Quality Changes Of Grass Carp Muscle During Partial-frozen And Iced Storage

In China, with the rapid development of aquaculture, cold chain, supermarket and retailmarket, the changes in national consumption concept and the improvement of living standards,sales of fresh aquatic products produced into fish fillets, chunks and steaks in small packagesare also growing. Moreover, the demand for aquatic products of high freshness and security inmarket is also increasing. Therefore, timely and effective preservation technology isparticularly important.The objective of the present study was to make a thorough comparision about the effectof partial-frozen and iced storage on the quality of grass carp, the typical fresh water fishspecies in China, mainly by biochemical and physical changes during storage. To evaluate thebiochemical properties, we measured the protein structure, autolytic degredation products(ADP), pH value, total volatile basic nitrogen compounds (TVB-N). And to assess thephysical properties, the indexs of drip loss, water holding capacity (WHC), microscopicalhistology, macroscopical appearance, color and texture were determined.Comared with iced storage, partial-frozen storage can significantly delay the changes inbiochemical properties, such as pH and ADP values, and microbial spoilage. The shelf life ofthe first grade freshness by TVB-N value was3and1d for the partial-frozen and iced fishfillets, respectively. After21d of partial-frozen storage, the TVB-N value was still far belowthe threshold of second grade freshness; however, after15d of iced storage, the TVB-N valuewas closed to the the threshold of second grade freshness. Under both storage conditions,slight degradation of muscle proteins was observed; however, the extractability of salt solubleproteins initially increased, then remained stable, and finally decreased, and the insolublefractions formed aggregates through non-covalent bonds, which can be solubilzed by heatingin5%SDS. In addition, partial-frozen and iced storage affected the higher structure of themajor proteins in fish muscle in different ways: a shoulder peak appeared just before thethermal transition peak corresponding to myosin and then became more and more significantduring the following storage for the partial-frozen fish fillets, while for the iced fish fillets, themyosin peak became more and more sharp during storage; the peak corresponding tosarcoplasmic proteins remain almost unchanged during storage for the partial-frozen fishfillets, while for the iced fish fillets, sacoplasmic protein peak disappeared gradually duringstorage; under both storage conditions, the peaks corresponding to actin remained almostunchanged.Compared with the partial-frozen storage, iced storage can significantly slow down thechanges in physical properties, such as drip loss, WHC, whiteness and texture. Partial-frozenand iced storage affected the muscle tissue structures in different ways. From themacroscopical point of view, white spot was observed after10d of storage and became moreand more obvious during the following storage for the partial frozen fish fillets; while for theiced fish fillets, the color of grass carp muscle changed from transparent white to milky whiteduring storage. From the microscopical perspective, breaks within and between the myofiberswere observed during the initial6d of storage, hollow spot within the myobiber appeared after10d of storage, breaks between the myofibers and hollow spots within the myofibersbecame larger and larger during the folloing storage, and breakages of myofibers occurredafter21d of storage for the partial-frozen fish fillets; while for the iced fish fillets, breakswithin and between the myofibers were also observed during storage, but they were smallerthan the partial-frozen fish fillets by a day to day comparision. Moreover, separation betweenthe sarcolemma and myofiber was observed after21d of iced storage.