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Psychrophiles in Milk Held Two Days in Farm Bulk Cooling Tanks¹

Department of Bacteriology, University of Wisconsin, Madison

ABSTRACT

Predominant psychrophilic bacteria were isolated from 174 samples of every-other-day milk from bulk milk cooling tanks from 12 farms producing Grade A milk. The 220 isolates so obtained were identified as to genus and species as far as possible. For the most part they fell into the genera Aerobacter, Alcaligenes, Arthrobacter, Flavobacterium, Micrococcus, and Pseudomonas. Species of these genera are listed. Not all of the species could grow in pasteurized milk within three days at the cooling tank temperature of 38° F. (3.3° C.), although they formed colonies in seven days at 5° C. on plate count agar, and most of them could grow in pasteurized milk at 45° F. (7.2° C.) All of the species grew little, if any, in raw milk in the cooling tanks at 38° F.

During barn feeding and holding of the cows, Arthrobacter species usually were predominant in the every-other-day milks, with Pseudomonas second in frequency and Micrococcus third. While the cows were on pasture, bacteria of the genus Flavobacterium were most often predominant, followed by Arthrobacter, and Alcaligenes first was detected. There was an average sevenfold increase in total numbers of psychrophiles when the cows went on to pasture from barn housing. There seemed to be a tendency for a certain genus of psychrophile to predominate in milk from a given farm, although the method of holding and feeding, barn versus pasture, had an important influence.

¹ Published with the approval of the Director of the Wisconsin Agricultural Experiment Station.

² At present with The Quaker Oats Co., Barrington, Ill.

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