Published: October 2026

Authors: Brian McEvoy, Pierre Reppert, Katharina Sturm-Lüdtke, Bianka Ulbricht, Martin Meltke, and Ana Maksimovic 

Visit Science Direct where you can download and read the full article: The effectiveness of industrial gamma and e-beam sources, to inactivate B. pumilus spores in the presence of liquid water and considerations for validation of sterilization dose in accordance with ISO 11137

Abstract

This study aimed to evaluate the effectiveness of industrial electron beam (e-beam) and gamma irradiators in inactivating Bacillus pumilus spores at varying levels of water availability. Commercially available B. pumilus spore strips were exposed to three hydration conditions: dry, immersed in saturated NaCl solution, or immersed in purified water, and subsequently irradiated. Gamma operated at a dose rate of 1 kGy/h, while the dose rate of e-beam was 1800 kGy/h. The results showed that gamma and e-beam irradiation were equally effective (p = 0.111) in inactivating dry B. pumilus spores. These findings suggest that, provided the validated minimum dose is delivered, the dose may be transferred between industrial radiation sources without compromising product sterility, assuming no liquid water is present. In contrast, e-beam was more efficient than gamma in inactivating spores in saturated NaCl solution (p = 0.007) and spores in water (p = 0.009). It was observed that spore resistance increased with increasing water availability. The protective effect of water may have important implications for dose qualification strategies in sterilization processes. In the industrial sterilization of medical devices, the dose is often qualified by using Method 1/VDmax25, defined in ISO 11137. Method 1/VDmaxdose setting relies on the assumption that the product bioburden exhibits resistance to ionizing radiation that is consistent with the Standard Distribution of Resistance (SDR). If due to the presence of water, the resistance of the bioburden exceeds that of the SDR, Method 2 of ISO 11137-2:2013 should be used instead.

Highlights

• The radiation resistance of spores increased with increasing water availability
• E-beam and gamma were equally efficient to inactivate dry spores.
• E-beam was more efficient than gamma to inactivate spores in aqueous solution.
• The selection of a dose setting method is influenced by water availability.