ICE CRYSTAL FORMATION IN PRESSURE SHIFT FREEZING OF ATLANTIC SALMON (SALMO SALAR) AS COMPARED TO CLASSICAL FREEZING METHODS

Abstract

Atlantic salmon were frozen either by pressure shift freezing (PSF) at 100 MPa (−8.4C), 150 MPa (−14C) and 200 MPa (−20C) or by conventional air freezing (CAF) at −30C and glycol/water bath freezing (GBF) at −20C. Temperature and phase transformations of fish were monitored during the freezing processes, and microstructures of ice crystals formed were evaluated for size, shape and location. The mean (± standard deviation) cross-section area of the ice crystals were: 11000 ± 7600, 280 ± 340, 260 ± 300, 63 ± 62, and 23 ± 22 μm2 for salmon subjected to CAF, GBF and PSF at 100, 150 and 200 MPa, respectively, as compared with that of the muscle fibers (7200 ± 2500 μm2). The roundness of the fish muscle fibers was 0.67 ± 0.07, while the ice-crystal roundness were: 0.38 ± 0.14, 0.55 ± 0.21, 0.57 ± 0.18, 0.63 ± 0.14 and 0.71 ± 0.14 for the salmon, respectively. CAF created larger and irregular ice crystals, and resulted in irreversible damage to muscle tissues. Due to its higher freezing rate, GBF produced smaller ice crystals than CAF, but the cross-section area and roundness values had larger deviations. The PSF process produced large amounts of fine and regular intracellular ice crystals that were homogeneously distributed throughout the salmon. Microscopic images clearly