Study On The Production Of L-lactic Acid By Microbial Fermentation

L-lactic acid,a kind of very important organic acid,is widely used in food,leather,medicine and chemical industries.Moreover,L-lactic acid could be used as monomers in the synthesis of poly lactic acid?PLA?.Therefore,L-lactic acid has a wide range application.However,the production of L-lactic acid using fermentation method still takes long time,or the production of L-lactic acid has a low optical purity and the low yield,or need the large energy consumption,resulting in high cost of lactic acid production,which has been a bottleneck restricting the development of lactic acid fermentation industry.In this paper,microbial fermentation of L-lactic acid was studied.Firstly,a strain with high-yield of L-lactic acid was selected.And then the optimization of fermentation medium and culture conditions were performed.Moreover,mutation breeding of strains was carried out.The aim of these work was to provide a good strain and cheap cottonseed meal replacing yeast powder for the industrial production of L-lactic acid.?1?Two strains with high-yield of lactic acid were screened from cereals and fermented foods.Morphological observation,VITEK 2 physiological and biochemical identification and 16 S rRNA sequence analysis showed that the two strains were all Lactobacillus rhamnosus,and named Lactobacillus rhamnosus DLF-15037 and Lactobacillus rhamnosus DLF-15038,respectively.The lactic acid yield of the strain DLF-15038,which was screened from the hot cabbage,was the highest,and the L-lactic acid was 100% L-lactic acid by the L-/D-lactic acid kit.?2?The culture medium and culture conditions of strain DLF-15038 were optimized by single factor and response surface experiment.It is found that the cheap cottonseed meal could partially replace yeast powder with 15 g/L cottonseed meal and 10 g/L yeast powder for the compound nitrogen source,the optimum inorganic salt concentration of CH3 COONa 3 g/L,KH2PO₄ 2 g/L,MnSO₄ 0.3 g/L,MgSO₄ 0.2 g/L.And it reduces fermentation costs.The optimum fermentation conditions in the flask were as follows: the volume of liquid 100 mL/250 mL,the fermentation temperature 37?,the rotation speed 184 r/min,the age of seed 11 h and inoculation amount 4%;The optimum fermentation conditions in 5 L fermentor were as follows: pH 6.0,the flow rate 0.1 vvm.The batch and fed-batch fermentation were carried out,the concentration of L-lactic acid were 158.24 g/L and 165.15 g/L for 72 h,and the yield were 2.20 g/?L·h?and 2.29 g/?L·h?,respectively.The conversion of glucose to lactic acid were 90.36% and 93.34%,respectively.?3?L.rhamnosus DLF-15038 was used as the wild strain,and UV and DES mutagenesis were carried out.The mutagenic conditions were as follows: the concentration of bacteria of 10-6,UV mutagenesis time 3 min;1% DES mutagenesis time 20 min,38?.After 6 passages,the mutant strain was found to have good genetic stability and named it Lactobacillus rhamnosus DLF-15041,providing a good strain for lactic acid fermentation.Finally,the batch and fed-batch fermentation are carried in a 5 L fermentor,the concentration of L-lactic acid were 167.45 g/L and 173.26 g/L,and the yield were 2.33 g/?L·h?and 2.41 g/?L·h?,respectively.The conversion rates of glucose to lactic acid were 93.52% and 93.23%,respectively.