An effective strategy for improving the freeze-drying survival rate of Lactobacillus curvatus and its potential protective mechanism

Freeze-drying is a common method to prepare dried starter cultures with high stability and a long storage time. However, the bacterial cells suffer varying degrees of damage during the freeze-drying process. This study aimed to provide an effective strategy to improve the survival rate of Lactobacillus curvatus and to explore its potential protective mechanisms. The survival rates of bacterial cells in different cryoprotectants before and after freeze-drying were measured using the plate count method. The protective effects of composite cryoprotectants on bacterial cells were investigated from multiple perspectives, including key enzyme activities, the internal morphology of bacterial cells, and cell membrane integrity. Results showed that the freeze-drying survival rate of bacterial cells in the composite cryoprotectants consisting of trehalose, oleic acid and skimmed milk (85.38%) was significantly improved compared to that in PBS solution (2.04%). The addition of composite cryoprotectants could reduce damage to the cell wall and cell membrane, maintain the normal morphology of bacterial cells and cell membrane integrity, and improve key enzyme activities (lactate dehydrogenase, pyruvate kinase and ATPases) during the freeze-drying process. The viability of freeze-dried cultures was also monitored at different temperatures during storage, and the cell survival of freeze-dried cultures at −20 °C was the highest, reaching 61.13%. The findings obtained in this work offer a new method to improve the viability of Lactobacillus curvatus after freeze-drying, and may promote the wide use of freeze-dried cultures in the food fermentation industry.