Sustainable Valorization of Farmed Atlantic Salmon Processing By-Products: Supercritical CO2 Extraction of Lipid Fraction and pH-Shift Recovery of Proteins from the Residual Material

Farmed Atlantic salmon skin-on trimmings were valorized through a sequential process combining supercritical carbon dioxide (SC-CO2) extraction with protein recovery from the resulting defatted residue. Freeze-dried material was extracted at 40 °C and 100, 200, and 300 bar for 540 min to evaluate the effect of pressure on extraction yield. The highest yield among the tested conditions was obtained at 300 bar (28.08%), and this extract was selected for further characterization. It contained 98.55 g/100 g lipids, with oleic acid as the predominant fatty acid, together with EPA, DHA, and astaxanthin (215.43 µg/mL), and showed concentration-dependent ABTS radical-scavenging activity (IC50 ≈ 109 mg/mL). The corresponding defatted biomass was subsequently subjected to conventional and ultrasound-assisted pH-shift protein recovery. Ultrasound assistance increased protein yield from 36.70 to 47.91 g/100 g. Both isolates contained more than 91 g/100 g protein and had similar amino acid profiles, with essential amino acids accounting for approximately 41% of total amino acids. Ultrasound treatment also improved protein solubility, water-holding capacity, and ABTS radical-scavenging activity, whereas the conventionally recovered isolate showed higher oil-holding capacity. These results support the sequential recovery of lipid- and protein-rich fractions from the same salmon-processing by-product.