<scp>l</scp>‐Lactate oxidase‐mediated removal of <scp>l</scp>‐lactic acid derived from fermentation medium for the production of optically pure D‐lactic acid

<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>There has been an increasing demand for optically pure <jats:sc>d</jats:sc>‐lactic and <jats:sc>l</jats:sc>‐lactic acid for the production of stereocomplex‐type polylactic acid. The <jats:sc>d</jats:sc>‐lactic acid production from lignocellulosic biomass is important owing to its great abundance in nature. Corn steep liquor (CSL) is a cheap nitrogen source used for industrial fermentation, though it contains a significant amount of <jats:sc>l</jats:sc>‐lactic acid, which decreases the optical purity of <jats:sc>d</jats:sc>‐lactic acid produced.</jats:p></jats:sec><jats:sec><jats:title>Method and results</jats:title><jats:p>To remove <jats:sc>l</jats:sc>‐lactic acid derived from the CSL‐based medium, <jats:sc>l</jats:sc>‐lactate oxidase (LoxL) from <jats:italic>Enterococcus</jats:italic> sp. NBRC 3427 was expressed in an engineered <jats:italic>Lactiplantibacillus plantarum</jats:italic> (formally called <jats:italic>Lactobacillus plantarum</jats:italic>) strain KOLP7, which exclusively produces <jats:sc>d</jats:sc>‐lactic acid from both hexose and pentose sugars. When the resulting strain was applied for <jats:sc>d</jats:sc>‐lactic acid fermentation from the mixed sugars consisting of the major constituent sugars of lignocellulose (35 g L<jats:sup>–1</jats:sup> glucose, 10 g L<jats:sup>–1</jats:sup> xylose, and 5 g L<jats:sup>–1</jats:sup> arabinose) using the medium containing 10 g L<jats:sup>–1</jats:sup> CSL, it completely removed <jats:sc>l</jats:sc>‐lactic acid derived from CSL (0.52 g L<jats:sup>–1</jats:sup>) and produced 41.7 g L<jats:sup>–1</jats:sup> of <jats:sc>d</jats:sc>‐lactic acid. The <jats:sc>l</jats:sc>‐lactic acid concentration was below the detection limit, and improvement in the optical purity of <jats:sc>d</jats:sc>‐lactic acid was observed (from 98.2% to > 99.99%) by the overexpression of LoxL.</jats:p></jats:sec><jats:sec><jats:title>Conclusion and implications</jats:title><jats:p>The LoxL‐mediated consumption of <jats:sc>l</jats:sc>‐lactic acid would enable the production of optically pure <jats:sc>d</jats:sc>‐lactic acid in any medium contaminated by <jats:sc>l</jats:sc>‐lactic acid.</jats:p></jats:sec>