《食品化学》课程教学资源（书籍文献）Principles of Food Chemistry，Third Edition，John M. deMan, PhD

24PRINCIPLESOFFOODCHEMISTRYstorage the rates rapidly went up.In thethe sample kept at 0.70 all through the experiment, because the enzyme was partiallyregion of monomolecular adsorption,enzy-micreactionseitherdidnotproceedatall orinactive during storage.Nonenzymic browning or Maillard reac-proceeded at a greatly reduced rate, whereasin the region of capillary condensation thetions are one of the most important factorsreaction rates increased greatly.Ackerfoundcausing spoilage in foods. These reactionsthat for reactions in which lipolytic enzymeare strongly dependent on wateractivity andactivity was measured, the manner in whichreachamaximumrateatavalues of 0.6tocomponents of the food system were put into0.7(Loncin et al.1968).This is illustrated bycontact significantly influenced the enzymethebrowning of milkpowderkeptat 40°℃activity.Separation of substrate and enzymefor 10 days as a function of water activitycould greatly retard the reaction. Also, the(Figure 1-29). The loss in lysine resultingsubstrate has to be in liquid form; for exam-from the browning reaction parallels theple, liquid oil could be hydrolyzed at watercolor change, as is shown in Figure 1-30activity as low as 0.15, but solid fat was onlyLabuza et al. (1970) have shown that, evenslightlyhydrolyzed.Oxidizingenzymesat low water activities,sucrose may bewere affected by water activity in about thehydrolyzed toform reducing sugars that maysame way as hydrolytic enzymes, as wastake part in browning reactions.Browningshown by the example of phenoloxidasereactions are usually slow at low humiditiesfrom potato (Figure 1-28).When the lower aand increase to a maximum in the range ofvalues were increased to 0.70 after 9 days ofintermediate-moisture foods. Beyond thisstorage,the final values were lower than withrange the rate again decreases. This behaviorcow=0,70S0.7040,55CHANGEOFaw0,40&0.25115510STORAGETIME,DAYSFigure1-28 Enzymic Browning in the System Polyphenoloxidase-Cellulose-Catechol at25C andDifferent Water Activities.Lowerawvalues were changed to O.70 after 9 days.Source: From L.Acker,WaterActivityandEnzymeActivity,FoodTechnol.,Vol.23,pp.1257-1270,1969

storage the rates rapidly went up. In the region of monomolecular adsorption, enzy￾mic reactions either did not proceed at all or proceeded at a greatly reduced rate, whereas in the region of capillary condensation the reaction rates increased greatly. Acker found that for reactions in which lipolytic enzyme activity was measured, the manner in which components of the food system were put into contact significantly influenced the enzyme activity. Separation of substrate and enzyme could greatly retard the reaction. Also, the substrate has to be in liquid form; for exam￾ple, liquid oil could be hydrolyzed at water activity as low as 0.15, but solid fat was only slightly hydrolyzed. Oxidizing enzymes were affected by water activity in about the same way as hydrolytic enzymes, as was shown by the example of phenoloxidase from potato (Figure 1-28). When the lower a values were increased to 0.70 after 9 days of storage, the final values were lower than with the sample kept at 0.70 all through the exper￾iment, because the enzyme was partially inactive during storage. Nonenzymic browning or Maillard reac￾tions are one of the most important factors causing spoilage in foods. These reactions are strongly dependent on water activity and reach a maximum rate at a values of 0.6 to 0.7 (Loncin et al. 1968). This is illustrated by the browning of milk powder kept at 4O0C for 10 days as a function of water activity (Figure 1-29). The loss in Iysine resulting from the browning reaction parallels the color change, as is shown in Figure 1-30. Labuza et al. (1970) have shown that, even at low water activities, sucrose may be hydrolyzed to form reducing sugars that may take part in browning reactions. Browning reactions are usually slow at low humidities and increase to a maximum in the range of intermediate-moisture foods. Beyond this range the rate again decreases. This behavior % DECREASE IN TRANSMITTANCE STORAGE TIME, DAYS Figure 1-28 Enzymic Browning in the System Polyphenoloxidase-Cellulose-Catechol at 250C and Different Water Activities. Lower aw values were changed to 0.70 after 9 days. Source: From L. Acker, Water Activity and Enzyme Activity, Food Technol, Vol. 23, pp. 1257-1270, 1969. CHANGE OF QW

