Study On Fermentation Of L-Lactic Acid By Bacteria And Its Metabolic Flux Analysis

This paper focused on the breeding of an L-lactic acid producing bacteria Lactobacillus easei YBQ1-1, studying the characteristics, medium components, pH and metabolic network of the mutant YBQH2-14 obtained after being selected.Using UV irradiation, L.casei YBQH2-14 was selected in the different plate media such as the medium with high concentration of glucose, lactic acid , CaCO3 -Bromocresol purple and KMnO4 -KBr. After fermentation for 72 hours, YBQH2-14 produced L-lactic acid output of 93g/L, 48.5% higher than that of the parent strain.The optimization of fermentation media about the strain L.casei YBQH2-14 was studied. Different sorts of carbon resources were studied in the single factor experiment. The yield of L-lactic acid could reach 105g/L grown in rice saccharifing liquid medium. A detailed response surface methodology was carried out to optimize the C/N ratio of the fermentation medium using the experimental design. The first study tested two factors(organic nitrogen sources and inorganic nitrogen sources)and one medium point was performed. The second factorial design based on surface respon.se with four factors (glucose concentration, peptone concentration, NH4C1 concentration, (NH4)2HPO4 concentration). The statistical analysis of the data obtained from the factorial study showed that a C/N ratio of 12-13 and the glucose concentration of 120.13g/L, peptone concentration of 19.342 g/L, NH4Cl concentration of 4 g/L, (NH4)2HPO4 concentration of 2.0096 g/L were the best condition.The cultivation conditions of the strain L.casei YBQH2-14 was studied and the optimal fermentation temperature was 37 C , the results showed that the concentration of H+ are indirectness both on the growth of lactic acid bacteria and on the productivity of lactic acid.Combining EMP, TCA with compositive route of biological molecule and based on stoichiometric balance equations, metabolic networks of L.casei YBQH2-14 was constructed. By applying MFA (Metabolic Flux Analysis), the metabolic flux distribution in cell under different phases were obtained.The fermentation of lactic acid by rye bran flour was studied here. And an orthogonal experiment was designed with three factors and three levels. The optimal concentration of a-amylase, saccharifying enzyme, protease and cellulose was 15 u/g dry starch, 1 u/g raw, 15 u/g raw, 80 u/g dry starch, respectively. The yield of L-lactic acid could reach 25g/L.