The Quality Change Of Turbot Under Different Conditions And The Mechnism Of Effect Of Protein Oxidation On Quality

Turbot?Scophthatmus maximus?is an internationally recognized high-value food fish that is popular among consumers for its meat quality and high nutritional value.The turbot grows fast,has strong resistance to stress and is resistant to high-density culture.It has become one of the important aquaculture fish species in the northern coastal areas of China.The breeding area has spread all over the Shandong Peninsula,the Liaodong Peninsula and the Bohai Bay region.At present,China's turbot breeding industry is booming,and the storage,processing and comprehensive utilization of turbot will become one of the main directions for the development of the turbot industry.Comprehensive research on the nutritional quality characteristics of turbot and the quality change mechanism during processing and storage can provide a scientific theoretical basis for the intensive processing and storage and preservation technology of turbot.Therefore,this paper takes the turbot as the research object,and divides the part of the turbot to comprehensively evaluate the nutritional quality and texture characteristics of different parts of the turbot.The 0°C ice storage and4°C refrigeration were used to investigate the changes of quality characteristics of turbot under different storage temperatures.From the perspective of protein denaturation and degradation,the oxidation of turbot muscle protein in 0°C ice storage and 4°C cold storage conditions was tested.By constructing a hydroxyl radical oxidation system,the effects of protein oxidation on the tissue structure,texture and water holding capacity of the muscle tissue of turbot and the functional properties of the muscle protein of turbot were studied.The results are as follows:?1?The basic nutrients,collagen content,amino acid and fatty acid composition and texture characteristics of different parts of the turbot?back,abdomen,chest,tail,skirt and fish skin?were compared and analyzed for nutritional quality.The results showed that the crude protein content of different parts varied,the protein content of fish skin was the highest,reaching 29.04%,and the skirt was the lowest,being12.99%,which was significantly different from other parts?p<0.01?,The crude protein content of the back,abdomen and chest and tail was 18.76%,18.96%,17.91%and 18.39%,respectively;The crude fat content of the skirt is the highest,accounting for 17.47%of the wet weight,there are 27 kinds of fatty acids up to318.09 mg/g,and the content of polyunsaturated fatty acids is 3.9-6.8 times of other parts,among which linoleic acid is the highest?107.26 mg/g?,followed by DHA?64.39 mg/g?and EPA?26.61 mg/g?;the collagen content in the fish skin was significantly higher than the rest?p<0.01?,up to 224.69 mg/g,which could be used as a raw material for the preparation of collagen;18 amino acids were detected in each part.The first limiting amino acid in the back,abdomen,chest,tail and skirt is Met+Cys,and the first limiting amino acid in the skin is Trp;In addition,except for the fish skin,the amino acid composition of the other parts of the turbot is in line with the FAO/WHO reference model standard;by texture analysis,it is found that the hardness,chewiness and elasticity of the chest muscle are significantly higher than the rest?p<0.05?,so the taste is better.?2?Sensory evaluation,physical and chemical analysis?pH,TVB-N value,K value,TBA value,cooking loss rate,drip loss rate and texture?and microbiological detection?total number of colonies?have been performed to reveal the changes of quality characteristics of turbot fillets under 4°C refrigeration and 0°C ice storage conditions.The results showed that with the prolongation of storage time,the quality evaluation indexes of turbot fillets showed different trends.The V-type change trend of pH was observed,and the TVB-N value,K value,cooking loss rate,drip loss and total colony value all showed an upward trend,while sensory score,hardness,elasticity,cohesiveness and chewiness showed a downward trend.In addition,the change rate of each index of 0°C ice fillets was significantly lower than that of the 4°C cold storage group.Comprehensive analysis of data from different indexes found that the total number of colonies,TVB-N values and sensory scores can accurately reflect the quality changes of the turbot fillets.The shelf life of turbot slices at 4°C and 0°C was8 d and 12 d,respectively.?3?By measuring myosin protein oxidation index(carbonyl content,sulfhydryl content,Ca²⁺-ATPase activity,surface hydrophobicity and solubility),combined with total soluble protein,water-soluble protein and salt-soluble protein by SDS-PAGE electrophoresis,the oxidative denaturation and degradation of muscle protein of turbot under storage of 4°C and 0°C conditions were compared and analyzed.The results showed that with the prolongation of storage time,the carbonyl content in the turbot muscles of 4°C and 0°C increased,the surface hydrophobicity increased,while the sulfhydryl content,Ca²⁺-ATPase activity and solubility