The Effect of Acidity on Lactobacillus Acidophilus Cultures

Summary 1. The strains of L. acidophilus used in the experiments reported upon gave the highest plate counts at acidities of approximately 1.00 per cent. 2. There was some variation among strains, some yielding the maximum number of organisms at less than 1.00 per cent and one strain at an acidity considerably greater than 1.00 per cent. 3. A decrease in the number of organisms had taken place at an acidity of approximately 1.2 per cent in all but one strain. The strains examined seemed to differ slightly with respect to the acidity at which destruction became apparent. 4. At higher acidities of approximately 1.5, 1.6, 1.7 and 1.8 per cent, a still greater destruction of organisms took place. The acidity at which this greater destruction occurred varied with the strain. 5. The results indicated that the particular sample of each strain that gave the highest count before storage, usually gave the highest count of the series after storage at 9° or 24°C. 6. Refrigerator storage (9°C.) of typical strains of L. acidophilus resulted in a smaller loss in the numbers of organisms than did storage at 20° to 24°C, in both high and low acidities. 7. Two strains of L. acidophilus not previously reported upon also survived either refrigerator or room temperature storage for several days without any great destruction of organisms, provided that the original acidity was low. 8. It is desirable to determine the acidity at which the maximum number of organisms is secured for each strain and under the conditions that obtain. 9. The results obtained indicated that the acidity at which the maximum number of organisms was secured during incubation of a culture was an index as to the acidity at which to place cultures in refrigerator storage.