Characterization of a Novel Atmospheric Air Cold Plasma System for Treatment of Packaged Biomaterials

Abstract

<abstract><title><italic>Abstract.</italic></title> The technology of atmospheric plasma (ionized gas), widely used in material processing, offers one of the most significant breakthroughs in food and agri-material processing. This technology allows for the generation of bactericidal molecules very efficiently with low power requirements. Non-thermal atmospheric plasma has been used to effectively decontaminate surfaces, but it has received limited investigation for in-package decontamination. This study demonstrates the potential for in-package plasma treatment of model liquids within sealed packages. The advantage of in-package cold plasma treatment is that the bactericidal molecules are generated and contained in the package, allowing extended exposure to bacteria while reverting back to the original package gas within 24 h storage. This study treated liquid food surrogates containing bacteria and oxidation sensitive dye (methylene blue) inside sealed packages. The surrogates were placed in 96-well plates and were packaged in air and exposed directly and indirectly to the plasma field in a 2.2 cm high × 23 cm × 31 cm sealed polypropylene container for up to 5 min. Treatments were carried out using a prototype dielectric barrier discharge (DBD) operating at a voltage of 40 kV and frequency of 50 Hz. The UV-Vis emission spectra of the plasma revealed emission bands for nitrogen and oxygen species, including strong emission lines for excited states of the atomic species O, O⁺, N, and N⁺. The results from the evaluation of methylene blue suggest that direct exposure to the plasma ionization field produced a greater oxidative effect compared to indirect exposure. For 5 min treatment, direct exposure of methylene blue resulted in a 90% reduction in absorbance, and indirect exposure of methylene blue resulted in a 75% reduction in absorbance. These reductions may result from conversion of ozone into hydroxyl radicals, which reduces the methylene blue from dark blue color to clear. This process is irreversible. Additionally, bacterial studies examining treatment of E. coli ATCC 25922 suspended in maximum recovery diluent in 96-well plates found a 7 log₁₀ reduction after 50 s treatment and 24 h storage for both direct and indirect plasma exposure. Ozone concentrations measured immediately after 5 min of ionization were approximately 1600 ppm. The goal of this research is to maximize bacterial reductions and minimize quality (oxidative) loss for biomaterials inside a sealed package.