Improved processing procedures for sea-frozen deepwater redfish (Sebastes mentella)

Abstract

The deepwater redfish (Sebastes mentella) is a commercially important species in Iceland, often processed and frozen at sea. This study examined how different freezing methods, vertical and horizontal plate freezers, combined with antioxidant treatment (sodium erythorbate) or glazing, affect the visual quality and lipid stability of redfish during short-term frozen storage. The aim was to assess both quality differences and the feasibility of using vertical freezers, which may be more practical under the spatial and operational constraints onboard fishing vessels. Four groups were evaluated. Groups 1, 2, and 3 consisted of whole fish frozen using vertical plate freezers. Group 1 (control) was untreated, Group 2 was treated with sodium erythorbate, and Group 3 was glazed with ice. Group 4, processed in a horizontal plate freezer, was headed, treated with sodium erythorbate, packed in waxed boxes, and frozen, representing typical processing for that method. The color was assessed through sensory evaluation and CIELAB color measurements. Lipid degradation was evaluated using peroxide value (PV), free fatty acids (FFA), and thiobarbituric acid reactive substances (TBARS) measurements. Moisture and lipid content were also measured in muscle samples. All groups were also evaluated using hyperspectral imaging (HIS). Results from CIELAB and sensory evaluation showed significant differences in skin color between groups. Group 4 (horizontal freezer with antioxidant treatment) had the most favorable outcome, while Groups 2 and 3 (vertical freezer with antioxidant or glazing) showed slightly lower but acceptable color grades. Group 1 (vertical freezer, untreated) exhibited the most discoloration, differing significantly from Group 4 in both sensory and instrumental evaluations. However, lipid degradation and chemical composition showed no significant differences across groups during the storage period. HSI-based models were not able to reliably distinguish the tested quality parameters using the current analytical approach and, therefore, could not accurately predict color grading. The findings suggest that antioxidant use, and appropriate freezing methods can improve redfish quality, and vertical freezing, and vertical freezing combined with glazing or use of antioxidants may be acceptable for market grading. This may support more efficient onboard processing and competitiveness, though further research on long-term storage is recommended.