Optimization of cryoprotectants for Streptococcus thermophilus during freeze-drying using Box-Behnken experimental design of response surface methodology

Streptococcus thermophilus are widely used in the production of yogurt as starter culture with L. bulgaricus, maximization of survival of S. thermophilus during freeze-drying is of vital importance in its application. In the present study, the cryoprotectants were optimized to improve the survival of S. thermophilus during freeze-drying by Box-Behnken experimental design of Response Surface Methodology. The results showed that, with the mixture of 17% (w/w) sucrose, 17% (w/w) soluble starch and 0.4% (w/w) ascorbic acid. The viability of S. thermophilus could be well preserved when freeze-drying was employed. Moreover, the survival rate and viable counts of S. thermophilus were observed to (69.93 +/- 0.07) % and (2.79 +/- 0.03) x 10(11) cfu/g during freeze-drying with the optimal compound cryprotectants, respectively, which verified the adequacy of predicted value 70.19 % and 2.81 x 10(11) cfu/g, which demonstrated that the mixture of the cells in the presence of above three cryprotectants before freeze-drying can protect S. thermophilus cells from the adverse freeze-drying environment efficiently.