Selection and characteristics of the Streptococcus thermophilus spontaneous mutant strain of bacteriophage-resistant

Streptococcus thermophilus, one of the Yogurt fermentation strains, as the original strain commonly, using the high-density phage culture repeatedly, obtained 9 mutant strains of bacteriophage-resistant preliminary, determining their genetic stability, lysogeny, EOP and the yogurt fermentation properties under normal conditions, summing up all the characteristics. Select ones which was better than the wild-type strain in the genetic stability, growth and fermentation performance, and then fermentation with the better ones and the wild-type strain in higher concentrations phage environment further. Determination of phage resistance, viscosity, hardness, growth and acid of fermentation products, also detect yogurt whey separation, taste and after acidification. The result showed that 9 bacteriophage-resistants were all non-lysogenic bacteria and 6 of them had genetic stabilities, 3 were close to the wild-type strain in growth ability, the ability to produce acid, fermented milk viscosity, hardness. Using the 3 mutants fermentation under higher concentrations phage environment, two bacteria growth normal, and their acid production, viscosity, hardness, taste were all better than others, shorter curd, whey separation less and acidification relatively weak, they were suitable for industrial production.