A method for the study of the contamination from churns is suggested. It consists of allowing a small amount of an agar medium containing 2.5 percent air-dried agar to solidify in contact with the surface to be studied, the transferring of the agar preparation thus formed to a sterile petri dish and finally the counting of the colonies that develop on incubation. With surfaces nearly horizontal the agar is poured on, while with surfaces not nearly horizontal the medium is poured behind a glass plate held a short distance from the surface by a gasket. The results are expressed as the number of colonies developing per square centimeter of the agar preparations.
Hammer, B. W. and Olson, H. C.
"Bacteriology of butter III. A method for studying the contamination from churns,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 11
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol11/iss141/1