A novel strategy of feeding nitrate for cost-effective production of poly-γ-glutamic acid from crude glycerol by Bacillus licheniformis WX-02

Poly-γ-glutamic acid (γ-PGA) is a natural and multifunctional biopolymer. Glycerol, especially crude glycerol, was used as fermentation raw material for production of a variety of chemical products. Here, glycerol was used with nitrate addition for γ-PGA production by Bacillus licheniformis WX-02 at a 5 L fermenter scale. However, nitrite accumulation was discovered in batch fermentation, which resulted in low γ-PGA yield (19.58 g/L). Furthermore, based on transcription analysis and intracellular NADH measurement, nitrite accumulation was probably due to the low content of intracellular NADH in glycerol-based medium, which might be resulted from low efficiency of NADH generation rate and quick NADH consumption by nitrate reduction. Accordingly, different nitrate feeding strategies were designed and optimized in γ-PGA fermentation with glycerol, then applied in fermentation with crude glycerol. Finally, 35.63 g/L of γ-PGA was achieved with the constant-rate nitrate feeding of 0.38 g/(L h) by using crude glycerol, while 36.83 g/L of γ-PGA was achieved with pH-feedback controlling. Therefore, this study provided a novel strategy for cost-effective production of γ-PGA from crude glycerol.