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

Water sorption reduces the glass transition temperature (T g) of amorphous carbohydrate powders due to water plasticization. Caking of amorphous powder occurs when T g decreases below the storage temperature (T), that is, when the glass-to-rubber transition occurs. Although glass-to-rubber transition also occurs when T is greater than T g, knowledge regarding the caking of amorphous powders induced by T elevation is limited. Thus, caking properties were investigated using amorphous carbohydrate powders with varying water activity (a w) values prepared at 25 °C, stored at a higher temperature, and then returned to 25 °C (T-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 T-cycled powders with high a w values was similar to that of a w-cycled samples (dried powders were stored under various a w conditions and then returned to the dry condition via vacuum-drying) reported previously. T-cycled powders with a low a w 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-a w 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 a w values is proposed. The fracture stress increased with increases in T - T g, depending on the sample. The binding effect of water also contributed to the formation of a harder cake.