Reserach On Selection Of Bacteriophage-Resistant Mutants, Cultivation And Lyophilized Starters Of Streptococcus Thermophilus

In this study we started with Streptococcus thermophilus S2. The bacteriophage-resistant mutants were isolated by spontaneous mutation. After spontaneous mutation four phage-resistant mutants (KS21, KS22, KS23, KS24)were obtained. They proved to be non-lysogen by Mitomycin induction. The resistance of these mutants had not changed after twenty generatiuons and the ability of yogurt fermentation is very stable. One of the resistant mutants KS23, its total acid producing reached 109°T and the yogurt curd time was almost not changed. From the SDS-PAGE we can see the protein composition of the original strain and resistant mutants varied evidently. This may be the reason of the resistant.The most suitable carbon and nitrogen glucose, soy peptone, tryptone some plant juices, nutrition factor and salts were selected according to single factorial experiment respectively. Based on the result, design of Plackett-Burman was used to evaluate the influence factors. Soy peptone, corn juice and sodium acetate played the important roles in the medium. We can approach the optimal region by using Steepest Ascent. The optimal enrichment mediums were further optimized by Respon Sesurface methodology and the result as follows:glucose 1.5%,soy peptone 4.5%, tryptone 2%,yeast powder 0.5%, beef extract 1%, K2HPO40.5%, coin juice 40%,NaAC 0.3%, pH 6.8.The cell density of KS23 could reach 1.16×109cfu/mL in this medium on 37℃after 18 h.The results of the 3L automatic fermenter showed that:the optimal inoculum size was 3%.25%Na2CO3 solution was best to keep pH as 6.8.In this optimal condition, the cell density of S800 could reach 1.01×1010 cfu/mL within the 12 hour by adding feeding as 200mL 10% lactose and 20% yeast extract after fermenting 2h.In order to increase activity of freeze-drying starter, we research about several factors. KS23 fermented at 42℃in optimal medium contained 10% skimmed milk. The optimum temperature of preincubation is 42℃. In this way, we can get high activity lyophilized starter culture of yoghurt. The cell density of the freeze-drying production was 3.8×1010 cfu/g. When fermenting the milk with the starter, the milk could clabber after 210 min, the acidity was about 87°T.