Study On L-malic Acid Production By Aspergillu Flavus SFW-7

L-malic acid, which is one of the important intermediate product of tricarboxylic acid cycle in organism, has important physiological function, widely distributing in the cell of plant, animal and microorganism, having widespread use in the field of food, medicine, daily use chemical industry, and so on. The production methods of L-malic acid have chemical synthesis, enzymatic conversion, microorganism fermentation. Microorganism fermentation includes one-step fermentation, two-step fermentation. Compared with other methods, one-step fermentation has the advantages of environmental friendly, rich and recycled raw material, and so on, which is valued more and more by people. However, one-step fermentation also has the disadvantages of lower producing level of L-malic acid, producing heteroacid , which restrict seriously the industrial production of L-malic acid by one-step fermentation, so that it is meaningful to research the problem of improving producing acid level and the metabolize mechanism of L-malic acid. In this paper, we carried out a series of experiments, including optimizing medium components, adding carrier of oxygen, surface active agent and the inhibitor of key enzymes of part metabolic pathway. The main results were as follows:The influence of n-hexane, dodecane, perfluocarbon, tween-20 and tween-80 to the fermentation result of Aspergillu flavus SFW-7 was investigated. The result indicated the output of L-malic acid was maxium, up to 26g/L, when 1%(V/V) dodecane and 0.1%(W/V) tween-80 were added together.In the single factor test's foundation, the medium components of Aspergillus flavus SFW-7 accumulating L-malic acid was optimized by quadratic general rotary design, and a quadiatic regression equation was established. The results showed the order of significance that affected the output of L-malic acid as Mn²⁺> Fe²⁺ > Mg²⁺. The mutual interaction of these factors was not significant. The statistical optimization method was used to optimize the fermentation conditions. The best ion concentration were : Mn²⁺14 mg/L, Fe²⁺ 20 mg/L, Mg²⁺ 10 mg/L. The optimal value of the output of L-malic acid was 38.73 g/L.The metabolic mechanism of Aspergillus flavus SFW-7 accumulating L-malic acid was discussed preliminary from the limiting oxygen fermentation, the recruitment of calcium carbonate, the inhibitor of enzymatic of TCA cycle and glyoxylate cycle. It was concluded that CO2 fixing was the primary way accumulating L-malic acid, according to the variation of L-malic acid contents.