Crystallization of Amorphous Sugars

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Water sorption studies of dairy powders have often shown that during storage at above 40% relative humidity (RH) and room temperature, large amounts of water may be sorbed initially, but during further storage the sorbed water content is reduced with storage time. For example, as early as in 1926, Supplee (5) found a gradual decrease in sorbed water content of milk powder when the material was stored at 50% RH at room temperature. Such decrease in sorbed water content has often been suggested to be a result of time-dependent crystallization of amorphous lactose (e.g., 13, 35, 52-59). Similar crystallization behavior from the amorphous state has been found to apply to other sugars, including glucose and sucrose (7, 60), and the development of crystallinity can be followed from the loss of sorbed water (32, 60, 61).

Sugar crystallization occurs in both pure amorphous sugar preparations and sugar-containing foods. Development of crystallinity of amorphous lactose and other sugars, as a result of thermal and water plasticization, has been detected using such methods as differential scanning calorimetry (11, 32, 62, 63), isothermal microcalorimetry (58, 64, 65), and X-ray diffraction techniques (13, 14). However, there has been variation between the extent of crystallinity determined with different techniques, because lactose may crystallize in several crystal forms, depending on temperature and water content (13, 14, 66). Moreover, different techniques measure either the amount of one or more of the crystal forms or the overall crystallinity. We have also shown that the crystallization of amorphous sugars can be followed using Raman spectriscopy and Raman microscopy (16).

1. Effect of Physical State on Sugar Crystallization

Crystallization of amorphous compounds is known to be related to the glass transition (1, 2). Theoretically, crystallization ceases below the glass transition, as the molecules freeze in the solid, glassy state. Above the Tg, nucleation occurs rapidly, but the growth of crystals occurs slowly due to the high viscosity and slow diffusion (2). At temperatures below, but close to, the equilibium melting temperature, Tm, nucleation occurs slowly but crystal growth is fast, as the driving force for nucleation decreases but the mobility of the molecules increases (2, 3, 32). Therefore, the maximum rate of crystallization appears between the Tg and Tm (2, 3).

Relationships between crystallization behavior of sugars and their glass transition 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 when the temperature has been increased to above the Tg or the water content has been increased sufficiently to depress the Tg to below ambient temperature. The conditions allowing crystallization have then been obtained from the state diagrams, or a critical water activity or water content corresponding to that at which the

Amorphous Lactose Sorption

Figure 5 Water sorption and plasticization of amorphous lactose and skim milk powder. The glass transition of lactose (A) and skim milk powder (A) decrease in a similar manner with increasing water content. The arrows indicate the critical water contents and water activity at 24°C for lactose (O) and skim milk powder (•). (Data from Ref. 35.)

Figure 5 Water sorption and plasticization of amorphous lactose and skim milk powder. The glass transition of lactose (A) and skim milk powder (A) decrease in a similar manner with increasing water content. The arrows indicate the critical water contents and water activity at 24°C for lactose (O) and skim milk powder (•). (Data from Ref. 35.)

Tg is at ambient temperature has been defined (67). As shown in Figure 5, we have 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 powders at 24°C was 37%. However, the critical water content of skim milk was higher, being 7.6 g/100 g of solids, probably because of additional contribution to water sorption properties by the component proteins. Because the critical values for water activity and water content are related to the anhydrous Tg (Table 1), low-molecular-weight sugars tolerate very low amounts of water or no water at all in their structure. The critical values of disaccharides increase with increasing anhydrous Tg, and the critical values for polysaccharides are fairly high, approaching water activities, allowing the growth of microorganisms (67, 68).

2. Kinetics of Sugar Crystallization

Rates of sugar crystallization at isothermal conditions have been reported to increase with increasing storage RH (e.g., 13, 14, 35, 55-58, 60, 62, 64, 65, 69). Some studies have also found that rates of amorphous sugar crystallization increase with increasing storage temperature at constant water contents (70). These findings suggest that the crystallization of amorphous sugars above some critical water content or temperature may occur with increasing rates, as a result of both increasing temperature and increasing water content.

Table 1 Anhydrous Glass Transition Temperatures for Amorphous Carbohydrates

Critical m (g/100 g

Table 1 Anhydrous Glass Transition Temperatures for Amorphous Carbohydrates

Critical m (g/100 g

Pentoses

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