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Method for the Determination or Rates of Spore Inactivation at Ultra-High Temperatures¹

Departments of Dairy and Food Industries and Bacteriology, University of Wisconsin, Madison

ABSTRACT

In a procedure for determining the rates of destruction of heat-resistant bacterial spores at temperatures between 119–130 C in 3:1 whole milk concentrate or 0.067 M phosphate buffer, thermal inactivation tubes (Figure 1, 6 mm by 100 mm) were constructed from cleaned pyrex glass tubing. Bach tube was sealed and constricted in an oxygen flame about 15 mm from one end, to facilitate opening the tube and removing its contents at the time of subculturing. Sterilized tubes were filled aseptically by means of an automatic syringe adjusted to deliver 0.5 ml of the spore-inoculated medium. After filling, the remaining open end of each tube was heated in an oxygen flame until sealed.

The spore inoculum, sufficient to provide 10⁶ spores per milliliter, was uniformly dispersed by mixing for 20 min with a magnetic stirring device. Incorporation of air in the milk concentrate was reduced by mixing the inoculum in a partial vacuum.

¹ Published with the approval of the Director of the Wisconsin Agricultural Experiment Station. This work was supported in part by a grant from the U. S. Steel Corporation.