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

Abstract

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.