Thermal Death of Microorganisms

A Heuristic Tutorial

NB: This is a work in progress. It is posted in this incomplete form so that anyone who may come across it will have the opportunity to send me comments or suggestions. The document will eventually be made interactive, with simple calculations, etc. Figures and charts will also be incorporated. I will add proper credits and references in due course. I hope this material will be an aid to understanding the various terms employed, properly or improperly, in the industry.

Please send comments or suggestions to Daniel Bull, Ph.D.

9. Sterilization Cycles. (TOC)
Unfortunately, it will not do simply to calculate the necessary time for the required reduction of organisms at a given temperature using a measured thermal death constant K. This is because except under special circumstances, there is a significant time for heating and cooling of the solution. The heating time and the cooling time contribute to the killing of organisms, as well as to the denaturation of other heat sensitive materials. Theoretically, the problem is easily accommodated. Simply integrate the kinetic death model from the time at which the lethal temperature is first reached until the time when the solution is cooled below this temperature. Thus, since
	Eq. (20)
the reduction of organisms can be expressed as
	Eq. (21)
Taking logarithms:
	Eq. (22)
Equation (20) holds for vegetative organisms, for most of which A and E are constant over the temperature range of interest. Therefore, if we know A and E from experimental measurements (recall that Z = R/E, and that a plot of lnD vs T is linear with intercept on the lnD axis at ln(2.303/A) and slope R/E) we can specify any arbitrary total reduction of organisms if we know the temperature T as a function of time t.

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