Evaluation of Steam Sterilization Processes: Comparing Calculations Using Temperature Data and Biointegrator Reduction Data and Calculation of Theoretical Temperature Difference

Abstract

When calculating of the physical F_121.1°C-value by the equation

F_121.1°C = t · 10^T°121.1/z

the temperature (T), in combination with the z-value, influences the F_121.1°C-value exponentially.

Because the z-value for spores of Geobacillus stearothermophilus often varies between 6 and 9, the biological F-value (FBIO) will not always correspond to the F₀-value based on temperature records from the sterilization process calculated with a z-value of 10, even if the calibration of both of them are correct. Consequently an error in calibration of thermocouples and difference in z-values influences the F_121.1°C-values logarithmically.

The paper describes how results from measurements with different z-values can be compared. The first part describes the mathematics of a calculation program, which makes it easily possible to compare F₀-values based on temperature records with the FBIO-value based on analysis of bioindicators such as glycerin-water-suspension sensors. For biological measurements, a suitable bioindicator with a high D₁₂₁-value can be used (such a bioindicator can be manufactured as described in the article “A Method of Increasing Test Range and Accuracy of Bioindicators—Geobacillus stearothermophilus Spores”).

By the mathematics and calculations described in this macro program it is possible to calculate for every position the theoretical temperature difference (ΔT_th) needed to explain the difference in results between the thermocouple and the biointegrator. Since the temperature difference is a linear function and constant all over the process this value is an indication of the magnitude of an error. A graph and table from these calculations gives a picture of the run.

The second part deals with product characteristics, the sterilization processes, loading patterns. Appropriate safety margins have to be chosen in the development phase.