The In Vitro Screening Of Lactobacillus Gasseri And The Research Of High Cell Density Culture

The in vitro screening of lactic acid bacteria and the research of high-density culture processes are the key links in the development of starter and microecological preparations,which have important theoretical significance and practical application value for the industrialized production of fermented dairy products and the development of new microecological preparations.In this thesis,human-derived Lactobacillus gasseri is used as the research object,and excellent strains are obtained through in vitro acid and bile salt resistance screening tests.Further optimization of medium components,optimization of static culture conditions,and high-density culture technology,the main results obtained in this paper are as follows.(1)The results of Lactobacillus gasseri's in vitro acid and bile salt resistance tests show that the tolerance of 10 strains of Lactobacillus gasseri to simulated artificial gastric juice,artificial intestinal juice and bile salts is different,and L.gasseri IMAU FB017 has the most tolerance to simulated artificial gastric juice and intestinal juice,and the tolerance to gastric juice for 3 h,intestinal juice for 4 h and intestinal juice for 8 h were 89.45%?61.19%and 36.6%L.gasseri IMAUFB017,respectively.The bile salt lag time was 0.51h.(2)The formula of L.gasseri IMAU FB017 proliferation medium was optimized.The single-factor test,orthogonal test and response surface design were used to obtain the bacteria's proliferation medium formula:lactose 60.07 g/L,yeast extract 56.62 g/L,yeast peptone 18.87 g/L,sodium dihydrogen phosphate 19.46 g/L,disodium hydrogen phosphate 1.69 g/L,MgSO4·7H2O 1.0 g/L,MnSO4·5H2O 0.08 g/L,L-cysteine hydrochloride 0.2 g/L.(3)The optimization results of L.gasseri IMAU FB017 static culture conditions were:initial pH6.0,temperature 37?,inoculum size 1%.By using the optimized medium formula and the best static culture conditions,the number of viable cells in the fermentation broth reached(3.15±0.16)×109 CFU/ml,which was 2.86 times higher than that in MRS medium(8.15±0.42)×108 CFU/mL before optimization.(4)The optimization results of L.gasseri IMAU FB017 high-density culture process are:constant pH5.5,the alkali-regulating agent is NH3·H2O with a mass fraction of 25%and nitrogen gas.In the 5 L automatic mechanical stirring fermenter,the cell growth rate was fast,the logarithmic growth period was prolonged,the cell density was significantly increased,and the number of viable cells was up to(4.98±0.28)×109 CFU/ml,which was 5.11 times higher than that before high density culture(8.15±0.42)× 108 CFU/mL.From 10 strains of Lactobacillus gasseri,L.gasseri IMAU FB017 with good acid-resistant and bile-salt-resistant properties was optimized,and its medium formulation and static culture conditions were optimized to determine the high-density culture process,laying a theoretical foundation for the subsequent industrial production of L.gasseri IMAU FB017.