Effect of heat shock or cold shock treatment on the resistance of Lactococcus lactis to freezing and lyophilization

This study investigated the effect of heat shock or cold shock treatment on the resistance of commercial Lactococcus lactis ssp. lactis and L. lactis ssp. cremoris cheese starter bacteria to freezing and freeze-drying. The ability to withstand freezing at −60°C for 24 h was variable among lactococci, but could be significantly (P \u3c 0.05) improved in most strains by a 25-min heat shock at 42°C (L. lactis ssp. lactis) or 39°C (L. lactis ssp. cremoris) or by a 2-h cold shock at 10°C. In addition, stress treatments that improved lactococcal cryotolerance significantly (P \u3c 0.05) enhanced the resistance of most strains to lyophilization. Greater resistance to freezing or lyophilization was not detected when stress treatments were performed in broth that contained erythromycin, which indicated that stress-induced proteins were required for cell protection. Membrane fatty acid analysis of stress-treated cells suggested that enhanced resistance to freezing and lyophilization may be related to changes in cell membrane lipid composition. Heat-shocked cells had a higher 19:0 cyclopropane fatty acid content than control cells, and cold-shocked cells contained a lower ratio of saturated to unsaturated fatty acids. Other factors must also be involved in cell protection, however, because similar changes in membrane lipid composition were detected in strains whose resistance to freezing and lyophilization was not improved by heat or cold shock treatment