Lactic Acid Production With An Isolated High-thermotolerant Enterococcus Faecalis DUT1805

As a platform compound,lactic acid(α-hydroxypropionic acid)is widely used in food,chemical,medical and other fields.Degradable polylactic acid(PLA),which is synthesized as a monomer,is one of the substitutes for petrochemical resources,and its applications promote the improvement of the ecological environment.In recent years,the high production cost of lactic acid has restricted the large-scale application of bio-based lactic acid and its derivatives.This project intends to produce lactic acid by high-efficiency biotransformation through low-cost renewable biomass materials(rice flour,corn stover,etc.).Through the gradient heating strategy to optimize the high temperature fermentation performance of Enterococcus faecalis DUT1805,the high-temperature simultaneous conversion of biomass resources such as rice flour and corn stover can produce lactic acid,which provides a low-cost,renewable and environment-friendly technical route for the production of lactic acid.First,physiological and biochemical experiments and performance studies were performed on E.faecalis DUT1805.The best carbon source for the production of lactic acid was found to be glucose,and the fermentation was good at high temperature(50-55~℃).Using glucose as a substrate,the performance of the bacteria to produce lactic acid at high temperature was explored by gradient heating.The results showed that the optimal fermentation temperature,pH,glucose concentration and heating time were 50~℃,6.5,80 g/L and 6 h,respectively,and the optimal fermentation method was batch fermentation.Under this condition,65.41 g/L of lactic acid was finally obtained with a yield of 0.88 g/g and a production strength of 4.09 g/(L·h).Secondly,at 50~℃,lactic acid was produced by simultaneous saccharification and fermentation of rice flour using E.faecalis DUT1805.The results show that the lactic acid performance(lactic acid concentration,starch conversion rate,production intensity)of the raw material fermentation is about double that of the clinker process.At the time of raw meal fermentation,the final concentration of lactic acid was 73.36 g/L,the conversion with respect to starch was 0.89 g/g,and the production intensity was 2.06 g/(L·h).At the same time,the corn nitrogen dry powder is used to replace the organic nitrogen source in the MRS medium to reduce the nitrogen source cost in the fermentation medium.The final concentration of lactic acid was 73.75 g/L,the conversion to starch was 0.87 g/g,and the production intensity was 2.06 g/(L·h).The results showed that the fermentation performance was flat compared with the MRS medium,and the nitrogen source cost was reduced by 90%.Economic analysis shows that when rice flour is used as raw material and corn syrup dry powder is used as nitrogen source,the total cost of the three is 4,542 yuan/ton,which accounts for 35-45%of the market price of lactic acid.Finally,the fermentation of lactic acid from the high temperature synchronous saccharification corn stalk hydrolysate of E.faecalis DUT1805 was investigated.The results showed that the bacteria could metabolize mixed sugar(glucose,xylose)to produce lactic acid.When the ratio of glucose to xylose was 2:1,the fermentation performance was the best.Fermentation with corn stalk hydrolysate as carbon source and corn syrup dry powder as nitrogen source can obtain 21.59 g/L lactic acid,the yield relative to glucose is 0.93 g/g,and the production intensity is 0.86 g/(L·h).In summary,E.faecalis DUT1805 can produce lactic acid at high temperature using starch-based raw materials or cellulose-based raw materials,and at the same time can achieve open operation,saving raw material cost,equipment investment and operating cost.