3. Failure of the Model.  (TOC)

The unimolecular model is empirical, and has been shown to be reasonably accurate for many vegetative organisms. Nevertheless, there are many problems associated with the model. First, for most vegetative organisms, at the commonly used temperature for sterilization, viz. 121^oC, the value of K is rather large, usually falling between about 10 and 10¹³ min^-1. This makes accurate estimation of the values for K problematic.

More important, the model fails entirely over a wide temperature range for most endospores, which are heat resistant forms sometimes taken by many microorganisms. For these heat resistant spores, for moderate temperatures (above the lethal limit) there is usually a period where the viable spore count is reduced slightly, if at all. After a period, the spores are said to be activated, and are then sensitive to the elevated temperature. After activation, most spore forming organisms then do follow the first order model outlined here. Although it is still possible to define a thermal death constant for such organisms by measuring the slope of the plot of ln(N(t)/N(t₀)) versus t over the linear portion of the graph, it is misleading to attach too much significance to the number.