to its uncertainty.

Neutron and γ-ray scattering occur when energized neutrons interact with nuclei. Hydrogen atoms are the most eff ective at scattering neutrons; this forms the basis of a determination of moisture using neutron-scattering methods [9,30]. As with the NMR method discussed earlier, neutrons will scatter from any hydrogen nucleus, not just those associated with water. Th us, neu-tron-scattering methods are most appropriate for samples low in nonaqueous protons, such as inorganic substances [9,30]. Th e application of the neutron-scattering method to moisture deter-mination in foods is virtually nonexistent, because food is primarily composed of proton-rich organic material.

3.7.2.10 Summary of Indirect Methods

Th e indirect methods are generally faster than the direct methods for moisture determination. When done properly, the indirect methods can be as accurate and precise as the more standard methods. However, the accuracy and precision of the indirect methods depend on careful prepa-ration and analysis of known standards to establish reliable calibration curves. Although most indirect methods require a large capital investment in equipment, the potential application for rapid on-line quality control might make the investment worthwhile. Nevertheless, preparation of the standards and accurate calibration curves must be verifi ed by a specifi c direct method to establish a reliable indirect method of instrumentation that can achieve accurate and precise pre-dicted values.

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Moisture and Water Activity - Routledge Handbooks