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Heat Resistance of Certain Pathogenic Bacteria in Milk Using a Commercial Plate Heat Exchanger

Department of Food Science and Technology

Warren Litsky

and Institute of Agricultural and Industrial Microbiology, University of Massachusetts, Amherst 01002

ABSTRACT

This investigation was undertaken to determine the effect of rapid heating and cooling of milk, with very short holding time, on the thermal resistance of certain organisms important to milk pasteurization. Suspensions of strains of Escherichia coli, Staphylococcus aureus, Streptococcus pyogenes, Corynebacterium diphtheriae, Salmonella typhosa, and Shigella dysenteriae were inoculated into milk which subsequently was processed at varying temperatures with a commercial plate pasteurizer. The pasteurizer was modified slightly to reduce resident heating, holding, and cooling times to a minimum.

D values were determined from recovery data for each strain of organisms, and were used to calculate theoretical lethalities by an arbitrary standard of 15 D being equivalent to unit lethality. From these data, it was calculated that a temperature of 82.2C would adequately, pasteurize milk. Milk can be pasteurized successfully by faster rates of heating and cooling, without an intended holding time at temperatures attainable with commercial heat exchangers. However, consideration must be given to rates of heating and cooling milk by individual pasteurizers and to controlling instrument responses in establishing standards.