The Study On High Cell Density Cultivation And Freeze-drying Protection Of Streptococcus Thermophilus

The potential excellent strains in fermenting yogurt were screened from 289 S. thermophilus. S. thermophilus ND03 was chosen as a representative strain, and conducted with the study on medium optimization, high cell density culture and freeze-dried protection. The main conclusions are as follows:1 By detecting the fermentation characteristics such as pH, time, sensory evaluation, 12strains were screened from 289 S. thermophilus by comparing with the fermentation characteristics of S. thermophilus Yc, which was obtained from a commercial starter.2 Based on the medium AST, all ingredients were screened and their concentration was adjusted in order to form the optimum medium for S. thermophilus ND03. When cultivated in the optimum medium at 37℃for 8¹0h, the viable count of fermented broth can get to 1.02×10¹⁰CFU/mL, which was 14times as many as before.3 When cultivated in the optimum medium, conditions for static culture and process of high cell density fermentation in 5L liquid fermentor were optimized. With the best fermented conditions for S. thermophilus ND03, the cultivation time was 6⁶.5h which was lower than before for 2h. And viable count can reach 1.80×10¹⁰CFU/mL, increased 76% than before.4 The addition proportion of protective agents, the pre-freezing temperature and the freezing time of S. thermophilus ND03 were studied. The optimum addition proportion of protective agents was 1:6. And the best pre-freezing time was 6h placing in -40℃. The ingredients of protective agents for freeze-dried protection were optimized, including carbohydrates, polyols, polymer types and salts. Under the best effects of protective agents for protection in the freeze-dried process, the viable count of freeze-dried powder were 1.83×10¹¹CFU/g, and the survival rate of freeze-dried processing can reach 84.3%, which was 20% more than before.5 All of the other 12 strains of S. thermophilus were conducted with the high cell density fermentation and the freeze-dried powder production under the optimum medium, the best process of the high cell density culture and the best freeze-dried conditions according to the representative stain S. thermophilus ND03. The result showed that the optimum medium and process of the high-density fermentation can get to their requirements, and with the optimum protective process of freeze-drying protection, the survival rate of the strains can be promoted.