Protein Oxidation-induced Structure And Function Changes Of Common Carp (Cyprinus Carpio) Protein And Its Control Technology

The carp muscle is the research object in this study, the influence of the hydroxylradical-generating system (HRGS)(consisting of0.1mM FeCl3,0.1mM ascorbic acid and differentconcentrations H2O2) on the physicochemical properties, protein structural and functionalproperties of myofibrillar protein (MP) were detailed studied, and the effect of protein oxidationinduced by different washing conditions (unwashed, traditional washing, adding sodium ascorbatewashing, adding PG washing), and adding cryoprotectants（4%sucrose+4%sorbitol） and wheyprotein hydrolysate (control, adding the commercial cryoprotectans, adding cryoprotectans+PG,adding cryoprotectans+WPI hydrolysates) on the structural and functional properties ofmyofibrillar protein were also studied, in order to illustrate the mechanism of fish qualitydeterioration and protein denaturation caused by oxidation. The carbonyl and the bityrosinecontent, the total sulfhydryl content, the ATPase activity, the surface hydrophobicity, DSC andSDS-PAGE were adopted to study the structural changes of myofibrillar proteins. The solubility,turbidity, emulsifying capacity, gel properties (TPA, whiteness, water-bolding capacity, gelmicrostructure), gel rheological properties, were adopted to study the functional properties and gelproperties changes of myofibrillar protein. The results are as follows:1. Protein structure was significantly changed after being oxidised by HRGS. After oxidizingfor5h at0.1,1,5,10,20mM H2O2, the carbonyl content were increased by28.6%,47.6%,61.9%,71.4%and83.3%(P<0.05) contrast to control，the total sulfhydryl content decreased11.1%,19.2%,25.6%,37.9%and49.4%(P<0.05), respectively. In addition, the bityrosine content and thesurface hydrophobicity were increased with the increasing H2O2concentration and oxidation time.Protein thermal stability is determined by DSC. The result is: after oxidation, the thermal transitiontemperature and enthalpy values of myofibrillar protein decreased significantly, which indicatedthat oxidation affected protein thermal stability seriously. The SDS-PAGE electrophoresis patternsalso showed that the structure of oxidized protein was changed, which was proved by highmolecular weight compounds that could not penetrate the gel network appeared with increasingH2O2concentrations in the SDS-PAGE patterns of the oxidised samples. The above results showed that the structure of protein was damaged by HRGS.2. The protein structure was changed by protein oxidation, and which have some effects onthe functional properties. After oxidizing for5h at0.1,1,5,10,20mM H2O2in HRGS, the proteinsolubility were decreased by12.7%,19.5%,28.9%,34.3%and44.7%(P<0.05) contrast to control,emulsifying activity decreased by1.0%,4.8%,14%,18.5%and26.1%(P<0.05), emulsionstability decreased by1.3%,4.6%,12.4%,26.7%and31.1%(P<0.05). The protein turbidity levelsalso increased with the increasing H2O2concentration and oxidation time, indicating that MP waspolymerized.3. The gel properties of myofibrillar protein were also studied. The results showed that thetexture parameters (hardness, elasticity) of myofibrillar protein gel were reduced by proteinoxidation, and rheological parameters G′（storage modulus），G′′（loss modulus）also decreasedsignificantly. At the same time, the water binding capacity, whiteness of myofibrillar protein gelare also significantly reduced, the gel microstructure by electron microscopy further validated thisresults. After oxidizing for5h at0.1,1,5,10,20mM H2O2, the gel elasticity decreased by10.3%,13.2%,14.3%,22.2%and23.5%(P<0.05)， the gel hardness decreased by19.0%,24.6%,46.7%,50.7%and60.9%(P<0.05). The changes of the water binding capacity and the whiteness of gelswere as the same trend. The gel microstructure of fresh muscle protein is high-quality dense,homogeneous three-dimensional network structure, after oxidation，the gel structure is loose, thenetwork size increased, rough, broken, or even small clumps. With the increasing oxidation timeand H2O2concentration, gel properties of myofibrillar protein are damaged more serious. Afteroxidizing by different H2O2concentration and oxidation time，the G’and G’’ also decreased, whichindicated the gel elasticity of samples was reduced, the G’’/G’(loss factor) analysis also showedwhen the carp myofibrillar protein was oxidized, the gel was less elasticity. The above resultsshowed that the changes of protein structure caused by protein oxidation destructed the gel qulity.4. After oxidation by free radical, the correlation analysis of protein structure and functionindex showed a high correlation between them, indicating that the reduced functional propertieswere caused by the changes of protein structure. So protein oxidation should be reasonablecontroled in actual production.5. The myofibrillar protein structure of samples with different washing treatment was changedduring frozen storage. The carbonyl content and the bityrosine content increased significantly,indicating protein oxidation was occurred during frozen storage, which leads to the structural andfunctional changes. After frozen for90d, carbonyl content of the control group, the traditionalwashing group, adding sodium ascorbate washing group, adding PG washing group were increasedby39.2%,47.5%,62.8%and27.1%(P<0.05), respectively, showing that the addition of PG caninhibit the protein oxidation to a certain extent, and the addition of ascorbic acid sodium canpromote oxidation. The changes of other structural properties (SH content, ATPase activity, protein surface hydrophobicity, protein thermal stability and SDS-PAGE) and function (proteinturbidity, solubility, emulsifying properties, rheological properties, gel texture) analysis alsoproved this conclusion.6. The myofibrillar protein structures of carp surimi with different cryoprotectants andantioxidants were changed during frozen storage. The carbonyl content and the bityrosine contentwere increased, while the total sulfhydryl content was decreased during frozen storage, whichindicating protein oxidation also occurred for all the samples with adding different cryoprotectantsand antioxidants during frozen storage. After being frozen storage for90d, the carbonyl content ofthe control group, adding cryoprotectants group, adding cryoprotectants+PG group, addingcryoprotectants+WPI hydrolysates group were increased by55.7%,46.1%,25.6%and22.4%(P<0.05), while the sulfhydryl content decreased by34.5%,28.6%,19.2%and20.9%(P<0.05).Results showed that adding PG and WPI hydrolysis during frozen storage can inhibit proteinoxidation effectively. The analyses of other structural properties (ATPase activity, surfacehydrophobicity, thermal stability, SDS-PAGE) and function properties (protein turbidity,solubility, emulsifying properties, rheological properties, gel texture) were also proved thisconclusion. Because WPI hydrolysates was a natural antioxidant, so it can be used in frozen surimiprocess safely, in order to improve the quality of surimi.