The effect of fish freshness on myosin denaturation in flounder Paralichthys olivaceus muscle during frozen storage

The effect of fish freshness on myosin denaturation during the frozen storage of flounder muscle was studied. The muscle of instantaneously killed flounder was immediately stored at − 20 °C (pre-rigor mortis; day 0), or after storage at 0 °C for 2 days (rigor mortis) or 5 days (post-rigor mortis). Myosin denaturation in frozen stored muscle was studied by measuring Ca2+-ATPase activity, myosin solubility in the presence of salt (salt solubility), and monomeric myosin content. High ATPase activity was maintained in the day-0 samples of instantaneously killed flounder during the early phase of frozen storage. High salt solubility of myosin was also observed in these samples. However, the beneficial effect of frozen storage was lost when the storage period exceeded 2 months. There was no difference in myosin denaturation among the three groups of samples after 2 months of frozen storage. The myosin denaturation profile during the frozen storage of muscle was characterized by high salt solubility in the presence of Mg-ATP, even after ATPase inactivation, especially in the day-0 samples. The salt-soluble fraction contained a large amount of aggregated myosin, as revealed by (NH4)2SO4 fractionation. The monomeric myosin content was explained by Ca2+-ATPase inactivation in the frozen stored meat for all three groups. This study demonstrated that the high quality of flounder meat is maintained during short-term frozen storage, but that the beneficial effects of frozen storage are lost over longer periods of time.