3.3 Effects of Butyric Acid
In a typical ABE fermentation, acetic and butyric acids produced in
acidogenesis were reassimilated via the CoA transferase, which transfers
the CoA from acetoacetyl-CoA to acetate and butyrate to form acetyl-CoA
and butyryl-CoA, respectively, with acetone, ethanol, and butanol as the
final products (Long and Jones, 1984; Zhao et al., 2013). Previous
studies have shown that overexpressing ctf AB encoding the CoA
transferase could increase butanol production and the robustness of
solventogenic clostridia (Lu et al., 2017; Yu et al., 2015) and butyric
acid could upregulate ctf AB (Bahl et al., 1982; Lee SM et al.,
2008). In this study, butyric acid was thus supplemented in the feed
medium to promote solventogenesis in clostridial fermentation, resulting
in ~20% higher butanol yield (0. 24 g/g vs.
~0.20 g/g) compared to without butyric acid addition.
The increased butanol yield could also be attributed to reduced cell
growth and acetone and ethanol production, which resulted in a much
higher ratio of butanol present in the total solvents
(~68% vs. ~60% w/w in typical ABE
fermentation with C. acetobutylicum (Xu et al., 2015). Similar
results were also reported for C. beijerinckii (Lee SM et al.,
2008). The addition of butyric acid as a carbon source and precursor for
butanol biosynthesis inhibited the conversion of butyryl-CoA to butyric
acid and thus increased carbon flow toward butanol production. However,
butyric acid at >5 g/L strongly inhibited cell growth and
reduced cell viability and productivity in free cell fermentation (see
Figs. 2 and 3). Nevertheless, cells immobilized in the FBB were
resilient to butyric acid toxicity and could tolerate butyric acid at
~5 g/L, although butyric acid at 8 g/L completely halted
cell metabolism in the FBB (Fig. S2).