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.This is probably due to the relatively high degree of cross-linking and/or entanglement.Molasses, honey, and batters are examples of foodsystems in the viscous flow region at room temperature.A blend of incompatible polymers will phase-separate and show more thanone Tg, as indicated in Figure 6 for sodium caseinate water and fructose watersystems (16).From all these observations, we can conclude that food systemscan indeed be viewed as synthetic polymer systems.Air cells or pockets help soften some products, such as breakfast cereals,that would otherwise be too hard.Gas cells inside some products can be frommany sources: entrapped air during mixing, yeast leavening, chemical leavening,injected inert gases, carbon dioxide, ethanol.Those gas cells will eventually havethe same gaseous composition as that of the packaged environment.One areathat differs from the synthetic polymer system is the presence of fat in foodmatrices.In bakery products, fats and emulsifiers do not affect Tg, but they dodecrease the rubbery modulus (17).That is why they are sometimes called tender-izers instead of plasticizers.The tenderizing effect at serving temperature is aCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved. Figure 4 DMTA plot for gluten samples stored under different RH values.(FromRef.13.)function of the solid fat index, fat content, and fat crystalline form.Due to theirlubricating effect, fats and emulsifiers also enhance the perceived moistness ofbakery products.Moistness also results partly because the true moisture contentof the nonfat portion is higher than the apparent moisture content, which is basedupon the total system.No wonder that fat-free bakery products usually taste dryand not as tender.III.EFFECT OF RHEOLOGICAL PROPERTIES ON THESTABILITY OF MATERIAL BEFORE, DURING, ANDAFTER PROCESSINGIn addition to having high hedonic quality during consumption, any successfulproduct in the marketplace needs to be stable throughout distribution.Duringmanufacturing, ingredients and in-process intermediates should not have caking,Copyright 2003 by Marcel Dekker, Inc.All Rights Reserved. Figure 5 Annular shear test for amorphous sorbitol: (´) storage modulus G2 , ( ) lossmodulus G3 , and (*) loss tangent.(From Ref.15.)shrinkage, or other physical instability problems.In new-product development,significant effort is on stabilizing the ingredient during storage, the processingintermediates during manufacturing, and the finished products during storage,consumer preparation, and consumption.Directly and indirectly, rheologicalproperties affect physical, chemical, and microbiological stabilities.In this chap-ter, we will focus more on the physical stability aspect.How do rheological properties affect stability? Well, in the previous sectionwe discussed the effect of physical state on rheological properties.Thus, it isphysical state that determines stability.A state diagram shows Tg as a functionof water content and solubility as a function of temperature.It also shows infor-mation on various physical changes that may occur due to the metastable stateof amorphous food solids and their approach toward equilibrium (18).Roos (19)summarized the methods of applying state diagrams in food processing and prod-uct development.LeMeste et al.(20) reviewed the relationship between physicalstates and the quality of cereal-based foods.A product in the glassy state duringstorage will be more stable, while products stored at temperatures above the Tgcurve will undergo physical changes at a rate according to William LandelFerry (WLF) kinetics (4 8).The Gordon Taylor equation can relate the effectof composition on Tg (19, 21).Thus, by combining the WLF and Gordon TaylorCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved. Figure 6 DMTA plot of sodium caseinate and fructose at a ratio of 2:1 stored at 75%RH (16% water).(From Ref.16.)equations, one can express rheological properties, such as viscosity, of food mate-rials as a function of temperature and moisture content.However, the coefficientsin those equations are not yet readily available, even for common food systems.Some of the stability problems encountered in product development will be dis-cussed here.These include caking during storage and spray-drying, structuralcollapse during processing, and loss of crispness during storage.A.Caking During Storage and ProcessingAs described in Chapter 7, caking, or the loss of free flow, is one of the mostoften encountered stability problems associated with spray-dried or freeze-driedamorphous ingredients.Ingredient caking affects plant operation efficiency.Cak-ing of wheat flour usually is of no concern if it is stored properly.However,under humid conditions, e.g., at 90% relative humidity, mold growth or infesta-tion might occur even at room temperature (22).Copyright 2003 by Marcel Dekker, Inc.All Rights Reserved. Caking is a mobility-related phenomenon associated with the physical stateof the continuous structural matrix on the state diagram.When either storagetemperature is too high (e.g., above Tg) or moisture content is too high (e.g.,higher than Wg), a powder ingredient might loss its free-flowing property [ Pobierz caÅ‚ość w formacie PDF ]
