Thermal behavior, solubility and structural properties of soy concentrate hydrolyzed by new plant proteases

A soy concentrate prepared by alcoholic extraction of defatted soy flour was hydrolyzed with three plant proteases: hieronymin and macrodontin, cysteine proteases, and pomiferin, a serine protease. A commercial microbial protease (alcalase) was included for comparative purposes. Working at optimal conditions for each enzyme 5-15% degree of hydrolysis (DH) values were obtained. Hydrolysates exhibited a characteristic SDS-PAGE pattern: the plant proteases attacked the polypeptides of 75 and 11S proteins with different intensity and selectivity, especially the A and B polypeptides of the 11S protein. Intermediate molecular weight peptides (24 and 60 kDa) were produced as the hydrolysis progressed, Differential Scanning Calorimetry (DSC) thermograms of flour, concentrate and hydrolysates were analyzed to evaluate the thermal stability and denaturation enthalpies of the major proteins. An increase in the degree of protein denaturation resulting from enzymatic action and a lower thermal stability at low pH were detected. The surface hydrophobicity of all hydrolysates, unlike expected, did not increase. Solubility at pH 7.0 is closely related to the DH, independent of the protease used. Solubility at pH 4.5 appeared to be related to the extent of hydrolysis of polypeptide A by each protease.