Effects of Ingredients Used in Condensed and Frozen Dairy Products on Thermal Resistance of Potentially Pathogenic Staphylococci1

Abstract

A cell suspension of Staphylococcus aureus (196E) was injected into raw skim milk which contained different concentrations of sugar, serum solids, fat, stabilizers, and emulsifiers. The ingredient samples were exposed for the desired length of time in a constant-temperature water bath (60 C). Standard plate counts were made, and the number of surviving organisms was determined. Regression coefficients for each ingredient concentration were calculated and plotted against the per cent ingredient concentration to give an indication of protective action. Analyses of variance were conducted on bacterial counts to test the protective action of each ingredient. A comparison of the number of survivors in different sugar concentrations showed that with up to 14% sugar all the organisms were killed within 30 min. In sugar concentrations above 14%, the number of survivors increased regularly with each increase in sugar concentration up to 57%, which was the maximum used. In concentrations of serum solids above 9%, some organisms survived 35 min of heat treatment. Butter fat, stabilizer, and emulsifier did not offer any protective action in the concentrations observed.