Aerobacter (Enterobacter) aerogenes and Escherichia coli were inoculated separately into commercially produced samples of yogurt, buttermilk, sour cream, and cottage cheese. Inoculated products were stored at 7.2 C and were tested daily for up to 10 days to determine changes in numbers of coliforms and in pH values. The number of viable coliforms in yogurt declined dramatically and was markedly different from the initial value after only 24 hr of storage. Usually, survival of coliforms in yogurt did not exceed 3 days of holding. In buttermilk, most often a marked decline in numbers of coliforms was evident after 24 hr of storage. A substantial reduction in numbers (>50% of organisms present initially) of A. aerogenes B199 occurred in sour cream during the first 24 hr of storage, but a similar decline in numbers of E. coli and A. aerogenes FD was not evident until after 3 days of storage. Changes in numbers of E. coli and A. aerogenes in cottage cheese generally were not as rapid as in other products during the first days of storage. A few cottage cheese samples, however, did support rapid increases in test culture numbers. Because of the rapid decline in numbers of coliforms in yogurt, buttermilk, and sour cream, the provision in Standard Methods for the Examination of Dairy Products that permits examination of some of these products for up to 48 hr after manufacture seems inadvisable.
1Published with the approval of the Director of the Research Division of the College of Agricultural and Life Sciences, University of Wisconsin.