| Silver carp (Hypophthalmichthys molitrix) is a freshwater species living in temperate conditions (6-28℃) and its natural distribution is in Asia. It is generally cultured and marketed locally alive or fresh in most of the producing countries. China is the largest producer of silver carp in the world; however, its market price is relatively low compared to most other species, normally costs 8-10Yuan/kg. In order to keep the product longer and further extend the markets instead of just consuming it fresh, processing technologies for adding value and frozen storage are of significantly important for this fish species.In this thesis, the report is mainly focused on the study done on isolation of myofibrillar protein from silver carp; exposing the isolated myofibrillar protein to oxidative environment simulated using an iron oxidation model system to mimic the oxidative stress during processing and frozen storage. Also the impacts of the iron oxidation model system, fatty acid oxidation and frozen storage on protein functionality of silver carp myofibrillar protein isolate were evaluated.Oxidative damage to silver carp myofibrillar proteins isolate (MPI) was investigated by measuring changes in physico-chemical, and functional properties after exposure to iron-catalyzed oxidation system (IOS). Iron oxidized MPI exhibited an increase in carbonyl content and dityrosine, which were significant and negatively correlated with protein solubility (PS) (r=0.85), (r=0.80) and gel strength (GS) (r=0.95), (0.93), respectively, however the decrease in total thiol group content was significantly and positively correlated with PS (r=0.77) and GS (r=0.89). These led to significant changes (P<0.05) observed in the protein and functional properties of oxidized MPI. Sodium dodecyl sulfatepolyacrylamide gel electrophoresis demonstrated that IOS resulted in a major loss of myosin and actin associated with formation of protein polymers as supported by gel-permeation chromatography (GPC) results. The total fat content of silver carp MPI was 0.6%, it is mainly composed of unsaturated fatty acids of which about 34%constituted by eicosapentaenoic acid, (20:5ω-3), docosahexaenoic acid, (22:6ω-3), and linolenic, (18:3ω-3) as the mainω-3 poly-unsaturated fatty acids (PUFAs) as well as linoleic acid, (18:2co-6) and arachidonic acid, (20:4ω-6), the mainω-6 PUFAs in fish, unfortunately PUFAs are prone to oxidation producing reactive oxygen species capable of modifying protein structure. This study suggests that the decreased functionality of proteins in muscle foods exposed to an oxidative environment could be due to chemical and physical changes resulting from oxidation reactions.It was found that silver carp myofibrillar protein is susceptible to an iron-catalyzed oxidation causing a significant loss of its functionality, thus a study was carried out to determine the susceptibility of silver carp protein to natural oxidants (fatty acids or fatty acids oxidation products) in frozen storage with respect to its functionality. Frozen stored myofibrillar protein isolate with 0.6MNaCl or myofibrillar protein isolated from previous frozen stored whole fish and fish mince of silver carp at-18℃for 90 days were assessed for protein and lipid oxidation with regard to protein functionality. The addition of sodium chloride to 0.6M at pH6.5 improved protein functionality especially water holding capacity of frozen stored myofibrillar protein isolate from 5.3 to 6.4mL/g MPI after 90 days of frozen storage. However, the differential scanning calorimeter results showed that, sodium chloride significantly increased thermal susceptibility of myosin from 48.12 to 46.40℃with 0.1MNaCl and 0.6MNaCl frozen stored for 90 days, respectively. Whole fish frozen storage was more susceptible to oxidation compared to fish mince and myofibrillar protein isolate due to their different lipid content. On contrary, myofibrillar isolated from frozen whole fish showed significant changes in protein functionality may be due to great loss of amino acid such as cysteine, lysine, histidine and methionine during frozen storage.On the other hand investigation was done on stabilization and oxidation protection of silver carp MPI stored at-18℃,90 days and composed of MPI,8% cryoprotectants (4% sucrose and 4% sorbitol) with or without antioxidants (0.2% ascorbate,0.2%α-tocopherol, or their combination) and packed in a polyethylene bag, sealed under air. MPI without cryoprotectants and antioxidants was the control. Compared with the control, cryoprotectants increased PS (protein solubility), WHC (water holding capacity), OHC (oil holding capacity), EC (emulsification capacity) and GS (gel strength), also, the cryoprotectants and/or antioxidants decreased MPI oxidation susceptibility as well as susceptibility of myosin to thermal denaturation. After 30 days of frozen storage, there were no significant differences. ((P>0.05) of storage time on alteration of protein functionality and actin susceptibility to thermal denaturation between cryoprotectants and the control. Antioxidants minimized oxidation effects on MPI frozen storage.Therefore, the present study reveals that, improvement in the functional properties of silver carp MPI were greatly influenced by cryoprotectants and antioxidants, this reflect a great role played by sucrose/sorbitol and antioxidants in protecting native protein structure to a greater extent owing to impact of frozen storage and oxidation respectively.
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