19Waterof high viscosity that exists in a metastablepoint about halfway between Tm and T..Atsolid state (Levine and Slade 1992).A glasshigh cooling rates and a degree of supercool-is formed when a liquid or an aqueous solu-ing that moves the temperature to below T,tion is cooled to a temperature that is consid-no crystals are formed and a glassy soliderably lower than its melting temperature.results.During the transition from themoltenThis is usually achieved at high coolingstate to theglassy state,themoisture contentrates.The normal process of crystallizationplays an important role. This is illustrated byinvolves the conversion of a disordered lig-the phase diagram of Figure 1-22. When theuid molecular structure to a highly orderedtemperature is lowered at sufficiently highcrystal formation. In a crystal, atoms or ionsmoisture content, the system goes through aare arranged in a regular, three-dimensionalrubbery state before becoming glassy (Chir-array.In the formation of a glass, the disor-ife and Buera 1996). The glass transitiondered liquid state is immobilized into a disor-temperature is characterized by very highapparentviscosities of more than 105 Ns/m2dered glassy solid, which has the rheological(Aguilera and Stanley 1990).The rate of difproperties of a solid but no ordered crystal-linestructure.fusion limited processes is more rapid in theThe relationships among melting pointrubbery state than in theglassy state,and this(Tm), glass transition temperature (T,), andmay be important in the storage stability ofcrystallization are schematically representedcertain foods.The effect of water activity oninFigure 1-21.At low degree of supercool-the glass transition temperature of a numbering (just below Tm), nucleation is at a mini-of plantproducts(carrots,strawberries,andmum and crystal growth predominates. Aspotatoes)as well as some biopolymers (gela-the degree of supercooling increases, nucle-tin, wheat gluten, and wheat starch)is shownation becomes the dominating effect. Thein Figure 1-23 (Chirife and Buera 1996).Inmaximum overall crystallization rate is at athe rubbery state the rates of chemical reac-liquid9eCrystalGrowthCrystalNucleationlce + liquidTgCrystallizationRateglassTT.1000CONCENTRATION%Figure 1-21 Relationships Among CrystalFigure 1-22Phase Diagram Showing the EffectGrowth, Nucleation,and Crystallization RatebetweenMelting Temperature(Tm)and Glassof Moisture Content on Melting TemperatureTemperature (Tg)(Tm)and Glass Transition Temperature (T
of high viscosity that exists in a metastable solid state (Levine and Slade 1992). A glass is formed when a liquid or an aqueous solution is cooled to a temperature that is considerably lower than its melting temperature. This is usually achieved at high cooling rates. The normal process of crystallization involves the conversion of a disordered liquid molecular structure to a highly ordered crystal formation. In a crystal, atoms or ions are arranged in a regular, three-dimensional array. In the formation of a glass, the disordered liquid state is immobilized into a disordered glassy solid, which has the rheological properties of a solid but no ordered crystalline structure. The relationships among melting point (Tm), glass transition temperature (Tg), and crystallization are schematically represented in Figure 1-21. At low degree of supercooling (just below Tm), nucleation is at a minimum and crystal growth predominates. As the degree of supercooling increases, nucleation becomes the dominating effect. The maximum overall crystallization rate is at a point about halfway between Tm and Tg. At high cooling rates and a degree of supercooling that moves the temperature to below Tg, no crystals are formed and a glassy solid results. During the transition from the molten state to the glassy state, the moisture content plays an important role. This is illustrated by the phase diagram of Figure 1-22. When the temperature is lowered at sufficiently high moisture content, the system goes through a rubbery state before becoming glassy (Chirife and Buera 1996). The glass transition temperature is characterized by very high apparent viscosities of more than 105 Ns/m2 (Aguilera and Stanley 1990). The