25Water0.6EE0.50.40.30.20.400.530.680.75WATERACTIVITYFigure 1-29 Color Change of Milk Powder Kept at 40C for 10 Days as a Function of Water Activitycan be explained by the fact that, in the inter-those giving monolayer coverage appears tomediate range, the reactants are all dissolved,give maximum protection against oxidationand that further increase in moisture contentThishasbeendemonstratedbyMartinezandleads to dilution of the reactants.Labuza(1968)withtheoxidationof lipids inThe effect of water activityon oxidation offreeze-dried salmon (Figure 1-31).Oxida-fats is complex. Storage of freeze-dried andtion of the lipids was reduced as water con-dehydrated foods at moisture levels abovetent increased. Thus, conditions that are880.75SENE0.500.250.750.400.530.68WATERACTIVITYFigure1-30 Loss of FreeLysine in Milk Powder Kept at40°C for10Days as a Functionof WaterActivity.Source: From M.Loncin, J.J.Bimbenet, and J.Lenges, Influence of the Activity of Water ontheSpoilageofFoodstuffs,J.FoodTechnol.,Vol.3,pp.131-142,1968

can be explained by the fact that, in the inter￾mediate range, the reactants are all dissolved, and that further increase in moisture content leads to dilution of the reactants. The effect of water activity on oxidation of fats is complex. Storage of freeze-dried and dehydrated foods at moisture levels above those giving monolayer coverage appears to give maximum protection against oxidation. This has been demonstrated by Martinez and Labuza (1968) with the oxidation of lipids in freeze-dried salmon (Figure 1-31). Oxida￾tion of the lipids was reduced as water con￾tent increased. Thus, conditions that are WATER ACTIVITY Figure 1-29 Color Change of Milk Powder Kept at 4O0C for 10 Days as a Function of Water Activity YELLO W INDE X WATER ACTIVITY Figure 1-30 Loss of Free Lysine in Milk Powder Kept at 4O0C for 10 Days as a Function of Water Activity. Source: From M. Loncin, JJ. Bimbenet, and J. Lenges, Influence of the Activity of Water on the Spoilage of Foodstuffs, J. Food Technol, Vol. 3, pp. 131-142, 1968. LYSINE LOS S %

26PRINCIPLES OFFOOD CHEMISTRYm..YERI-ICXYGENPERKO.LIPID7660LAYEA-S2RHDAOOVEMOPOL700TIME-HOURSFigure 1-31 Peroxide Production in Freeze-Dried Salmon Stored at Different Relative Humidities.Source:From F.Martinez and T.P. Labuza, Effect of Moisture Content on Rate of Deterioration ofFreeze-DriedSalmon,J.Food Sci.,Vol.33,pp.241-247,1968of moisture with respect to relative humidityoptimal for protection against oxidation may(M/△RH),calculated from sorption isobe conducive to other spoilage reactions,therms,is related toproduct stabilitysuchasbrowning,Water activity may affect the properties ofThe interaction between water and poly-powdered dried product. Berlin et al. (1968)mer moleculesin gel formation has beenstudied the effect of water vapor sorption onreviewedbyBusk (1984).theporosity of milkpowders.When thepow-ders were equilibrated at 50 percent relativeWATERACTIVITYANDPACKAGINGhumidity (RH),the microporous structureBecause water activity is a major factorwasdestroyed.Thefreefatcontentwascon-siderably increased, which also indicatesinfluencing the keeping quality of a numberstructural changes.of foods, it is obvious that packaging can doOther reactions that may be influenced bymuchtomaintain optimalconditions for longwater activity are hydrolysis of protopectin,storage life. Sorption isotherms play ansplitting and demethylation of pectin, auto-important role in the selection of packagingcatalytic hydrolysis of fats, and the transfor-materials.Hygroscopicproducts always havemationofchlorophyll intopheophytina steep sorption isotherm and reach the criti-(Loncin et al. 1968).cal area of moisture content before reachingRockland(1969)has introduced theconexternal climatic conditions. Such foodscept of local isotherm to provide a closerhave to be packaged in glass containers withrelationship between sorptionisotherms andmoistureproof seals or in watertight plasticstability than is possible with other methods.(thick polyvinylchloride).For example, con-He suggested that the differential coefficientsider instant coffee, where the critical area is

