Commercial production of DVS yogurt starter cultures.

The interactions(including the symbiosis and antagonism),their abilities to produce lactic acid,viscosity,and amino acid between Streptococcus thermophilus strains and Lactobacillus delbrueckii subsp.bulgaricus strains were studied.It was seen that 4 paired strains were more suitable for the production of yogurt.For strains Y6-ST1,the best medium was 2.5% of tomato extract,1.0% of lactose,0.5% of yeast extract and 1.5% of peptone.If they were cultivated at 42 ℃ for 6h,The cell population was up to 1.40×109 mL-1 The optimal cryoprotectants to improve the viability of strains Y6-ST1 were investigated.A combination consisting of NFMS(18%),glycerin(2.0%),glutamate-Na(1.0%) and Sorbitan mono-oleate(0.5%) produced a very positive effect on promoting the viability of the paired strains LB2-ST1.Their viable cells after freeze-drying were 3.62×1011g-1 The optimal parameters for the production of yogurt starters were: temperature 42.3 ℃,pH=6.4,rotating speed 86.8 r/min,inoculation3%,feeding 1.0%,fermentation 6 h,freeze-drying-40 ℃,15 h.The cell of the production were1011g-1.