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.The stickiness, cak-ing, and collapse in a practical time scale occur above the Tg (24), where theviscosity seems to follow both the Fermi and WLF models.It should be noticedthat the viscosity of glassy materials has not been measured and the experimentalCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved.Figure 4 Water sorption and plasticization by a maltodextrin (M200) with viscosityand critical water activity and water content depressing the glass transition temperature,Tg, to 25°C.The main difference between the Fermi and WLF model predictions occursover the glass transition.(Data from Ref.39.)viscosity data available for carbohydrates cover only values up to 109 Pa-s (41,50, 51).III.CRYSTALLIZATION AND GLASS TRANSITIONThe driving force for crystallization is determined by the extent of supercoolingbelow the equilibrium melting temperature or the extent of supersaturation.Amorphous food systems can be considered as supercooled and supersaturatedmaterials, which have a temperature- and concentration-dependent driving forcefor crystallization.Some of the main factors controlling the rates of crystallizationare probably diffusion of molecules to nucleation sites and their ability to reorientthemselves into the crystal lattice structure.In concentrated systems, the diffusionof molecules may become restricted by high viscosity, and, thereby, the rates ofcrystallization are likely to decrease.In polymer systems, the rates of crystalliza-tion approach zero at temperatures close to the Tg (1).Copyright 2003 by Marcel Dekker, Inc.All Rights Reserved.A.Crystallization of Amorphous SugarsWater sorption studies of dairy powders have often shown that during storage atabove 40% relative humidity (RH) and room temperature, large amounts of watermay be sorbed initially, but during further storage the sorbed water content isreduced with storage time.For example, as early as in 1926, Supplee (5) founda gradual decrease in sorbed water content of milk powder when the material wasstored at 50% RH at room temperature.Such decrease in sorbed water content hasoften been suggested to be a result of time-dependent crystallization of amor-phous lactose (e.g., 13, 35, 52 59).Similar crystallization behavior from theamorphous state has been found to apply to other sugars, including glucose andsucrose (7, 60), and the development of crystallinity can be followed from theloss of sorbed water (32, 60, 61).Sugar crystallization occurs in both pure amorphous sugar preparations andsugar-containing foods.Development of crystallinity of amorphous lactose andother sugars, as a result of thermal and water plasticization, has been detectedusing such methods as differential scanning calorimetry (11, 32, 62, 63), isother-mal microcalorimetry (58, 64, 65), and X-ray diffraction techniques (13, 14).However, there has been variation between the extent of crystallinity determinedwith different techniques, because lactose may crystallize in several crystal forms,depending on temperature and water content (13, 14, 66).Moreover, differenttechniques measure either the amount of one or more of the crystal forms or theoverall crystallinity.We have also shown that the crystallization of amorphoussugars can be followed using Raman spectriscopy and Raman microscopy (16).1.Effect of Physical State on Sugar CrystallizationCrystallization of amorphous compounds is known to be related to the glass tran-sition (1, 2).Theoretically, crystallization ceases below the glass transition, asthe molecules freeze in the solid, glassy state.Above the Tg, nucleation occursrapidly, but the growth of crystals occurs slowly due to the high viscosity andslow diffusion (2).At temperatures below, but close to, the equilibium meltingtemperature, Tm, nucleation occurs slowly but crystal growth is fast, as the drivingforce for nucleation decreases but the mobility of the molecules increases (2, 3,32).Therefore, the maximum rate of crystallization appears between the Tg andTm (2, 3).Relationships between crystallization behavior of sugars and their glasstransition have been studied by several authors (e.g., 2, 7, 10, 11, 13, 14, 32,34).The assumption in such studies has been that crystallization occurs whenthe temperature has been increased to above the Tg or the water content has beenincreased sufficiently to depress the Tg to below ambient temperature.The condi-tions allowing crystallization have then been obtained from the state diagrams,or a critical water activity or water content corresponding to that at which theCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved.Figure 5 Water sorption and plasticization of amorphous lactose and skim milk powder.The glass transition of lactose ( ) and skim milk powder ( ) decrease in a similar mannerwith increasing water content.The arrows indicate the critical water contents and wateractivity at 24°C for lactose ( ) and skim milk powder ( ).(Data from Ref.35.)Tg is at ambient temperature has been defined (67).As shown in Figure 5, wehave obtained the critical water content for amorphous lactose using the GordonTaylor equation with Tg1 of 97°C and k of 6.7 (35) to be 6.7 g/100 g of solids.The corresponding critical RH for amorphous lactose and lactose in milk powdersat 24°C was 37%.However, the critical water content of skim milk was higher,being 7 [ Pobierz caÅ‚ość w formacie PDF ]