Effects of Water Activity and Temperature on the Caking Properties of Amorphous Carbohydrate Powders.

Water sorption reduces the glass transition temperature ( ) of amorphous carbohydrate powders due to water plasticization. Caking of amorphous powder occurs when decreases below the storage temperature (), that is, when the glass-to-rubber transition occurs. Although glass-to-rubber transition also occurs when is greater than , knowledge regarding the caking of amorphous powders induced by elevation is limited. Thus, caking properties were investigated using amorphous carbohydrate powders with varying water activity ( ) values prepared at 25 °C, stored at a higher temperature, and then returned to 25 °C (-cycled samples) for storage. Maltodextrin and glucose mixtures at weight ratios of 0, 0.1, and 0.2 glucose were employed. The caking behavior of -cycled powders with high values was similar to that of -cycled samples (dried powders were stored under various conditions and then returned to the dry condition via vacuum-drying) reported previously. -cycled powders with a low value, by contrast, were resistant to caking even in the rubbery state. This suggests that water molecules support the progression of caking as the binder under high- conditions. To analyze the hydration level at which water molecules begin to act as a binder for caking, determination of the multilayer adsorbed water content and multilayer adsorbed values is proposed. The fracture stress increased with increases in - , depending on the sample. The binding effect of water also contributed to the formation of a harder cake.