Food Analysis

1 Food Analysis edited byS. Suzanne NielsenPurdue UniversityWest Lafayette, IN, USAF ourth EditionABCII partCompositional Analysisof Foods6chapterMoisture and Total SolidsAnalysisRobert L. Bradley, of Food Science, University of Wisconsin,Madison, WI 53706, Importance of Moisture Moisture content of Forms of Water in Sample Collection and Oven Drying General Removal of Decomposition of Other Temperature Types of Pans for Oven Handling and Preparation Control of Surface Crust Formation(Sand Pan Technique) Forced Draft Vacuum Microwave Infrared Nielsen,Food Analysis ,Food Science Texts Series, DOI ,c Springer Science+Business Media, LLC 20108586 Part II Compositional Analysis of Rapid Moisture Analyzer Distillation Reflux Distillation with Chemical Method: Karl Fischer Physical Dielectric Infrared Freezing Water Comparison of Nature of Intended Study Practice References104 Chapter 6 Moisture and Total Solids INTRODUCTIONM oisture assays can be one of the most importantanalyses performed on a food product and yet one ofthe most difficult from which to obtain accurate andprecise data.

2 This chapter describes various methodsfor moisture Analysis their principles, procedures,applications, cautions, advantages, and disadvan-tages. Water activity measurement also is described,since it parallels the measurement of total moistureas an important stability and quality factor. With anunderstanding of techniques described, one can applyappropriate moisture analyses to a wide variety offood Importance of Moisture AssayOne of the most fundamental and important analyticalprocedures that can be performed on a food productis an assay for the amount of moisture (1 3). The drymatter that remains after moisture removal is com-monly referred to astotal solids. This analytical valueis of great economic importance to a food manufac-turer because water is an inexpensive filler. The fol-lowing listing gives some examples in which moisturecontent is important to the food is a quality factor in the preservationof some products and affects stability in(a)Dehydrated vegetables and fruits(b)Dried milks(c)Powdered eggs(d)Dehydrated potatoes(e)Spices and is used as a quality factor for(a)Jams and jellies to prevent sugar crystalliza-tion(b)Sugar syrups(c)Prepared cereals conventional, 4 8%;puffed, 7 8% moisture is used for convenience inpackaging or shipping of(a)Concentrated milks(b)Liquid cane sugar (67% solids) and liquidcorn sweetener (80% solids)(c)Dehydrated products (these are difficult topackage if too high in moisture)(d)Concentrated fruit (or solids) content is often specifiedin compositional standards ( , Standards ofIdentity)(a)Cheddar cheese must be 39% moisture.

3 (b)Enriched flour must be 15% moisture.(c)Pineapple juice must have soluble solids of Brix (conditions specified).(d)Glucose syrup must have 70% total solids.(e)The percentage of added water in of the nutritional value of foodsrequire that you know the moisture data are used to express results ofother analytical determinations on a uniformbasis [ , dry weight basis (dwb), rather thanwet weight basis (wwb)]. Moisture content of FoodsThe moisture content of foods varies greatly as shownin Table6-1(4). Water is a major constituent of mostfood products. The approximate, expected moisturecontent of a food can affect the choice of the methodof measurement. It can also guide the analyst in deter-mining the practical level of accuracy required whenmeasuring moisture content , relative to other Forms of Water in FoodsThe ease of water removal from foods depends on howit exists in the food product. The three states of waterin food products water:This water retains its physical prop-erties and thus acts as the dispersing agent forcolloids and the solvent for water:This water is held tightly or isoccluded in cell walls or protoplasm and is heldtightly to of hydration:This water is bound chem-ically, for example, lactose monohydrate; alsosome salts such as Na2SO4 on the form of the water present in afood, the method used for determining moisture maymeasure more or less of the moisture present.

4 Thisis the reason for official methods with stated proce-dures (5 7). However, several official methods mayexist for a particular product. For example, the AOACI nternational methods for cheese include: , vacuum oven; , forced draft oven; ,microwave oven; , distillation (5). Usually, thefirst method listed by AOAC International is preferredover others in any Sample Collection and HandlingGeneral procedures for sampling, sample handlingand storage, and sample preparation are given inChap. 5. These procedures are perhaps the greatestpotential source of error in any Analysis . Precau-tions must be taken to minimize inadvertentmois-ture losses or gainsthat occur during these II Compositional Analysis of Foods6-1tableMoisture content of Selected FoodsFood ItemApproximatePercent Moisture(Wet WeightBasis)Cereals, bread, and pastaWheat flour, bread, enriched (wheat flour) flakes , dry, productsMilk, reduced fat, fluid, 2% , plain, low cheese, low fat or 2% milk cream, and oilsMargarine, regular, hard, corn, , with , salad, or cooking0 Fruits and vegetablesWatermelon, , raw, California , raw, with , American type, , with peel, , microwaved, cooked in skin,flesh and beans, green, , poultry, and fishBeef, ground, raw, 95% , broilers and fryers, lightmeat, meat and skin, , flatfish (flounder and solespecies), , whole, raw, , black, , all types, dry roastedwith butter, smooth style, with , granulated0 Sugar, , strained or US Department of Agriculture, Agricultural Research Service(2009)

