## 3 Results and Discussion  
### Lipid Oxidation  
MDA levels increased over the storage period in all treatments, however it was more pronounced in AUV1 and AUV3, and less pronounced in OSP, OSUV1 and OSUV3 (\(p<0.05\); Table 2). On day 0,no difference was observed in MDA levels among all treatments. On days 3, 6 and 9, AUV3 exhibited the highest MDA levels followed by AUV1 in comparison to AP (\(p<0.05\)), indicating that UV-C radiation affected the lipid oxidation in a dose-dependent manner. On the other hand, OSP, OSUV1 and OSUV3 had similar MDA levels (\(p>0.05\)), which were lower than AP on days 3, 6 and 9 (\(p<0.05\); Table 2).  
Lipid oxidation is initiated by factors such as temperature, light, enzymes, reactive oxygen species (ROS) or presence of pro-oxidants, which remove a hydrogen atom from an unsaturated fatty acid and, in the presence of O\({}_{2}\), it results in a complex free-radical chain reaction [6; 28]. It leads to formation of primary compounds like peroxide radicals (ROO-) and hydroperoxides (ROOH), which due to their instability result in the formation of secondary compounds such as aldehydes, ketones and hydrocarbons, which are stable and responsible for off-flavor and meat discoloration during refrigerated storage, mainly the aldehydes [6; 28; 29]. Rainbow trout is a farmed freshwater fish species with high susceptibility to lipid oxidation due to its fatty acid profile, which contains approximately 82.60% of unsaturated fatty acids and 15.80% of saturated fatty acids [2]. These facts may explain the increase of lipid oxidation over the storage period found in this study. Similarly, [PERSON] et al. [12] and [PERSON] et al. [30] observed an increase in the MDA levels in farmed rainbow trout fillets stored at 4 \({}^{\circ}\)C under aerobic conditions for 22 and 15 days, respectively.  
The increase of MDA levels by both UV-C doses may be attributed to the pro-oxidant effect of UV-C radiation generating ROS and accelerating the lipid oxidation [11]. In agreement with our findings, [PERSON] et al. [9] observed a dose-dependent effect of two UV-C doses (0.10 and 0.30 J/cm\({}^{2}\)) on an increase of MDA levels in refrigerated tilapia fillets for 23 days. Likewise, [PERSON] et al. [16] reported the same effect on lipid oxidation due to the application of UV-C doses at 0.80 and 1.60 J/cm\({}^{2}\) in sea bass fillets stored at 4 \({}^{\circ}\)C for 11 days. The effectiveness of the O\({}_{2}\) scavenger against lipid oxidation is due to its ability to reduce the O\({}_{2}\) concentration inside the package (<0.01%) and to minimize the free-radical chain reaction induced by ROS [17; 18; 31]. Similarly, O\({}_{2}\) scavenger was able to prevent the lipid oxidation of fish species stored under refrigeration, such as rainbow trout fillets [7], fresh coobia steaks [10], Indian oil sardine [19], and dried sardine [32].  
Regarding combined treatments, there is a lack of studies concerning the effect of O\({}_{2}\) scavenger in foods in the worst scenario, like in fish species subjected to oxidation-inducing treatments. [PERSON] et al. [9] observed that O\({}_{2}\) scavenger prevented the lipid oxidation induced by UV-C radiation  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multirow{2}{*}{**Treatments \({}^{\text{\text{\text@underline{c}}}}\)**} & \multicolumn{4}{c}{**Days of Storage**} \\ \cline{3-6}  & & **0** & **3** & **6** & **9** \\ \hline \multirow{6}{*}{Lipid oxidation (mg MDA \({}^{\text{\text@underline{V}}}\)/kg fish)} & AP & 0.117 \(\pm\) 0.010 \({}^{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\texttexttexttexttexttexttexttexttexttexttexttext  
in tilapia fillets stored under refrigeration, regardless of the dose (0.10 and 0.30 J/cm\({}^{2}\)), corroborating with our results. Nevertheless, it is worth highlighting that, when compared with tilapia, trout flesh has a higher amount of unsaturated fatty acids and is more biochemically unstable, and therefore is more prone to lipid oxidation [2; 3; 13].