THE ROLE OF PSYCHROPHILIC BACTERIA IN THE KEEPING QUALITY OF COMMERCIALLY PASTEURIZED AND HOMOGENIZED MILK*

Abstract

Commercially pasteurized and homogenized milk, collected during the summer, fall, and winter months and stored at 40° F and 33° F, was studied for keeping quality. When the storage temperature was lowered from 40° to 33° F, the keeping quality was found to be extended 11 to 14 days. The keeping quality of the milk stored at 40° F, when based upon the development of psychrophilic bacteria up to the 50,000 per milliliter standard, was lower than when based on the deterioration of flavor to a score of 37. The keeping quality of the milk stored at 33° F was found to be approximately the same when judged by either criterion. The keeping quality of the milk samples collected in the fall and winter was poorer than those collected in the summer. This difference was greater when the samples were stored at 33° F than when stored at 40° F. No oxidized flavors were encountered in these milk samples. The deterioration of flavor was closely associated with the growth of psychrophilic bacteria in all samples.