The utensils and equipment with which dairy products come in contact on farms and in dairy plants constitute a very important source of the microorganisms in these materials. Because their construction makes them especially difficult to clean, certain pieces of equipment are of much greater significance in this connection than others. In butter plants the churns are a particularly important source of organisms and, moreover, this contamination occurs subsequent to the pasteurization of the cream so that none of the organisms added by it are destroyed.
Olson, H. C. and Hammer, B. W.
"Bacteriology of butter V. Studies on the microorganisms in churns,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 13
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol13/iss159/1