Improvement of emulsifying properties of soy protein through selective hydrolysis: Interfacial shear rheology of adsorption layer

Abstract

The interfacial viscoelastic behavior of adsorption layers at the oil/water interface for soy protein isolate (SPI), β-conglycinin (7S) and two hydrolysates of soy protein were investigated using a rotational rheometer equipped with Du Noüy ring geometry. Two kinds of soy protein hydrolysates were prepared by selective and limited hydrolysis. The limited hydrolysis products of soy protein (soy protein hydrolysates, SPH) and sodium caseinate, with flexible structures, quickly adsorbed at the oil/water interface and formed fluid-like interface with low interfacial shear modulus. The SPI, 7S and selective hydrolysis products of soy protein (reduced-glycinin, RG) exhibited the viscoelastic properties of globular proteins and formed interfacial films with high elastic and viscous moduli. The dynamic time sweep experiments showed that the adsorption of RG was faster than that of the SPI and 7S, which could help to prevent coalescence during the formation of emulsion, and was expected to result in the formation of smaller droplet size. Moreover, RG exhibited the highest adsorbed amount, indicating that it was less spread out at the surface and thus reduced the chance of protein unfolding. The microstructures of the emulsions suggested that the emulsifying properties of RG and 7S were comparable to those of sodium caseinate. This study indicates that selective hydrolysis may significantly improve the emulsifying and rheological properties of soy protein, and provides useful information for the preparation of high emulsifying soy protein products.