Abstract: In this paper, we optimized the conditions for ultrasound-assisted enzymatic preparation of peptides from Tibetan sheep placental protein (TSPP) using a combination of one-factor-at-a-time method and response surface methodology (RSM). Degree of hydrolysis (DH), peptide yield (PY) and antioxidant capacity were considered as the response variables. Six proteases and their binary combinations were screened for their suitability for the hydrolysis of TSPP preceding the optimization. Furthermore, the amino acid composition and molecular mass distribution of the hydrolysate prepared under the optimized conditions were determined. The results indicated that the simultaneous addition of papain and neutral protease was found to be the best hydrolysis scheme. The effects of sonication time, temperature, hydrolysis time and ultrasonic power on DH decreased in that order. The optimal hydrolysis parameters were obtained as follows: sonication time 16.5 min, temperature 25.5 ℃, ultrasonic power 438 W, and hydrolysis time 4.9 h, resulting in DH, PY and antioxidant capacity of 55.22%, 18.52% and 1.08 mmol/L Trolox equivalent, respectively. The resulting hydrolysate was rich in glutamic acid, aspartic acid and glycine and peptides below 2 000 Da accounted for 95.02% of the total peptides, the majority of which were small oligopeptides. In conclusion, the ultrasound-assisted enzymatic hydrolysis process could not only had improved hydrolysis efficiency but also produce small peptides with excellent antioxidant activity.

Key words: Tibetan sheep placental protein, ultrasonic, hydrolysis with enzyme combinations, response surface methodology, antioxidant capacity, molecular mass distribution, amino acid composition