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The fish industry has a significant quality problem due to the loss of fillets as a result of texture problems. Management of texture problems has not been successful due to the lack of characterization of muscle changes associated with fish fillet texture, and lack of knowledge of fish muscle proteases postmortem. Within the SEAFOODplus project 4.3 ‘LIPIDTEXT’ the relationship of fillet composition (especially lipid content) to texture will be examined and which proteases cause soft texture problems will be determined.

The fish industry has a significant quality problem due to the loss of fillets as a result of texture problems. The economic consequence of salmon fillet texture problems, for example, is about 6% of total revenue amounting to approximately 40million €/yr. Other species for the fresh fillet market also suffer losses due to soft texture and gaping, including various species such as cod, tilapia and sea bass. Management of texture problems has not been successful due to the lack of characterization of muscle changes associated with fish fillet texture, and lack of knowledge of fish muscle proteases postmortem. Within the SEAFOODplus project 4.3 ‘LIPIDTEXT’  the relationship of fillet composition (especially lipid content) to texture will be examined and which proteases cause soft texture problems will be determined. Richard Taylor (INRA) and Flemming Jessen (DIFRES) are the responsible scientists for the research.

Preliminary Results

It has been verified that the cathepsin fluorescent substrate activity used for rat and cattle works on rainbow trout, brown trout and salmon. Activity of fish cathepsins B+L was approximately 10 times that found in bovine muscle and equal to rat muscle activity, and fish cathepsin B activity was 10 times bovine and 5 times rat.

Most interesting is that in mammals cathepsin is in the lysosomal fraction, whereas in fish approximately 90% was non-lysosomal. Some localization has been done by electron microscopy cytochemistry and there is usually cathepsin activity associated with lipid droplets (and much more lipid in fish muscle than mammalian), and also unusual extra-cellular lamellar lipid. Previous work has shown that fish connective tissue is degraded within days postmortem, and that connective tissue changes are associated with soft texture.
These results indicate that fish connective tissue may be rapidly degraded by the soluble cathepsins during postmortem storage, and that the lipid content and localization of lipid within the muscle is important in this process.

Figure: Two hour postmortem trout muscle showing sarcomeres (Sarc) and large lipid droplets (Lipid) in the extracellular space. The arrows indicate acid phosphatase activity as demonstrated by the black precipitate.