This paper summarizes the concept of F0 and its related parameters (D, z). Essential notions on sterilization kinetics are explained. The idea of physical and biological “equivalent time” is presented and its application in moist-heat sterilization processes is discussed.
It has been experimentally shown that the reaction of thermal degradation of microorganisms obeys
the laws of 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
If N0 is the initial microbial population, after an exposure time t to defined and steady conditions,
the population N will be expressed by each one of the following formulas:
Log (N0 / N) = t / D;
Log N0 – Log N = t / D;
N = N0 * 10 (- t /D)
Parameter D in these formulas is physically a time and is defined decimal (or decadal) decay (or reduction). This parameter expresses in a quantitative way the resistance of a microbial species to a sterilizing treatment; for this reason, the Parenteral Drug Association of USA (PDA) calls it resistance value. In a thermal sterilization
process, D-value is the time required to reduce to one tenth of the initial value the population of:
· a specified microbial species
· prepared in a defined way
· treated on a specific substrate
· underspecified and ideally steady process conditions.
Resistance value D is:
· expressed in minutes and tenths of minutes
· referred always and explicitly to a temperature, that is usually indicated as foot index: DT, for example D121, D134
· independent of the residual amount of the microbial population.
If liquid H2O (superheated water or condensing steam) is steadily in contact with the microorganisms to inactivate, the process is called moist heat sterilization. As well as on all the above conditions, in this type of process the resistance value D depends very strongly on the temperature T. If there is no contact between microorganisms and liquid H2O, the process is called dry heat sterilization. In this type of process, D-values
useful for industry can be obtained only at notably higher temperature than in the case of moist heat sterilization, and their dependency on temperature is less sensible.
- author: Mascherpa Vittorio