3.1 Batch Fermentation Kinetics
Butyric acid is a strong growth inhibitor and plays a critical role in regulating cell metabolism in ABE fermentation, which switches from acidogenesis to solventogenesis when the butyric acid concentration reaches a critical level in the culture medium (Bahl et al., 1982). Previous studies have shown that adding a small amount of butyric acid in the culture medium could induce early solventogenesis and increase butanol production (Huang et al., 2004; Lee SM et al., 2008). The effect of butyric acid on cell growth was first investigated in serum bottles and the results are shown in Figure 2 . As expected, cell growth was strongly inhibited by butyric acid and no growth was observed at 10 g/L of butyric acid. Then, batch fermentation kinetics with medium initially containing 4 g/L butyric acid in a stirred-tank bioreactor with pH controlled at ~5.0 was investigated. After a 12-h lag phase, cells grew to reach the maximum OD of ~2.0 at 34 h and then decreased gradually, whereas both butyric acid consumption and solvent production started when cell growth ceased at 34 h (Figure 3 ). About 13.1 g/L butanol, 5.6 g/L acetone, and 0.9 g/L ethanol were produced from ~50 g/L glucose and ~2 g/L butyric acid consumed in the fermentation ended at ~311 h, which was much longer than the typical ~72 h for batch ABE fermentation with glucose but without butyric acid addition in the medium (Figure S1 ). The butanol yield of ~0.26 g/g glucose obtained in the fermentation with 4 g/L butyric acid was significantly higher than that obtained in typical ABE fermentation (~0.20 g/g glucose), whereas the productivity was much lower at ~0.05 g/L∙h (vs. ~0.2 g/L∙h) due to the strong inhibition by butyric acid resulting in a much lower cell density of <2.0 OD (vs. Max. OD of ~14 reached in batch fermentation without butyric acid added in the culture medium). The results indicated that the addition of butyric acid could maintain stable butanol production in ABE fermentation, but the optimal butyric acid concentration must be explored in order to minimize its inhibition effect on cell growth and reactor productivity.