The effect of the pH on thermal aggregation and gelation of soy proteins

Abstract : Thermal aggregation and gelation of soy protein globulins was studied over a wide range of protein concentrations (1–95 g/L), pH values (5.8–6.8) and heating temperatures (65–95 °C). The size and structure of the aggregates was characterized by light scattering techniques and cryo-transmission electron microscopy (Cryo-TEM). At low concentrations or short heating times irregular spherical particles were formed with a hydrodynamic radius that increased with decreasing pH from 30 to 40 nm. The density of these particles increased with decreasing pH from 0.1 to 0.2 g/cm3. At longer heating times or at higher protein concentrations, these particles randomly aggregated into self-similar aggregates. At a critical heating time, gelation was observed for C = 40 g/L, which was characterized by oscillatory shear measurements. The gel stiffness increased sharply with increasing protein concentration, but was almost independent of the pH. The aggregation and gelation rate increased with increasing protein concentration and decreasing pH. The temperature dependence (65–85 °C) of the gelation rate was characterized by an activation energy of 180 kJ/mol independent of the pH. The effect of aggregation on the relationship between the pH and the charge density was determined by potentiometric titration.