In situ extractive fermentation for the production of hexanoic acid from galactitol by Clostridium sp. BS-1

Clostridium sp. BS-1 produces hexanoic acid as a metabolite using galactitol and enhanced hexanoic acid production was obtained by in situ extractive fermentation with Clostridium sp. BS-1 under an optimized medium composition. For medium optimization, five ingredients were selected as variables, and among them yeast extract, tryptone, and sodium butyrate were selected as significant variables according to a fractional factorial experimental design, a steepest ascent experimental design, and a Box–Behnken experimental design. The optimized medium had the following compositions in modified Clostridium acetobutyricum (mCAB) medium: 15.5 g L−1 of yeast extract, 10.13 g L−1 of tryptone, 0.04 g L−1 of FeSO4·7H2O, 0.85 g L−1 of sodium acetate, and 6.47 g L−1 of sodium butyrate. The predicted concentration of hexanoic acid with the optimized medium was 6.98 g L−1, and this was validated experimentally by producing 6.96 g L−1 of hexanoic acid with Clostridium sp. BS-1 under the optimized conditions. In situ extractive fermentation for hexanoic acid removal was then applied in a batch culture system with the optimized medium and 10% (v/v) alamine 336 in oleyl alcohol as an extractive solvent. The pH of the culture in the extractive fermentation was maintained at 5.4–5.6 by an acid balance between production and retrieval by extraction. During a 16 day culture, the hexanoic acid concentration in the solvent increased to 32 g L−1 while it was maintained in a range of 1–2 g L−1 in the medium. The maximum rate of hexanoic acid production was 0.34 g L−1 h−1 in in situ extractive fermentation.