Effect of Preliminary Incubation (55 F/18 HR) on Microflora of Raw Bulk Tank Milk, with Some Observations on Microflora Milking Equipment

Abstract

In view of the interest in preliminary incubation (PI 55F/18 hr) of raw milk samples before testing, a study of the microflora of milk before and after PI was undertaken. Fifty-seven milk samples were collected from individual milk producers during March-July, 1963. When 18 of the samples were collected, pulsating rinses were made of the milking units. Standard plate counts were made on the raw milk, the raw milk after PI, and the rinse solutions. Colonies were picked from plates and classified for shape, Gram-staining reaction, catalase production, lactose fermentation, and ability to form spores. Results indicate that, in general, the effect of PI on the microflora of raw milk is to decrease the percentage of micrococci, or streptococci, or both, and increase the percentage of Gram-negative rods. Observations on the microflora of milking equipment showed that micrococci were dominant in 50%, streptococci in 28%, and Gram-negative rods in 22% of the rinses. The type of sanitizing agent affected the microflora of milking equipment.