Abstract: The dynamic analysis of protease hydrolysis of milk protein in pH naturally changed and controlled conditions was studied and the physico-chemical properties, especially the bitter taste, of hydrolysates with the former method was further investigated to select the right protease for less bitter hydrolysates and the proper hydrolysis technique. The two indexes, degree of hydrolysis and 10% trichloroacetic acid-soluble nitrogen(TCA-SN) content, were used to compare the four kinds of proteolysis in naturally changed pH and controlled conditions. The solubility, viscosity, bitter taste and protein hydrolyzed conditions of the four hydrolysates produced by the pH naturally changed technique were further studied to find their physicochemical properties. The results showed: There is little difference on hydrolysis degree and TCA-SN content between the two techniques; Compared with the milk protein, the hydrolysates show good solubility in acidic condition. Viscosity does not change much with the improvement of concentration. Hydrolysates produced by Novo Neutrase and Papain are the most bitter while by Novo Protamex less bitter. SDS-PAGE showed that the two neutrases mainly hydrolyze milk protein casein. In a word, these four enzymes can be used to hydrolyze milk protein in the naturally changed pH conditions in order to simplify the hydrolyzed process, to give the hydrolysates good properties.

Key words: degree of hydrolysis, trichloroacetic acid-soluble nitrogen, milk-protein peptide, pH, physico-chemical