rate of diffusion limited processes is more rapid in the rubbery state than in the glassy state, and this may be important in the storage stability of certain foods. The effect of water activity on the glass transition temperature of a number of plant products (carrots, strawberries, and potatoes) as well as some biopolymers (gelatin, wheat gluten, and wheat starch) is shown in Figure 1-23 (Chirife and Buera 1996). In the rubbery state the rates of chemical reacCrystal Nucleation Crystal Growth Crystalization Rate glass liquid ice + liquid TEMPERATURE 0C Figure 1-21 Relationships Among Crystal Growth, Nucleation, and Crystallization Rate between Melting Temperature (Tm) and Glass Temperature (Tg) Figure 1-22 Phase Diagram Showing the Effect of Moisture Content on Melting Temperature (Tm) and Glass Transition Temperature (Tg) CONCENTRATION %
20PRINCIPLES OFFOODCHEMISTRYtion appear to behigher than in theglassylar weight food polymers,proteins,andstate (Roos and Karel 1991e).polysaccharides are high and cannot beWhen water-containing foods are cooleddetermined experimentally,because of ther-below the freezing point of water, ice may bemaldecomposition.Anexampleofmeasuredformed and the remaining water is increas-T。values for low molecular weight carbohy-drates is given in Figure 1-24. The value ofingly highindissolved solids.When theT,for starch is obtained by extrapolation.glass transitiontemperature is reached, theremaining water is transformed into a glassThe water present in foods may act as aIce formation during freezing may destabi-plasticizer.Plasticizers increase plasticityand flexibility offood polymers as a result oflize sensitive products by rupturing cell wallsand breaking emulsions.The presence ofweakening of the intermolecular forces exist-glass-forming substances may help preventing between molecules. Increasing waterthis from occurring. Such stabilization ofcontent decreases T..Roos and Karel (1991a)frozenproductsisknownascryoprotection,studied the plasticizing effect of water onthermal behavior and crystallization of amor-and the agents areknown as cryoprotectants.When water is rapidly removed from foodsphous food models. They found that driedfoods containing sugars behave like amorduring processes such as extrusion,drying,or freezing,a glassy state may be producedphous materials, and that small amounts of(Roos 1995). The T, values of high molecu-water decrease Tg to room temperature with9080Wheat StarchGefatin70Wheat Gluten60c50403020100-10-20SStrawberry30CPotatoes40-50Carrots-60--70-0.50.60.70.20.30.40.80.9Water activityFigure 1-23 Relationship Between Water Activity (aw) and Glass Transition Temperature (T) ofSome Plant Materials and Biopolymers. Source:Reprinted with permission fromJ.Cherifeand M.delPinarBuera,Water Activity,WaterGlass Dynamics and the Control of Microbiological Growth inFoods,CriticalReviewFoodSci.Nutr.,Vol.36,No.5,p.490,1996.CopyrightCRCPress,BocaRaton,Florida
tion appear to be higher than in the glassy state (Roos and Karel 199Ie). When water-containing foods are cooled below the freezing point of water, ice may be formed and the remaining water is increasingly high in dissolved solids. When the glass transition temperature is reached, the remaining water is transformed into a glass. Ice formation during freezing may destabilize sensitive products by rupturing cell walls and breaking emulsions. The presence of glass-forming substances may help prevent this from occurring. Such stabilization of frozen products is known as cryoprotection, and the agents are known as cryoprotectants. When water is rapidly removed from foods during processes such as extrusion, drying, or freezing, a glassy state may be produced (Roos 1995). The Tg values of high molecular weight food polymers, proteins, and polysaccharides are high and cannot be determined experimentally, because of thermal decomposition. An example of measured Tg values for low molecular weight carbohydrates is given in Figure 1-24. The value of Tg for starch is obtained by extrapolation. The water present in foods may act as a plasticizer. Plasticizers increase plasticity and flexibility of food polymers as a result of weakening of the intermolecular