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

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)×10 11 cfu/g during freeze-drying with the optimal compound cryprotectants, respectively, which verified the adequacy of predicted value 70.19 % and 2.81×10 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.