Page71.3ApproachtotheStudyofFoodChemistryIt is desirable to establish an analytical approach to the chemistry of food formulation, processing, and storage stability, so thatfactsderivedfromthestudyofonefoodormodelsystemcanenhanceourunderstandingofotherproducts.Therearefourcomponents to this approach: (a) determining those properties that are important characteristics of safe, high-quality foods, (b)determiningthosechemical andbiochemicalreactionsthathaveimportant influencesonlossofqualityand/orwholesomenessoffoods, (c) integrating the first two points so that one understands how the key chemical and biochemical reactions influencequality and safety,and (d) applying this understanding to various situations encountered during formulation, processing, andstorage offood.1.3.1Quality and Safety AttributesIt is essential to reiterate that safety is the first requisite of any food. In a broad sense, this means a food must be free of anyharmfulchemical ormicrobialcontaminantatthetimeofitsconsumptionForoperationalpurposesthisdefinitiontakesonamore applied fom. In the canning industry,commercial"sterility as applied to low-acid foods means the absence of viablespores ofClostridium botulinum.This in turn can be translated into a specific set ofheating conditionsfora specific productina specific package. Given these heating requirements, one can then select specific time-temperature conditions that willoptimizeretention of quality attributes. Similarly, in a product such as peanut butter, operational safety can be regarded primarily as theabsence of aflatoxinscarcinogenic substances produced by certain species ofmolds. Steps taken to prevent growth of themold inquestionmayormaynotinterferewithretentionofsomeotherqualityattribute,nevertheless,conditionsproducingasafeproduct must beemployed.A list of quality attributes of food and some alterations they can undergo during processing and storage is given in Table 1.Thechanges that can occur, with the exception ofthose involving nutritive value and safety,are readily evident to the consumer.1.3.2ChemicalandBiochemicalReactionsMany reactions can alter food quality or safety. Some of the more important classes of these reactions are listed in Table 2Eachreactionclasscaninvolvedifferentreactantsorsubstratesdependingonthespecificfoodandtheparticularconditionsforhandling, processing,or storage.They are treated as reaction classes because the general nature ofthe substrates or reactants issimilarfor allfoods.Thus,nonenzymic browning involvesreaction ofcarbonyl compounds,whichcan arisefrom existingreducing sugars orfrom diverse reactions, such as oxidation of ascorbic acid, hydrolysis of starch, or oxidation oflipids.Oxidation may involvelipids,proteins,vitamins,or pigments,and morespecifically,oxidation oflipids may involvetriacylglycerols in one food or phospholipids in another.Discussion of these reactions in detail will occur in subsequent chaptersofthis book.1.3.3EffectofReactionsontheQualityandSafetyofFoodThe reactions listed in Table 3cause the alterations listed in Table 1.Integration of the information contained in both tables canleadtoanunderstandingofthecausesoffooddeterioration,Deteriorationoffoodusuallyconsistsofaseriesofprimaryeventsfollowed by
Pag e 7 1.3 Approach to the Study of Food Chemistry It is desirable to establish an analytical approach to the chemistry of food formulation, processing, and storage stability, so that facts derived from the study of one food or model system can enhance our understanding of other products. There are four components to this approach: (a) determining those properties that are important characteristics of safe, high-quality foods, (b) determining those chemical and biochemical reactions that have important influences on loss of quality and/or wholesomeness of foods, (c) integrating the first two points so that one understands how the key chemical and biochemical reactions influence quality and safety, and (d) applying this understanding to various situations encountered during formulation, processing, and storage of food. 1.3.1 Quality and Safety Attributes It is essential to reiterate that safety is the first requisite of any food. In a broad sense, this means a food must be free of any harmful chemical or microbial contaminant at the time of its consumption. For operational purposes this definition takes on a more applied form. In the canning industry, “commercial” sterility as applied to low-acid foods means the absence of viable spores of Clostridium botulinum. This in turn can be translated into a specific set of heating conditions for a specific product in a specific package. Given these heating requirements, one can then select specific time-temperature conditions that will optimize retention of quality attributes. Similarly, in a product such as peanut butter, operational safety can be regarded primarily as the absence of aflatoxins—carcinogenic substances produced by certain species of molds. Steps taken to prevent growth of the mold in question may or may not interfere with retention of some other quality attribute; nevertheless, conditions producing a safe product must be employed. A list of quality attributes of food and some alterations they can undergo during processing and storage is given in Table 1. The changes that can occur, with the exception of those involving nutritive value and safety, are readily evident to the consumer. 