Novel pH control strategy for efficient production of optically active l-lactic acid from kitchen refuse using a mixed culture system

Uninvestigated control factors of meta-fermentation, the fermentative production of pure chemicals and fuels in a mixed culture system, were examined for production of optically pure l-lactic acid (LA) from food waste. In meta-fermentations by pH swing control, l-LA production with 100% optical purity (OP_l-LA) was achieved even using unsterilized model kitchen refuse medium with preferential proliferation of l-LA-producing Bacillus coagulans, a minor member in the seed, whereas agitation decreased OP_l-LA drastically. pH constant control shortened the fermentation time but decreased OP_l-LA and LA selectivity (S_LA) by stimulating growth of heterofermentative Bacillus thermoamylovorans. Deliberately switching from pH swing control to constant control exhibited the best performance for l-LA production: maximum accumulation, 39.2 g L^-1; OP_l-LA, 100%; S_LA, 96.6%; productivity, 1.09 g L^-1 h^-1. These results present a novel pH control strategy for efficient l-LA production in meta-fermentation based on a concept different from that of pure culture systems.