Abstract: The enzymatic hydrolysis of Qula casein with either alkaline protease or trypsin was optimized based on degree of hydrolysis (DH). The effects of hydrolysis time, temperature, pH, substrate concentration and enzyme concentration on DH were investigated by one-factor-at-a-time method. Subsequently, response surface methodology was used to optimize four significant variables. The resulting hydrolysates were comparatively evaluated for their 1,1-diphenyl-2-picrylhydrazyl (DPPH), superoxide anion and hydroxyl radical scavenging capacity, Fe2+ and Cu2+ chelating ability, and reducing power. The results showed that the optimal hydrolysis time, temperature, substrate concentration, pH and enzyme concentration were 3.8 h, 49.8 ℃, 60 g/L, 8.5 and 140 U/g for alkaline protease; and 2.5 h, 47.8 ℃, 35 g/L, 7.5 and 2 900 U/g for trypsin, yielding the maximum DH value of 24.25% and 13.57%, respectively. The alkaline protease hydrolysate had significantly lower superoxide anion radical scavenging capacity and Fe2+ chelating ability (P < 0.01) but higher hydroxyl radical scavenging capacity than the trypsin hydrolysate (P > 0.05). For both hydrolysates, Cu2+ chelating ability, DPPH radical scavenging capacity and reducing power increased with increasing concentration from 1 to 5 mg/mL, and Cu2+ chelating ability was lower than Fe2+ chelating ability (P > 0.05). Significant difference (P < 0.01) was found as far as DPPH radical scavenging capacity and reducing power were concerned. In summary, the two proteases had different impacts on the antioxidant activity of hydrolysates and the alkaline protease hydrolysate better antioxidant properties.

Key words: Qula casein, alkaline protease, trypsin, enzymatic hydrolysis process, antioxidant activity