Acetate and ethanol production from H2 and CO2 by Moorella sp. using a repeated batch culture

The growth inhibition of Moorella sp. HUC22-1 by un-dissociated acetic acid was analyzed using a non-competitive inhibition model coupled with a pH inhibition model. In the cells grown on H2, and CO2, the inhibition constant, Ksub(p) of the un-dissociated acetic acid was 6.2 mM (164 mM as the total acetate at pH 6.2, pKa=4.795, 55 deg C), which was 1.5-fold higher than that obtained in cells grown on fructose. When a pH-controlled batch culture was performed using a fermentor at pH 6.2 with H2, and CO2, a maximum of 0.92g/l of dry cell weight and 339 mM of acetate were produced after 220h, which were 4.4- and 6.8-fold higher than those produced in the pH-uncontrolled batch culture, respectively. In order to reduce acetate inhibition in the culture medium, a repeated batch culture with cell recycling was performed at a constant pH with H2 and CO2 At a pH of 6.2, the total acetate production reached 840 mmol/l-reactor with 4.7 mmol/l-reactor of total ethanol production after 420h. When the culture pH was maintained at 5.8, which was the optimum for ethanol production, the total ethanol production reached 15.4mmol/l-reactor after 430h, although the total acetate production was decreased to 675mmol/l-reactor.