Gelation properties and interactions of fish proteins

Abstract

This chapter compares the rheological properties of fresh and frozen lean, and fatty fish and relates the rheological changes to physico-chemical properties. It also identifies biochemical mechanisms responsible for texture changes. Major differences in texture are observed by small deformation rheology for lean fish cod and haddock. The texture changes were attributed to the denaturation of the myofibrillar proteins, particularly myosin, as assessed by differential scanning calorimetry. Protein denaturation leads to a progressive increase in the formation of salt-insoluble, SDS-insoluble and SDS-β-mercaptoethanol insoluble aggregates with increased time and temperature of storage, indicating the presence of strong covalent linkages on prolonged storage of fish. Differences in the amino acid composition of the aggregates are also observed, confirming the presence of myosin in the aggregates. In addition to the isolation of aggregates in model systems, covalently linked aggregates in whole fillets are also visualized by atomic force microscopy and electron microscopy/immunocy to chemistry. However, after one year of storage, differences are not exhibited between cod, a fomaldehyde producing lean fish and haddock, which is also a gadoid fish that produces negligible formaldehyde. Although formaldehyde may be important in the early stages of storage in the formation of the non-covalently bound aggregates other factors play a major role in the toughening of fish during frozen storage. The results strongly indicate that texture changes in frozen fish may be attributed primarily to changes in fish due to freezing conditions and also to protein–lipid interactions.