forces existing between molecules. Increasing water content decreases Tg. Roos and Karel (199Ia) studied the plasticizing effect of water on thermal behavior and crystallization of amorphous food models. They found that dried foods containing sugars behave like amorphous materials, and that small amounts of water decrease Tg to room temperature with Wheat Starch Gelatin Wheat Gluten Strawberry Potatoes Carrots Glas s Transitio n Temp., C Water activity Figure 1-23 Relationship Between Water Activity (aw) and Glass Transition Temperature (Tg) of Some Plant Materials and Biopolymers. Source: Reprinted with permission from J. Cherife and M. del Pinar Buera, Water Activity, Water Glass Dynamics and the Control of Microbiological Growth in Foods, Critical Review Food ScL Nutr., Vol. 36, No. 5, p. 490, © 1996. Copyright CRC Press, Boca Raton, Florida
21Water250200Starchspos23025TgCurveGABIsotheri2101506 10 600/6)1904040Maltohexaos208315MaltotrioseQM200S12350R24100Maltose58.0000.640.0030.0010.002-501/M0.2T00.40.60.8WATERACTIVITYFigure 1-24Glass Transition Temperature(T,)Figure 1-25 Modified State Diagram ShowingforMaltose,Maltose Polymers,and ExtrapoRelationshipBetween Glass TransitionTemper-lated Value for Starch.M indicates molecularature (T,), Water Activity (GAB isotherm), andweight.Source:Reprinted withpermission fromWaterContent for an ExtrudedSnack FoodY.H.Roos,Glass Transition-Related Physico-Model.Crispness is lost as water plasticizationChemical Changes in Foods, Food Technology,depresses Tto below 24℃.Plasticization isVol.49,No.10,p.98,1995, Institute of Foodindicated with critical values for water activityTechnologists.and water content. Source:Reprinted with per-mission from Y.H. Roos, Glass Transition-Related Physico-Chemical Changes in Foods.theresultofstructuralcollapseandformationFood Technology, Vol. 49, No. 10, p.99, 1995,Institute ofFoodTechnologists.of stickiness.RoosandKarel (199le)reportalinearity between water activity (aw)and T, inthe aw range of 0.1 to 0.8. This allows predic-tion of T. at the aw range typical of dehy-drated and intermediate moisture foodsshowed an increased reaction rate as waterRoos(1995)has useda combined sorptioncontent increased.isotherm and state diagram to obtain criticalwater activity and water content values thatWaterActivity and Reaction Rateresult in depressing T, to below ambientWater activity has a profound effect on thetemperature (Figure 1-25). This type of plotrate of many chemical reactions in foods andcan be used to evaluate the stability of low-on the rate of microbial growth (Labuzamoisture foods under different storage condi-1980).This information is summarized intions. When the T. is decreased to belowTable 1-9.Enzyme activity is virtually non-ambient temperature, molecules are mobi-lized because of plasticization and reactionexistent in the monolayer water(awbetween0 and 0.2).Not surprisingly,growth ofrates increase because of increased diffusion,microorganisms at this level of aw is also vir-which in turn may lead to deterioration. Roostually zero.Molds and yeasts start to grow atand Himberg(1994)andRoos etal.(1996)awbetween 0.7 and 0.8, the upper limit ofhave described howglass transition tempera-capillarywater.Bacterial growthtakesplacetures influence nonenzymatic browning inwhen aw reaches 0.8, the limit of looselymodel systems. This deteriorative reaction
Figure 1-24 Glass Transition Temperature (Tg) for Maltose, Maltose Polymers, and Extrapolated Value for Starch. M indicates molecular weight. Source: Reprinted with permission from Y.H. Roos, Glass Transition-Related PhysicoChemical Changes in Foods, Food Technology, Vol. 49, No. 10, p. 98, © 1995, Institute of Food Technologists. the result of structural collapse and formation of stickiness, Roos and Karel (199Ie) report a linearity between water activity (aw) and Tg in the aw range of 0.1 to 0.8. This allows prediction of Tg at the aw range typical of dehydrated and intermediate moisture foods. Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of lowmoisture foods under different storage conditions. When the T is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction WATER ACTIVITY Figure 1-25 Modified State Diagram Showing Relationship Between Glass Transition Temperature (Tg), Water Activity (GAB isotherm), and Water Content for an Extruded Snack Food Model. Crispness is lost as water plasticization depresses Tg to below 240C. Plasticization is indicated with critical values for water activity and water content. Source: Reprinted with permission from Y.H. Roos, Glass TransitionRelated Physico-Chemical Changes in Foods, Food Technology, Vol. 49, No. 10, p. 99, © 1995, Institute of Food Technologists. showed an increased reaction rate as water content increased. Water Activity and Reaction Rate Water activity has a profound effect on the rate of many chemical reactions in foods and on the rate of microbial growth (Labuza 1980). This information is summarized in Table 1-9. Enzyme activity is virtually nonexistent in the monolayer water (aw between O and 0.2). Not surprisingly, growth of microorganisms at this level of aw is also virtually zero. Molds and yeasts start to grow at aw between 0.7 and 0.8, the upper limit of capillary water. Bacterial growth takes place when aw reaches 0.8, the limit of loosely TEMPERATURE ( 0C) Starch Maltohexaosa Maltotriose Maltose Tg Curve GAB Isotherm 1/M TEMPERATURE ( 0C) WATER CONTENT (g/100 g of Solids)
22PRINCIPLES OF FOOD CHEMISTRYTable1-9ReactionRates inFoodsasDeterminedbyWaterActivityLooselyBoundMonolayerReactionWaterCapillary WaterWaterLowEnzyme activityZeroHighZeroLowHighMold growthLowZeroHighYeast growthZeroZeroHighBacterial growthHighZeroHydrolysisRapidincreaseZeroHighNonenzymicbrowningRapidincreaseHighHighLipid oxidationRapidincrease'Growth starts at aw of 0.7 to 0.8.bound water. Enzyme activity increasesence of awon chemical reactivity has beengradually between aw of 0.3 and 0.8, thenreviewed by Leung (1987). The relation-increases rapidly in the loosely bound watership between water activity and rates of sev-area (aw 0.8 to 1.0). Hydrolytic reactions anderal reactions and enzyme activity is pre-nonenzymic browning do not proceed in thesented graphically in Figure 1-26 (Bone1987).monolayer water range of aw (0.0 to 0.25)However, lipid oxidation rates are high inWater activity has a major effect on thethis area, passing from a minimum at aw 0.3texture of some foods,as Bourne (1986)hasto 0.4, to a maximum at aw 0.8. The influ-showninthecaseofapples.Covalent-1Free(Solute& Capilfary)StabllltyIsothermBrowningo-ReactlonROBBISNMolstorptlonIsothermFree FattyAcidso102030405060708090100WaterActivity (% R.H.)Figure 1-26Relationship Between Water Activity and a Number of Reaction Rates.Source:Reprinted withpermissionfromD.P.Bone,PracticalApplicationsofWaterActivityandMoistureRelationsin Foods,in WaterActivity:TheoryandApplicationtoFood,L.B.RocklandandL.RBeuchat,eds.,p.387,1987,bycourtesyofMarcelDekker, Inc
bound water. Enzyme activity increases gradually between aw of 0.3 and 0.8, then increases rapidly in the loosely bound water area (aw 0.8 to 1.0). Hydrolytic reactions and nonenzymic browning do not proceed in the monolayer water range of aw (0.0 to 0.25). However, lipid oxidation rates are high in this area, passing from a minimum at aw 0.3 to 0.4, to a maximum at aw 0.8. The influence of aw on chemical reactivity has been reviewed by Leung (1987). The relationship between water activity and rates of several reactions and enzyme activity is presented graphically in Figure 1-26 (Bone 1987). Water activity has a major effect on the texture of some foods, as Bourne (1986) has shown in the case of apples. Water Activity (% R.H.) Figure 1-26 Relationship Between Water Activity and a Number of Reaction Rates. Source: Reprinted with permission from D.R Bone, Practical Applications of Water Activity and Moisture Relations in Foods, in Water Activity: Theory and Application to Food, L.B. Rockland and L.R. Beuchat, eds., p. 387, 1987, by courtesy of Marcel Dekker, Inc. Free Fatty Acids Moisture Content Relativ e Activit y Autoxldatlon Microorganis m Proliferation Free (Solute A Capillary) Ionic Covalent Stability Isotherm Browning Reaction Amlhocyanwn Degradation Table 1-9 Reaction Rates in Foods as Determined by Water Activity Reaction Enzyme activity Mold growth Yeast growth Bacterial growth Hydrolysis Nonenzymic browning Lipid oxidation Monolayer Water Zero Zero Zero Zero Zero Zero High Capillary Water Low Low* Low* Zero Rapid increase Rapid increase Rapid increase Loosely Bound Water High High High High High High High 'Growth starts at aw of 0.7 to 0.8
