The vital role of citrate buffer in acetone–butanol–ethanol (ABE) fermentation using corn stover and high-efficient product recovery by vapor stripping–vapor permeation (VSVP) process

BACKGROUND Butanol is not only an important solvent and chemical intermediate in food and pharmaceutical industries, but also considered as an advanced biofuel. Recently, there have been resurging interests in producing biobutanol especially using low-cost lignocellulosic biomass, but the process still suffers from low titer and productivity. The challenge for the bioconversion approach is to find an effective way of degrading materials into simple sugars that can then be converted into fuels by microorganisms. The pretreatment of lignocellulosic biomass is the great important process in influencing butanol production and recovery, finally determining its eco-feasibility in commercialization. RESULTS The effects of various strengths of citrate buffer on enzymatic hydrolysis and acetone-butanol-ethanol fermentation using corn stover or glucose as feedstock were investigated. The strengths of citrate buffer in the range of 20-100 mM had no effect on enzymatic hydrolysis, but greatly influenced the performance of ABE fermentation using corn stover hydrolysate. When 30 mM citrate buffer was used for enzymatic hydrolysis, the fermentation broth with the maximum butanol and ABE concentrations of 11.2 and 19.8 g/L were obtained from 30.9 g/L glucose and 9.7 g/L xylose, respectively, which was concentrated to 100.4 g/L butanol and 153.5 g/L ABE by vapor stripping-vapor permeation process. Furthermore, using glucose as sole carbon source, there were no cell growth and ABE production in the P2 medium with 80 or 100 mM citrate buffer, indicating that higher concentrations of citrate buffer had deleterious effect on cell growth and metabolism due to the variation of cells internal pH and cell membrane permeability. To mimic in situ product recovery for ABE fermentation, the VSVP process produced the condensate containing 212.0-232.0 g/L butanol (306.6-356.1 g/L ABE) from fermentation broth containing ~10 g/L butanol (~17 g/L ABE), the performance of which was more effective than pervaporation and gas stripping. CONCLUSIONS As it has significant impact on butanol fermentation, the strength of citrate buffer is of great importance in lignocellulosic butanol fermentation. Compared with pervaporation and gas stripping, the VSVP process has great potential for efficient butanol recovery in biobutanol production.