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Thermal Deactivation Studies on Pathogenic Bacteria in Milk and Various Milk Products. I. Corynebacterium diphtheriae ATCC No. 296¹

Department of Bacteriology and Public Health, University of Massachusetts, Amherst

ABSTRACT

Corynebacterium diphtheriae No. 296 was shown to have mean D values of 6.65, 1.68, 0.00547, 0.00314, 0.00140, and 0.00084 min. in milk at temperatures of 120, 125, 154, 156, 158, and 160° F., respectively. The mean D values for the organism in chocolate milk were 5.60, 1.18, 0.00758, 0.00556, 0.00407, and 0.00230 min. at respective temperatures of 120, 125, 148, 150, 152, and 154° F. Mean D values of 5.06, 0.72, 0.00309, 0.00238, and 0.00175 min. were found in cream for respective temperatures of 120, 125, 136, 138, and 140° F. In ice cream mix the organism was found to have mean D values of 15.0, 6.0, 0.00083, 0.00038, 0.00037, and 0.00027 min. for temperatures of 120, 125, 162, 164, 166, and 168° F., respectively.

According to the theory of logarithmic destruction of bacteria by heat, sterilization can not be achieved in a sample of infinite size. Therefore, for purposes of calculation, one must set arbitrary limits in order to determine a given process value. If one accepts a 10 D value process as being adequate for proper pasteurization, a holding time of 0.01 min. (0.6 sec.) at 160.5° F. will reduce the concentration of viable cells of C. diphtheriae No. 296 to the arbitrarily chosen lower limit of acceptability in milk and ice cream mix, which were found to provide more protection to the test organism than did chocolate milk and cream containing 40% fat.

¹ Contribution No. 1245 from the Massachusetts Agricultural Experiment Station. This investigation was supported in part by Research Grant E-1173 from the National Institute of Allergy and Infectious Diseases, Public Health Service.