The fish muscle structure provides the inherently soft fillet, especially during the cooking, as the connective tissue in fish can be solved at low cooking temperatures. However, compared to the mammalian fish muscle undergo less myofibrillar changes during the post-mortem process (Delbarre-Ladrat, et al., 2006). The post-mortem changes in beef and sheep muscle structure can be different with fish muscle in I band. (Taylor, Fjaera, & Skjervold, 2002a) observed an extensive break in costamers and I band of mammalian after 7 days while I band was almost un-change at this time. On the other hand, (Geesink, Morton, Kent, & Bickerstaffe, 2000; Olson, Jr, & Stromer, 1976) revealed that a significant break in purified myofibers of mammalian and fish during the post-mortem changes. It might be linked to the mechanical disruption of fish myofibers which are connected to the fish texture. Muscle disruption makes more unfolded proteins especially close to the tail which is prone more to cross-link aggregate. It can be the main reason that handling of fish during the rigor mortis is done carefully. During the first 24h after death the detachments of endomysium from myofiber demonstrated the role of these breaks on the textural parameters of fish fillet (Taylor, Fjaera, & Skjervold, 2002b). Endomysium in fish muscle, as a part of connective tissue is stable during the rigor-mortis changes but due to the activities of calpain enzyme, it is separated from the myofibers and beside the breaks of myocommata lead to gaping in fish fillet. Despite substantial investigates regarding the protein alteration during the post-mortem changes still some mechanisms of protein degradation is not clear. For example, R-connectin as a cytoskeletal protein can be converted to ?-connectin as it has been reported in carp and rainbow trout, which indicate the R-connectin degradation. (Tsuchiya, Kita, & Seki, 1992). Furthermore, the degradation of dystrophin has been commented in sea bass during storage at 4°C by (Papa et al., 1997), which indicates total degraded dystrophin protein in 48h after death.
As we have mentioned before troponin T has an important role in actomyosin connection. (Geesink, et al., 2000) observed a 31-kDa band after 7 days storage of salmon fillet at +4oC, which indicates it as a degraded product of troponin-T. In addition, electron microscopy investigation revealed that during the post-mortem changes, degradation of nebulin and titin in the Z-disc and M-line can disrupt the junction between myofibrils proteins (Taylor, Geesink, Thompson, Koohmaraie, & Goll, 1995).
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.