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 treatments (rapidly in liquid nitrogen and more slowly in cold air at -30 degrees C), then stored at -80 and -30 degrees C for 9 months. Survival was determined by plate counting with and without bile salts or sodium chloride at the highest tolerated concentration for each strain, to distinguish the undamaged population from the total population. Fermentative activity was measured as total lactic acid production by thawed cultures under standard conditions. Higher survival rates and greater activity were always obtained by storing cultures at -80 degrees C, but most strains stored at -30 degrees C also survived well. Analysis of variance revealed that the viability of the frozen cultures was affected more by storage temperature than by cooling rate. Selective media were unable to distinguish the active population from the total surviving population. The correlation between values for activity and survival with selective media was poor.