| The compound antioxidant peptide was prepared by two-step hydrolysis of zein-soy protein isolate (mass ratio of 1∶ 1) with malt flour and alkaline protease Alcalase. The second stage hydrolysis conditions of Alcalase were optimized by determining hydrolysis degree, soluble protein content and antioxidant activities. Moreover, the relative molecular weight distribution, stability and the amino acid composition of compound antioxidant peptide were investigated. The results showed that the optimal conditions for the preparation of compound antioxidant peptide were obtained as follows: the first stage hydrolysis conditions of substrate mass fraction 10%, pH 5.5, temperature 50 ℃, and mass ratio of enzyme (malt flour) to substrate 30%, hydrolysis time 3 h; the second stage hydrolysis conditions of substrate mass fraction 10%, pH 85, temperature 60 ℃, mass ratio of enzyme (Alcalase) to substrate 0.75% and hydrolysis time 3 h. Under these conditions, the hydrolysis degree, soluble protein content, hydroxyl radical scavenging rate and ferrous ion chelation ability were 36.32%，36.83 mg/mL，40.01% and 86.83%, respectively. Meanwhile, the relative molecular weight of the compound antioxidant peptide was mainly distributed in the range of 300-6 500 Da (92.44%), and the essential amino acid index value was 802 (n=9), which indicated that the peptide prepared was a high-quality protein source. The compound antioxidant peptide had thermal stability, and it was stable in neutral and alkaline conditions. However, acidic condition(pH 3-5), 4 ℃ storage and repeated freezing-thaw reduced the antioxidant activity of the compound antioxidant peptide. |