1.3.2 Chemical and Biochemical Reactions Many reactions can alter food quality or safety. Some of the more important classes of these reactions are listed in Table 2. Each reaction class can involve different reactants or substrates depending on the specific food and the particular conditions for handling, processing, or storage. They are treated as reaction classes because the general nature of the substrates or reactants is similar for all foods. Thus, nonenzymic browning involves reaction of carbonyl compounds, which can arise from existing reducing sugars or from diverse reactions, such as oxidation of ascorbic acid, hydrolysis of starch, or oxidation of lipids. Oxidation may involve lipids, proteins, vitamins, or pigments, and more specifically, oxidation of lipids may involve triacylglycerols in one food or phospholipids in another. Discussion of these reactions in detail will occur in subsequent chapters of this book. 1.3.3 Effect of Reactions on the Quality and Safety of Food The reactions listed in Table 3 cause the alterations listed in Table 1. Integration of the information contained in both tables can lead to an understanding of the causes of food deterioration. Deterioration of food usually consists of a series of primary events followed by
Page8TABLE1Classification ofAlterations That Can Occur in Food During HandlingProcessing,or StorageAttributeAlterationTextureLoss of solubilityLoss ofwater-holding capacityTougheningSofteningFlavorDevelopment of.Rancidity (hydrolytic or oxidative)Cooked or caramel flavorsOther off-flavorsDesirable flavorsColorDarkeningBleachingDevelopmentofotheroff-colorsDevelopmentofdesirablecolors(e.g.,browningofbakedgoods)Nutritive valueLoss,dgradation oraltered bioavailabilityofproteins,lipidsvitamins,mineralsSafetyGeneration oftoxic substancesDevelopment of substances that are protective to healthInactivationoftoxicsubstancessecondary events, which, in turn, become evident as altered quality attributes (Table 1). Examples of sequences of this type areshowninTable3.NoteparticularlythatagivenqualityattributecanbealteredasaresultofseveraldifferentprimaryeventsThe sequences in Table 3 can be applied in two directions. Operating from left to right one can consider a particular primaryevent,theassociatedsecondaryevents,andtheeffectonaTABLE2 Some Chemical and Biochemical Reactions ThatCan Lead to Alteration ofFood QualityorSafetyExamplesTypes of reactionNonenzymic browningBaked goodsEnzymic browningCut fruitsOxidationLipids (off-flavors), vitamin degradation, pigmentdecoloration,proteins (loss of nutritive value)HydrolysisLipids, proteins, vitamins, carbohydrates, pigmentsMetal interactionsComplexation (anthocyanins),loss of Mg fromchlorophyll,catalysis ofoxidationCis trans, nonconjugated→conjugatedLipid isomerizationLipid cyclizationMonocyclic fatty acidsLipid polymerizationFoaming during deep fat fryingProtein denaturationEgg white coagulation,enzyme inactivationProtein cross-linkingLoss ofnutritive value during alkali processingPolysaccharide synthesisIn plants postharvestAnimaltissuepostmortem,planttissuepostharvestGlycolytic changes
Pag e 8 TABLE 1 Classification of Alterations That Can Occur in Food During Handling , Processing , or Storag e Attribute Alteration Texture Loss of solubility Loss of water-holding capacity Toug hening Softening Flavor Development of: Rancidity (hydrolytic or oxidative) Cooked or caramel flavors Other off-flavors Desirable flavors Color Darkening Bleaching Development of other off-colors Development of desirable colors (e.g ., browning of baked g oods) Nutritive value Loss, deg radation or altered bioavailability of proteins, lipids, vitamins, minerals Safety Generation of toxic substances Development of substances that are protective to health Inactivation of toxic substances secondary events, which, in turn, become evident as altered quality attributes (Table 1). Examples of sequences of this type are shown in Table 3. Note particularly that a given quality attribute can be altered as a result of several different primary events. The sequences in Table 3 can be applied in two directions. Operating from left to right one can consider a particular primary event, the associated secondary events, and the effect on a TABLE 2 Some Chemical and Biochemical Reactions That Can Lead to Alteration of Food Quality or Safety Types of reaction Examples Nonenzymic browning Baked g oods Enzymic browning Cut fruits Oxidation Lipids (off-flavors), vitamin deg radation, pig ment decoloration, proteins (loss of nutritive value) Hydrolysis Lipids, proteins, vitamins, carbohydrates, pig ments Metal interactions Complexation (anthocyanins), loss of Mg from chlorophyll, catalysis of oxidation Lipid isomerization Cis trans, nonconjug ated conjug ated Lipid cyclization Monocyclic fatty acids Lipid polymerization Foaming during deep fat frying Protein denaturation Eg g white coag ulation, enzyme inactivation Protein cross-linking Loss of nutritive value during alkali processing Polysaccharide synthesis In plants postharvest Glycolytic chang es Animal tissue postmortem, plant tissue postharvest
