Optimization of the production of poly-γ-glutamic acid by Bacillus amyloliquefaciens C1 in solid-state fermentation using dairy manure compost and monosodium glutamate production residues as basic substrates

Poly-γ-glutamic acid (γ-PGA) is a polymer with uses in foods, cosmetics, medicine and agriculture. The medium for the production of γ-PGA by Bacillus amyloliquefaciens C1 was optimized by response surface methodology using agro-industrial wastes in solid-state fermentation (SSF). The optimal SSF medium (20 g substrates with 50% initial moisture) for producing γ-PGA was determined to contain 5.51 g dairy manure compost, 1.91 g soybean cake, 0.57 g corn flour, 2.15 g monosodium glutamate production residues, 1.5 g wheat bran, 0.5 g rapeseed cake, 0.1 g citric acid, 0.05 g MgSO4·7H2O and 0.03 g MnSO4·H2O. In this medium the strain produced up to 0.0437 g γ-PGA per gram of substrates when cultured for 48 h at 37 °C. SDS–PAGE showed that the molecular weight of the γ-PGA was more than 130 kDa. Due to the high-yields observed and the low-cost nature of the optimal medium, this study indicates a possibility to establish economical large-scale production of γ-PGA.