Preparation process of black soybean polypeptide

The invention discloses a preparation process of black soybean polypeptide. The preparation process includes: smashing black soybean, dissolving the smashed black soybean into water, stirring, adjusting pH to 8.5-9.5, heating to 70-90 DEG C, extracting, collecting supernate in a centrifugal manner, and repeating extraction once; adding alkaline protease into the obtained black soybean protein solution, adjusting pH to 9.0, and performing enzymolysis at the temperature of 40-50 DEG C for 1-2 hours; adjusting pH to 7.0, adding neutral protease, and performing enzymolysis at the temperature of 55-65 DEG C for 30-50 minutes; adjusting pH to 9.0, adding alkaline protease, and performing enzymolysis at the temperature of 40-50 DEG C for 1-2 hours. The preparation process has the advantages that black soybean protein is extracted through a water extraction method, the alkaline protease enzymolysis is performed twice, the neutral protease enzymolysis is performed between the two alkaline protease enzymolysis, enzymolysis time is short, and black soybean polypeptide conversion rate is increased greatly.