3.5. Butanol production by C. acetobutylicum XY16 using the hydrolysate generated by GVL/water solution
Recently, plenty of works have been reported to use biomassderived sugars for butanol fermentation [14]. However, few of the hydrolysates could be directly used in microbial fermentation and further detoxification step should be performed, which would greatly decrease the economic feasibility of biobutanol production process [19]. Due to the low concentration of inhibitors in hydrolysate, we proposed to use the hydrolysate generated by GVL/water solution for butanol production using C. acetobutylicum XY16. The hydrolysate was diluted by concentrated P2 medium to a final sugar concentration of 41 and 61 g/L. Glucose was first utilized due to the carbon catabolite repression (a common phenomenon observed in microorganism) [32–34], displaying sequential sugar ultilization when grown in the hydrolysate (Fig. 3a). Xylose utilization was observed when glucose and arabinose were exhausted at 42 h and cells cultured in 500 ml glass fermentor grew to the highest cell density (OD600) of 7.65. The cells produced 14.26 g/L ABE solvents, including 4.1 g/L acetone, 9.3 g/L butanol and 0.86 g/L ethanol. The solvent productivity and yield reached 0.24 g/(L h) and 0.35 g/g. The solvent concentration and yield was similar to batch fermentation using pure glucose as carbon source, while the solvent productivity decrease 18%. All supplied sugars were exhausted after 60 h fermentation, suggesting that the hydrolysate with 41 g/L total sugars could be potential carbon resource for butanol fermentation. Increasing the initial sugar concentration to 61 g/L led to poor cell growth and reduced solvent production (Fig. 3b), which might be caused by the higher concentration of NaCl and GVL in culture medium. The cells slowly grew to its maximum cell density of 4.55, which was 59% of that in the fermentation with 41 g/L sugars. As a result, the cells only consumed 26 g/L glucose and produced 7.6 g/L solvents containing 2.3 g/L acetone, 4.8 g/L butanol and 0.5 g/L ethanol. The solvent productivity and yield were both at a low level of 0.12 g/(L h) and 0.29 g/g. Most of the sugars including 20 g/L glucose and 15 g/L xylose were left in the culture