## 3 Results and Discussion
Causing a pH decrease inside a bacterial cell by ionization of the undissociated acid molecules or a disturbance of the substrate transport by changing the permeability of the cell envelope could be a mechanism of action that causes cell inactivation. In addition, pH values below 4.0 have a negative effect on the growth of most food-borne microorganisms. Due to the chemical behavior of PTW and PPA, whether used separately or in combination, acidification is expected on the sample surface and thus in the direct microbial environment [31, 47, 48, 46]. Another reason for the inactivation of microorganisms by PTW and PPA can be moisture in the PPA and PTW processes, which is provided by the environment/compressed air or by the water itself. [PERSON] et al. (2003) have reported on the influence of humidity in the air on bacterial inactivation [49]. They observed an obvious dependence of microbiological inactivation of _E. coli_ on air humidity. The microorganism under investigation showed a maximum inactivation rate at 43% relative humidity, which is frequently found in laboratory/room air. The significance of plasma gas humidity for the inactivation of _Bacillus subtilis_ endospores and _Aspergillus niger_ conidiospores was investigated by [PERSON] et al. (2008) [50]. They showed a reduction of up to 5 log\({}_{10}\) cycles within 7 s at 0% relative humidity and of up to 4 log\({}_{10}\) cycles at 80% relative humidity for the spores. The third and perhaps most important explanation could result from the chemical composition of PTW and PPA. Against this background, there are antimicrobial active reactive nitrogen species and a few reactive oxygen species that come into the fore and have been detected in our PTW and PPA. Therefore, experimental studies of the chemical composition of PPA and PTW are crucial for understanding the biochemical reactions responsible for microbial inactivation. Chemical analysis showed that reactive species (RONS) were detectable in both PTW and PPA. The chemical species detected in PPA were nitric oxide (NO), nitrogen dioxide (NO\({}_{2}\)), nitrous acid (HNO\({}_{2}\)), nitric acid (HNO\({}_{3}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) and water (H\({}_{2}\)O). In the PTW, nitrite (NO\({}_{2}\)-), nitrate (NO\({}_{3}\)-) and very small amounts of hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) were measured. Nitrogen monoxide (NO) is produced by nitrogen (N\({}_{2}\)) and oxygen (O\({}_{2}\)), both of which are contained in the compressed air used as working gas. In the presence of O\({}_{2}\), 'NO reacts further to form nitrogen dioxide ('NO\({}_{2}\)). The long-lived radicals 'NO and 'NO\({}_{2}\) are known for their antimicrobial effectiveness. In addition, peroxynitrite (ONOO\({}^{-}\)), which is believed to have a strong antimicrobial activity, could be a product of the reaction of NO with a superoxide radical (O\({}_{2}\)-) [45; 51]. The analysis of O\({}_{2}\)- in PPA will be part of future investigations. With regard to PTW, other reactions responsible for the mechanism of action may occur, as H\({}_{2}\)O may be an additional reaction component. 'NO, O\({}_{2}\) and H\({}_{2}\)O react to form NO\({}_{2}\)- and hydrogen (H\({}^{+}\)). If NO\({}_{2}\), instead of NO, reacts with O\({}_{2}\) and H\({}_{2}\)O, H\({}^{+}\) and NO\({}_{3}\)- are the reaction products. The PTW used in our experiments showed a strong acidification, which could be a consequence of the radicals 'NO and 'NO\({}_{2}\), which react with H\({}_{2}\)O to nitrous acid (HNO\({}_{2}\)) and nitric acid (HNO\({}_{3}\)). Normally, HNO\({}_{2}\) decomposes to H+ and NO\({}_{2}\)-, but a pH below 2.75 could lead to the spontaneous formation of 'OH and 'NO [45; 51]. Many of the detected and postulated ions, radicals and molecules are known for their strong antimicrobial activity. In combination with the antimicrobial potential of a pH shift into an acidic environment, the occurrence of such compounds, which are also influenced by pH, may lead to the observed antimicrobial mechanism of action of PPA and PTW.