Water Sorption Isotherm and Critical Water Activity of Amorphous Water-Soluble Carbohydrates Characterized by the Glass Transition Temperature.

Water-soluble carbohydrates commonly exist in an amorphous state in foods and undergo glass-rubber transition (glass transition) at the glass transition temperature (T_g). The critical water content (W_c) and critical water activity (a_wc) are the water content and water activity (a_w) at which the glass transition occurs at 298 K (typical ambient temperature), respectively. For amorphous water-soluble carbohydrates, W_c can be predicted from the T_g of anhydrous solid (T_gs) using previously reported equations. However, an approach for predicting a_wc is still lacking. This study aimed to establish an a_wc-predictive approach for amorphous water-soluble carbohydrates based on T_gs. First, the water sorption isotherms of four hydrogenated starch hydrolysates were investigated, and the results were analyzed using the Guggenheim-Anderson-de Boer (GAB) model. Second, the effect of T_gs on the GAB parameters (C, K, and W_m) was evaluated using the T_gs values reported in previous literatures. C and W_m decreased and increased logarithmically, respectively, with increasing 1/T_gs. K was fixed to 1 (constant), as it showed little variation. These results enabled the prediction of the GAB parameters from T_gs. The GAB model could then predict a_wc from W_c, which was determined using the previously established equations. The predicted a_wc values were in good agreement with the experimentally determined a_wc. Additionally, we demonstrated that this a_wc-prediction approach is also applicable to amorphous water-soluble electrolytes and partially water-insoluble carbohydrates. Thus, this approach can be used for the quality control of amorphous water-soluble carbohydrates and carbohydrate-based foods.