Characterization of a Novel Cold Atmospheric Air Plasma System for Treatment of Packaged Liquid Food Products

Abstract

The technology of atmospheric plasma (ionized gas), widely used in material processing, offers one of the most significant breakthroughs in food processing and safety. The technology allows the generation of bactericidal molecules very efficiently with low power requirements. Non-thermal atmospheric plasma has been used to effectively decontaminate surfaces, but has received limited investigation for in-package decontamination. This study demonstrates the potential for in-package plasma treatment of food products in 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 hr storage. The 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 x 31 cm sealed polypropylene container for up to five minutes of treatment. 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 plasma were collected and analyzed. The spectra showed 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 produces a greater oxidative effect compared to indirect exposure. For five minutes 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 appears to be non-reversible. Additionally, bacterial studies examining treatment of E. coli ATCC 25922 suspended in maximum recovery diluent inside of 96 well plates found a 7 log reduction after 50 s treatment and 24 hours storage for both direct and indirect plasma exposure. Ozone concentrations measured immediately after five minutes of ionization were approximately 1600 ppm. The goal of this research is to maximize bacterial reductions and minimize quality (oxidative) loss for liquid food products inside a sealed package.