Study On Frozen Denaturation Of Miofibrillar Protein Of Silver Carp (Hypophthalmichthys Molitrix) Muscle

Silver carp is a kind of important economic freshwater fish in China, with rich nutritionand huge production. As its marketing time is highly concentrated and fresh sales are limited,frozen storage is usually used as a long-term preservation method for further processing.Although various microorganisms and endogenous enzymes are efficiently reduced atdecreased temperatures, quality losses are inevitable, especially in texture, flavor and color.During freezing and frozen storage, the generation and development of ice crystal, increasedconcentration of cell solutes and so on, can cause frozen denaturation of muscle proteins.Storage temperature and time are main factors affecting frozen denaturation of muscleproteins from fish species. This reserach aimed at studying quality changes in muscle,physicochemical and structural changes of myofibrillar protein from silver carp underdifferent temperatures (-18℃,-50℃) and time (12weeks) of frozen storage, in order toimprove qualities of frozen carp products and provide theoretical supportings for utilizingmuscle proteins of silver carp effectively.Changes in quality characteristics of silver carp muscle during frozen storage were firststudied by measuring water retention ability, color, texture, gel-forming ability, thermalstability and other indicators. Quality losses of silver carp muscle occurred with the extensionof frozen time: the drip loss significantly increased and water holding capacity (WHC)decreased, indicating the longer the time of frozen storage, the worse the water retentionability of fish muscle would be. Compared with fresh silver carp, L*and b*values of frozensamples significantly increased, so that the acceptance of fish color reduced. During frozenstorage, the hardness of silver carp muscle declined by58.01%(-50℃) and69.61%(-18℃)respectively, and the texture became loose. The gel-forming ability reduced and gel quality ofmuscle was significantly worse. By the results of DSC and SDS-PAGE electrophoresis,myofibrillar protein was main protein denaturated in frozen storage.During frozen storage, denaturation of myofibrillar protein can cause a series of changesin physicochemical properties. Effects of frozen temperature and time on physicochemicalproperties of myofibrillar protein were investigated by analyzing protein content, Ca2+-ATPaseactivity, total sulfhydryl content, active sulfhydryl content and gel elasticity. Temperatures andtime of frozen storage had great impacts on physicochemical properties of myofibrillarprotein. With the extension of storage time, the protein content in fish muscle, Ca2+-ATPaseactivity, total sulfhydryl content, active sulfhydry content and gel elasticity of myofibrillarprotein all showed two-stage decreases; storage temperatures greatly affectedphysicochemical properties, the higher the frozen temperature, the quicker the rates ofphysicochemical changes. These physicochemical changes would lead to deterioration ofwater retention, texture, gelling and other quality characteristics, resulting in subsequentdeclines in qualities of processed products.Strutural changes in myofibrillar protein during frozen storage were determined by avariety of means liake DSC, SDS-PAGE, UV-absorption, CD, intrinsic fluorescence andRaman spectroscopy, in order to preliminary interpret the mechanism of frozen denaturation of proteins at the molecular level. DSC indicated that myosin was easier to change duringfrozen storage than actin, and the rod-like tail portion of myosin was more susceptible tofrozen denaturation than the two globular heads portion of myosin. CD and Raman spectraindicated an increase of β-sheets and random coil structures at the expense of α-helices, whichcaused a decline in tightness and stability in protein conformation. During frozen storage,when the unfolding of protein polypeptide chains occurred, some buried hydrophobic aminoacids (tyrosine, tryptophan) exposed, then protein aggregated through increased hydrophobicinteractions and disulfide bonds (formed by oxidation of active sulfhydryl groups duringfrozen storage), thus causing frozen denaturation of myofibrillar proteins from silver carpmuscle.During frozen storage at different temperatures, types of structural changes inmyofibrillar proteins of silver carp muscle were similar; however, magnitudes of changes intheir physicochemical characteristics and in the quality of fish muscle were significantlydifferent. Frozen denaturation of muscle proteins stored at-50℃(ultra-low temperature) wasmuch less serious than those stored at-18℃(conventional frozen temperature in China). Thisconfirmed that ultra-low temperature storage could significantly inhibit frozen denaturation ofmuscle proteins.