Modeling the inactivation of Bacillus subtilis spores during cold plasma sterilization

Cold plasma sterilization is an emerging non-thermal technology that is receiving great attention in the food processing area. Plasma is a neutral ionized gas composed of reactive gas species that inactivate bacteria or spores in a variety of food materials without compromising the main physico-chemical characteristics of the food. Survival curves of Bacillus subtilis spores were obtained after spore strip samples containing an initial spore population of 1.5–2.5 × 106 cfu/strip were subjected to plasma treatment. The shape of the survival curves was clearly not linear indicating that spores exhibited a spectrum of inactivation resistances to the plasma treatment. A Weibull model was used to describe these curves. In order to capture the effects of the typical variability in the concentration of the inactivating reactive gas species during plasma processing, time-varying concentrations were incorporated in the calculating approach. The result was an ordinary differential equation (ODE) that was numerically solved using MATLAB. This approach was successfully applied to describe the survival of B. subtilis spores during plasma processing as well as data obtained from the literature for B. atrophaeus. Ozone was assumed the lethal reactive gas species responsible for spore inactivation. Modeling plasma processing is of great interest because it may provide an accurate estimation of time and conditions required for a complete plasma-based sterilization process.