Screening Of L-Lactic Acid Highly Producing Rhizopus Oryzae Strain Adapting To The Sweet Potato Starch

Lactic acid is a kind of widely used organic acid. Increasing its fermentation production depends on the microbial strain to some extend, which can be obtained by screening from environment and mutagenization.According to the strain screening theory and metabolic controlling fermentation theory, the screening of L-Lactic acid high- producing mutant of Rhizopus Oryzae and the key enzyme Lactate Dehydrogenase(LDH) during the fermentation were systematically studied. The study includes the isolation and mutagenization, the stability of the mutants,the properties of LDH and the effect of changing the fermentation conditions on the activity of LDH. The sweet ptato starch were used as carbon source.A series of convenient methods for selection of L-Lactic acid high-producing mutant were developed. These methods have enhanced the efficiency and solved the problem that the filamentous fungi overspreads on the plate. By using the ultraviolet radiation (UV) and diethyl sulfate (DES) to mutagenize Rhizopus Oryzae As.3.819, two mutant strains 40S-5-1 and 40M-4-2-1 with higher yield of L-Lactic acid were obtained. The L-Lactic acid yields are 130.7g/L and 126.4g/L respectively. The starch conversion rates are 83.9% and 81.1% respectively, while the yields and the starch conversion rates of the parent strain are 117.4g/L and 75.3%.Lactate Dehydrogenase (LDH) is a key enzyme which catalyzes the formation of lactic acid from pyruvic acid during the fermentation with Rhizopus oryzae. The properties of its partially purified extract were studied. The results show that LDH extract has optimum reaction pH of 7.4 and optimum temperature of 30-50C. Mg2+ and Ca2+ increase the activity of LDH, but K+ and Zn2+ reduce its activity. The km constants based on the substrates of NADH and pyruvate are 7.22x10-4mol/L and 1.24x 10-3mol/L respectively.The kinetic properties of LDH during the fermentation were also studied. We found that there are some relations between the activity of LDH and the yield of L-Lactic acid and the starch conversion.