Optimization of d-lactic acid fermentation by Sporolactobacillus kofuensis SB7-2 to increase productivity

Abstract

D-lactic acid now gains many interest in both research and industry levels since the discovery of the superior properties of stereocomplex polylactic acid. Nonetheless, the commercial production of D-lactic acid is still limited compared to the existing facilities for L-lactate to be widely used in food and pharmaceutical industries. Thus, this leaves the room for future research and development for pursuing industrial production. Recent studies reported fermentation of D-lactic acid by Sporolactobacillus sp. including S. laevolacticus and S. nakayamae but to date there is no D-lactic acid fermentation study of S. kofuensis found in the literatures. The in-house isolate, SB7-2, screened from the tree bark collected from the natural habitat in Saraburi was tested for its ability to produce D-lactic acid. From bacterial identification, SB7-2 acquired the high similarity percentage to S. kofuensis (99.86 %). From the preliminary results, S. kofuensis SB7-2 could produce D-lactic acid at the final titer up to 112 g/L with the remarkable yield of 0.93 g/g and productivity of 1.6 g/L·h from simple medium containing 120 g/L glucose during anaerobic cultivation in the shaken flask. To develop the valid fermentation platform for this isolate, several key successful fermentation factors were optimized both in the shake flask and the stirred fermentor. Those inclued the age of slant, inoculum size, initial glucose concentration, preculture time, mixing, and oxygen. With the optimized conditions determined in this study, the highest D-lactic acid of 98.95 g/L with the yield 0.96 g/g and the productivity of 3.97 g/L·h was obtained within 24 h. This productivity was 2.5 times higher than that from the preliminary result in the shake flask culture.