Acetic acid was produced from anaerobic fermentation of lactose by the co-culture ofStreptococcus lactisandClostridium formicoaceticumat 35° C and pHs between 7.0 and 7.6. Lactose was converted to lactic acid, and then to acetic acid in this mixed culture fermentation. The overall acetic acid yield from lactose was about 95 at pH 7.6 and 90 at pH 7.0. The fermentation rate was also higher at pH 7.6 than at pH 7.0. In batch fermentation of whey permeate containing about 5 lactose at pH 7.6, the concentration of acetic acid reached 20 g/l within 20 h. The production rate then became very slow due to end-product inhibition and high Na+concentration. About 30 g/l acetate and 20 g/l lactate were obtained at a fermentation time of 80 h. However, when diluted whey permeate containing 2.5 lactose was used, all the whey lactose was converted to acetic acid within 30 h by this mixed culture
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