Time-temperature effects on Salmonellae and Staphylococci in foods. III. Thermal death time studies.

Thermal death time studies were conducted at 5 F intervals from 130 to 150 F with strains of salmonellae and enterotoxigenic staphylococci. Heat-resistant Salmonella senftenberg strain 775W, Staphylococcus aureus strains 196E and Ms149, and non-heat-resistant Salmonella manhattan were studied in custard, chicken à la king, and ham salad. The F(140) values (minutes of exposure at 140 F required to effect 100% destruction) were as follows: S. senftenberg 775W in custard 78, and chicken à la king 81.5; S. manhattan in custard 19, and chicken à la king 3.1; S. aureus 196E in custard 59, and chicken à la king 47; S. aureus Ms149 in custard 53, and chicken à la king 40. The end points of survival-kill at all the test temperatures for both salmonellae and staphylococci in ham salad were considerably less than for the other foods studied.D(140) values (minutes of exposure at 140 F required to effect a 90% reduction in numbers) were also calculated from the data and presented. Values for z(F) and z(D) (slope of the thermal-death-time and decimal-reduction-time curves) are also presented and discussed in relation to type of food, organism, and temperature. These data indicate that heating perishable foods of the type studied to 150 F and holding every particle of food at this temperature for at least 12 min reduces 10 million or less salmonellae or staphylococci per gram to nondetectable levels. The same degree of destruction is achieved in similarly contaminated foods when held at 140 F for 78 to 83 min. On the basis of the calculation procedures employed, it is estimated that 45-min exposure at 140 F would be necessary to reduce 1,000 organisms per gram to nondetectable levels.