Fish muscle is organized as tissue blocks of myomeres attached to myocommata. This structure makes the flesh inherently soft, especially when cooking, because fish connective tissue is soluble at low cooking temperatures. However, there is very little structural change in fish myofibrils post-mortem, and in fact they are much more stable than mammalian. Beef and sheep structural changes are well characterized (Taylor, Geesink, Thompson, Koohmaraie, & Goll, 1995) and show significant breaks in I bands and costameres after 7 days of storage. In marked contrast, fish I bands are almost never broken (Taylor, Fjaera, & Skjervold, 2002). As mentioned above the cytoskeletal proteins of fish and mammals are degraded within days post-mortem, so it is surprising that fish myofibrils are structurally stable. A possible explanation is that these results are based on electron microscopy of carefully handled samples. In fact, when fish (Geesink, Morton, Kent, & Bickerstaffe, 2000) and mammalian (Olson, Jr, & Stromer, 1976) myofibers are purified and the myofiber breaks quantified, both show extensive breaks post-mortem. Therefore, mechanical disruption is needed to demonstrate the fragility of fish myofibers. The myofiber fragmentation index correlates with fish texture. Both fish (Papa et al., 1997) and mammals (Taylor, et al., 1995) show myofiber to connective tissue (endomysium) detachments within 24h post-mortem. Quantification of these breaks demonstrates that they are associated with fish fillet texture, and probably account for much of the early texture changes (Taylor, et al., 2002). As mentioned above the costamere degradation and endomysium detachment is due to calpain activity against cytoskeletal proteins. The connective tissue of mammal and fish, especially the endomysium, is very stable post-mortem, but it is detached from the myofibers. Endomysium detachment is due to cytoskeletal breaks as mentioned above, but not to degradation of the connective tissue. The endomysium, at least in mammals, is also heat stable. Perimysium shows some weakening by scanning electron microscopy of muscle extracts but it is rare to see extensive breaks. There are breaks within the myocommata after about 5 days of storage, breaks which are associated with gaping (Bremner, 1992). The myocommata is also mechanically weakened during storage, and is very heat labile (Lavety, Afolabi, & Love, 1988). It is the fragility of the connective tissue which contributes to fillet gaping and long term storage texture changes.
Known post-mortem protein changes in fish muscle include the degradation of some large cytoskeletal proteins, whereas the sarcoplasmic protein profiles seem to be rather unaffected by spoilage (Rehbein, 1997). Despite substantial and intensive studies, there are still many uncertainties regarding protein changes during deterioration. For instance, cytoskeletal protein such as, R-connectin, is converted to ?-connectin and a 400 kDa fragment, thereby indicating the degradation of R-connectin. In carp muscle R-connectin is completely converted into ?-connectin within 2 days at 0 °C, whereas in rainbow trout muscle this conversion of R-connectin is slower and not complete after 4 days (Tsuchiya, Kita, & Seki, 1992). A rapid degradation of dystrophin has been reported in sea bass muscle stored at 4 °C (Papa, et al., 1997), with a total degradation of the protein 48 h after death. Degradation of nebulin has also been reported (Papa, et al., 1997; Tsuchiya, et al., 1992). During 7 days of storage at 5 °C of salmon a 31-kDa band appeared, which has been interpreted as a troponin-T degradation product (Geesink, et al., 2000). This is supported by the fact that titin, a giant muscle protein spanning from the Z-line to M-line region, and nebulin, which runs parallel with the thin filaments to the Z-line (Bandman & Zdanis, 1988), both have been shown to degrade faster in tender meat than in tough meat (Fritz & Greaser, 1991; Hufflonergan, Parrish, & Robson, 1995). Moreover, electron microscopy studies of the myofibrils during tenderization have shown that the attachment of the sarcolemma to myofibrils and the junctions between myofibrils at the level of the Z-disk and M-line are disrupted post-mortem (Taylor, et al., 1995), which suggests that proteolysis of the costamere proteins desmin, dystrophin, and vinculin, which attach the myofibrils to the sarcolemma, is taking place.
In general, post-mortem degradation of muscle proteins is an important factor in the meat tenderization process (Koohmaraie, 1996), as post-mortem degradation of several structural proteins including troponin T, nebulin, titin, vinculin, desmin, dystrophin, and troponin T has been demonstrated using one-dimensional SDS-PAGE and immuno-blotting (Hopkins & Thompson, 2002). Thus, differentially altered protein could be used as a biochemical marker of muscle degradation and textural change for predicting freshness and quality of fish by proteomic methodologies.