The protective effect of freezing temperatures on different lactic acid bacteria and its mechanism

The freezing temperature during the freeze-drying process is critical to the freeze-drying survival rate of lactic acid bacteria. However, variations among bacterial strains limit the generalization of the freeze-drying process, and few studies have examined how different strains tolerate and respond to freezing temperatures. This study explored the effect of freezing temperature on the viability of various lactic acid bacteria and its underlying mechanisms. The results indicated that the survival rates of all three strains were highest at a freezing temperature of-196 degrees C, while their storage stability and fermentation performance reached optimal levels at freezing temperatures below-80 degrees C. Further analysis revealed that when the freezing temperature dropped below-80 degrees C, damage to the integrity and functionality of the bacterial cell membranes significantly decreased. Moreover, freezing temperature significantly impacted enzyme activity. Specifically, Limosilactobacillus fermentum showed the greatest change, as its lactate dehydrogenase and ATPase activities increased by 1.23 and 1.45 times, respectively. Additionally, freezing temperatures also caused significant changes in cyclic fatty acid content. In summary, these findings offer theoretical insights for optimizing the freeze-drying process of active lactic acid bacteria and present a strategy to enhance their viability.