Improved performance in γ-polyglutamic acid production by Bacillus subtilis LX on industrial scale by impeller retrofitting and its unstructured microbial growth kinetics model

We conducted industrial scale gamma-polyglutamic acid (gamma-PGA) production by Bacillus subtilis (B. subtilis) LX and modeled its microbial growth kinetics based on a logistic regression. We found that the use of a three-layer impeller including a lower semicircular disc impeller and two-layers of six-wide-leaf impellers were able to both increase gamma-PGA yields and decrease fermentation time as compared with two-layer Rushton impellers. Indeed, our results revealed that the optimal gamma-PGA yield (20.67 +/- 2.19 g/L) was obtained after 40 hr in the impeller retrofitted fermenter, and this yield was 29.7% higher than that in Rushton impellers fixed fermenter. The microbial growth kinetics of B. subtilis LX in this system were established, and the model was consistent with the experimental data (R-2 = 0.924) suggesting that it was suitable for describing the microbial growth kinetics underlying gamma-PGA production on an industrial scale. In addition, biomass yield (Yx/s-glucose), gamma-PGA yield (Yp/s-glucose), gamma-PGA yield (Yp/s-glutamate), and the correlation between gamma-PGA production and B. subtilis LX (Y-p/x) were found to be 0.043, 0.133, 0.743, and 3.090 g/g, respectively, in the impeller retrofitted fermenter, as compared with 0.036, 0.103, 0.629, and 2.819 g/g, respectively, in the two-layer Rushton impeller fermenter.