Effects of complex cryoprotectant on the freeze-drying survival of Lactobacillus acidophilus FMNS-10 and its protective mechanisms

Freeze-drying is a common method for preserving bacteria cells, but it can cause cell damage in the process of protection. Therefore, it is necessary to explore a cryoprotectant to improve cell survival during the process. This research aimed to explore the optimal cryoprotectant formulation of L. acidophilus FMNS-10 and its protection mechanism. The optimal cryoprotectant consisted of 10.7 % skim milk, 3 % sodium L-glutamate and 10 % D-trehalose, resulting in a survival rate of 91.69 % for L. acidophilus FMNS-10 after freeze-drying ( p  < 0.05). Under this condition, the integrity of cell wall and membrane of L. acidophilus FMNS-10 was close to that of the fresh strain, which could be attributed to preventing ice crystal formation. Cryoprotectants demonstrated a significant protective effect on enzymatic activities. In the cryoprotectant-free group, the activities of β-galactosidase, L-LDH, Na + /K + -ATPase and the concentration of Ca 2+ decreased by 36.19 %, 51.82 %, 89.40 %, and 41.90 %, respectively. However, the cryoprotectant group were only 7.10 %, 1.66 %, 7.41 %, and 4.10 % reductions compared to the cryoprotectant-free group ( p  < 0.05). These results indicated that the optimal cryoprotectant improved the survival rate of L. acidophilus FMNS- 10 by inhibiting the loss of enzyme activity. In addition, the optimal cryoprotectant significantly mitigated the DNA damage induced by freeze-drying. The reduction in enzyme and DNA damage may be attributed to complex cryoprotectants maintaining the stability of their three-dimensional structures and reducing structural disruption caused by freezing. Further analysis of storage stability revealed that the viable count of L. acidophilus FMNS-10 exceeded 10 7  CFU/mL when stored at −20 °C for 180 days. These findings provide a comprehensive theoretical foundation for how to freeze-dry Lactobacillus acidophilus.