Breeding Research Of High Yield L-lactic Acid Bacteria Strains By X Ray-heavy Ion Beam Compound Mutagenesis

L-lactic acid, one of the three kinds structure of lactic acid. It is widespread in the human body, animals, plants, and microorganisms. It has important applications in food etc, especially as the prerequisite of the biodegradable material poly L-lactic acid. Now, the mainly way of production is fermentation and stains is key. Good stains may raise yield to save the lost of production. It is same important mean to production of L-lactic acid to research a way that gets good stains by compound mutagenesis and fermentation conditions. So this paper main research was that the Lactobacillus thermophilus irradiated by X-ray were screened by calcium carbonate-bromocresol purple plate and submerged fermentation in shaking flask and got height purity of acid produce mutant stain. The mutant strains irradiated by heavy ion compound mutagenesis again and got final mutant stain. In the end, there was the initial research of fermentation conditions to the final mutant stain. The research studies were as follows:(1)The results of the original stain irradiated by X-ray showed that a stable high-yield mutant strain, named SR7, could produce 22g/L of L-lactic acid on average with the yield increase of 15.04%, compared to the original strain. It was also proved that SR7 strain has good genetic stability by repeating experiments.(2)The result of SR7 irradiated by heavy ion showed that a stable high-yield mutant strain, named SRZ50, could produce 24.5 g/L of L-lactic acid on average with the yield increase by 11%, compared to the original strain. It was also proved that SR7 strain has good genetic stability by repeating experiments.It was also proved that SRZ50 strain has good genetic stability by repeating experiments. and has made the qualitative appraisal of its.(3)The optimal process of the fermentation medium of final mutant strains SRZ50 by the Box-Behnken design optimization is:6.18%glucose, 0.33%yeast powder, 0.22%sodium acetate, 0.1%potassium hydrogen phosphate. And the initial culture conditions were studied, fermentation temperature is 50 ℃, pH 7.0, fungus age is 18 h, fluid volume for 50 mL/250 mL, 5% inoculated quantity, fermentation way of 100 r/min 4 h after static culture, concentration of calcium carbonate was 2.5%.The final mutant strains SRZ50 produce high-yield high-pure L-lactic acid. 72 hfermentation produce acid amount to 41.18g/L, sugar acid conversion rate is 86%, sugar utilization rate of 90%.