Abstract: In order to improve the utilization rate of millet bran as a protein source and to provide references for the intensive processing of millet bran, solid state fermentation of millet bran by Bacillus natto was optimized for increased production of antioxidant peptides. Firstly, one-factor-at-a-time experiments were carried out. Secondly, the fermentation conditions with a significant effect on the total antioxidant capacity (T-AOC) of aqueous extracts from fermented millet bran were selected using Plackett-Burman design for further optimization using response surface methodology (RSM). The polypeptide content and 1,1-diphenyl-2-trinitrobenzene hydrazine (DPPH) radical scavenging ability of the aqueous extract from millet bran fermented under optimized conditions were determined. Results showed that the optimum fermentation conditions were as follows: average particle diameter of millet bran, 0.22 mm (between 60 and 80 mesh); inoculum size, 0.4 mL; initial pH, 6.7; fermentation time, 26 h; and fermentation temperature, 35 ℃. Experiments conducted under these conditions yielded an extract with a T-AOC of (344.51 ± 8.02) U/g, and a polypeptide yield of (68.37 ± 0.92) mg/g millet bran. The results also showed that the bioactive peptides had strong DPPH free radical scavenging activity with an IC50 value of 0.12 mg/mL. Therefore, this study proves that bioactive peptides obtained from fermented millet bran have a high antioxidant capacity.

Key words: millet bran, Bacillus natto, solid state fermentation, response surface methodology, antioxidant peptides