Synthesis Of L-lactic Acid. Lb.paracasei Hd1.7 Optimization Of Fermentation Conditions And Metabolic Flux Analysis

L-lactic acid, as one of the three important organic acids, which is nontoxic and has anti-bacteria ability, has been comprehensive used in many fields and has an increasing need year by year. Its polymer—polylactic acid has good mechanical and physical function,as well as biodegradable ability. The polylactic acid has been widely used in many fields, such as biomedicine, medicine, clothing, and wrapping. With these good characters, polylactic acid has got highly concerned and will have promising market in the coming future.We isolated the bacteria from pickled cabbage juice which has high optimal purity, which is named Lb.paracasei HD1.7. And its production of L-lactic acid is highly improved after the initial optimization. So this bacteria is highly considered to be worth of researching and exploring.This paper has optimized the culture component and fermentation condition. After comparing some regular culture which is generally for lactic acid bacteria, we drew the conclusion that MRS is the best culture for Lb.paracasei HD1.7. We have optimized the carbon source, organic and inorganic nitrogen source, phosphorus source and metal ion. The outcome of lactic acid after optimization is 23.8g/L, which is 2.4 times than that of before optimization.Besides, we also optimized the inoculum age, inoculation rate, fermentation temperature which have big effect on lactic acid production. We drew the conclusion that in the condition of alternating temperature, transferring the 16 hours seed culture to fermentation culture in the proportion of 5 percent, which pH is controlled by 5 mol/L NH3·H2O, the lactic acid yield is 90g/L, and the transformation efficiency of glucose is 90 percent. Altering temperature is the innovation in this research, and its result shows that the yield of lactic acid in 42℃+37℃, compared to 37℃condition, is almost the same. While in the former condition, the fermentation cycle is reduced by 16 hours, which has significant influence in the industry. Besides, we studied on the fed-batch glucose's effect on the lactic acid's yield. In the end, the outcome of lactic acid reached 112g/L.At last, we used the newly emerging metabolic method, combined with traditional fermentation strategy, to look for some shortcuts to improve the lactic acid's yield. Firstly, we constructed the metabolic network of Lb.paracaseiHD1.7, and then made use of stechiometry to calculate the metabolic fluxes of all the intermediate products at different time. According to the result, we concluded that Phosphoenolpyruvate PEPCK, PFK and LDH are the key knots which control the outcome of lactic acid. If we could target these enzymes and make use some genetic method to modify or restrict them, we may get some strains which have higher lactic acid outcome, in this way, we can get over the weakness of traditional fermentation.