Changes Of Myofibrillar Protein In Snakehead During Short-term Refrigeration And Its Application

As one of the important components in tissue of snakehead,muscle protein contains serious amount of myofibrillar protein.During refrigeration,myofibrillar proteins underwent some degree of denaturation and degradation,which leaded to the changes of the quality of fish and surimi gel.In this thesis,myofibrillar protein was firstly studied.The changes in structural and physicochemical properties of protein were investigated by ultraviolet spectroscopy,fluorescence spectroscopy,light scattering and other technologies.The results showed that during the short-term refrigeration process,the reductions of protein concentration,Ca²⁺-ATPase activity,and total sulfhydryl content respectively,while surface hydrophobicity increasing.All indicated 1)myofibrillar protein degradation,and 2)myosin denaturation with secondary structure distribution modification.The results of the SDS-PAGE gel electrophoresis indicated that refrigeration caused degradation of myofibrillar protein and polymerization of protein molecules.The maximum absorption peak of ultraviolet spectroscopy displayed blue shift with 1nm,while the absorption peak was gradually increased.The maximum emission wavelength of fluorescence spectroscopy displayed red shift with 1nm and the fluorescence intensity gradually reduced.All indicated that short-term refrigeration caused the microenvironment changes of the protein.Moreover,the particle size of myofibrillar protein gradually increased,and became compact.Then,the freshness of snakehead was characterized through sensory,microbial,physical and chemical evaluations respectively.The mechanism of quality decline was explained from the perspective of protein changes,which provided a reference for the effective preservation of freshwater fish such as snakehead.The results showed that during the refrigeration process,microorganisms used the nutrients of snakehead to multiply in large quantities.Nitrogen-containing substances such as proteins were gradually decomposed into alkaline substances,so that the volatile base nitrogen content increased,and the pH value arose after dropping.The ATP was gradually decomposed to increase the K value.The free water content between muscle tissues decreased and the brightness L^*decreased.Fat oxidation,which increased thiobarbituric acid value,and promoted the oxidation of Fe²⁺in hemoglobin,resulting in the decrease in redness a^*.The oxidation of protein leaded to the decrease of hemoglobin and heme in snakehead,which leaded to the rise of b^*.In the early stage of refrigeration,muscle contracted and hardened,then the structure of myocyte cytoskeletal protein and extracellular matrix were degraded,causing fish hardness,springiness and resilience declined.Denaturation and degradation of proteins leaded to a large number of exposure of hydrophobic amino acids on the surface,which results in the decrease of gumminess,cohesiveness and chewiness of the fish respectively.Finally,DWS diffusion spectroscopy was used to monitor the growth process of gels in snakehead surimi system.The results showed that the initial surimi system was a viscous fluid of which rheological behavior was close to that of Newtonian fluid.With the increase of temperature,the network structure gradually matured to form a gel.As the time of refrigeration extended,myosin light chain subunits dissociated and intermolecular crosslinked,which results in the reduction of heating temperature required for the formation of the surimi sample from pure viscous fluid to elasticity.Moreover,the viscosity,deformation resistance and recovery of the surimi system were more stable with the extension of the refrigerating time in the late heating stage.