## 3 Results and Discussion  
### Inactivation of Salmonella and Listeria monocytogenes  
This study aimed to assess the inactivation efficiency against _L. monocytogenes_ and _Salmonella_ inoculated on clean or protein-soiled stainless steel (SS), using an air-fed low-voltage piezoelectric CAP device at exposure times up to 300 s, and different distances between the plasma source and the contaminated surface. CAP treatment was able to significantly reduce the levels of both pathogens. The log reduction results in Table 1 demonstrate that pathogen inactivation on the SS surface is time-dependent. The analysis of variance indicates that the treatment time was the factor with the greatest influence \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Bacterial Species**} & \multirow{2}{*}{**Treatment Time (s)**} & \multicolumn{6}{c}{**Reduction (log CFU/mL) (Mean \(\pm\) SD)**} \\ \cline{3-8}  & & \multicolumn{3}{c}{**10 mm Distance**} & \multicolumn{3}{c}{**20 mm Distance**} \\ \cline{3-8}  & & \multicolumn{2}{c}{**Protein-Soiled SS**} & \multicolumn{2}{c}{**Clean SS**} & \multicolumn{2}{c}{**Protein-Soiled SS**} & \multicolumn{2}{c}{**Clean SS**} \\ \hline _Listeria_ & 60 & 0.81 a & 0.21 & 1.51 a & 0.29 & 0.48 a & 0.22 & 1.01 a & 0.24 \\ _monocytogenes_ & 120 & 1.49 b & 0.26 & 2.33 b & 0.29 & 1.02 b & 0.26 & 1.52 b & 0.21 \\  & 180 & 1.95 c & 0.13 & 3.01 c & 0.41 & 1.34 b & 0.51 & 2.03 c & 0.39 \\  & 240 & 2.46 d & 0.27 & 3.47 d & 0.38 & 2.25 c & 0.12 & 3.23 d & 0.24 \\  & 300 & 2.73 d & 0.28 & 3.59 d & 0.34 & 2.52 c & 0.35 & 3.32 d & 0.28 \\ \hline _Salmonella_ & 60 & 0.84 a & 0.20 & 1.52 a & 0.50 & 0.80 a & 0.18 & 1.25 a & 0.17 \\ Typhimurium & 120 & 1.34 b & 0.24 & 2.42 b & 0.20 & 1.04 a & 0.22 & 1.68 b & 0.27 \\  & 180 & 1.94 c & 0.22 & 3.01 c & 0.23 & 1.38 b & 0.18 & 2.25 c & 0.25 \\  & 240 & 2.56 d & 0.33 & 3.49 d & 0.22 & 2.36 c & 0.23 & 3.44 d & 0.20 \\  & 300 & 2.67 d & 0.23 & 3.51 d & 0.08 & 2.32 c & 0.26 & 3.54 d & 0.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Log reduction of _L. monocytogenes_ and _Salmonella_ after CAP treatment from 0 to 300 s, at two different distances (10 and 20 mm), and inoculated on clean stainless-steel (SS) or protein-soiled SS. The results are expressed as the mean \(\pm\) SD. The superscript letters indicate different subsets for each treatment; the same letter indicates no significant difference when \(\alpha=0.05\) and \(n=6\). (F \(>\) 1000, \(p\) \(<\) 0.001), followed by the cleanliness of the surface (F \(>\) 900, \(p\) \(<\) 0.001) and finally, the distance from the source (F \(>\) 152, \(p\) \(<\) 0.001) for both strains. Our results are in agreement with previous studies, where higher levels of inactivation were achieved as a function of treatment time [3]. In a previous study, a high-voltage input CAP device at 13.7 kHz showed reductions of \(>\)3.55 log CFU/mL on \(L\). _monocytogenes_ and 2.06 log CFU/mL on \(S\). Typhimurium, after 175 and 105 s, respectively, on SS when the initial inoculum was approximately 8 log CFU/mL. Another study that used an RF plasma system (13.6 MHz) reported a time-dependent antimicrobial activity against \(E\). _coli_ K12 inoculated on SS, achieving up to 5 log reduction after 30 min of treatment, with an initial inoculum of log 7 [19]. Another report using a high-voltage (10 kV) CAP device with a frequency of 2 kHz also showed 2.80, 3.43, and 4.17 log bacterial reductions after 5, 10, and 20 min, respectively, against \(E\). _coli_ inoculated on SS [29]. The present study showed that similar reductions of 3.51 and 3.59 log CFU/mL were achieved for _Salmonella_ and \(L\). _monocytogenes_, respectively, on SS after 5 min of treatment using the piezoelectric CAP device. It has been found that the air-fed CAP system employed in this study produces NO\({}_{2}\), O\({}_{3}\), and NO at temperatures below 35 \({}^{\circ}\)C [22].