Page9TABLE 3 Cause-and-Effect Relationships Pertaining to Food Alterations During Handling,Storage,andProcessingSecondary eventPrimarycausativeeventAttributeinfluenced(seeTable1)Hydrolysis of lipidsTexture, flavor, nutritive valueFree fattyacids reactwith proteinHydrolysis ofSugars react with proteinsTexture, flavor, color, nutritivevaluepolysaccharidesOxidation of lipidsOxidationproducts reactwithmanyotherTexture,flavor,color,nutritiveconstituentsvalue; toxic substances can begeneratedBruising of fruitCellsbreak,enzymes arereleased,oxygenTexture, flavor, color,nutritiveaccessiblevalueHeating ofgreenTexture,flavor,color,nutritiveCell walls and membranes lose integrity,vegetablesvalueacidsarereleased,enzymesbecomeinactiveHeating ofmuscle tissueProteins denature and aggregate, enzymesTexture, flavor,color,nutritivebecome inactivevalueCis →trans conversionsEnhanced rate of polymerization duringExcessivefoamingduringdeepfatin lipidsdeepfatfiryingfrying,diminished bioavailabilityof lipidsquality attribute. Alternatively, one can determine the probable cause(s) of an observed quality change (column 3, Table 3) byconsidering all primary events that could be involved and then isolating,by appropriate chemical tests, the key primary event.The utility of constructing such sequences is that they encourage one to approach problems of food alteration in an analyticalmanner.Figure1isasimplisticsummaryofreactionsandinteractionsofthemajorconstituentsP02, HEATOXIDIZEDPPEROXIDESLCATALYSTSHEATOFF-FLAVORSPIGMENTSSTRONGACIDOFF-COLORSVITAMINSSTRONG BASEREACTIVELOSSOFCFLAVORSCARBONYLSNUTRITIVEVALUELOSSOF/INTERMEDLATETEXTUREWATERACTIMITYAMBIENTTOELEVATEDTEMPERATUREPFIGURE1Summaryofchemical interactions amongmajorfood constituents:L,lipid pool(triacylglycerols,fattyacids,andphospholipids);C,carbohydratepool(polysaccharides,sugars,organicacids,andsoon),P,proteinpool (proteins,peptides,amino acids,and otherN-containing substances)
Pag e 9 TABLE 3 Cause-and-Effect Relationships Pertaining to Food Alterations During Handling , Storag e, and Processing Primary causative event Secondary event Attribute influenced (see Table 1) Hydrolysis of lipids Free fatty acids react with protein Texture, flavor, nutritive value Hydrolysis of polysaccharides Sug ars react with proteins Texture, flavor, color, nutritive value Oxidation of lipids Oxidation products react with many other constituents Texture, flavor, color, nutritive value; toxic substances can be g enerated Bruising of fruit Cells break, enzymes are released, oxyg en accessible Texture, flavor, color, nutritive value Heating of g reen veg etables Cell walls and membranes lose integ rity, acids are released, enzymes become inactive Texture, flavor, color, nutritive value Heating of muscle tissue Proteins denature and ag g reg ate, enzymes become inactive Texture, flavor, color, nutritive value Cis trans conversions in lipids Enhanced rate of polymerization during deep fat frying Excessive foaming during deep fat frying ; diminished bioavailability of lipids quality attribute. Alternatively, one can determine the probable cause(s) of an observed quality change (column 3, Table 3) by considering all primary events that could be involved and then isolating, by appropriate chemical tests, the key primary event. The utility of constructing such sequences is that they encourage one to approach problems of food alteration in an analytical manner. Figure 1 is a simplistic summary of reactions and interactions of the major constituents FIGURE 1 Summary of chemical interactions among major food constituents: L, lipid pool (triacylg lycerols, fatty acids, and phospholipids); C, carbohydrate pool (polysaccharides, sug ars, org anic acids, and so on); P, protein pool (proteins, peptides, amino acids, and other N-containing substances)
