Optimization of stress medium enhance hydroxyl radical inhibition by water-soluble protein from germinated millet

A growth-promoting medium was developed to enhance the production of a hydroxyl radical inhibitory water-soluble protein from germinated millet. The single factor test indicated that H2O2 plays a key role in the inhibition activity. An optimal medium composition, consisted of H2O2, gibberellin, Protamex and Tween-40, was achieved using statistical experimental designs. A fractional factorial design was applied to determine the key factors that affected the hydroxyl radical scavenging activity, and a central composite experimental design and response surface methodology were used to optimize those factors, respectively. A second-order polynomial prediction equation was obtained, that could determine the effects of H2O2 and Protamex on the response. It was found that a low concentration of H2O2 and Protamex could enhance the inhibition of hydroxyl radicals. The optimal medium composition for growth-promoting was as follows: H2O2 1.56 mL/100 mL, gibberellin 0.2595 mg/L, Protamex 0.25 mg/mL and Tween-40 0.7 mL/100 mL. With the optimal medium, the highest hydroxyl radical inhibition (58.16%) was achieved.