Kinetic analysis and pH-shift control strategy for poly(γ-glutamic acid) production with Bacillus subtilis CGMCC 0833

Production of poly(γ-glutamic acid) (γ-PGA) by Bacillus subtilis CGMCC 0833 was investigated. It was influenced by the utilization of extracellular substrate glutamate, which was a relatively low-efficiency procedure in γ-PGA fermentation. Then it was discovered that the glutamate utilization was greatly dependent on the medium pH. However, the optimal pH for glutamate utilization was not consistent with that for the cell growth. Therefore, in order to obtain the optimal production of γ-PGA, a two-stage pH-shift control strategy was proposed. At first 24 h, pH was controlled at 7.0 to obtain the maximum biomass, and then shifted to 6.5 to obtain high level of glutamate utilization and γ-PGA production. By applying such a pH-shift control strategy, the glutamate utilization increased from 24.3 to 29.5 g/l, the intracellular glutamate, which was the direct substrate, also increased, and consequently the yield of γ-PGA increased from 22.2 to 27.7 g/l, increased by 24.8%. This was the first report of using a two-stage pH-shift control strategy in γ-PGA production, and it was proved to be an effective way for the optimization of γ-PGA production in this strain.