Assessment of the Inaccuracy Inherent in the F0 and A0 Concepts of Microbial Inactivation
Part 1: F0 concept
Rudolf Friedrich Bliema and Werner Georg Nowakb
Laboratorium für Betriebshygiene and Institut für Angewandte Mikrobiologiea, and Institut für Mathematikb, Universität für Bodenkultur, Vienna (Austria)
Theory and practice of moist heat sterilisation are based on the premise that microbial inactivation strictly follows first-rate kinetics, i.e. the exponential model. Experimental data however indicate otherwise. Nevertheless, with the simplicity of its mathematical procedures the exponential model has become the established standard for inactivation processes. In moist heat sterilization the error between model and experimental phenomenon is largely lost in the enormous margin of safety, assuming microbial loads of 106or 1012with Geobacillus stearothermophilus. Without this margin of safety the procedures using the first-order reaction rate function and the Arrhenius function would hardly be acceptable under today’s expectations of quality management.
Moist-heat procedures below the atmospheric boiling point of water, more commonly known as disinfection procedures, typically indicate obviously non-exponential behaviour and have there-fore been largeley excluded from the application of the exponential model. However, this model is now also being adopted, albeit in a limited form referred to as the A0 concept, which is the F0 procedure with general exposure times at the reference temperature of 80 °C.
This report offers an indication of errors inherent in the exponential model itself and in its application, especially with respect to the use of z-values. This parameter neither correlates with the thermostability nor with the pathogenicity of organisms. z-values are not as assumed temperature-independ-ent constants, but vary by over 2 °C over a range of 30 °C. In addition, there is no single common z-value, as the standard value of 10 implies. With sterilization processes the choice of the appropriate z-value is based on the indicator organism G. stearothermophilus. In contrast, the choice of a z-value of 10 for A0 calculations, in not linked to an indicator organism for disinfection processes, but rather assumes a general applicability of the value of 10. Alternatively, the widespread use of PCs now allows the use of new mathematical models and procedures providing far improved levels of accuracy.
Key words A0 concept • Disinfection • F0 concept • Operational hygiene • Quality assurance • Sterilisation