The Research Of Producing L-lactic Acid Using Fiber Material

L-lactic acid and its derivatives are widely applied in food, bio-medicine andchemical industry. In recent years, with bio-degradable plastic, poly-L-lactic acidextensively being used, the demand of L-lactic acid has increased dramatically. In currenttechnology, the simultaneous saccharification and fermentation（SSF） is the main methodfor producing high purity L-lactic acid at home and abroad. Howerever, there are somedisadvantages in simultaneous saccharification and fermentation（SSF）. For example, thedifferent optimal temperature for saccharification and fermentation, the feedback inhibitionof the final product, the increasing cost due to adding amylases or cellulases. Therefore, theconsolidated bioprocessing（CBP） has tremendous potential.In CBP, one kind ofmicroorganism was used to degrade cellulose and produce L-lactic acid. This processingnot only reduces the cost of production, but solves environmental pollution from cropresidues. But the strain is a limiting factor in CBP. In this paper,a strain of enterococcusfaecalis CTB374-1with celluose degradation ability was separated and identificated in ourgroup’s previous research. To improve the L-lactic acid of the strain using cellulose as rawmaterils. The following works were done:（1）The percentages of cellulose, hemicellulose and lignin in straws of corn, cotton,wheat, soybeans, millet and sorghum were determined. Understanding the characteristicsof different fiber materials is a base of applying them in the production of L-lactic acid.（2）The strain CTB374-1was mutated with UV. The optimal time of UV mutagenesiswas determined for30s by single factor experiment, and a high-yield L-lactic acid strainCTB374-1-8was screened. The yield of L-lactic acid of CTB374-1-8was15.22g·L^-1when glucose was used as carbon source, which was30.37%higher than the strainCTB374-1.（3）The optimal culture conditions and fermentation medium of L-lactic acidprouucedwere studied. The optimal culture conditions were determined by single factorexperiment, and the results were as following: fermentation temperature37℃;inoculation volume5%; fermentation time60h; initial pH7.5; aerobic fermentation; cornstraw as carbon source. The fermentation medium components were optimized byPlackett-Burman and Box-Behnken experiments.The results showed that peptone, citric acid triamide and K2HPO4were the main factor effecting the production of L-lactic acid.The optimal concentrtins for above three factors were as following: peptone11.71g·L^-1,citric acid triamide2.29g·L^-1, K2HPO42.39g·L^-1. Under above culture conditions andmedium, the maximum production of L-lactic acid was1.173g·L^-1with corn straw ascarbon source,which was more than86%improvement.