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A Study of Methods for Bacterial Analyses of Market Milk

Leslie H. Cooledge

Department of Bacteriology, Michigan State College, East Lansing, Michigan

ABSTRACT

1. When plating methods are used, no one medium should be expected to give results which indicate correctly the condition of all grades of milk.
   
2. The pH score proved more efficient as a means of detecting slight changes in the history of milk than did the plating methods tried.
   
3. The methods as a whole were efficient in detecting slight changes in the history of the samples as follows:
   In samples with bacterial counts under 25,000—45.8 per cent efficient.
   In samples with bacterial counts between 25,000 and 100,000—71.4 per cent efficient.
   In samples with bacterial counts between 100,000 and 1,000,000—85.0 per cent efficient.
   In samples with bacterial counts over 1,000,000—97,0 per cent efficient.
   It seems that the medium giving highest average counts depends upon the predominant groups present in the sample studied. This may account for divergent results obtained by various workers.
   
4. In 81 comparisons of the pH score with the reductase ratings and the bacterial plate counts, the pH score checked as well with the number of bacteria per cubic centimeter as did the reductase rating, although neither is an absolute measure of the bacterial content.
   

Professor Cooledge died in May, 1925. This paper was nearly ready for publication and is presented with some slight changes designed to clarify certain points which seemed a little obscure.—G. L. A. Ruehle.