Thermal Behavior and Nonfreezing Water of Soybean Protein Components

Thermal denaturation and hydration of two soybean protein components were studied using differential scanning calorimetry (DSC). Results showed that temperature of denaturation (T_d) of both 11S and 7S globulins decreased sharply with an increase in water content. Enthalpy of denaturation (ΔH_d) of 11S increased with increasing water content at first, and then leveled off at high water content. However, ΔH_d of both 7S and 11S components in 7S samples first increased and then decreased at high water content. The preparation method of samples influenced the ΔH_d value significantly but had little effect on the T_d. Nonfreezing water was determined from the DSC results. It increased in both 11S and 7S as water content increased but was more abundant in 7S, probably because of different compositions and structures. Threshold value of water content for the appearance of freezing water was 0.30–0.32 h (g of water/g of protein, mass ratio) for 11S. The water absorbed by both 11S and 7S during denaturation increased quickly at low water contents and remained almost constant at high water contents. The results were attributed to different structure and conformation of proteins before and after denaturation.