SURVIVAL AND RESIDUAL ACTIVITY OF LACTOBACILLUS-ACIDOPHILUS FROZEN CULTURES UNDER DIFFERENT CONDITIONS

Eight strains of the Lactobacillus acidophilus complex were subjected, as concentrated cultures in skim milk, to different freezing treatmen ts (rapidly in liquid nitrogen and more slowly in cold air at -30 degr ees C), then stored at -80 and -30 degrees C for 9 months. Survival wa s determined by plate counting with and without bile salts or sodium c hloride at the highest tolerated concentration for each strain, to dis tinguish the undamaged population from the total population. Fermentat ive activity was measured as total lactic acid production by thawed cu ltures under standard conditions. Higher survival rates and greater ac tivity were always obtained by storing cultures at -80 degrees C, but most strains stored at -30 degrees C also survived well. Analysis of v ariance revealed that the viability of the frozen cultures was affecte d more by storage temperature than by cooling rate. Selective media we re unable to distinguish the active population from the total survivin g population. The correlation between values for activity and survival with selective media was poor.