optimal for protection against oxidation may be conducive to other spoilage reactions, such as browning. Water activity may affect the properties of powdered dried product. Berlin et al. (1968) studied the effect of water vapor sorption on the porosity of milk powders. When the pow￾ders were equilibrated at 50 percent relative humidity (RH), the microporous structure was destroyed. The free fat content was con￾siderably increased, which also indicates structural changes. Other reactions that may be influenced by water activity are hydrolysis of protopectin, splitting and demethylation of pectin, auto￾catalytic hydrolysis of fats, and the transfor￾mation of chlorophyll into pheophytin (Loncin et al. 1968). Rockland (1969) has introduced the con￾cept of local isotherm to provide a closer relationship between sorption isotherms and stability than is possible with other methods. He suggested that the differential coefficient of moisture with respect to relative humidity (AM/ARH), calculated from sorption iso￾therms, is related to product stability. The interaction between water and poly￾mer molecules in gel formation has been reviewed by Busk (1984). WATER ACTIVITY AND PACKAGING Because water activity is a major factor influencing the keeping quality of a number of foods, it is obvious that packaging can do much to maintain optimal conditions for long storage life. Sorption isotherms play an important role in the selection of packaging materials. Hygroscopic products always have a steep sorption isotherm and reach the criti￾cal area of moisture content before reaching external climatic conditions. Such foods have to be packaged in glass containers with moistureproof seals or in watertight plastic (thick polyviny!chloride). For example, con￾sider instant coffee, where the critical area is Figure 1-31 Peroxide Production in Freeze-Dried Salmon Stored at Different Relative Humidities. Source: From F. Martinez and T.P. Labuza, Effect of Moisture Content on Rate of Deterioration of Freeze-Dried Salmon, J. Food ScL, Vol. 33, pp. 241-247, 1968. TIME - HOURS 0IdH Ox «3d N30AXO •'"> MiCROLlTERS OXYGEN P£R GRAM LlPiO

27WaterThe initial moisture content of B is X,andat about50percentRH.Under these condi-tions the product cakes and loses itsafter equilibration with A, the moisture con-flowability. Other products might not betent is X.The substances A and B will reachhygroscopic and no unfavorable reactionsa mean relative humidityof about40percent,occur at normal conditions of storage.Suchbut not a mean moisture content. If this wereproducts can be packaged in polyethylenea dry soup mix and the sensitive componentcontainers.was a freeze-dried vegetable with a moistureThere are some foods where the equilibcontent of 2percent andthe other compo-rium relative humidity is above that of thenent, a starch or flour with a moisture con-external climatic conditions.The packagingtent of 13 percent, the vegetable would bemoistened to up to 9 percent. This wouldmaterial then serves the purpose of protect-ing the product from moisture loss.This isresult in rapid quality deterioration due tothe case with processed cheese and bakednonenzymic browning reactions. In this case,goods.the starch would have to be postdried.Differentproblems mayarisein compositeSalwin and Slawson(1959)found that sta-foods, such as soup mixes, where several dis-bility in dehydrated foods was impaired iftinct ingredients are packaged together. Inseveral products were packaged together.AFigure1-32,for example,substanceB withtransfer ofwater couldtakeplacefrom itemsthe steep isotherm is more sensitive to mois-of higher moisture-vapor pressure to thoseof lower moisture-vapor pressure. Theseture, and is mixed in equal quantities withsubstance A in an impermeable package.*authors determined packaging compatibilityby examining the respective sorption iso-therms.They suggested a formula for calcu-*The initial relative humidity of A is 65 percentlation of the final equilibrium moisturecontent of each component from the iso-and of B, 15 percent.B专-X1-A----651540REL.HUM.%Figure 1-32 Sorption Isotherms of Materials A and B

at about 50 percent RH. Under these condi￾tions the product cakes and loses its flowability. Other products might not be hygroscopic and no unfavorable reactions occur at normal conditions of storage. Such products can be packaged in polyethylene containers. There are some foods where the equilib￾rium relative humidity is above that of the external climatic conditions. The packaging material then serves the purpose of protect￾ing the product from moisture loss. This is the case with processed cheese and baked goods. Different problems may arise in composite foods, such as soup mixes, where several dis￾tinct ingredients are packaged together. In Figure 1-32, for example, substance B with the steep isotherm is more sensitive to mois￾ture, and is mixed in equal quantities with substance A in an impermeable package.* *The initial relative humidity of A is 65 percent and of B, 15 percent. The initial moisture content of B is X1, and after equilibration with A, the moisture con￾tent is X2. The substances A and B will reach a mean relative humidity of about 40 percent, but not a mean moisture content. If this were a dry soup mix and the sensitive component was a freeze-dried vegetable with a moisture content of 2 percent and the other compo￾nent, a starch or flour with a moisture con￾tent of 13 percent, the vegetable would be moistened to up to 9 percent. This would result in rapid quality deterioration due to nonenzymic browning reactions. In this case, the starch would have to be postdried. Salwin and Slawson (1959) found that sta￾bility in dehydrated foods was impaired if several products were packaged together. A transfer of water could take place from items of higher moisture-vapor pressure to those of lower moisture-vapor pressure. These authors determined packaging compatibility by examining the respective sorption iso￾therms. They suggested a formula for calcu￾lation of the final equilibrium moisture content of each component from the iso￾MOISTUR E % REL HUM. % Figure 1-32 Sorption Isotherms of Materials A and B

