The effect of adding NaCl on thermal aggregation and gelation of soy protein isolate

Thermal aggregation and gelation of soy protein isolate (SPI) was studied at fixed charge density over a wide range of protein concentrations (1–95 g/L), NaCl concentrations (0–0.5 M) and temperatures (30–85 °C). Gelation was studied with oscillatory shear measurements and aggregation with light scattering. At all conditions, self-similar aggregates were formed by random association of elementary units consisting of dense SPI particles with radii between 30 nm and 50 nm that were formed in a first step of the thermal aggregation process. Two distinct irreversible aggregation processes were identified: one dominating between 30 and 45 °C and one dominating above 65 °C. Addition of salt led to faster aggregation and gelation, but did not influence the gel stiffness at steady state. The size and density of the elementary units of the aggregates increased with increasing NaCl concentration and confocal laser scanning microscopy images showed increasingly heterogeneous gels structures. The effect of varying the ionic strength on thermal aggregation is compared with that of varying the net charge density of the proteins.