Production of d-lactic acid from hardwood pulp by mechanical milling followed by simultaneous saccharification and fermentation using metabolically engineered Lactobacillus plantarum

This study focused on the process development for the d-lactic acid production from cellulosic feedstocks using the Lactobacillus plantarum mutant, genetically modified to produce optically pure d-lactic acid from both glucose and xylose. The simultaneous saccharification and fermentation (SSF) using delignified hardwood pulp (5-15% loads) resulted in the lactic acid titers of 55.2-84.6 g/L after 72 h and increased productivities of 1.77-2.61 g/L/h. To facilitate the enzymatic saccharification of high-load pulp at a fermentation temperature, short-term (⩽10 min) pulverization of pulp was conducted, leading to a significantly improved saccharification with the suppressed formation of formic acid by-product. The short-term milling followed by SSF resulted in a lactic acid titer of 102.3 g/L, an optical purity of 99.2%, and a yield of 0.879 g/g-sugars without fed-batch process control. Therefore, the process presented here shows promise for the production of high-titer d-lactic acid using the L. plantarum mutant.