Production of Optically Pure D-Lactic Acid by the Combined use of Weissella sp. S26 and Bacillus sp. ADS3.

Optically pure D-lactic acid was produced from glucose, xylose, or starch by the combined use of Weissella sp. S26 and Bacillus sp. ADS3, two native bacterial strains isolated from Singapore environment. Weissella sp. S26 was used to ferment various sugars to lactic acid rich in D-isomer followed by sterilization of the broth and inoculation of Bacillus sp. ADS3 cells to selectively degrade acetic acid (if any) and L-lactic acid. In a simultaneous saccharification and fermentation of starch by Weissella sp. S26 in 1 L of modified MRS medium containing 50 g/L starch at 30 °C, lactic acid reached 24.2 g/L (23.6 g/L of D-isomers and 0.6 g/L of L-isomers), and acetic acid was 11.8 g/L at 37 h. The fermentation broth was sterilized at 100 °C for 20 min and cooled down to 30 °C followed by inoculation of Bacillus sp. ADS3 (10 %, v/v), and the mixture was kept at 30 °C for 115 h. Acetic acid was completely removed, and L-lactic acid was largely removed giving an optical purity of D-lactic acid as high as 99.5 %.