The pasteurized milk coming from certain of the Iowa market milk plants contains larger numbers of bacteria than would be expected from the numbers present in the raw milk. These large numbers are sometimes due to contamination following pasteurization, but comparatively high counts are also encountered with carefully pasteurized milk taken directly from the pasteurizer so that there is no opportunity for contamination from the cooler, bottler, etc.

In order to determine whether or not certain milk supplies contain comparatively large percentages of heat resistant organisms, the pasteurization efficiencies¹ secured with milk coming from different farms were studied. The trials were conducted on milk coming to two small Iowa market milk plants. Most of the samples examined were morning milk delivered to the plants shortly after production; these were selected so that the results would apply primarily to organisms getting into the milk from various sources rather than to those that had grown in the milk.