Effect of surface roughness in model and fresh fruit systems on microbial inactivation efficacy of cold atmospheric pressure plasma

Cold Atmospheric Pressure Plasma (CAPP) is an ionized gas consisting of charged and neutral particles, and radiation of varying wavelengths. Due to its relatively low temperature, CAPP is considered as a potential non-thermal decontamination technique. The goal of this research was to assess the suitability of CAPP for fresh produce decontamination as affected by surface roughness. The specific objectives were: i) to isolate and investigate the effect of surface roughness on microbial inactivation efficacy of CAPP using a model system, ii) to understand the extent to which surface roughness affects the microbial inactivation efficacy of CAPP in fresh produce, and iii) to detect and relatively quantify active plasma species in CAPP. CAPP from filtered dry air was generated using a plasma jet. Optical Emission Spectroscopy (OES) was used to quantify the relative concentrations of plasma species. Closed coat sandpapers with roughness (quantified via parameter of root mean square deviation Pq, measured using Confocal Laser Scanning Microscopy (CLSM)) ranging from, 6 µm to 16 µm were selected as model system. Sandpapers were inoculated with Enterobacter aerogenes (reportedly non-pathogenic, surrogate to Salmonella spp), left to dry for two hours, and treated with plasma for eight minutes twelve seconds. Based on their measured Pq values using CLSM, apples, oranges, and cantaloupes surfaces were selected and similar microbial inoculation, processing and analysis were performed. Model system results showed a 0.52 log higher inactivation of E. aerogenes (2.08 log inactivation) on the smoothest sandpaper and the difference was statistically significant from roughest sandpaper. Fruit surfaces results showed 1.25 log higher inactivation on apples (1.86 log inactivation) which were the smoothest and the difference was statistically significant from the roughest cantaloupes. As the surface roughness increased, the microbial inactivation efficacy of CAPP decreased. However, the results from fruit surfaces showed high variability, and were not predictable from the sandpaper data. Emission spectrum from OES indicated the presence of reactive oxygen and nitrogen species with potential to cause microbial inactivation. In conclusion, microbial inactivation efficacy of CAPP is affected by factors beyond surface roughness and further research is needed to determine its suitability for fresh produce decontamination.M.S.Includes bibliographical referencesby Siddharth Bhid