23WaterWATERACTIVITYANDFOODwater activity can be obtained by drying orSPOILAGEbyaddingwater-solublesubstances,suchassugar to jams or salt to pickled preserves.The influence of water activity on foodBacterial growth is virtually impossiblebelow a water activity of 0.90. Molds andquality and spoilage is increasingly beingrecognized as an important factor (Rocklandyeasts are usually inhibited between 0.88 andand Nishi 1980).Moisture content and water0.80,although some osmophile yeast strainsactivityaffecttheprogressof chemical andgrowatwateractivitiesdownto0.65.microbiological spoilage reactions in foods.Most enzymes are inactive when the waterDried or freeze-dried foods, which haveactivity falls below 0.85. Such enzymesgreat storage stability, usually have waterinclude amylases,phenoloxidases,and percontents in the range of about 5 to 15 per-oxidases.However,lipases mayremaincent. The group of intermediate-moistureactive at values as low as 0.3 or even 0.1foods, such as dates and cakes, may have(Loncin et al.1968).Acker (1969)providedmoisture contents in the range of about 20 toexamples of the effect of water activity on40 percent. The dried foods correspond tosomeenzymicreactions.Amixtureofthe lower part of the sorption isotherms.Thisground barley and lecithin was stored at dif-includes water in the monolayer and multi-ferentwater activities,and theratesoflayer category.Intermediate-moisture foodshydrolysis were greatly influenced by thehave water activities generally above 0.5,value of a (Figure 1-27). When the lower aincluding the capillary water. Reduction ofvalues were changed to 0.70 after 48 days ofs 0,70160:% SISATOHAH400,650.60200,450.35: 0.25204060STORAGE TIME,DAYSFigure1-27Enzymic Splitting of Lecithin ina Mixtureof BarleyMalt and LecithinStored at 30°Cand Different Water Activities. Lower aw values were changed to 0.70 after 48 days. Source: From L.Acker,WaterActivity andEnzymeActivity,FoodTechnol.,Vol.23,pp.1257-1270,1969
WATER ACTIVITY AND FOOD SPOILAGE The influence of water activity on food quality and spoilage is increasingly being recognized as an important factor (Rockland and Nishi 1980). Moisture content and water activity affect the progress of chemical and microbiological spoilage reactions in foods. Dried or freeze-dried foods, which have great storage stability, usually have water contents in the range of about 5 to 15 percent. The group of intermediate-moisture foods, such as dates and cakes, may have moisture contents in the range of about 20 to 40 percent. The dried foods correspond to the lower part of the sorption isotherms. This includes water in the monolayer and multilayer category. Intermediate-moisture foods have water activities generally above 0.5, including the capillary water. Reduction of water activity can be obtained by drying or by adding water-soluble substances, such as sugar to jams or salt to pickled preserves. Bacterial growth is virtually impossible below a water activity of 0.90. Molds and yeasts are usually inhibited between 0.88 and 0.80, although some osmophile yeast strains grow at water activities down to 0.65. Most enzymes are inactive when the water activity falls below 0.85. Such enzymes include amylases, phenoloxidases, and peroxidases. However, lipases may remain active at values as low as 0.3 or even 0.1 (Loncin et al. 1968). Acker (1969) provided examples of the effect of water activity on some enzymic reactions. A mixture of ground barley and lecithin was stored at different water activities, and the rates of hydrolysis were greatly influenced by the value of a (Figure 1-27). When the lower a values were changed to 0.70 after 48 days of HYDROLYSIS, % STORAGE TIME, DAYS Figure 1-27 Enzymic Splitting of Lecithin in a Mixture of Barley Malt and Lecithin Stored at 3O0C and Different Water Activities. Lower aw values were changed to 0.70 after 48 days. Source: From L. Acker, Water Activity and Enzyme Activity, Food Technol, Vol. 23, pp. 1257-1270, 1969