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To prolong the preservation time of Lactobacillus rhamnosus and improve the cell survival rate, the cryoprotectants formulation was optimized by response surface experiments with the survival rate as the index. The optimal combination of cryoprotectants is 11.1% trehalose, 9.1% glycerin, 3.5% sodium glutamate, and 15.7% skimmed milk powder. The freeze-dried powder could be stored stably at −20 °C for 60 days and the survival rate was enhanced to 97.8% compared to the 22% survival rate of the control group. Further study of the freeze-dried powder revealed that the cell membrane was intact and could be maintained for at least 60 days, and the cell surface morphology was smooth and fully covered by the cryoprotectants, and the Na+/K+-ATPase activity was only decreased by 4.7%. These results demonstrated a mechanism whereby the composite cryoprotectants could reduce the damage to cells by ice crystal formation through enhanced protection of cell membranes, thus obtaining the effect of extended preservation time and improved survival rate, which provided a theoretical basis for the freeze-drying of Lactobacillus rhamnosus.
Chen et al. (Fri,) studied this question.