Interaction between microorganisms and cold atmospheric pressure plasma: experimental and numerical studies
Creators: name: Arserim, Ender Hikmet
Publisher: name: No Publisher Supplied
Year: 2023

description: Plasma is an ionized gas, which consists of charged and neutral particles. Various reactive species such as reactive oxygen species and reactive nitrogen species in plasma can be effectively used for microbial inactivation. Atmospheric pressure plasma processing which is a novel technology that can be used as a surface treatment to inactive microorganisms. Although the efficacy of plasma on microbial inactivation has been well studied experimentally, the prediction of plasma mediated microbial inactivation kinetics would help optimize the impact of plasma processing on surface microbial inactivation. The overarching goal of the study was to develop a microbial inactivation model as a function of reactive species concentrations and validate inactivation predictions with experimental results. One of the specific goals of the study was to evaluate the microbial inactivation efficacy of cold plasma using a pathogen (Salmonella) surrogate Enterobacter aerogenes deposited on a glass surface to obtain inactivation kinetics. Cold plasma generated by a custom-made floating electrode dielectric barrier discharge (FE-DBD) plasma at three different frequencies (1 kHz, 2 kHz, 3.5 kHz) and by a commercial atmospheric pressure plasma jet (APPJ) at 22.5 kHz was used for microbial inactivation experiments. Four different treatment times were applied for FE-DBD plasma and APPJ processing. Correlations based on exponential model and Weibull model were developed to fit the microbial inactivation data. Other goals of the study were to numerically predict reactive species concentration and distribution within FE-DBD and APPJ systems. A two-dimensional axisymmetric numerical simulation model based on COMSOL® Multiphysics was used to predict reactive species concentrations and distributions within an FE-DBD and APPJ systems. Microbial inactivation kinetics as a function of numerically predicted reactive species was developed. using rates from the literature and the results were compared with experimental data.The results showed that the FE-DBD plasma treatment achieved a microbial reduction of (4.6±0.2) log CFU/surface at 3.5 kHz, (5.1±0.09) log CFU/ surface at 2 kHz, and (5.1±0.05) log CFU/ surface at 1 kHz in 2 min, 3 min, and 6 min, respectively. Although the inactivation rate of DBD plasma treatment increased with increasing frequency, the inactivation rate of E. aerogenes per pulse was not significantly different at three different frequencies. Exposing E. aerogenes to APPJ for 5, 10, 15, and 20 min at a distance of 7.7 cm inactivated the population of E. aerogenes by (1.3±0.2, 1.9±0.3, 2.9±0.3 and 3.4±0.3) log CFU/surface (0.0254 m x 0.0254 m), respectively. In our studies, the DBD plasma system resulted in higher microbial inactivation efficacy compared to the plasma jet system. The predicted values were 4.0 log CFU/surface, 4.1 log CFU/surface, and 4.6 log CFU/surface at 1 kHz, 2 kHz, and 3.5 kHz, respectively for the FE-DBD system and 2.9 log CFU/surface for plasma jet after 20 minutes of exposure. A maximum one log difference for FE-DBD and 0.44 log for plasma jet was observed between the predictions for microbial inactivation and the experimental results. The difference might be due to synergistic interactions between plasma species, UV component plasma, and/or the electrical field effects, which could not be included in the numerical simulation.The results from this study will help predict the microbial inactivation efficacy of in a cold plasma application. The model might help in designing plasma equipment to optimize plasma process parameters.