Racemization of l-lactic acid in pH-swing open fermentation of kitchen refuse by selective proliferation of Lactobacillus plantarum

We have shown that stable lactic acid fermentation of model kitchen refuse occurs with intermittent pH adjustment under nonsterilized conditions. Nonetheless, the optical activity of the accumulated lactic acid was low, which is disadvantageous for the production of high-quality poly-l-lactic acid. Here, we attempt to increase optical purity by introducing l-lactic acid-producing strains under nonsterilized conditions and demonstrate that the inoculation of Lactobacillus rhamnosus or Lactococcus lactis, both of which are l-lactic acid producers, is partially effective in the early fermentation stage, but does not improve the final optical purity of the accumulated lactic acid. We confirmed by fluorescence in situ hybridization using group-specific and species-specific 16S rDNA probes that this is due to the selective proliferation of naturally existing L. plantarum. L. plantarum KY-1, which is isolated from model kichen refuse, showing lactic acid racemase activity, as well as d-lactate dehydrogenase activity, in its membrane fraction. We conclude that racemase activity associated with L. plantarum is the main cause of decreased optical purity in the accumulated lactic acid.