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Psychrophilic Bacteria. III. Population Levels Associated with Flavor or Physical Change in Milk^{1, 2,}

Department of Dairy Industries, University of Minnesota, St. Paul

ABSTRACT

Fifty-seven psychrophilic cultures were used in a study to determine population levels present at the time a flavor or physical change in pasteurized milk was detected. Populations associated with a detectable change varied among genera and among species within a genus. Generally, these levels were in a range of 5 to 20 million/milliliter at the time a change was observed; more active cultures were below and less active ones were above this range. The most commonly observed flavor defect was unclean. Slight changes detectable organoleptically were not detectable by chemical measurement of fat or protein hydrolysis. One culture of Alcaligenes viscolactis produced ropiness, with no detectable flavor change. This occurred at population levels below 5 million/milliliter. When oxidized and strong feed flavors were present, these flavors were markedly reduced by growth of each of several cultures to a population level just below that at which a flavor defect characteristic of the particular culture occurred. Comparative studies of six cultures very active in causing off flavors at 6 and 20 C indicated that approximately the same population levels were associated with change at both temperatures.

¹ Scientific Journal Series Paper No. 5647, Minnesota Agricultural Experiment Station, St. Paul.

² This work was supported, in part, by the Minneapolis-St. Paul Quality Control Committee.