Inactivation kinetics of Geobacillus stearothermophilus spores during the sterilization in steam-NCGs (steam-air) mixtures

Little is known about the steam content in mixtures of steam and NCGs (Non-Condensable Gases), also referred to as steam-air mixtures, required for successful sterilization and about the contribution of additional liquid water in the sterilization chamber to this process. In this study, sterilization efficiency was assessed with Geobacillus stearothermophilus spores in different steam-NCGs mixtures and under different temperatures and holding times. For each experiment, water, air and bioindicators (BIs) were placed in a gas-tight metal box which then underwent one pre-defined sterilization cycle. The initial water amount, sterilization temperatures and holding times were varied. The volume fractions of steam were calculated by applying thermodynamic laws, and the steam distribution was simulated using Computational Fluid Dynamics (CFD). The inactivation of the BIs was represented as a function of different steam-NCG mixtures. The main finding is that the steam volume in the gas phase is of little importance, but the amount of liquid water available to wet BIs during sterilization is crucial. This study represents the first investigation on bacterial inactivation during the sterilization process in different steam-NCG mixtures using different amounts of liquid water. The previously unknown inactivation kinetics results may prove useful for developers and researchers and enhance medical safety.