Changes in the texture and structure of cod and haddock fillets during frozen storage

Changes in the texture of commercially important lean cod (which produces substantial formaldehyde), and haddock fillets (which produces negligible formaldehyde), stored at −10 and −30°C for up to 30 weeks were measured. For both species, hardness measured by large deformation rheology and elastic modulus (G′) by small deformation techniques, which reflect muscle toughening, increased with a higher storage temperature and prolonged time of storage, in a similar way. Differential scanning thermal analysis of both frozen cod and haddock muscle indicated alteration in the transition temperature and enthalpy due to a higher rate of protein denaturation at −10°C compared with muscle stored at −30°C. Protein denaturation and texture changes were accompanied by a decrease in protein solubility, an increase in hydrophobicity and the formation of non-covalently and covalently linked aggregates. The similarities in the texture and biochemical changes in both frozen cod and haddock indicates that formaldehyde is not a major factor in muscle toughening in frozen fish.