## 3 Results and Discussion
### Volatile Compounds and Relationship to Consumer Evaluation of Freshness
This is again in accordance with the preference of the consumers who chose MAP-PAD seabass fillets, at both high quality and end of high-quality shelf-life time points, preferring these to MAP samples. Aldehydes, such as decanal and octanal, are present in the same quadrants of the PCA with seabass fillets at the high quality shelf-life time point, and indeed are characteristic of them (Figure 3) as also reported in previous research [41; 42]. Specifically, decanal is negatively correlated with hexanal, 2,3-pentanedione, and 1-penten-3-ol, which are associated with fish rancidity [43]; in addition, octanal is positively correlated with MAP samples at the high quality time point and with consumers' perceived freshness, suggesting their potential as freshness markers of seabass. The presence of Z-4-heptenal, hexanal, 2,3-pentanedione, 3-hydroxy-2-butanone, 1-octen-3-ol, and 2-penten-1-ol in the 4 th quadrant of the PCA plot of seabream fillets and respectively, hexanal, 2,3-pentanedione, and 1-penten-3-ol in the 1 st quadrant of the PCA plot of seabass fillets, indicates that these VOCs are most closely associated with the late days of storage (End) and mainly with MAP samples (Figures 2 and 3); this is also justified by the fact that these compounds are involved with oxidation of polyunsaturated fatty acids [43]. In confirmation, these compounds were indeed found to increase (\(p<0.05\)) or tend to increase their individual concentrations with storage (Figures 4 and 5, Supplementary Tables S7 and S8). These compounds are hereafter referred to as target compounds. Moreover, similar findings have previously been found for them in a study including various time storage intervals [18]. Z-4-heptenal, found in this study to be a target volatile for gilthead seabream, is responsible for off-flavor in fish. It can be either derived from lipid oxidation of n-3 PUFA or produced via 2,6-nonadienal that is produced by the action of 12-lipoxygenase on eicosapentaenoic acid (EPA, 20:5n-3) [44,45], and has already been identified in gilthead seabream [46]. Hexanal, found here to be an important freshness determinant for both species, has already been identified in European seabass and proposed as a spoilage marker by [PERSON] et al. [41]. Another characteristic target compound in both species associated with fish rancidity is 2,3-pentanedione, which is formed during storage [43], as is the case for 3-hydroxy-2-butanone [47]. The 1-octen-3-ol is a noteworthy contributor to off-flavors due to its low odor score, and has been reported to derive from the oxidation of arachidonic acid by 12-lipoxygenase [43,44]. As for 1-penten-3-ol, it is a characteristic alcohol detected in many fish species [47,48,49] and is produced during storage from EPA following reactions with 15-lipoxygenase and hydroperoxide lyases [48]. Furthermore, it is described as giving a fish-like odor to fish products during storage [50]. Moreover, 2-penten-1-ol has been recently  
Figure 4: Concentration (ng/g) of target compounds identified via HS-SPME-GC-MS analysis in seabream fillets, packed in active packaging with (MAP-PAD) or without (MAP) a pad, at the high quality (HQ) time point or the end of high-quality (End) shelf-life. (*) stands for \(p\) < 0.05; (**) stands for \(p\) < 0.01; (***) stands for \(p\) < 0.001. Figure 5: Concentration (ng/g) of target compounds identified via HS-SPME-GC-MS analysis in seabass fillets, packed in active packaging with (MAP-PAD) or without (MAP) a pad, at the high quality (HQ) time point or the end of high-quality (End) shelf-life. (*) stands for \(p\) < 0.05; (**) stands for \(p\) < 0.01; (***) stands for \(p\) < 0.001.  
characterized as a potential spoilage marker for red seabream due to its increasing levels during storage [51] and has also been identified as a potent odorant in raw sandine [52]. Finally, among all conditions, the highest total average VOC content was observed in packaging without pad at the end of high quality, as expected (Supplementary Figure S1).