Cell-recycle continuous fermentation of Enterococcus faecalis RKY1 for economical production of lactic acid by reduction of yeast extract supplementation.

Both lactic acid productivity and cell growth were linearly correlated with yeast extract supplementation in batch fermentation. During conventional continuous operation, although fresh feed was introduced into the bioreactor with a significantly low dilution rate (0.04 h(-1)), the amount of yeast extract employed was not enough to maintain the growth of microorganism. However, when the fresh feed contained 100 g/l glucose and 2 g/l yeast extract during cell-recycle continuous operation at a dilution rate of 0.04 h(-1), more than 90 g/l lactic acid was continuously produced, with the average productivity of 3.72 g/l·h. In this experiment, 82 g of yeast extract (77% of reduction yield) could be reduced for the production of 1 kg of lactic acid compared with batch fermentation of a similar volumetric productivity.