## 3 Results and Discussion
### Effects of OM Treatment on Protein Oxidation  
#### 3.3.1 Free Amino Acids  
Free amino acids are correlated to the nutritional value and flavor quality of meats [35]. In Table 3, complete data regarding the concentration of free amino acids in pork and fish samples treated by OM and CW are listed. For pork samples, no significant difference was found in the 11 amino acids in the CW vs. OM comparison. However, significantly higher (\(p<0.05\)) levels of Glycine (Gly), Alanine (Ala), and Proline (Pro) in the OM group were detected, which may originate from the higher proteolysis due to the change of protein secondary structure. As [PERSON] et al. [13] reported, ozone treatment led to a lower amount of \(\alpha\)-helical and higher amount of \(\beta\)-sheet and random coil, which subsequently promoted protein unfolding and proteolysis [36]. However, significantly lower (\(p<0.05\)) concentrations of Cysteine (Cys), Methionine (Met), Lysine (Lys), and Isoleucine (Ile) were determined in the OM group. [PERSON] et al. [37] also reported a significantly lower (\(p<0.05\)) content of Cys, Met, and Lys in shrimp tropomyosin after 15 min of high-intensity ultrasound treatment, which induced free radicals in water, in comparison with the control group (without treatment). It may be explained by the fact that some amino acids, especially some sulfur amino acids such as Cys and Met, are susceptible to ROS [38], and some amino acids, especially Lys, are involved in the formation of more stable oxidized products by bonding covalently with some oxidized products [39]. Previous studies have explored the oxidation of amino acids caused by ozone, which indicated that \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Free Amino Acid**} & \multicolumn{2}{c}{**Pork**} & \multicolumn{2}{c}{**Fish**} \\ \cline{2-5}  & **CW** & **OM** & **CW** & **OM** \\ \hline Asp & 13.28 \(\pm\) 2.26 & 12.91 \(\pm\) 0.15 & 10.44 \(\pm\) 0.10 & 11.01 \(\pm\) 0.94 \\ Ser & 39.58 \(\pm\) 1.47 & 37.34 \(\pm\) 0.70 & 34.73 \(\pm\) 1.58 & 28.67 \(\pm\) 1.61 \({}^{\star}\) \\ Glu & 50.77 \(\pm\) 4.40 & 56.11 \(\pm\) 1.43 & 20.98 \(\pm\) 0.96 & 24.61 \(\pm\) 3.04 \\ Gly & 46.84 \(\pm\) 1.10 & 53.01 \(\pm\) 0.80 \({}^{\star}\) & 484.23 \(\pm\) 16.79 & 531.03 \(\pm\) 9.21 \({}^{\star}\) \\ His & 20.12 \(\pm\) 1.24 & 19.17 \(\pm\) 0.35 & 1121.46 \(\pm\) 22.93 & 1145.86 \(\pm\) 24.65 \\ Arg & 36.52 \(\pm\) 6.61 & 42.91 \(\pm\) 6.78 & 102.73 \(\pm\) 6.02 & 116.61 \(\pm\) 9.07 \\ Thr & 1735.14 \(\pm\) 76.58 & 1736.48 \(\pm\) 5.25 & ND & ND \\ Ala & 307.21 \(\pm\) 9.41 & 354.44 \(\pm\) 2.11 \({}^{\star}\) & 176.12 \(\pm\) 13.00 & 291.46 \(\pm\) 31.39 \({}^{\star}\) \\ Pro & 24.33 \(\pm\) 3.17 & 66.95 \(\pm\) 1.26 \({}^{\star}\) & 138.46 \(\pm\) 6.94 & 125.32 \(\pm\) 13.42 \\ Cys & 30.28 \(\pm\) 4.78 & 19.36 \(\pm\) 0.39 \({}^{\star}\) & ND & ND \\ Tyr & 23.12 \(\pm\) 3.36 & 18.81 \(\pm\) 0.44 & 4.45 \(\pm\) 1.44 & 4.88 \(\pm\) 0.76 \\ Val & 27.74 \(\pm\) 6.66 & 25.20 \(\pm\) 5.60 & 21.51 \(\pm\) 2.13 & 22.19 \(\pm\) 2.47 \\ Met & 12.38 \(\pm\) 1.19 & 7.12 \(\pm\) 0.20 \({}^{\star}\) & 8.74 \(\pm\) 1.16 & 8.51 \(\pm\) 1.07 \\ Lys & 23.89 \(\pm\) 2.39 & 13.76 \(\pm\) 0.15 \({}^{\star}\) & 139.4 \(\pm\) 14.09 & 146.34 \(\pm\) 17.03 \\ Ile & 17.38 \(\pm\) 2.04 & 11.58 \(\pm\) 0.29 \({}^{\star}\) & 29.81 \(\pm\) 2.49 & 23.75 \(\pm\) 2.34 \({}^{\star}\) \\ Leu & 30.55 \(\pm\) 2.27 & 27.53 \(\pm\) 0.48 & 44.29 \(\pm\) 3.08 & 44.74 \(\pm\) 3.53 \\ Phe & 14.67 \(\pm\) 1.32 & 12.64 \(\pm\) 0.50 & 15.39 \(\pm\) 1.16 & 13.49 \(\pm\) 0.57 \\ \hline Total FAA & 2453.78 \(\pm\) 98.51 & 2515.33 \(\pm\) 7.25 & 2352.74 \(\pm\) 87.92 & 2538.46 \(\pm\) 108.78 \\ \hline \hline \end{tabular} \({}\)  
* ND indicates that the compound was not detected. For each species of muscle foods, \({}^{\star}\) indicates that the values in the same row differ significantly (\(p<0.05\)) between the two different washing treatment groups.  
\end{table}