Isolation of thermophilic l-lactic acid producing bacteria showing homo-fermentative manner under high aeration condition

By applying non-sterile open fermentation of food waste, various thermotolerant l -lactic acid-producing bacteria were isolated and identified. The predominant bacterial isolates showing higher accumulation of l -lactic acid belong to 3 groups of Bacillus coagulans , according to their 16S rRNA gene sequence similarities. B. coagulans strains M21 and M36 produced high amounts of l -lactic acid of high optical purity and lactic acid selectivity in model kitchen refuse medium and glucose–yeast extract–peptone medium. Other thermotolerant isolates resembling to Bacillus humi , B. ruris , B. subtilis , B. niacini and B. soli were also identified. These bacteria produced low amounts of l -lactic acid of more than 99% optical purity. All isolated strains showed the highest growth rate at temperatures around 55–60°C. They showed unique responses to various oxygen supply conditions. The majority of isolates produced l -lactic acid at a low overall oxygen transfer coefficient ( K L a ); however, acetic acid was produced instead of l -lactic acid at a high K L a . B .  coagulans M21 was the only strain that produced high, consistent, and reproducible amounts of optically pure l -lactic acid (>99% optical purity) under high and low K L a conditions in a homo-fermentative manner.