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Effect of pH and Brine Concentration on the Thermal Resistance of PA 3679 in a Processed Cheese Spread^{1, 2,}

Departments of Food Science and Microbiology and Public Health Michigan State University, East Lansing

ABSTRACT

Tests conducted on a processed cheese spread inoculated with PA (putrefactive anaerobe) 3679 and packaged in TDT (thermal death time) cans indicated that only a small amount of heat would prevent gas production as measured by can expansion, whereas a considerably greater amount of heat would kill the spores. The average D₂₅₀ values of PA 3679 in cheese spread at pH 5.50, 6.25, and 7.00 were 0.67, 1.01, and 1.21 min., respectively, with a z value of 18° F.

The results suggest that a surviving spore population in the range of 10² to 10⁵ per gram is necessary for gas production by this organism in the processed cheese spread tested. As the processing temperature increased, the calculated number of spores necessary to initiate gas production decreased.

The thermal resistance of a spore suspension of PA 3679 in neutral phosphate buffer was equivalent to a D₂₅₀ value of 0.98 with a z value of 17.5° F., as measured with the thermoresistometer technique and subsequent subculturing in liver infusion broth.

¹ Michigan Agricultural Experiment Station, Journal Article No. 2785.

² This manuscript describes a portion of the research contained in a thesis prepared by the senior author in partial fulfillment of the requirements for the Doctor of Philosophy degree at Michigan State University.

³ Present address: Borden Company, Dixon, Illinois.