Production of L-lactic acid by simultaneous saccharification and fermentation using unsterilized defatted rice bran as a carbon source and nutrient components

Abstract

On the basis of growth rate at low pH, yield of lactic acid from glucose, and optical purity of lactic acid produced, we selected lactic acid bacteria favorable for production of optically pure L-lactic acid from defatted rice bran without sterilization. Of 21 strains tested, strains Nos. 13 and 16 produced 27-29 kg m-3 of lactic acid with high optical purity from 100 kg m-3 of unsterilized defatted rice bran in simultaneous saccharification and fermentation (SSF) with MRS medium at pH 4.5, a level at which the growth of indigenous lactic acid bacteria in defatted rice bran was suppressed. In a SSF process using strain No. 16 in which Mcllvaine buffer (pH 4.5) was used instead of MRS medium, no growth of indigenous lactic acid bacteria was observed in defatted rice bran, and 28 kg m -3 of lactic acid with 92% L-type content was produced from 100 kg m-3 of unsterilized defatted rice bran. In SSF using McIlvaine buffer (pH 4.5), the protein fraction of defatted rice bran was found to play a significant role as a nitrogen source for the growth of lactic acid bacteria. By increasing the initial cell concentration to OD660 = 1.0 for SSF using strain No. 16 and McIlvaine buffer (pH 4.5), the proportion of L-lactic acid produced was enhanced to 95%.