Effect of a non-thermal atmospheric pressure plasma jet on four different yeasts

Non-thermal atmospheric pressure plasmas are known to be an effective method for the inactivation of microorganisms. However, yeasts have proved to be more resistant to provided treatments. We investigated the influence of an Argon plasma jet on the inactivation of four different yeasts; Saccharomyces cerevisiae, Schizosaccharomyces pombe, Candida parapsilosis and Magnusiomyces magnusii. Results from direct plasma treatment on the yeasts in distilled water, as the most effective technique we found, are presented here. The surviving yeasts after 5, 7, and 10 min plasma exposures were evaluated visually and quantitatively. Quantitative results showed strong reductions in the survival rates after the plasma treatment. The longest plasma exposure time yielded the least survival. Moreover, how long the inactivations lasted was evaluated by assessing the treated samples after 1 h and again after 2 h. The survival rates were further decreased within these storage times. In the 10 min treated-2 h stored samples, the rates dropped to 5% or even below, depending on the yeast species. High concentrations of reactive oxygen and nitrogen species in the plasma-air-water interacting system were detected by optical emission spectroscopy of the plasma jet and also by measuring concentrations of H₂O₂, NO₂ ⁻ and NO₃ ⁻ in plasma activated water (as the surrounding liquid for the yeasts). The conductivity and pH of the treated water was measured as well. The level of the conductivity increased along with the decrease in pH. Synergistic effects of these chemical reactive species in acidified water resulted in the yeast inactivation. (paper)