5 USDA National Nutrient Database for Standard Ref-erence. Release 22. Nutrient Data Laboratory Home Page, , any exposure of a sample to the openatmosphere should be as short as possible. Any heat-ing of a sample by friction during grinding shouldbe minimized. Headspace in the sample storage con-tainer should be minimal because moisture is lost fromthe sample to equilibrate the container environmentagainst the sample. It is critical to control temperaturefluctuations since moisture will migrate in a sample tothe colder part. To control this potential error, removethe entire sample from the container, reblend quickly,and then remove a test portion (8,9).To illustrate the need for optimum efficiency andspeed in weighing samples for Analysis , Bradley andVanderwarn (10) showed, using shredded Cheddarcheese (2 3 g in a aluminum foil pan), thatmoisture loss within an analytical balance was astraight line function.

6 The rate of loss was relatedto the relative humidity. At 50% relative humidity, itrequired only 5 s to lose moisture. This timedoubled at 70% humidity or moisture lossin 10 s. While one might expect a curvilinear loss,the moisture loss was actually linear over a 5-minstudy interval. These data demonstrate the necessity ofabsolute control during collection of samples throughweighing, before OVEN DRYING METHODSI noven drying methods, the sample is heated underspecified conditions, and the loss of weight is usedto calculate the moisture content of the sample. Theamount of moisture determined is highly dependenton the type of oven used, conditions within the oven,and the time and temperature of drying. Various ovenmethods are approved by AOAC International fordetermining the amount of moisture in many foodproducts. The methods are simple, and many ovensallow for simultaneous Analysis of large numbers ofsamples.

7 The time required may be from a few minutesto over 24 General Removal of MoistureAny oven method used to evaporate moisture has asits foundation the fact that the boiling point of wateris 100 C; however, this considers only pure water atsea level. Free water is the easiest of the three formsof water to remove. However, if 1 molecular weight(1 mol) of a solute is dissolved in L of water, theboiling point would be raised by C. This boil-ing point elevation continues throughout the mois-ture removal process as more and more removal is sometimes best achieved in atwo-stage process. Liquid products ( , juices, milk)are commonly predried over asteam bathbefore dry-ing in an oven. Products such as bread and field-driedgrain are often air dried, then ground and oven dried,with the moisture content calculated from moistureChapter 6 Moisture and Total Solids Analysis89loss at both air and oven drying steps. Particle size,particle size distribution, sample sizes, and surfacearea during drying influence the rate and efficiency ofmoisture Decomposition of Other FoodConstituentsMoisture loss from a sample during Analysis is afunction of time and temperature.

8 Decompositionenters the picture when time is extended too muchor temperature is too high. Thus, most methods forfood moisture Analysis involve a compromise betweentime and a particular temperature at which limiteddecomposition might be a factor. One major prob-lem exists in that the physical process must sepa-rate all the moisture without decomposing any of theconstituents that could release water. For example,carbohydrates decompose at 100 C according to thefollowing reaction:C6H12O6 6C+6H2O[1]The moisture generated in carbohydrate decomposi-tion is not the moisture that we want to other chemical reactions ( , sucrose hydrol-ysis) can result in utilization of moisture, which wouldreduce the moisture for measurement. A less seriousproblem, but one that would be a consistent error, isthe loss ofvolatile constituents, such as acetic, pro-pionic, and butyric acids; and alcohols, esters, andaldehydes among flavor compounds.

9 While weightchanges in oven drying methods are assumed to bedue to moisture loss, weight gains also can occur dueto oxidation of unsaturated fatty acids and certainother and Hulett (11) determined that mois-ture was retained in biological products to at least365 C, which is coincidentally the critical temperaturefor water. Their data indicate that among the decom-position products at elevated temperatures were CO,CO2,CH4,andH2O. These were not given off atany one particular temperature but at all temperaturesand at different rates at the respective temperature plotting moisture liberated against tempera-ture, curves were obtained that show the amountof moisture liberated at each temperature ( ).Distinct breaks were shown that indicated the tem-perature at which decomposition became of these curves showed any break before 184 , proteins decompose at temperatures some-what lower than required for starches and of the flat portion of each curve to 250 C6-1figureMoisture content of several foods held at vari-ous temperatures in an oven.

10 The hyphenatedline extrapolates data to 250 F, the true mois-ture content . [Reprinted with permission from(11) Nelson OA and Hulett GA. 1920. Themoisture content of Industrial :40 45. Copyright 1920, AmericanChemical Society.]gave a true moisture content based on the assump-tion that there was no adsorbed water present at thetemperature in Temperature ControlDrying methods utilize specified drying tempera-tures and times, which must be carefully , there may be considerable variability oftemperature, depending on the type of oven used formoisture Analysis . One should determine the extentof variation within an oven before relying on datacollected from its the temperature variation in three typesof ovens:convection(atmospheric),forced draft,andvacuum. The greatest temperature variation exists ina convection oven. This is because hot air slowly cir-culates without the aid of a fan. Air movement isobstructed further by pans placed in the oven.