1. Introduction.  (TOC)

This short treatise will examine some of the concepts in common use regarding the thermal destruction of microorganisms. The discussion will be confined to the common microorganisms, bacteria, Actinomycetes, yeasts, and molds. Consideration will be given to several of the frequently encountered quantities used as measurements of the sensitivity of a given organism, and to the limitations of such measurements. I will also present some tentative materials regarding more rigorous measurements, especially as regards sterilization of liquids. This is an introductory text, and should not be used for design. More exhaustive treatments are available in the engineering literature, most of which is not online. Only a minimal knowledge of differential equations is assumed, and statistical aspects, which are actually very important in the design of real systems, are avoided entirely.

Microorganisms, like all living things, are sensitive to heat, or thermal energy, and will not survive if subjected to sufficient quantities of heat at sufficient intensity. How this happens is not a simple phenomenon. Consider a single microorganism. To kill the microorganism, heat must be transferred to it in sufficient quantity to raise the temperature to a lethal level. Then the organism must be held at that level for sufficient time to ensure destruction of the life force, however that may be defined.

This narrative description of the death of a microorganism is not satisfying. To be useful, we must have a quantitative description. For example, what is "temperature," and what is a "lethal level?" What is "heat" and what does it mean to "transfer" heat? Finally, how can we know what it "sufficient time?" To prevent the discussion from becoming only metaphysical, let us admit from the start that we must use a meta-language to describe physical phenomena, and that that is what we are doing.

Temperature, for our purposes, is a measure of the vibrational energy of molecules. Temperature is thus an intensive thermodynamic variable. If the vibrational energy of the molecules in a microorganism is high enough, then the organism will eventually die after some time, unless the vibrational energy, or temperature, is lowered before that happens. This is a statistical phenomenon. We can only say that there is a certain probability that the organism will die after a given time at a given temperature. In addition, below some threshold temperature level characteristic of the organism, death will not occur from this cause, even after infinite time. There is disagreement about what causes death, but this writer is of the opinion that irreversible denaturation of some enzyme or enzymes critical for DNA or RNA replication is at the heart of the matter.

Equipped with this knowledge, we can now discuss rationally the death of microorganisms, and the related phenomenon, sterilization.