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.All Rights Reserved. fitted to the crystallization data.However, it was confirmed that the crystallizationat a constant water content proceeded with a rate controlled by the T Tg.Butin closed containers, where water was released by the crystallizing material andsubsequently sorbed by the remaining amorphous material, crystallization oc-curred extremely rapidly.This was also reported by Kim et al.(74), who foundthat the release of water as a result of crystallization caused a significant increasein the rate of nonenzymatic browning of whey powders stored in sealed contain-ers.Obviously, crystallization of sugars in foods stored in sealed containers maycause a rapid deterioration of the materials at abusing storage conditions.The Avrami equation was found to fit the crystallization data of amorphouslactose and lactose in skim milk, both produced by freeze-drying, when the in-crease in crystallinity during storage above the Tg was followed using powderX-ray diffraction detection of crystallinity (13, 14).The half-time of crystallinity,as defined by Eq.(11), was found to be a convenient parameter for the evaluationof the storage stability of lactose-containing materials.The data for half-time ofcrystallization reported by Jouppila et al.(13, 14) and Kedward et al.(63) suggestthat the maximum rate of lactose crystallization may occur at a T Tg of 60100°C (Fig.6).The rate may also be affected by water content and materialcomposition; i.e., the rates may be different at equal T Tg at various watercontents and significantly dependent on other components.It seems that lactosecrystallization in skim milk occurs at a substantially lower rate, even at a veryhigh T Tg, when compared with pure amorphous lactose.ln 0.5t1/2 n (11)" kA number of food materials contain several amorphous carbohydratesmixed with other food components.It has been reported that the presence ofother compounds with amorphous sugars may delay sugar crystallization.Thisis even true when anomers of the same sugar are present in the material, becausethey may have different solubilities and crystals formed may often contain onlyone of the anomers.According to Iglesias and Chirife (61), the addition of poly-saccharides delayed the crystallization of amorphous sucrose.Saleki-Gerhardtand Zografi (32) found that pure amorphous sucrose crystallized more rapidlythan in a mixture with lactose, trehalose, or raffinose at 32.4% RH and 30°C.Induction time for sucrose crystallization increased with increasing amount oflactose, raffinose, and trehalose.When the amount of lactose, raffinose, or treha-lose exceeded 10%, no crystallization was observed during two weeks of storage.The delayed crystallization of amorphous sucrose in the presence of other sugarsor corn syrup solids may be a result of effects on the Tg, effects of molecularcharges, and effects of steric hindrance (75).It should also be noticed that lactosehydrolysis in dairy powders may result in delayed crystallization; e.g., we haveCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved. (a)(b)Figure 6 Half-time (a) and reciprocal half-time (b) for crystallization of amorphouslactose and sucrose, as a function of temperature difference between storage temperatureand glass transition temperature, T Tg.Left axis: lactose, square (Ref.63), and sucrose,circle (Ref.63).Right axis: lactose, up-triangle (Ref.14), and lactose in freeze-dried skimmilk, down-triangle (Ref.13).Copyright 2003 by Marcel Dekker, Inc.All Rights Reserved. found no crystallization in freeze-dried skim milk containing hydrolyzed lactose(57).Therefore, sugar crystallization in food products may be controlled by addi-tion of other sugars in addition to control by temperature and water content.3.Extent of Sugar CrystallizationAnhydrous amorphous sugars are likely to crystallize to a full extent when ex-posed to conditions favoring crystallization.However, in the presence of water,the extent of crystallization may become limited by solubility.In the presenceof other compounds, crystallization may be limited by intermolecular interactionsbetween various compounds.It has been found that the extent of isothermal sugar crystallization is depen-ent on the storage relative humidity (13, 14, 64, 65).Sebhatu et al.(65) foundthat the heat of lactose crystallization at 25°C was slightly lower for amorphouslactose stored at 57% and 100% RH than for amorphous lactose stored at 75%and 84% RH, indicating that less lactose crystallized at 57% and 100% RH thanat 75% and 84% RH.According to Briggner et al.(64), the heat of crystallizationof amorphous lactose at 25°C when stored at 75% and 85% RH was slightlylower than that of amorphous lactose stored at 53% and 65% RH.The highestheat of crystallization was measured for lactose stored at 65% RH (64).Theresults of these studies suggest that not only was the extent of lactose crystalliza-tion dependent on the storage relative humidity, but the extent of crystallizationincreased with storage relative humidity to a maximum value and then decreasedwhen lactose was stored at a higher relative humidity.We have reported (14)fairly low extents of crystallization of amorphous lactose when stored at 24°C and44.4% RH [ Pobierz caÅ‚ość w formacie PDF ]
