γ-PGA Fermentation by Bacillus subtilis PG-001 with Glucose Feedback Control pH-stat Strategy

Poly-γ-glutamic acid (γ-PGA) is an important biopolymer with many applications due to its biodegradable and non-toxic characteristics. γ-PGA is produced industrially by fermentation of Bacillus species. The optimal pH range for producing γ-PGA by Bacillus subtilis PG-001 was firstly studied by glucose fed-batch fermentation with non-controlled pH. Result showed that both cell growth and γ-PGA synthesis were repressed when pH was lower than pH 6. Further investigation with γ-PGA fed-batch fermentation showed that pH 6.5 is more suitable for γ-PGA fermentation than pH 7. Under comparable consumption of glutamic acid and glucose, 11.8 g/L γ-PGA and 0.7 g/g yield were achieved by fermentation at pH 6.5, which was significantly higher than 10.5 g/L and 0.56 g/g yield of fermentation at pH 7. In addition, γ-PGA degradation during later phase of fermentation was repressed at pH 6.5 as 9238cP of final broth viscosity was achieved from fermentation at pH 6.5 while it was only 346 cP for fermentation at pH 7. Finally, a glucose feedback control pH-stat strategy was performed for reducing alkali consumption during γ-PGA fermentation, which further increased final γ-PGA concentration to 15.5 g/L with much higher viscosity (11458 cP); meanwhile the consumption of alkali decreased 57%. The fed-batch γ-PGA fermentation with glucose feedback control pH-stat strategy showed high feasibility for industrial scaling-up.