The Mechanism On Deterioration Of Texture Quality Of Cooked Fish Caused By Stored Time Extension Of Refrigerated Prepared Fish

With the acceleration of the pace of modern life,in addition to nutrition and safety,consumers’requirements for food pay more attention to their convenience,fastness,and good taste.Refrigerated prepared foods are popular with consumers because of these characteristics,especially freshwater fish products.At present,most of the refrigerated prepared fish are simply packaged after slaughtering and cleaning,and sold in a fresh-keeping rack at 4°C.Although the fish does not deteriorate during the shelf life,there is still a large difference in taste after cooking.Our group has previously studied the mechanism of quality deterioration of refrigerated prepared fish in raw and cooked states,but it lacks the mechanism of the quality difference of refrigerated prepared fish with different storage time after cooking.Therefore,in order to clarify the influence mechanism of storage time of refrigerated prepared fish on texture quality during its shelf life,It is necessary to systematically study the dynamic changes of the quality of refrigerated prepared fish during cooking.Mandarin fish is used as the research object in this study.First,the changes of quality characteristics of refrigerated prepared fish with different storage time during cooking are studied.Then,the changes of muscle structure and structural protein of raw and cooked fish meat with different storage time are researched.The variation in protein conformation and water state of refrigerated fish with different storage time during cooking are investigated.The specific conclusions are as follows:（1）The quality characteristics of refrigerated prepared fish with different storage time during cooking were studied,the results showed that with the increase of storage time and cooking,the hardness,cohesiveness,chewiness and shearing force of fish muscle generally decreased,while the cooking loss,cohesiveness and springiness increased.The stress relaxation time of fish muscle during storage was mainly affected by the interactions between myofibrillar proteins,but the stress relaxation time during cooking was mainly related to the protein-protein interactions on the surface of fish.The principal component analysis results showed that the quality characteristics of mandarin fish muscle could be explained well by the first and second principal components.The first principal component（PC-1）mainly reflected the information of the chewiness,adhesiveness,hardness,stress and shear force,which could be used to describe the palatability of fish muscle.The second principal component（PC-2）mainly manifested the cohesiveness and springiness information,which could be used to describe the internal structural characteristics of fish muscle.The results of TEM showed that the sarcomere length of fish muscle increased and then decreased during post-mortem storage,which could influence the tenderness of fish muscle.The destruction degree of sarcomere structure of cooked fish with different storage time was related to the integrity of sarcomere structure before cooking.（2）The changes of protein degradation in uncooked or cooked fish muscle with different storage time were researched.The results showed that titin was degraded initially during post-mortem storage,accompanied with an increase of the I-band length.Subsequently,the degradation of desmin induced a gradual breakage of the myofibrils.The following degradation of nebulin accelerated the destruction of N₂-lines,which occurred with a slightly fuzzy Z-disc.Finally,TPM began to degrade,and the remaining desmin was also degraded further.Besides,the overall change pattern of structural proteins during cooking was as follows:titin degraded firstly and then the degradation rate of nebulin accelerated due to the thermal effect,while compared with the nebulin,the degradation rate of desmin was a bit slow due to the inactivation of endogenous enzymes,finally the tropomyosin in myosin-actin complex released and degraded gradually because of the prolonged storage time and thermal effect.The content analysis of lost fluid showed that the content of hydroxyproline in the lost fluid gradually increased with the extension of storage time and cooking time,the connective tissue in the muscle gradually weakened,and collagen was continuously dissolved out due to its denaturation.Correlation analysis results showed that the palatability of fish muscle during the early storage time（1-3 days）was mainly affected by the degradation of titin,nebulin and desmin,while the internal structural characteristics of fish muscle were mainly influenced by the degradation of tropomyosin in the later storage time（4-5 days）.For cooked fish,the changes in palatability and internal structural characteristics were mainly affected by the degradation of high-molecular-weight structural protein titin and nebulin during the early storage time,plus the collagen degradation also influenced the palatability of fish muscle.（3）The changes of protein conformation and water migration of refrigerated prepared fish with different storage time during cooking were studied.The change of the content of strongly bound water could well reflect the texture quality of fish muscle stored for 1-3 days,and the change of the fluidity of immobilized water and the content of free water could well reflect the texture quality of fish muscle in the later storage period（4-5 days）.After heat treatment of the fish muscle stored for 1-3 days,with the extension of storage time,titin and nebulin in the cooked fish stored for three days were almost completely degraded,the sarcomere structure was destroyed,and the secondary and tertiary structure of protein unfolded further during cooking,and the tryptophan and tyrosine residues gradually shift from exposure to embed,caused the interaction with hydrogen bond weakened,theα-helix gradually transformed into random coils,and the protein structure tend to be disordered.Therefore,the strong binding water content of the fish muscle increased in the later period of cooking,which eventually caused the increase of cohesiveness and cooking loss rate and the decrease of shear force.However,after heat treatment of fish muscle stored for 4-5 days,the degradation degree of structural proteins was high.Titin and nebulin have been completely degraded,which not only aggravated the damage of the sarcomere structure and protein structure,but also loosen the protein network and reduced the ability of water retention.Therefore,the fluidity of immobilized water increased and the free water content decreased,finally influenced the texture quality of fish muscle.