Study On Screening And Fermentation Conditions Of Strain Producing L-lactic Acid

The paper focused on selections and breeding of the lactic acid bacteria producing L-lactic acid with homolactic fermentation under high temperature from the nature. Firstly the fermentation characteristics, fermentation culture medium and cultivation conditions of this lactic acid bacteria was studied. Then this lactic acid bacteria was identified. Finally, in order to enhance the ability of bacteria producing L-lactic acid ,the bacteria was mutated by UV. The detailed contents were as followed:1. Based on the bacteria characteristic which is high producing L-lactic acid and substrate feedback principle, the enrichment culture medium, the selectivity culture medium and the route of selection were designed. In the summer abroadly collected the samples, 28 strains utilizing glucose producing L-lactic acid were obtained, the yield of L-lactic acid exceed 60g/L under the temperature 52℃ in 48 hours. By the second screening and the experiment of fermentation characteristics, 1 strains utilizing glucose, which had a higher yield of L-lactic acid 90.85g/L, were determined, named LB-p.The growth curve of strain and the downtrend of pH were determined. The results of selection proved: L-lactci acid calcium added 60g/L to enrich culture medium and separate culture medium was the best, the capability of strain was better.2. The optimization of fermentation media about the strain LB-p was researched. Different sorts of carbon and nitrogen resources, growth factors, metal ions of inorganic salt were studied based on the single factor experiment. Three ingredients' ( rich saccharifing liquid, peptone, yeast power) ratio of medium was optimized through Response Surface Analysis (Central Composite Design). The functional relationship between main factors and response value (L-lactic acid yield) was established, and the regression equation was obtained. The regression equation was :Y=90.39+1.17X₁+5.36X₂+3.10X3-1.48X₁²-1.92X₂²-1.55X₃²+0.66X,*X₃-1.39X₂*X₃.Through analyzing the regression equation, the optimal concentrations of the factors were obtain: rich saccharifying liquid 125.25g/L, peptone 8.79g/L, yeast power 3.58g/L. And the optimal medium was as followed: 125.25g/L, peptone 8.79g/L, yeast power 3.58g/L, K₂HPO₄ 3g/L, beef grease 4g/L, MgSO₄0.05g/L, water 1000ml. The yield of L-lactic acid reached 94.783g/ L, the ratio of glucose about 95%.3. The cultivation conditions of the strain LB-p were studied and the optimal conditions were as followed: the inoculation size 8%, the fermentation temperature 52°C, the age 16h, the quantity of flask 150ml/250ml, the neutralizer CaCO₃.4. The strain was judged to be Lactobaillus tyermophius by the study of LB-p colobial, cell morphology, physiochemical characteristic and fermentation speciality ect.