Effects Of Transglutaminase Modification On Gel Properties And In Vitro Digestibility Of Myofibrillar Protein Of Silver Carp (Hypophthalmichthys Molitrix)

Silver carp（Hypophthalmichthys molitrix）is a kind of high protein and low fat food.However,due to the poor performance of gel,the processing quality of its products is low and the benefit is not high.Myofibrillar proteins（MP）play a major role in the gelation of surimi.Adding transglutaminase（TGase）can effectively improve the quality of freshwater surimi.In this study,the effects of TGase modification on the properties of MP gel and in vitro digestion of silver carp were investigated.The effects of conformation,gel properties and in vitro digestion characteristics of MP gel induced by TGase were the main content of this study.The purpose was to provide theoretical support for the application of TGase in freshwater fish products,improving the utilization rate of freshwater fish and the high-value utilization of freshwater fish industry in China.The results were as follows:（1）In this chapter,MP gels with different amount of TGase addition levels（0,5,10,15,20,30 U/g protein）were prepared,and the optimum TGase addition level was determined by using degree of crosslinking as the characterization method.The results showed that the crosslinking degree increased and then stabilized with the increase of TG addition level.When the addition level was 15 U/g protein,the MP crosslinking degree reached the maximum,which was 52.63%.Therefore,in the later three chapters,the addition amount of TG was 15 U/g protein.（2）In this chapter,MP gels with different crosslinking times（0,5,10,15,30 and60 min）were prepared,meanwhile MP without TGase was used as the control group（CK group）to explore the changes in protein conformation,including the following indicators:degree of crosslinking,chemical force,sulfhydryl content,secondary structure,surface hydrophobicity,endogenous fluorescence spectrum analysis and SDS-PAGE analysis.The results showed that the degree of crosslinking of MP gel induced by TGase（TG group）was significantly higher than that of CK group（P<0.05）,which increased gradually during the crosslinking process,among which the degree of crosslinking of TG-30 sample was the highest（78.87%）.The contents of ionic bond and hydrogen bond decreased,disulfide bond was the highest,total sulfhydryl group and active sulfhydryl group decreased,indicating that ionic bond and hydrogen bond were not the main forces during gelation,but disulfide bond was one of the main forces.SDS-PAGE analysis showed that the gels of CK group were mainly maintained by disulfide bonds,MHC of TG group was maintained byε-（γ-Gln）-Lys heteropeptide bonds,and actin was maintained by disulfide bonds.Analysis of secondary structure changes showed thatα-helix content decreased from 29.55%to 25.63%,andβ-folding content increased from 21.31%to 25.77%in TG group.Therefore,in the gelation process,MP gradually transforms from intramolecular aggregation to intermolecular aggregation.（3）In this chapter,MP gels with different crosslinking times（0,5,10,15,30 and60 min）were prepared to explore the influence of TGase on the properties of MP gels,including the following indicators:gel strength and texture analysis,color,yield,water capacity,water migration and water distribution and observation of scanning electron microscope.The results showed that the gel strength,hardness,cohesion and chewiness of MP gels induced by TGase were significantly improved（P<0.05）,which 433.71g·mm,277.15 g,0.71,184.33 g,0.44 to 671.18 g·mm,511.55 g,0.83,514.55 g,0.67,and yield decreased from 53.60%to 36.86%,respectively.The gel gradually changed to high hardness gel.The water holding capacity was increased from 73.03%to 81.62%,and the T₂ transverse relaxation time was shortened.The T₂₂ value decreased while T₂₃value increased,and T_2b+T₂₁ value did not change significantly（P>0.05）.Comprehensive analysis showed that TG-30 was the best of the gel performance.The correlation between protein conformation and gel properties was also analyzed.The results showed that disulfide bond andβ-folding content were significantly positively correlated with crosslinking degree,gel strength,hardness,chewiness,resilience,whiteness,yield and water holding capacity（P<0.05）.Theα-helix content was negatively correlated with gel strength,hardness,spring,chewiness,resilience,whiteness,yield and water retention（P<0.05）.（4）In this chapter,two groups of MP gels with different crosslinking time were simulated for gastrointestinal digestion in vitro,and the changes of MP gel digestive characteristics were explored.The results showed that after gastric digestion,the amino content of TG group decreased from 1394.83 to 1017.55 mg/m L with the increase of crosslinking degree.The amino content of all samples in TG group was lower than CK group.Although there were significant differences in crosslinking degree among TG-10,TG-15,TG-30 and TG-60 samples（P<0.05）,there was no significant difference in amino content in digestive juice（P>0.05）.After intestinal digestion,the difference of amino content between TG group and CK group decreased.The results of SDS-PAGE of supernatant after digestion were as follows:（a）Gastric digestion product:the MW of proteins was concentrated below 52 KDa,but there was a difference in molecular weight distribution between the two groups.CK group:There were more proteins at 40 KDa.The longer time of crosslinking,the thinner of bands.TG group:There were more proteins at 4.6-10 KDa.The 40 KDa band corresponding to CK group was not displayed,because its peptide MW was greater than 40 KDa.（b）Gastrointestinal digestive products:Protein bands are almost gone.The total reducing power of MP gel gastrointestinal digestive fluid was low.The DPPH·clearance rate of gastric digested products in TG group was higher than that of gastrointestinal digested products.The hydroxyl radical clearance rate of gastrointestinal digestive products in TG group and CK group was lower than that of gastric digestive products.