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.All Rights Reserved. fitted to the crystallization data.However, it was confirmed that the crystallizationat a constant water content proceeded with a rate controlled by the T Tg.Butin closed containers, where water was released by the crystallizing material andsubsequently sorbed by the remaining amorphous material, crystallization oc-curred extremely rapidly.This was also reported by Kim et al.(74), who foundthat the release of water as a result of crystallization caused a significant increasein the rate of nonenzymatic browning of whey powders stored in sealed contain-ers.Obviously, crystallization of sugars in foods stored in sealed containers maycause a rapid deterioration of the materials at abusing storage conditions.The Avrami equation was found to fit the crystallization data of amorphouslactose and lactose in skim milk, both produced by freeze-drying, when the in-crease in crystallinity during storage above the Tg was followed using powderX-ray diffraction detection of crystallinity (13, 14).The half-time of crystallinity,as defined by Eq.(11), was found to be a convenient parameter for the evaluationof the storage stability of lactose-containing materials.The data for half-time ofcrystallization reported by Jouppila et al.(13, 14) and Kedward et al.(63) suggestthat the maximum rate of lactose crystallization may occur at a T Tg of 60100°C (Fig.6).The rate may also be affected by water content and materialcomposition; i.e., the rates may be different at equal T Tg at various watercontents and significantly dependent on other components.It seems that lactosecrystallization in skim milk occurs at a substantially lower rate, even at a veryhigh T Tg, when compared with pure amorphous lactose.ln 0.5t1/2 n (11)" kA number of food materials contain several amorphous carbohydratesmixed with other food components.It has been reported that the presence ofother compounds with amorphous sugars may delay sugar crystallization.Thisis even true when anomers of the same sugar are present in the material, becausethey may have different solubilities and crystals formed may often contain onlyone of the anomers.According to Iglesias and Chirife (61), the addition of poly-saccharides delayed the crystallization of amorphous sucrose.Saleki-Gerhardtand Zografi (32) found that pure amorphous sucrose crystallized more rapidlythan in a mixture with lactose, trehalose, or raffinose at 32.4% RH and 30°C.Induction time for sucrose crystallization increased with increasing amount oflactose, raffinose, and trehalose.When the amount of lactose, raffinose, or treha-lose exceeded 10%, no crystallization was observed during two weeks of storage.The delayed crystallization of amorphous sucrose in the presence of other sugarsor corn syrup solids may be a result of effects on the Tg, effects of molecularcharges, and effects of steric hindrance (75).It should also be noticed that lactosehydrolysis in dairy powders may result in delayed crystallization; e.g., we haveCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved. (a)(b)Figure 6 Half-time (a) and reciprocal half-time (b) for crystallization of amorphouslactose and sucrose, as a function of temperature difference between storage temperatureand glass transition temperature, T Tg.Left axis: lactose, square (Ref.63), and sucrose,circle (Ref.63).Right axis: lactose, up-triangle (Ref.14), and lactose in freeze-dried skimmilk, down-triangle (Ref.13).Copyright 2003 by Marcel Dekker, Inc.All Rights Reserved. found no crystallization in freeze-dried skim milk containing hydrolyzed lactose(57).Therefore, sugar crystallization in food products may be controlled by addi-tion of other sugars in addition to control by temperature and water content.3.Extent of Sugar CrystallizationAnhydrous amorphous sugars are likely to crystallize to a full extent when ex-posed to conditions favoring crystallization.However, in the presence of water,the extent of crystallization may become limited by solubility.In the presenceof other compounds, crystallization may be limited by intermolecular interactionsbetween various compounds.It has been found that the extent of isothermal sugar crystallization is depen-ent on the storage relative humidity (13, 14, 64, 65).Sebhatu et al.(65) foundthat the heat of lactose crystallization at 25°C was slightly lower for amorphouslactose stored at 57% and 100% RH than for amorphous lactose stored at 75%and 84% RH, indicating that less lactose crystallized at 57% and 100% RH thanat 75% and 84% RH.According to Briggner et al.(64), the heat of crystallizationof amorphous lactose at 25°C when stored at 75% and 85% RH was slightlylower than that of amorphous lactose stored at 53% and 65% RH.The highestheat of crystallization was measured for lactose stored at 65% RH (64).Theresults of these studies suggest that not only was the extent of lactose crystalliza-tion dependent on the storage relative humidity, but the extent of crystallizationincreased with storage relative humidity to a maximum value and then decreasedwhen lactose was stored at a higher relative humidity.We have reported (14)fairly low extents of crystallization of amorphous lactose when stored at 24°C and44.4% RH [ Pobierz caÅ‚ość w formacie PDF ]