###### Abstract  
The pressurized liquid extraction (PLE) technique was used to obtain protein extracts with antioxidant capacity from salmon muscle remains, heads, viscera, skin, and tailfins. A protein recovery percentage \(\approx\)28% was obtained for all samples except for viscera, which was \(\approx\)92%. These values represented an increase of 1.5-4.8-fold compared to stirring extraction (control). Different SDS-PAGE profiles in control and PLE extracts revealed that extraction conditions affected the protein molecular weight distribution of the obtained extracts. Both TEAC (Trolox equivalent antioxidant capacity) and ORAC (oxygen radical antioxidant capacity) assays showed an outstanding antioxidant activity for viscera PLE extract. Through liquid chromatography coupled with electrospray ionization triple time-of-flight (nanoESI qQTOF) mass spectrometry, 137 and 67 peptides were identified in control and PLE extracts from salmon viscera, respectively None of these peptides was found among the antioxidant peptides inputted in the BIOPEP-UMP database. However, bioinformatics analysis showed several antioxidant small peptides encrypted in amino acid sequences of viscera extracts, especially GPP (glycine-proline-proline) and GAA (glycine-alanine-alanine) for PLE extracts. Further research on the relationship between antioxidant activity and specific peptides from salmon viscera PLE extracts is required. In addition, the salmon side streams studied presented non-toxic levels of As, Hg, Cd, and Pb, as well as the absence of mycotoxins or related metabolites. Overall, these results confirm the feasible use of farmed salmon processing side streams as alternative sources of protein and bioactive compounds for human consumption. prematized liquid extraction; salmon; side streams; peptides; protein; SDS-PAGE; antioxidant capacity; mycotoxins; heavy metals +
Footnote †: journal: Applied Soft Matter  
make them candidate substrates for protein and oil recovery. Salmon side streams also showed relevant levels of essential amino acids (21-35%) as well as oleic acid (39-42%) and omega-3 fatty acids (19-21%) [5; 6]. In addition, peptides with functional and bioactive properties are also found in several marine side streams [7; 8; 9]. For instance, peptides from salmon trimmings and pectoral fins have exhibited antihypertensive and antioxidant activities [4; 10]. Antioxidant peptides from the viscera of sardinella, black pomfret, and mackerel have also been reported [9]. Therefore, salmon side stream materials could be considered a promising source of valuable compounds from the European circular economy point of view [11]. The valorization of seafood discards has been gaining attention over the last years, as their nutritional and bioactive compounds can now be extracted more efficiently using green technologies [12; 13]. Pressurized liquid extraction (PLE) is currently considered an environmentally friendly technique to recover bioactive compounds from food matrices, as water is the most preferred solvent for the extraction process [14]. PLE is based on the use of high pressure and temperature to improve the extraction performance [15; 16]. The possibility of applying different extraction conditions has made PLE a useful tool to optimize the extraction of high-added-value compounds from a wide variety of matrices, including marine sources and related side streams. For instance, PLE-assisted extraction was recently used to obtain aqueous protein extracts with in vitro antioxidant capacity from several side streams of rainbow trout, sole, sea bass, and sea bream [17; 18; 19]. Protein extraction from macro- and micro-algae using PLE has been also investigated [20]. In addition to healthy nutritional properties, any starting material that can be used in the food industry must be free of potentially harmful substances. In this sense, farmed fishes can be exposed to mycotoxins from plant-based feed [21; 22], as well as toxic metals from the aquaculture environment [23].