Poly (γ-glutamic acid) Production Enhancement in Submerged Fermentation of Bacillus Licheniformis ATCC 9945a Using Optimization of Operating Variables and Glutamate Feeding

Poly (γ-glutamic acid) is a versatile biopolymer that can be used on an industrial scale if efficient methods are developed to increase production. In this study, first, based on the central composite design method of the response surface module, the effect of operational variables including temperature in the range of 30-44 °C, pH 4.5-8.5, and stirring in the range of 600-1000 rpm on poly (γ-glutamic acid) production was investigated in the batch fermentation of Bacillus licheniformis ATCC 9945a for the first time. Under optimal conditions viz. T of 37.4 °C, pH of 6.6, and agitation rate of 784.2 rpm, 15.5 g/L γ-PGA was obtained. According to the statistical analyses, adjusted R2 was 0.9572, and analysis of variance explicated that T-T, pH-pH, and agitation-agitation effects indicated the lowest p-values and had the most significant influence on biopolymer synthesis. Under the optimal conditions, glutamate (a novel feed) pulse feeding (as poly (γ-glutamic acid)-based monomer) was optimized, for the first time, using the one-factorial method to achieve a maximum of 42.13 g/L of biopolymer production (highest in comparison with others’ studies of this strain) by the two-pulsed feeding method. The chemical confirmation and novel physical characterization of the powdered product indicated a pure poly (γ-glutamic acid) sample suitable for biological, biomedical, and biopharmaceutical applications.