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Staphylococci in Colby Cheese¹

Department of Food Science, Michigan Agricultural Experiment Station, East Lansing

ABSTRACT

Colby cheese was manufactured from raw milk inoculated with a coagulase-positive Staphylococcus aureus and from noninoculated raw milk obtained from cows with subclinical mastitis. Organisms were enumerated on plate count agar (PCA) and on Staphylococcus Medium No. 110 (S-110) on samples of milk, curd, and whey taken during the cheese-making process and on samples of cheese taken at 15-day intervals during storage for 4 mo. at 10° C. Many representative colonies were picked from S-110 medium and examined for coagulase reaction.

At the time the curd was cut, the counts on S-110 medium were 13 to 32 times greater in the curd than in the whey. Between cutting and hooping the curd, the populations on S-110 medium increased 6.5 to 29 times, with the greatest increase occurring in the cheese made from the mastitic milk. The maximum populations on S-110 medium occurred in the curd when hooped or in the cheese when one day old and were high enough to cause the cheese to be a potential source of clinical levels of enterotoxin, regardless of decreases in populations during subsequent ripening.

As the aging period progressed the percentage of coagulase positive colonies picked from S-110 medium gradually declined, but not uniformly, and coagulase-positive colonies were obtained from cheese cured for 120 days at 10° C. At the end of the 120-day holding period the populations on S-110 medium varied from 0.07 to 1.7% of the corresponding counts on the first day and the highest percentage of surviving organisms occurred in the cheese made from the mastitic milk. These results, together with the greater multiplication previously mentioned, indicate that the staphylococci naturally present in milk are better adapted to the environment of the cheese-making and curing process than the laboratory cultures of S. aureus which were inoculated into the milk.

¹ Michigan Agricultural Experiment Station Journal Article No. 2689.