Study on Elimination of Exogenous D-Lactic Acid by Escherichia coli L-Lactic Acid Engineering Bacteria for Improving Optical Purity
D-lactic acid,an isomer of L-lactic acid,can cause metabolic acidosis,intestinal barrier damage or other unknown hazards,so there is a need to reduce the amount of D-lactic acid in L-lactic acid used as a food acidifier. In this study,the sodium nitrate is added to enhance the elimination of exogenous D-lactic acid by Escherichia coli L-lactic acid engineering bacteria under anaerobic conditions as well as the deletion of L-lactate dehydrogenase genes (lldD gene,ykgEFG gene) to reduce their consumption of L-lactic acid.Through shaking flask fermentation experiment,it is proven that the engineering bacterium HBUT-LB,which has knocked out the L-lactate dehydrogenase gene,can effectively remove 1 g/L exogenous D-lactic acid after adding 20 mmol/L sodium nitrate.Fermentation is carried out in a 7 L fermenter using inexpensive raw materials containing D-lactic acid. The consumption rate of D-lactic acid increases by 460％ with adding 20 mmol/L sodium nitrate compared to without adding sodium nitrate,and the optical purity of L-lactic acid increases from 99.32％ to 99.99％ in the engineering bacterium HBUT-LB. At the same time,there is no sodium nitrate and sodium nitrite residue in the final fermentation broth. The results show that the addition of sodium nitrate during the fermentation with engineering strain HBUT-LB can effectively eliminate the exogenous D-lactic acid,improve the optical purity of L-lactic acid,which has industrial application value in refining L-lactic acid with ultra-high optical purity.
optical purity of L-lactic acidL-lactate dehydrogenase genesodium nitrate
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