Effect of different freezing processes on the microstructure of Atlantic salmon (Salmo salar) fillets

Salmon fillets were frozen either by pressure shift freezing (PSF, 200 MPa, − 18 °C or 100 MPa, − 10 °C) or by air-blast freezing (ABF, − 30 °C, 1 m/s or 4 m/s) or direct-contact freezing, and then stored at − 20 °C for 6 months. The influence of these treatments on the microstructure of Salmon fillets was studied. The equivalent diameter of the intracellular ice crystals were 14.69 ± 4.11, 5.52 ± 2.11, and 30.65 ± 6.31 μm for the samples subjected respectively to PSF at 100, 200 MPa and ABF (− 30 °C, 4 m/s) after 2 days of storage. Smaller and more regular intracellular ice crystals were observed in fillets frozen by PSF (200 MPa) compared with PSF (100 MPa), ABF and direct-contact frozen ones. Significant differences were observed between the size of the ice crystals obtained after conventional freezing process and PSF. Large and extracellular ice crystals were observed in fillets frozen by ABF (1 m/s) and direct-contact frozen. Minimal changes in the size of ice crystals were observed during a 3 months storage.

Industrial relevance

This paper compares different freezing methods and subsequent frozen storage with respect to their effect on microstructures of salmon fillets. Pressure shift freezing at 200 MPa was superior to conventional freezing regarding small and regular ice crystal formation. Interestingly, during frozen storage for up to 3 months the high quality product obtained via pressure freezing at 200 MPa could be retained. For longer storage periods lower pressures (100 MPa) seem sufficient to achieve stable ice crystals.