The strain Streptococcus thermophilus are widely used in the ways of probiotic formulations form or starter culture powder depending on the function of the strain and viability during production process. The pros and cons of freeze-drying on the viability and storage of starter lactic acid bacteria are well documented. In the present study, we focused on the optimization of freeze-drying medium with addition of cryoprotectants for Streptococcus thermophilus. The interactive effects of sucrose, soluble starch and ascorbic acid on the survival rate as well as the viable counts of S. thermophilus during freeze-drying were investigated by Box-Behnken experimental design. The results showed that, with the mixture of 17 g/100mL sucrose, 17 g/100mL soluble starch and 0.4 g/mL ascorbic acid, the viability 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) ×1011 cfu/g during freeze-drying using the optimal compound cryprotectants, respectively, which verified the adequacy of predicted value 70.19 % and 2.81×1011 cfu/g. Our results demonstrated that the mixture of the cells in the presence of above three cryprotectants before freeze-drying protected S. thermophilus cells from the adverse freeze-drying environment efficiently.
Key words: Streptococcus thermophilus; freeze-drying; cryoprotectants; Box-Behnken experimental design
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