## 3 Results and Discussion  
### Chemical Parameters  
Chemical parameters of apple samples are reported in Table 1. Water content and maturity index (SSC/TA) were not affected by the treatment and did not show any significant difference compared to the control sample at all treatment times.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Time (min)**} \\ \cline{2-5}  & **0** & **10** & **20** & **30** \\ \hline Water content (\%) & 83.73a & 83.53a & 84.84a & 83.63a \\ MI (SSC/TA) & 34.88a & 33.08a & 37.68a & 34.54a \\ pH & 3.73a & 3.64b & 3.56b & 3.65b \\ L\({}^{*}\) (4 h) & 77.89a & 76.93a & 76.87a & 79.48b \\ a\({}^{*}\) (4 h) & 0.59a & 0.08a & \(-\)0.56a & \(-\)2.49b \\ b\({}^{*}\) (4 h) & 26.75a & 27.03a & 27.16a & 24.54b \\ \hline \hline \end{tabular} Values followed by different letters within the same row are significantly different at a \(p<0.05\) level.  
\end{table}
Table 1: Physico-chemical parameters of Pink Lady apples, as affected by plasma treatment time.  
Slight acidification was observed upon plasma exposure, independently of treatment time. While some other quality parameters, such as color and texture, have been studied in a wide range of food products, the impact of plasma on product pH has not been investigated often. No differences were observed by [PERSON] et al. [22] in milk or other model liquid systems treated with argon plasma. However, [PERSON] et al. [23] found a strong decrease of PBS buffer pH from 7.3 to 3-4, as well as an increase of conductivity as a consequence of pulsed plasma exposure. Slight acidification was observed also in pork loins treated with a DBD device, with helium and oxygen as the working gas [24]. The authors suggested that those changes were due to the dissolution of acidogenic molecules, such as nitrogen oxides, generally generated in plasma discharges in the food system.  
Color was measured four hours after treatment in order to evaluate a possible effect on the browning of fresh-cut apples. No differences were observed up to 20 min of treatment; however, samples exposed for 30 min were characterized by higher L* values and lower a* and b* values compared to the untreated sample.  
These results confirm those obtained in a previous study of plasma effects on browning inhibition [25], where with the same experimental conditions, a reduction in browning was observed after 30 min of treatment by image analysis. In the same study, an inhibition of polyphenoxidase (PPO) activity proportional to treatment time up to 57% after a 30 min treatment was reported.  
Reduction of enzymatic activity upon plasma exposure has been observed in various studies [26; 27], and has been attributed to the oxidation of the reactive side-chain of the amino acids by plasma radicals, particularly OH, O2-; HOO and NO, that promote a change in the secondary protein structure and the modification of some amino acids' side-chains of the enzyme [28]. In particular, [PERSON] et al. [25] found a variation in the relative amounts of \(\alpha\)-helix structures and \(\beta\)-sheet content upon plasma exposure which was strongly correlated to the loss of enzymatic activity. Generally, the inhibitory effect is dependent on the type of enzyme considered and on the matrix effect [29].