Staphylococci in Colby Cheese

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 months at 10[degree]C. At the time the curd was cut, the counts on S-110 medium were 13-32 times greater in the curd than in the whey. Between cutting and hooping the curd, the populations on S-110 medium increased 6.5-29 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 1 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 10oC. 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 1st 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.