Five brands each of commercially produced cultured buttermilk, sour cream, and unflavored yogurt were inoculated with Staphylococcus aureus to contain initial numbers of approximately 102 and 105 organisms per gram. Inoculated products were stored at 7 and 23 C and were tested daily to determine numbers of viable staphylococci. Generally, products which contained initial numbers of 102 organisms per gram were free of viable staphylococci, or nearly so, within 24 hr regardless of storage temperature. When products were inoculated with higher numbers of the organism, survival was greatest in sour cream which was followed in order by buttermilk and yogurt. Yogurt samples with approximately 105 organisms per gram, initially, were free of viable staphylococci by the 2nd to 4th day of storage, whereas the organisms persisted in sour cream until the 4th to 7th day. Survival was markedly higher in one brand of sour cream and somewhat so in another. Temperature of storage had no obvious effect on survival of staphylococci in buttermilk or yogurt but survival was greater in sour cream stored at 7 C than at 23 C. The pH values of the products remained reasonably stable throughout storage.
1Supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison and by Public Health Service Grant No. FD00009-05 from the Food and Drug Administration.