In vitro antioxidant activity of the casein hydrolysates modified by plastein reaction in the presence of phenylalanine

Papain was used to hydrolyze casein to prepare casein hydrolysates with a degree of hydrolysis of 9.6% and a scavenging activity on DPPH· radical of 38.7%. Phenylalanine was added into the hydrolysates to carry out a plastein reaction catalyzed by papain. When reaction time was fixed at 5h,response surface method was used to optimize plastein reaction conditions,which were selected as follows:reaction temperature 30℃,substrate concentration of 50%(w/v) ,enzyme addition 3kU/g proteins,phenylalanine addition 0.74mol/mol free amino groups of the hydrolysates. Five modified casein hydrolysates were thus prepared and evaluated for their antioxidant properties,with the original casein hydrolysates,mixture of casein hydrolysates and phenylalanine,modified casein hydrolysates without phenylalanine addition as controls. The evaluation results showed that the modified casein hydrolysates had better antioxidant properties including scavenging activity on DPPH· radical or hydroxyl radical and reducing powder,compared to these controls. It was also found that the improved antioxidant properties of the modified casein hydrolysates were not related to the reaction time and phenylalanine addition applied in the preparation.