Synergistic combination of cryoprotectants for high freeze-dried survival rate and viable cell counts of Streptococcus thermophilus

Vacuum freeze-drying is one of the common methods to prepare probiotic powder. However, the low temperature and low moisture content could lead to the death of probiotics. The method of adding cryoprotectants is efficient to improve the survival rate of probiotics during vacuum freeze-drying. The synergistic combination of different cryoprotectants could play a better protective effect. However, the principle of the synergistic combination of different cryoprotectants during vacuum freeze-drying is not clear. In this study, literature review and analysis were firstly used to reveal commonly used cryoprotectants for coccus. Taking the price of cryoprotectants into account, the combination is preliminarily determined as sucrose/skim milk/sodium glutamate. Then, taking Streptococcus thermophilus 937 as the research target strain, the effect of this cryoprotectant combination was verified by a single-factor screening experiment and concentration optimization of cryoprotectants. It was found that the preferred cryoprotectant combination for Streptococcus thermophilus 937 is 6% sucrose/8% skim milk/4% sodium glutamate, with the freeze-dried survival rate and viable counts being 90.59% and 1.78 × 1011CFU/g, respectively. Meanwhile, this cryoprotectant combination also had a good protective effect on the other two cocci, Streptococcus thermophilus Grx02 and Enterococcus faecium 218. This result might have guiding and reference significance on the vacuum freeze-drying of coccus.