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.This is probably due to the relatively high degree of cross-linking and/or entanglement.Molasses, honey, and batters are examples of foodsystems in the viscous flow region at room temperature.A blend of incompatible polymers will phase-separate and show more thanone Tg, as indicated in Figure 6 for sodium caseinate water and fructose watersystems (16).From all these observations, we can conclude that food systemscan indeed be viewed as synthetic polymer systems.Air cells or pockets help soften some products, such as breakfast cereals,that would otherwise be too hard.Gas cells inside some products can be frommany sources: entrapped air during mixing, yeast leavening, chemical leavening,injected inert gases, carbon dioxide, ethanol.Those gas cells will eventually havethe same gaseous composition as that of the packaged environment.One areathat differs from the synthetic polymer system is the presence of fat in foodmatrices.In bakery products, fats and emulsifiers do not affect Tg, but they dodecrease the rubbery modulus (17).That is why they are sometimes called tender-izers instead of plasticizers.The tenderizing effect at serving temperature is aCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved. Figure 4 DMTA plot for gluten samples stored under different RH values.(FromRef.13.)function of the solid fat index, fat content, and fat crystalline form.Due to theirlubricating effect, fats and emulsifiers also enhance the perceived moistness ofbakery products.Moistness also results partly because the true moisture contentof the nonfat portion is higher than the apparent moisture content, which is basedupon the total system.No wonder that fat-free bakery products usually taste dryand not as tender.III.EFFECT OF RHEOLOGICAL PROPERTIES ON THESTABILITY OF MATERIAL BEFORE, DURING, ANDAFTER PROCESSINGIn addition to having high hedonic quality during consumption, any successfulproduct in the marketplace needs to be stable throughout distribution.Duringmanufacturing, ingredients and in-process intermediates should not have caking,Copyright 2003 by Marcel Dekker, Inc.All Rights Reserved. Figure 5 Annular shear test for amorphous sorbitol: (´) storage modulus G2 , ( ) lossmodulus G3 , and (*) loss tangent.(From Ref.15.)shrinkage, or other physical instability problems.In new-product development,significant effort is on stabilizing the ingredient during storage, the processingintermediates during manufacturing, and the finished products during storage,consumer preparation, and consumption.Directly and indirectly, rheologicalproperties affect physical, chemical, and microbiological stabilities.In this chap-ter, we will focus more on the physical stability aspect.How do rheological properties affect stability? Well, in the previous sectionwe discussed the effect of physical state on rheological properties.Thus, it isphysical state that determines stability.A state diagram shows Tg as a functionof water content and solubility as a function of temperature.It also shows infor-mation on various physical changes that may occur due to the metastable stateof amorphous food solids and their approach toward equilibrium (18).Roos (19)summarized the methods of applying state diagrams in food processing and prod-uct development.LeMeste et al.(20) reviewed the relationship between physicalstates and the quality of cereal-based foods.A product in the glassy state duringstorage will be more stable, while products stored at temperatures above the Tgcurve will undergo physical changes at a rate according to William LandelFerry (WLF) kinetics (4 8).The Gordon Taylor equation can relate the effectof composition on Tg (19, 21).Thus, by combining the WLF and Gordon TaylorCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved. Figure 6 DMTA plot of sodium caseinate and fructose at a ratio of 2:1 stored at 75%RH (16% water).(From Ref.16.)equations, one can express rheological properties, such as viscosity, of food mate-rials as a function of temperature and moisture content.However, the coefficientsin those equations are not yet readily available, even for common food systems.Some of the stability problems encountered in product development will be dis-cussed here.These include caking during storage and spray-drying, structuralcollapse during processing, and loss of crispness during storage.A.Caking During Storage and ProcessingAs described in Chapter 7, caking, or the loss of free flow, is one of the mostoften encountered stability problems associated with spray-dried or freeze-driedamorphous ingredients.Ingredient caking affects plant operation efficiency.Cak-ing of wheat flour usually is of no concern if it is stored properly.However,under humid conditions, e.g., at 90% relative humidity, mold growth or infesta-tion might occur even at room temperature (22).Copyright 2003 by Marcel Dekker, Inc.All Rights Reserved. Caking is a mobility-related phenomenon associated with the physical stateof the continuous structural matrix on the state diagram.When either storagetemperature is too high (e.g., above Tg) or moisture content is too high (e.g.,higher than Wg), a powder ingredient might loss its free-flowing property [ Pobierz caÅ‚ość w formacie PDF ]