Study On Preparing Technology Of The Lactic Acid Bacteria Direct Vat Set And Its Application

Along with the rapid development of our country economy,people require the higher quality of the yogurt products.Starter culture is one of the key factors that affect the quality of yogurt.So the development of lactic acid bacteria(LAB)direct vat set(DVS)with good storage stability,containing higher number of viable bacteria will promote the development of yogurt industry in China.In this paper,Streptococcus thermophilus L10 and Lactobacillus plantarum MP166 were used as test bacterial strain.The optimum culture conditions of Streptococcus thermophilus L10 and Lactobacillus plantarum MP166 were studied and the compounds of freeze-drying protective agent were screened in the same time.In the experiment,the stability of LAB DVS in storage period was examined and the starter culture has been applicated in fermention of soymilk.The main results were as follows:(1)We screened a strain of Streptococcus thermophilus L10 from commercially available yogurt.The optimizing proliferation mediums for this strain were as follows: the optimum buffer salts were ammonium citrate and sodium acetate,the optimum trace element were MgSO4·7H2O and MnSO4·4H2O,the optimum growth factor was carrot juice.The culture temperature,initial pH and inoculation that effect the growth of this strian had been studied.The result indicated that the optimal culture condition of this strain were cultivate temperature 37℃,initial p H6.6,inoculation 5%.The growth curve of Streptococcus thermophilus L10 was determined under this culture condition and indicated that the highest bacterial counts of this strain was 2.39×109cfu/mL and the best harvesting time was 10 h.(2)The formula of the freeze-dried protectants for Streptococcus thermophilus L10 was optimized by the methods of single factor and box-behnken design(BBD).The optimized formula was as follows: lactose 5.58%,tryptone 6.27%,trehalose 7.73%.The freeze-dried survival rate of the Streptococcus thermophilus L10 can reach 77.85±1.46% with this formula protectants.The bacteria powder were storaged in 4℃ for eight weeks and the density of living bacterium was maintained more than 1010 cfu/g in this period.(3)We have optimized the proliferation medium of Lactobacillus plantarum MP166 and the result were as follows: the optimum buffer salts were ammonium citrate and sodium acetate,the optimum trace element were MgSO4·7H2O and MnSO4·4H2O,the optimum growth factor was carrot juice.The culture temperature,initial pH and inoculation that effect the growth of Lactobacillus plantarum MP166 were studied and the result indicated that the optimal culture condition were cultivate temperature 37℃,initial pH6.4,inoculation 3%.The growth curve of Lactobacillus plantarum MP166 was determined under this culture condition and indicated that the highest bacterial counts of this strain was 3.25×109cfu/mL and the best harvesting time was 16 h.(4)The formula of the freeze-dried protectants for Lactobacillus plantarum MP166 was optimized by the mothods of single factor and box-behnken design(BBD).The optimized formula was as follows: lactose 9.05%,tryptone 8.79%,trehalose 11.21%.The freeze-dried survival rate of the Lactobacillus plantarum MP166 can reach 96.25±1.54% with this formula protectants.The bacteria powder are storaged in 4℃ for eight weeks and the density of living bacterium was maintained more than 1011 cfu/g during this period.(5)The result of co-fermentation of the Streptococcus thermophilus L10 and Lactobacillus plantarum MP166 were showed: the shorter fermentation time,the higher viscosity,no whey separated and a comprehensive evaluation of 85.8 points.In conclusion,the combination LAB DVS have the potential application to the fermented soymilk.In this study,we optimized the technology for high-density cultivation of Streptococcus thermophilus L10 and Lactobacillus plantarum MP166,the technology for creating its LAB DVS.This study could provide powerful science basis for industrialized production of LAB DVS.