Study of thermal properties and heat-induced denaturation and aggregation of soy proteins by modulated differential scanning calorimetry.

The thermal properties and heat-induced denaturation and aggregation of soy protein isolates (SPI) were studied using modulated differential scanning calorimetry (MDSC). Reversible and non-reversible heat flow signals were separated from the total heat flow signals in the thermograms. In the non-reversible profiles, two major endothermic peaks (at around 100 and 220 degrees C, respectively) associated with the loss of residual water were identified. In the reversible profiles, an exothermic peak associated with thermal aggregation was observed. Soy proteins denatured to various extents by heat treatments showed different non-reversible and reversible heat flow patterns, especially the exothermic peak. The endothermic or exothermic transition characteristics in both non-reversible and reversible signals were affected by the thermal history of the samples. The enthalpy change of the exothermic (aggregation) peak increased almost linearly with increase in relative humidity (RH) in the range between 8 and 85%. In contrast, the onset temperature of the exotherm decreased progressively with increase in RH. These results suggest that the MDSC technique could be used to study thermal properties and heat-induced denaturation/aggregation of soy proteins at low moisture contents. Associated functional properties such as water holding and hydration property can also be evaluated.