Effects of strains and growth conditions on autolytic activity and survival to freezing and lyophilization of Lactobacillus delbrueckii ssp. lactis isolated from cheese

Fifty-one strains of Lactobacillus delbrueckii ssp. lactis isolated from Swiss cheeses were tested for autolytic activity and survival rate to freezing and lyophilization. All lactobacilli, including high autolytic reference strains, showed autolytic activities in the range from 15% to 40%, which were lower than for the Propionibacterium (83±6.76%) and Bacillus (75%) control strains. All lactobacilli survived freezing well (higher than 50%), whereas resistance to lyophilization in the range 0.49–63% was strain-dependent. No correlation between autolytic activity and survival after freezing or lyophilization was shown for lactobacilli strains. Lb. delbrueckii ssp. lactis FAM 10991 was selected for its high autolytic activity and low survival to lyophilization to study the effects of pH and CO2 during batch fermentation. Batch fermentation at low pH of 5 or without pH control with CO2 produced cells exhibiting 20-fold lower autolytic activity and 20–45-fold higher survival to lyophilization than for cells grown at pH 6.