## 3 Results and Discussion
### Effect of HP on the Physicochemical Characteristics of Sea Bass Fillets
It seems to  
Figure 3: (**a**) Values of lightness (L*) of untreated European sea bass fillets and after HP processing at different pressure–temperature–time conditions, (**b**) visual appearance compared to lightness (L*) values of untreated and HP-treated fish fillets, and (**c**) evolution of lightness (L*) during storage for fish samples treated at different pressure levels at 25 \({}^{\circ}\)C for 10 min. Each bar shows the mean and the standard deviation of four replicates (\(n=4\)). be recognized as the main issue in the commercialization of HP in the seafood industry and the reason why HP treatment is only used commercially in the muscles of fish with fairly white flesh, such as cod products (ALKORTA company in Basque Country, Spain). As depicted in Figure 3a, neither the temperature nor the processing time affected the color parameter of the lightness (L*) of the fish samples. These findings are in accordance with the results reported by [PERSON] et al. [17, 29] for cold smoked salmon and fresh gilthead sea bream treated with various combinations of pressure, temperature, and holding time (220, 250, and 330 MPa at 3, 7, 15, and 25 \({}^{\circ}\)C for 5 and 10 min). The color coordinates a and b (indices of redness and yellowness, respectively) were not affected by any of the processing parameters, including the pressure level. Finally, the color parameters did not change over the storage time except for small increases and decreases, which were likely related to sample variability (Figure 3c). As far as lipid oxidation is concerned, this was evaluated through the TBARs index, representing the formation of secondary oxidation products, with the maximum acceptable level set at 1 mg MDA/kg of product. Lipid oxidation developed only in the case where fish fills were aerobically stored, since no evolution of lipid oxidation was observed during the corresponding storage period for samples stored under vacuum conditions. As can be seen in Figure 4, the untreated sea bass fillets exhibited the highest TBARs values, followed by the samples treated at 100 and 200 MPa. Regarding the samples that were treated at pressures above 300 MPa, a lag phase in the development of secondary lipid oxidation products was observed. At the same time, these samples had significantly lower TBARs values than those treated at lower pressures (100-200 MPa), indicating the inhibitory effect of HP treatment on the formation of secondary lipid oxidation products. Previous studies have reported that the HP treatment of seafood at pressures above 300 MPa may lead to the inactivation of the endogenous prooxidative enzymes, which leads to oxidation development. The increased TBARs values of the samples that were treated below 200 MPa can be explained by the pressure-induced denaturation of iron-containing proteins, increasing the free metal iron [20, 57]. The formation of secondary oxidation products significantly increased during the storage time for all tested samples, following zero-order kinetics. The kinetic parameters of lipid oxidation were estimated through linear regression (Table 4, \(R^{2}\) > 0.95). The lipid oxidation rate of untreated fillets was significantly (\(p\) < 0.001) higher than that of the HP-treated fillets, exhibiting a similar shelf life to what was calculated based on microbiological data  
Figure 4: Evolution of lipid oxidation (in terms of malonic aldehyde concentration, mg MDA/kg product) of untreated European sea bass fillets and after HP processing at different pressures and ambient temperature for 10 min, and under aerobic storage. Each bar shows the standard deviation of four replicates (\(n\) = 4). considering a TBARs index level of acceptability of 1 mg MDA/kg product [33] (10 \(\pm\) 0.8 vs. 9.7 \(\pm\) 1.0 d based on microbiological and oxidation data, respectively).