Screening Of Eminent Rhizopus Oryzae Irradiated By Synchronous Radiation In L-lactic Acid Production And Study On Fermentative Character

The fermentation of L-lactic acid by Rhizopus oryzae has such advantages as simple requirement for nutrition and high optical pure product, but the disadvantages of low output and high content of by-product also exist. Therefore, it is meaningful to sceen eminent strain. Soft X-ray of synchronous radiation, which is continuous, adjustable and monochromatic, has the Ck Nk Ok wave-length radiation. It is extensively applied in biology. The soft X-ray of synchronous radiation (wavelength 2.3nm 3.2nm 4.4nm)was used in this paper to irradiate on Rhizopus oryzae As3.819. The results showed that the influence of the vacuum time on mortality was obvious. The results of YPD plate screening indicated that the mortality of Nk soft X-ray was lower than Ck soft X-ray, mutagenic strains obtained from Ck soft X-ray appeared in high radiation dose ranges, and mutants obtained from Nk soft X-ray appeared in low radiation dose ranges; the mortality of Ok soft X-ray was the lowest and mutants appeared in different radiation dose ranges.Three mutants were selected through the YPD plate bromomcresol green plate flat screen and lactic acid plate. Initially, the C-60 mutant was determined as the best strain by fermentation. By the unifactor and orthogonal design of experiment, the mutant's optimum fermentation medium is determined. The medium compositions were: 120g/L Glucose, 1g/L (NH₄)₂SO₄, 0.3g/L MgSO₄, 0.3g/L KH₂PO₄, 0.44g/L ZnSO₄. Under this best fermentation condition, the lactic acid yield of C-60 mutant reaches 98.4g/L, the content of alcohol is 5.1g/L. Hereditary stability test demonstrated that the productities of mutant strain from 1st to 7st serial passage were stable.Comparing the fermentation results of C-60 mutant and parent strain by 5L bio-reactor, the Alcohol Dehydrogenase(ADH) activity of C-60 mutant is 14U/ml, decreasing by 26.3%; the ethanol yield is 5.2 g/L, decreasing by 23.8%. The Lactate dehydrogenase(LDH) activity is 17U/ml, increasing by 6.3%; and the lactic acid yield is 98.1 g/L, only inceasing by 2%. The fumaric acid yield is 0.05g/L, decreasing by 73.7%. The result of this experiment proves that C-60 mutant has good fermentative character. The effect of soft X-ray of synchronous radiation on Rhizopus oryzae and feasibility of screening method were confirmed furtherly.