Study On The Breeding Of L-Lactic Acid High-Producing Strain And Its Metabolism

This paper focused on the breeding of L-lactic acid producing bacteria, studying the characteristics and metabolism of the mutant strain.Using UV light, Lactobacillus casei G was mutated in order to get high lactic acid endurance strain. The mutant Gil-3-7 could accumulate 48g/L L-lactic acid in production medium. After the optional production medium being preliminary developed, the L-lactic acid production could reach 58g/L. The ability of growth and producing lactic acid of the mutant were better than G.Studying the characteristics and metabolism of Gil-3-7 were also an important part of this paper. The outcome indicated that the concentration of lactic acid is the main reason of the growth inhibition of lactic acid bacteria and declining the productivity of lactic acid. This result would be enhanced with the descending of pH. The result that comes from the concentration of H+ is indirectness both on the growth of lactic acid bacteria and on the productivity of lactic acid. The component of production medium was an important factor in affecting the growth of bacteria and acid production. The adding of fructose and lactose could both stimulate the growth of bacteria and acid production. By studying the effect of yeast extract and peptone, the outcome could prove two points. One was that increasing the concentration of yeast extract and peptone could raise the L-lactic acid production. The other was that the effect of peptone is greater than yeast extract. Metal ions are also important components of production medium, but different metal ions have different effects. Two conclusions were obtained from the research. One was that increasing the concentration of Mg2+ could raise L-lactic acid production because Mg2+ could stimulate many enzyme of BMP. The other was that Mn2+ could enhance the growth of bacteria, so increased the L-lactic acid production.The pyruvate had a significant beneficial effect on the glucose conversion and L-lactic acid production. From the results, we could see that 30g/L of pyruvate can increase the L-lactic acid production significantly in the environment of 90g/L of glucose production medium. During the 72h fermentation, pyruvate can increase the L-lactic acid production greatly when glucose added at 24h and 42h after the beginning of the fermentation, and the L-lactic acid production can reach 72g/L and 76g/L.