28PRINCIPLES OFFOOD CHEMISTRYtherms of the mixed food and its equilibriumalsoonthenondissociablecarboxylandimino groups of the peptidebonds.The bind-relative humidity:ingofwater is stronglyinfluencedbythepHof meat.The effect of pH on the swelling or(W -S, -aw1)+(W2 -S2 aw2)unswelling (that is, water-binding capacity ofaw(W,.S,)+(W2.S2)proteins)is schematically represented in Figure 1-34 (Honkel 1989).The second portionof the curve corresponds to multilayer ad-wheresorption, which amounts to another 4 to 6W,= gram solids of ingredient 1percent of water.Hamm (1962) considereds.= linear slope of ingredient 1these two quantities of water to represent the= initial aw of ingredient 1awlreal water of hydration and found them toamount to between50 and 60g per 100 g ofprotein. Muscle binds much more than thisWATERBINDINGOFMEATamount of water.Meat with a protein contentof 20 to22percent contains74 to 76percentAccording to Hamm (1962), the water-water,so that100g of protein binds aboutbinding capacity of meat is caused by the350 to 360 g of water. This ratio is evenmuscleproteins.Some 34percent of thesehigher in fish muscle.Most of this water isproteins are water-soluble. The main portionmerely immobilized-retained by the net-of meatproteinsisstructuralmaterial.Onlyabout 3 percent of the total water-bindingcapacity of muscle can be attributed towater-soluble (plasma)proteins.The mainwater-binding capacity of muscle can be24attributed to actomyosin,the main compo-nent of the myofibrils.The adsorption iso-20therm of freeze-dried meat has the shape%shown in Figure 1-33.The curve is similarto the sorption isotherms of other foods and16consists of three parts. The first part corre-sponds to the tightly bound water,about 412percent, which is given off at very low vaporpressures. This quantity is only about one-8fifth the total quantity required to cover thewhole protein with a monomolecular layer.This water is bound under simultaneous lib-eration ofa considerable amount ofenergy,3to 6 kcal per mole of water. The binding ofthis water results in a volume contraction of0204060801000.05 mL per g of protein. The binding isREL.HUM.%localized at hydrophilic groups on proteinsFigure 1-33 Adsorption Isotherm of Freeze-such as polar side chains having carboxylDried Meatamino, hydroxyl, and sulphydryl groups and

therms of the mixed food and its equilibrium relative humidity: _ (W1- Vg w l O + (W2-52-flw2,) (W1-S1)H-(W2-S2) where W1 = gram solids of ingredient 1 S1 = linear slope of ingredient 1 aw[' = initial aw of ingredient 1 WATER BINDING OF MEAT According to Hamm (1962), the water￾binding capacity of meat is caused by the muscle proteins. Some 34 percent of these proteins are water-soluble. The main portion of meat proteins is structural material. Only about 3 percent of the total water-binding capacity of muscle can be attributed to water-soluble (plasma) proteins. The main water-binding capacity of muscle can be attributed to actomyosin, the main compo￾nent of the myofibrils. The adsorption iso￾therm of freeze-dried meat has the shape shown in Figure 1-33. The curve is similar to the sorption isotherms of other foods and consists of three parts. The first part corre￾sponds to the tightly bound water, about 4 percent, which is given off at very low vapor pressures. This quantity is only about one￾fifth the total quantity required to cover the whole protein with a monomolecular layer. This water is bound under simultaneous lib￾eration of a considerable amount of energy, 3 to 6 kcal per mole of water. The binding of this water results in a volume contraction of 0.05 mL per g of protein. The binding is localized at hydrophilic groups on proteins such as polar side chains having carboxyl, amino, hydroxyl, and sulphydryl groups and also on the nondissociable carboxyl and imino groups of the peptide bonds. The bind￾ing of water is strongly influenced by the pH of meat. The effect of pH on the swelling or unswelling (that is, water-binding capacity of proteins) is schematically represented in Fig￾ure 1-34 (Honkel 1989). The second portion of the curve corresponds to multilayer ad￾sorption, which amounts to another 4 to 6 percent of water. Hamm (1962) considered these two quantities of water to represent the real water of hydration and found them to amount to between 50 and 60 g per 100 g of protein. Muscle binds much more than this amount of water. Meat with a protein content of 20 to 22 percent contains 74 to 76 percent water, so that 100 g of protein binds about 350 to 360 g of water. This ratio is even higher in fish muscle. Most of this water is merely immobilized—retained by the net- WATE R CONTEN T % REL HUM. % Figure 1-33 Adsorption Isotherm of Freeze￾Dried Meat