Optimization of production conditions for antioxidant peptides from walnut protein meal using solid-state fermentation

Solid-state fermentation (SSF) of walnut protein meal (WPM) by Bacillus subtilis was optimized for the maximum degree of hydrolysis (DH) and reducing power (RP) using response surface methodology (RSM). Optimum fermentation conditions were fermentation time of 82.01 h, inoculum concentration of 10.40, and water content of 1.50 mL/g. Optimized values were 41.80% and 0.78 for DH and RP, respectively. Walnut peptides (WP) were ultrafiltration membrane fractionated. The WPs-II (molecular weight < 5 kDa) fraction showed the highest RP value. WPs-II showed good DPPH free radical scavenging and Fe2+ chelating activities, higher than for GSH (l-glutathione reduced) at the same concentration. Amino acid composition analysis of WPs-II showed that Asp, Glu, and Arg were the major amino acids playing important roles in the antioxidant activity. WPs is an efficient antioxidant suitable for use as a food additive and as a pharmaceutical agent.