Survival During Frozen and Subsequent Refrigerated Storage of Lactobacillus acidophilus Cells as Influenced by the Growth Phase

Cells of Lactobacillus acidophilus propagated in peptonized milk nutrient broth maintained at pH 5 were harvested during the late logarithmic (log) phase and at 6 h into the stationary phase of growth. Concentrated cultures were prepared from each age of cells, frozen, and stored at –196°C. The concentrated cultures were assayed for numbers of total and bile-tolerant lactobacilli, β-galactosidase activity, and ability to assimilate cholesterol before and after storage at –196°C. Nonfermented acidophilus milk, prepared following 28 d of storage at –196°C from each concentrated culture, was tested before and after 0, 7, 14, 21, and 28 d of storage at 7°C. No decrease in numbers of total or bile-tolerant lactobacilli, β-galactosidase activity, or amount of cholesterol assimilated was observed for any of three strains studied during storage at –196°C. There were no significant differences between the two harvest times during storage at this temperature for any of the three strains. However, when the cells were suspended in milk and stored at 7°C, the numbers of total and bile-tolerant lactobacilli declined over time as did the β-galactosidase activity and the ability to assimilate cholesterol. The cells from L. acidophilus 223, harvested 6 h into the stationary phase, exhibited significantly less decrease in total numbers during extended storage at 7°C than did the cells harvested in the late logarithmic phase. There was little or no difference in the decline in numbers of total lactobacilli between the two harvest times for strains 606 and 107. All strains decreased significantly in β-galactosidase activity and the ability to assimilate cholesterol during storage in milk at 7°C with no significant differences between the two harvest times.