Functional properties of soybean isolate protein as influenced by its critical overlap concentration

The critical overlap concentration ( c *) of soy protein isolate (SPI) was studied at a wide range of temperatures (10–90 °C). Results obtained indicate that temperature and c * were directly proportional. The c * determined experimentally was similar to the theoretical value that was calculated by the inverse of the intrinsic viscosity, [ η ]. At c *, SPI solutions can be divided into the dilute and semi-dilute regimes. The [ η ] values were estimated by four semi-empirical equations: the Huggins equation, the Kraemer equation, the Tanglertaibul-Rao equation, and the Oliver-Ward equation, of which the Oliver-Ward equation was found to be the most suitable. The aggregation behavior of the SPI was confirmed by association with the start of molecular entanglement at c *. Turbidity, foam capacity (FC) and foam stability (FS) increased significantly when solutions concentration reached c * at all experimental temperatures. The rheology measurement results indicated onset of gel formation when protein concentration reached c * at 90 °C, which can be considered as the gel formation concentration at that temperature. At concentrations below c *, the SPI solutions were unable to form gels and had much lower water holding capacities (WHC) than gels prepared at c *. These findings suggested that c * was related to a number of functional characteristics of SPI, including FC, FS and WHC, and can be used to estimate the concentration required for protein gel formation.