Effect of sorbed water on the thermal stability of soybean protein

A soybean protein isolate (SPI), and its beta-conglycinin and glycinin componets were obtained from defatted soybean flour by applying dissolution and precipitation based on the difference in their solubility depending on each isoelectric point. The purity evaluated by SDS-PAGE of the beta-conglycinin and glycinin preparations was about 84% and 80%, respectively, resulting in a clear difference in the pH dependence on solubility. A BET plot derived from the water sorption isotherm at 25 degrees C showed that the amount of the monolayer adsorption of these preparations was about 6-9%, the value for the beta-conglycinin preparation being about 1.5 times higher than that for the glycinin preparation. The beta-conglycinin and glycinin preparations were respectively denatured at around 75 degrees C and 86 degrees C in the presence of excess water, whereas the denaturation temperature of both preparations was markedly increased by decreasing sorbed water content below 40%, corresponding well with the unfrozen water content.