Thermal Denaturation of Glycinin as a Function of Hydration

Hydration effects on the thermal stability of glycinin (soybean 11S protein) were examined by differential scanning calorimetry (DSC). In a model system with pure glycinin, the denaturation temperature (Td) decreased with increasing moisture. Between 22 and 44% moisture, two endotherms were observed, where the lower-temperature endotherm became progressively reduced in magnitude with a concomitant increase in a higher-temperature transition. At 45.5% moisture, only a single endotherm was observed. The regression curves over the entire moisture range from 2 to 66% were derived as asymptote functions, where M equals the percentage total moisture. Equations were developed from the curves, and the relationship between Td and moisture were: Td = 92.4 + 196.5e-O.0681\1 and, for the low-temperature endotherm, 82.4 + l44.3e-o.o68M. By interaction of lIS protein with either ethanol, a neutral detergent (Triton X-100) or 40% sucrose, both oneand twoendotherm curves were generated. Such calorimetric behavior is indicative of nonequilibrium denaturation and supports the notion that structure reorganization during DSC is water content-dependent. Our findings suggest that either glycinin's acidic/basic subunits or a change in secondary protein structure may give rise to two endotherms.