Improved lactic acid productivity by an open repeated batch fermentation system using Enterococcus mundtii QU 25

Decreasing the production cost of polymer-grade lactic acid is a challenge that the polylactic acid industry must surmount to be competitive with producers of petrochemical-derived plastics. In this study, we aimed to improve lactic acid productivity by investigating different fermentation systems. Enterococcus mundtii QU 25, previously reported by our group as a thermotolerant lactic acid bacterium, homofermentatively metabolizes glucose with the production of high optically pure l-lactic acid. The strain showed high lactic acid concentration and yield by controlled pH (7.0) and at a high temperature (43 °C). In a batch fermentation system, the optimal initial glucose concentration was 100 g L^-1, at which high cell mass was achieved with production of 82.4 g L^-1 of lactic acid at yield of 0.858 g g^-1 glucose consumed. However, the productivity was approximately 2.0 g L^-1 h^-1. Subsequently, open repeated batch or fed-batch fermentation with recycled cells of the previous fermentation was conducted for 11 runs to enhance the productivity. As a result, lactic acid productivity was enhanced-up to 5.5-fold higher than that of the conventional batch culture, and fermentation was completed within 6 h with almost the same yield (0.761-0.832 g g ^-1 glucose consumed) and lactic acid concentration (81.6-84.5 g L^-1). Up to 132 g L^-1 lactic acid at yield of 0.853 g g^-1 glucose consumed and productivity of 6.99 g L^-1 h ^-1 were obtained in open repeated fed-batch fermentation. This indicates that strain QU 25 has the potential for industrial-scale production of lactic acid due to its high productivity, high yield of substrate conversion, and high final concentration of lactic acid produced under non-sterile conditions.