Poly-(γ-glutamic acid) Production and Optimization from Agro-Industrial Bioresources as Renewable Substrates by Bacillus sp. FBL-2 through Response Surface Methodology
We optimized culture conditions using Bacillus sp. FBL-2 as a poly-(γ-glutamic acid) (PGA)
producing strain isolated from cheonggukjang. All experiments were performed under aerobic conditions
using a laboratory scale 2.5 L fermentor. We investigated the effects of fermentation parameters
(temperature, pH, agitation, and aeration) and medium components (glutamic acid, citric acid, and yeast
extract) on poly-(γ-glutamic acid) production, viscosity, and dry cell mass. A non-optimized fermentation
method (1.5 vvm, 350 rpm, and 37 °C) yielded PGA, viscosity, and dry cell mass at levels of 100.7 g/L,
483.2 cP, and 3.4 g/L, respectively. L-glutamic acid, citric acid, and yeast extract supplementation enhanced
poly-(γ-glutamic acid) production to 175.9 g/L. Additionally, the production of poly-(γ-glutamic acid) from
rice bran and wheat bran was assessed using response surface methodology (central composite rotatable
design). Agricultural by-products (rice bran and wheat bran) and H2SO4 were selected as factors,
and experiments were performed by combining various component concentrations to determine optimal
component concentrations. Our experimentally-derived optimal parameters included 38.6 g/L of rice bran,
0.42% of H2SO4, 28.0 g/L of wheat bran, and 0.32% of H2SO4. Under optimum conditions, rice bran
medium facilitated poly-(γ-glutamic acid) production of up to 22.64 g/L, and the use of wheat bran medium
yielded up to 14.6 g/L. Based on a validity test using the optimized culture conditions, poly-(γ-glutamic
acid) was produced at 47.6 g/L and 36.4 g/L from these respective mediums, and both results were higher
than statistically predicted. This study suggests that rice bran can be used as a potential alternative substrate
for poly-(γ-glutamic acid) production
