Optimization of a cryoprotective medium to increase the viability of freeze-dried Streptococcus thermophilus by response surface methodology

- S. thermophilus shows different survival rates in varied medium after freeze-drying.
- S. thermophilus STX2's freezing-drying resistance is best displayed with skim milk.
- Three factors produce synergistic protective effects on S. thermophilus STX2 growth.

Streptococcus thermophilus normally exhibits different survival rates in different bacteria medium during freeze-drying. In this study, response surface methodology (RSM) was applied on the design of experiments for optimizing the cryoprotective medium. Our results showed that the most significant factor influencing the resistance of S. thermophilus STX2 to freeze-drying was skim milk, followed by sodium glutamate, and then glycerol. These three factors may have interactive effects and produce synergistic protective effects on the growth of S. thermophilus STX2. Regression analysis indicated that the optimal concentrations of these variables were determined as: glycerol 79.60 g/L, sodium glutamate 77.40 g/L, and skim milk 116.40 g/L. S. thermophilus STX2 freeze-dried in a medium with the optimal formulation obtained a best cell viability up to 93.58%.