Page 10TABLE4 Important Factors Governing the Stability ofFoods During Handling,Processing,and StorageProductfactorschemical properties ofindividual constituents (includingcatalysts),oxygen content,pH,wateractivity,gandwgEnvironmental factorstemperature T),time (t).composition ofthe atmosphere,chemical, physical or biological treatments imposed,exposure to light,contamination,physicalabuseNote.Wateractivityp/po,wherepisthepartialpressureofwatervaporabovethefoodanboisthevaporpressureofpurewater,gistheglasstransitiontemperatureWistheproductwatercontentatof food. The major cellular pools of carbohydrates, lipids, proteins, and their intermediary metabolites are shown on the left-hand side of the diagram. The exact nature of these pools is dependent on the physiological state ofthe tissue at the time ofprocessing or storage, and the constituents present in or added to nontissue foods. Each class of compound can undergo its owncharacteristic type of deterioration. Noteworthy is the role that carbonyl compounds play in many deterioration processes. Theyarise mainlyfrom lipid oxidation and carbohydrate degradation,and can lead tothedestruction ofnutritional value,to off-colorsand to off-flavors. Of course these same reactions lead to desirable flavors and colors during the cooking of many foods.1.3.4 Analysis of Situations Encountered During the Storage and Processing of FoodHaving before us a description of the attributes of high-quality, safe foods, the significant chemical reactions involved in thedeterioration of food, and the relationship between the two, we can now begin to consider how to apply this information tosituations encountered during the storage and processing of food.The variables that are important during the storage and processing of food are listed in Table 4. Temperature is perhaps the mostimportant of these variables because of its broad influence on all types of chemical reactions. The effect of temperature on anindividual reaction can be estimated from the Arrhenius equation, k = Ae-AE/RT, Data conforming to the Arrhenius equation yielda straight line whenlogk is plottedversus1/T.Arrhenius plots inFigure2representreactions important infood deterioration.ItisevidentthatfoodreactionsgenerallyconformtotheArrheniusrelationshipovera limitedintermediatetemperaturerangebutthatdeviationsfromthisrelationshipcanoccurat highorlowtemperatures[21].Thus,itis importanttorememberthattheArrheniusrelationshipforfoodsystemsisvalidonlyoverarangeoftemperaturethathasbeenexperimentallyverified.Deviations from the Arrhenius relationship can occur because of the following events, most of which are induced by either highorlowtemperatures:(a)enzymeactivitymaybelost,(b)thereactionpathwaymaychangeormaybeinfluencedbyacompetingreaction(s),(c)thephysical stateofthesystemmaychange(e.g.,byfreezing),or(d)oneormoreofthereactantsmaybecomedepleted.Another important factor in Table 4 is time. During storage of a food product, one frequently wants to know how long the foodcan be expected to retain a specified level of quality.Therefore, one is interested in time with respect to the integral of chemicaland/ormicrobiologicalchangesthatoccurduringaspecifiedstorageperiod,andinthewaythesechangescombinetodeterminea specified storage life for the product. During processing, one is often
Pag e 10 TABLE 4 Important Factors Governing the Stability of Foods During Handling , Processing , and Storag e Product factors: chemical properties of individual constituents (including catalysts), oxyg en content, pH, water activity, T g and W g Environmental factors: temperature (T), time (t), composition of the atmosphere, chemical, physical or biolog ical treatments imposed, exposure to lig ht, contamination, physical abuse Note. W ater activity = p/p 0, where p is the partial pressure of water vapor above the food and p 0 is the vapor pressure of pure water; T g is the g lass transition temperature; W g is the product water content at T g. of food. The major cellular pools of carbohydrates, lipids, proteins, and their intermediary metabolites are shown on the lefthand side of the diagram. The exact nature of these pools is dependent on the physiological state of the tissue at the time of processing or storage, and the constituents present in or added to nontissue foods. Each class of compound can undergo its own characteristic type of deterioration. Noteworthy is the role that carbonyl compounds play in many deterioration processes. They arise mainly from lipid oxidation and carbohydrate degradation, and can lead to the destruction of nutritional value, to off-colors, and to off-flavors. Of course these same reactions lead to desirable flavors and colors during the cooking of many foods. 