decreased.In addition,the rate of change in the indicators of 4°C fish is significantly faster than that of 0°C.The results of SDS-PAGE electrophoresis showed that protein in the muscle of the turbot was mainly composed of myosin heavy chain,actin and tropomyosin.The bands in total soluble protein did not change significantly,while the bands of myosin heavy chain,actin and tropomyosin in salt-soluble proteins and the bands of water-soluble protein of 76-78 kDa and 91-93 KDa changed significantly with the prolongation of storage time.Compared with 4°C,the degradation rate of muscle protein in turbot was slower under 0°C ice storage conditions during the same storage time.It can be seen that the muscle protein of turbot has obvious oxidative degeneration and degradation under cold storage at 4°C and 0°C,and the ice deposit has a certain inhibitory effect on the oxidation and degradation of muscle protein of turbot.?4?In vitro simulated oxidation using hydroxyl radical oxidation system?0.01mmol/L FeCl₃,0.1 mmol/L ascorbic acid,0,0.5,1,5,10 and 20 mmol/L H₂O₂?was carried out to study the effects of protein oxidation on water retention,texture and tissue structure of turbot muscle.The results show that when the concentration of H₂O₂ is low,the oxidation effect of hydroxyl radical oxidation system on the muscle protein of turbot is not obvious.When the concentration of H₂O₂ in the oxidation system is more than 1mmol/L,the carbonyl content,dimer tyrosine content and surface hydrophobicity of myofibrillar protein are significantly increased with the increase of concentration,while the total sulfhydryl content is significantly reduced.Compared with the control group,the carbonyl content,dimer tyrosine content and surface hydrophobicity of the muscles of the 20 mmol/L H₂O₂ group increased by147.89%,335.69%and 166.73%,respectively,while the total sulfhydryl content decreased by 43.08%.The microstructure analysis of the tissue showed that the muscle tissue of the control group was a tightly connected network structure,and the muscle fibers were densely arranged without gaps.With the increase of H₂O₂concentration in the oxidation system,the muscle tissue gap of the turbot increased significantly,the muscle fibers broke,and the muscle fiber structure became dense and loose.In addition,the water-holding and texture properties of turbot muscles gradually decreased with the increase of H₂O₂ concentration.When the concentration of H₂O₂ in the oxidation system was 20 mmol/L,the cooking loss rate,drip loss rate,hardness,elasticity,cohesiveness and chewiness were 36.01%,5.82%,4005.68 g,0.743,0.588 and 384.19,respectively.The above results indicate that protein oxidation can destroy the structural and functional properties of the muscle myofibrillar protein of the turbot,leading to deterioration of fish quality and affecting its processing applicability.?5?In vitro oxidative oxidation of turbot myofibrillar proteins was carried out by hydroxyl radical oxidation system to study the effect of oxidation on the functional properties?carbonyl content,dimeric tyrosine content,surface hydrophobicity,solubility,chemical forces,EAI,ESI and strength,whiteness and water-holding capacity of gel?of myofibrillar proteins.When the concentration of H₂O₂ in the oxidation system is low,the oxidation effect of the hydroxyl radical oxidation system on myofibrillar protein is not obvious;When the concentration of H₂O₂ in the oxidation system is more than 1mmol/L,the carbonyl content,dimer tyrosine content and surface hydrophobicity of myofibrillar protein increased significantly with the increase of concentration,while the total sulfhydryl content decreased significantly.Compared with the control group,the carbonyl content,polytyrosine content and surface hydrophobicity of myofibrillar protein in the 10 mmol/L H₂O₂group increased by 161.27%,234.31%and 164.82%,respectively,while the total sulfhydryl content decreased by 29.23%.Myofibrillar protein chemical force changes significantly with increasing protein oxidation,among them,the ionic bond and the hydrogen bond content are gradually decreased,and the disulfide bond content and the hydrophobic interaction are gradually increased.Compared with the control group,the content of myofibrillar protein ionic bond and hydrogen bond in the 10mmol/L H₂O₂group decreased by 65.08%and 64.36%,respectively,while the disulfide bond content and hydrophobic interaction increased by 135.50%and102.54%,respectively.With the increase of H₂O₂ concentration in the oxidative system,the myofibrillar protein solubility and protein gel strength,whiteness and water retention showed a downward trend,and the 10 mmol/L group decreased by38.51%,27.78%,7.67%and 13.24%,respectively.During the oxidation process,EAI and ESI increased first and then decreased,and the 0.5 mmol/L group increased by6.16%and 4.40%,respectively,while the 10 mmol/L group decreased by 46.53%and49.13%,respectively.The above results indicate that the hydroxyl radical oxidation system promotes the oxidation of myofibrillar proteins and destroys their structural and functional properties.