Effect of tryptic digestion on emulsifying properties of soy protein.

Abstract

The effect of tryptic digestion on the solubility and emulsifying properties of heat-denatured and native soy protein (crude glycinin) was studied as a function of pH. Fractionation of the tryptic digests of heat-denatured soy protein was achieved by centrifugation and ultra-filtration. The resulting precipitate (PPT), high molecular weight fraction (HMF) and low molecular weight fraction (LMF) were constituted mainly with polypeptide. They had molecular weights of approximately 30, 000, 20, 000 and less than 10, 000, respectively. The isoelectric region of HMF and that of DNG (digest of native glycinin) shifted to the acidic side, and the solubility of PPT and LMF was not influenced by pH. Emulsifying properties, such as emulsifying capacity (EC) and emulsion stability (ES) after 30 min and 24 hr, were measured. It was found that the emulsifying properties were improved by tryptic hydrolysis but the low molecular weight digests showed poorer emulsifying properties. Among these digests, digestion intermediates (HMF and DNG) showed the highest emulsifying properties. The behavior of EC and ES after 30 min of these digests was related to that of their solubility and these values showed the minimum in the isoelectric region. Conversely, the ES value after 24 hr was maximum in this region.