## 2 Results and Discussion  
### Total Antioxidant Capacity  
The results of total antioxidant capacity, determined using the Trolox equivalent antioxidant capacity (TEAC) and oxygen radical antioxidant capacity (ORAC) methods in control and PLE extracts of salmon side streams, are shown in Figure 1. TEAC values in PLE extracts were 734 \(\pm\) 38, 472 \(\pm\) 7, 3739 \(\pm\) 209, 147 \(\pm\) 37, and 704 \(\pm\) 42 \(\mu\)M Trolox Equivalents (Eq) for muscle, head, viscera, skin, and tailfins, respectively, whereas TEAC values in the corresponding control extracts were 776 \(\pm\) 32, 322 \(\pm\) 18, 778 \(\pm\) 26, 206 \(\pm\) 12, and 324 \(\pm\) 22 \(\mu\)M Trolox Eq. Regarding the ORAC assay, the values of total antioxidant capacity were higher in PLE extracts than in control extracts for all samples. ORAC values (\(\mu\)M Trolox Eq) in PLE extracts were 4586 \(\pm\) 241 (muscle), 3567 \(\pm\) 63 (heads), 7772 \(\pm\) 1174 (viscera), 1244 \(\pm\) 94 (skin), and 2620 \(\pm\) 78 (tailfins), whereas control ORAC values were \(3005\pm 217\), 797 \(\pm\) 73, 2451 \(\pm\) 139, 599 \(\pm\) 19, and 736 \(\pm\) 39, respectively. Therefore, PLE-assisted extraction improved the antioxidant capacity (ORAC) compared to conventional extraction for all salmon side streams. The increases were 1.5-, 4.5-, 3.2-, 2-, and 3.6-fold for muscle, head, viscera, skin, and tailfins, respectively. As for TEAC, the antioxidant capacity of PLE extracts also increased compared to the controls for head (1.5), viscera (4.8), and tails (2.2), whereas the muscle and skin values remained without significant changes. The highest antiradical activity was observed in PLE extracts of viscera for both antioxidant assays. These results are slightly different to those obtained for PLE extracts of sea bass and sea bream by-products, in which muscle PLE extracts showed the highest values of antioxidant capacity determined by both TEAC and ORAC methods [18; 19]. The antioxidant capacity of viscera PLE extracts from sea bass and sea bream were similar to those of head PLE extracts. These differences may be due to the fact that seabass and sea bream are a more closely related species compared to salmon.  
On the other hand, the different antioxidant capacity exhibited by the protein extracts obtained is probably related to both the size and the amino acid composition of the protein fragments of each salmon side stream. Several authors have suggested that hydrophobic  
Figure 1: Total antioxidant capacity determined by TEAC and ORAC in control and PLE extracts from salmon muscle, head, viscera, skin, and tailfin. TEAC: trolox equivalent antioxidant capacity. ORAC: oxygen radical absorbance capacity. PLE: pressurized liquid extraction. \(\upmu\)M Trolox Eq (micromolar trolox equivalent). Results of TEAC (n = 3) and ORAC (n =6) are expressed as mean \(\pm\) standard deviation. Different lowercase letters in the bars indicate statistically significant differences (\(p<0.05\)) among samples.  
amino acids could contribute to the total antioxidant activity of protein fragments [7; 9]. In this way, glycine and glutamic acid have been reported as the most abundant polar amino acids in salmon heads, skin, and viscera [5]. Hydrophobic amino acids such as alanine, proline, leucine, and valine were also found in relevant quantities. In addition, the molecular weight of fish peptides (0.5-1.5 kDa) has been associated with antioxidant properties [7; 26]. According to this, the outstanding antioxidant capacity shown by PLE viscera extracts could mean the presence of bioactive peptides with some of the aforementioned amino acids in their sequence.