Essentials of Steam Sterilization Kinetics

ABSTRACT

Steam sterilization kinetics represent the base for a correct understanding of industrial practice
of moist-heat sterilization, but is often a neglected topic. This article by the well-known Italian sterilization
practitioner Dario Pistolesi and his former assistant Vittorio Mascherpa provides a simple but rigorous
introduction to it and is preliminary and propaedeutic to any useful discussion of the concept of equivalent
time Fo. Basic mathematical relationships and concepts as D and z parameters are the object of this article.
KEYWORDS: moist-heat sterilization, steam sterilization kinetics, equivalent time Fo, temperature.

GENERALS

Let us suppose to immerse in pressurized saturated steam, at constant temperature, a system contaminated by a micro biological species (which we assume, for the sake of simplicity, to be pure and homogeneous): e.g. a vial containing an aqueous suspension of a certain sporogenous microorganism. It has been experimentally shown that, under the above conditions, the reaction of thermal degradation of the microorganism at issue obeys
the laws of chemical reactions. Using N to indicate the number of microorganism present in the system at a given moment, the variation of this number as the function of a chosen time t of exposure to the selected sterilization temperature can be written as:

dN/dt  = – KN

where K is a constant which is typical of the species and conditions of the chosen microorganism.
The degradation reaction, i.e. the sterilization reaction, therefore develops like a first order
chemical reaction (i.e. like a chemical decomposition reaction) in which the reaction rate is proportional, in each moment, only to the amount of product still to be degraded (or decomposed). This seems to be obvious for dry sterilization, but less rigorous for steam sterilization, in which the water vapour molecules also seem to take part in the reaction. Actually, this bimolecular reaction is of the first order, since the steam is present in high excess all the reaction long and its concentration may be regarded as constant.