Experiments for d-lactic acid production with fermentation

Abstract

Lactic acid is one of the oldest known chiral chemicals. Its L-enantiomer is widely produced and used both in the nature and in human society. The renaissance ofthe L-lactic acid production was forced by the biodegradable plastic production on lactic acid basis which produces PLA.This polymer has good biodegradability, but poor mechanic properties, thus its characteristic is under strong improvements. During this investigations it was established that the stereocomplex polymers from L and D enantiomers has higher melting point resulting in broader application. Unfortunately the production of D-lactic acid was not solved since it is not the natural formof lactic acid. For this reason we wanted to examine the possibilities for D-lactic acid fermentation with Lactobacillus coryniformis. We adapted a new technique for cultivation in microtiterplates. First we successfully optimized the media composition in term of carbon source (glucose) and nitrogen source (yeast extract) and obtained an optimal glucose=67.5 g/L and YE=27 g/L concentration by Gauss-surface fitting. Next we noticed (on observation that the final product titer never surpassed 65 g/L but a lot of glucose remained) that this fermentation is under strong product inhibition. We also examined the inhibitory effect of different form of product lactate (i.e. with Na+, Ca2+ and NH4+). We obtained that all the examined products cause total inhibition on cell growth at 90–120 g/L concentration but they differ in their effect between 30–60 g/L: the strongest inhibition occurred at NH4-LA (both 30 and 60 g/L), the Na-LA has lower inhibition range, but its effect is also equal at 30 and 60 g/L and finally Ca-LA showed hardly inhibition at 30 g/L but strong inhibition at 60 g/L.