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.