Screening Of High Diacetyl-Producing Lactococcus Lactis Ssp. Lactis Bv. Diacetylactis Mutants And Optimization Of The Fermentation Conditions

Fermentated milk was widely enjoyed all over the world by its particular salubrious effects, abundant nutrition, freshing sourness and tender texture. This is a promising field for development of functional foods in the future lies in its important functions on the host for its health benefit and prophylactic effects. Diacetyl is considered to be one of the most important elements in the aroma and flavor of fermentated milk. So the important aim of following scientific research should rest with how to enhance the ability of Lactococcus lactis ssp.lactis biovar diacetylactis to metabolize diacetyl.Firstly we studied the biological features of Lactococcus lactis subsp. lactis biovar diacetylactis. Then we studied how to enhance its ability of diacetyl-metabolizing by two aspects of screening and optimizing the fermentating conditions. Endly, the multiplication culture of L.diacetylactis was developed.All these offers a better basis for industrialization production. The main research results were as follows:The growth curve of strain X67 was tested by monitoring OD₆₀₀ of GM17 broth, the final period of logarithm growing of X67 was 10-12 hours.Then NTG was employed to mutate X67. When X67 was disposed by 1.25g/L NTG 60 min, its lethal rate reach 75.5%. A new strain NT3008 was obtained when strain X67 disposed continuously three times by NTG, which ability of diacetyl-producing could reach 0.78 mg/L, 13.93 times more than before. Pass-generation test showed that the heredity character of mutant strain was sTab..The fermenting conditions were optimized using single factor experiments, that is to say to confirm the optimum diacetyl producing temperature, the pH value and the concentration of the factor of the sodium citrate in fermentating culture. The optimum conditions were as follows: reaction temperature: 30℃, pH 6.5, the concentration of the sodium citrate in the culture: 0.75g/L . Under optimum conditions the diacetyl production level was at 0.86 mg/L。Endly based on single factors experiment, the optimum based cultures of L. diacetylactis was determined as 10% skim milk. And then sources of carbon, nitrogen and yeast extract power were optimized with orthogonal design, It showed that the optimal culture components of L. diacety were 10%skim milk, 0.5% pancreas peptone, 0.25%yeast extract powder, 0.2%DL-Valine, 1%sodium...