1.3.4 Analysis of Situations Encountered During the Storage and Processing of Food Having before us a description of the attributes of high-quality, safe foods, the significant chemical reactions involved in the deterioration of food, and the relationship between the two, we can now begin to consider how to apply this information to situations encountered during the storage and processing of food. The variables that are important during the storage and processing of food are listed in Table 4. Temperature is perhaps the most important of these variables because of its broad influence on all types of chemical reactions. The effect of temperature on an individual reaction can be estimated from the Arrhenius equation, k = Ae -DE/RT . Data conforming to the Arrhenius equation yield a straight line when logk is plotted versus 1/T. Arrhenius plots in Figure 2 represent reactions important in food deterioration. It is evident that food reactions generally conform to the Arrhenius relationship over a limited intermediate temperature range but that deviations from this relationship can occur at high or low temperatures [21]. Thus, it is important to remember that the Arrhenius relationship for food systems is valid only over a range of temperature that has been experimentally verified. Deviations from the Arrhenius relationship can occur because of the following events, most of which are induced by either high or low temperatures: (a) enzyme activity may be lost, (b) the reaction pathway may change or may be influenced by a competing reaction(s), (c) the physical state of the system may change (e.g., by freezing), or (d) one or more of the reactants may become depleted. Another important factor in Table 4 is time. During storage of a food product, one frequently wants to know how long the food can be expected to retain a specified level of quality. Therefore, one is interested in time with respect to the integral of chemical and/or microbiological changes that occur during a specified storage period, and in the way these changes combine to determine a specified storage life for the product. During processing, one is often
Page llNONENZYMICINENOEENZYMECATALYZEDd0°℃1(OK-)TEMPERATUREFIGURE2Conformityof importantdeteriorative reactionsin food to the Arrhenius relationship. (a)Abovea certainvaluedftheremaybedeviations from linearity due to a changein the path ofthe reaction. (b) As thetemperature is lowered below the freezingpointofthe system,the ice phase (essentiallypure)enlarges and the fluid phase, whichcontains all the solutes, diminishes.Thisconcentrationofsolutesintheunfrozenphasecandecrease reactionrates (supplementtheeffectofdecreasingtemperature)orincreasereaction rates (opposethe effectofdecliningtemperature),dependingonthenatureofthe system (see Chap.2). (c) For an enzymicreactionthere is atemperature inthevicinityofthefreezing pointof water where subtlechanges, such as the dissociation ofanenzyme complex, can lead to a sharpdeclinein reactionrate.interested in thetime it takes to inactivatea particularpopulation of microorganisms or in howlong it takesforareaction toproceed to a specified extent. For example, it may be of interest to know how long it takes to produce a desired brown color inpotato chips during frying. To accomplish this, attention must be given to temperature change with time, that is, the rate oftemperature change (dT/di). This relationship is important because it determines the rate at which microorganisms are destroyedand the relative rates of competing chemical reactions. The latter is of interest in foods that deteriorate by more than one means,such as lipid oxidation and nonenzymic browning. If the products of the browning reaction are antioxidants, it is important toknowwhethertherelativeratesofthesereactionsaresuchthatasignificant interactionwill occurbetweenthem
Pag e 11 FIGURE 2 Conformity of important deteriorative reactions in food to the Arrhenius relationship. (a) Above a certain value of T there may be deviations from linearity due to a chang e in the path of the reaction. (b) As the temperature is lowered below the freezing point of the system, the ice phase (essentially pure) enlarg es and the fluid phase, which contains all the solutes, diminishes. This concentration of solutes in the unfrozen phase can decrease reaction rates (supplement the effect of decreasing temperature) or increase reaction rates (oppose the effect of declining temperature), depending on the nature of the system (see Chap. 2). (c) For an enzymic reaction there is a temperature in the vicinity of the freezing point of water where subtle chang es, such as the dissociation of an enzyme complex, can lead to a sharp decline in reaction rate. interested in the time it takes to inactivate a particular population of microorganisms or in how long it takes for a reaction to proceed to a specified extent. For example, it may be of interest to know how long it takes to produce a desired brown color in potato chips during frying. To accomplish this, attention must be given to temperature change with time, that is, the rate of temperature change (dT/dt). This relationship is important because it determines the rate at which microorganisms are destroyed and the relative rates of competing chemical reactions. The latter is of interest in foods that deteriorate by more than one means, such as lipid oxidation and nonenzymic browning. If the products of the browning reaction are antioxidants, it is important to know whether the relative rates of these reactions are such that a significant interaction will occur between them