Abstract: Antioxidant peptides were prepared from highly denatured soybean meal by successive ultrasonic-assisted cellulase hydrolysis and protease hydrolysis in this study. The optimization of the process conditions was carried out using one-factor-at-a-time method and response surface methodology. The optimal conditions for ultrasonic-assisted cellulase hydrolysis were determined as follows: ultrasonic power 300 W, irradiation time 20 min, substrate concentration 8.36 g/100 mL, enzyme dosage 666 U/g, and pH 4.1. Under these conditions, the contents of soluble peptides and polysaccharides in the hydrolysate were (18.51 ± 0.36)% and (10.83 ± 0.32)%, respectively. The hydrolysate was then further hydrolyzed with alkaline protease, and the optimal hydrolysis conditions were obtained as follows: enzyme dosage 61 900 U, pH 9, hydrolysis duration 3 h, and temperature 56.4 ℃. The contents of soluble polypeptide and polysaccharide in the final product prepared under the optimized conditions were (25.47 ± 0.81)% and (13.22 ± 0.49)%, respectively. After purification by ethanol precipitation, DEAE-cellulose 52 ion exchange chromatography and Sephadex G-25 gel chromatography, the hydrolysis product was tested for antioxidant activity. The yield of purified antioxidant polypeptides was 2.18%. The ferric reducing power and superoxide anion radical scavenging capacity of the polypeptides at 1 mg/mL was determined to be (0.495 ± 0.042) mmol/g and (17.02 ± 0.22) U/g, respectively.

Key words: highly denatured soybean meal, ultrasonic-assisted enzymatic hydrolysis, antioxidant products