Study On Quality Deterioration Phenomenon And Its Mechanism Of Giant Freshwater Prawn (Macrobrachium Rosenbergii) During Frozen Storage

Giant freshwater prawns(Macrobrachium rosenbergii),which are known as Malaysian prawns,are widely consumed for the distinct flavors and high nutritional values.There are only 4 to 5 months in a year can giant freshwater prawns survive.The current processed products mainly are frozen whole prawns and peeled prawns,which aviod the seasonal characteristics of giant freshwater prawns directly.Meanwhile,under the traditional quick-freezing process and the conditions of frozen storages,the muscle of shrimps become increasingly soft and inelastic with the extension of freezing,which seriously affects the qualities of prawn products.Therefore,it is urgent to carry out a research to study the deterioration problems of frozen prawns.In this paper,the relationships between endogenous protease activities and degradation of prawn proteins,as well as their correlations with texture changes during the frozen storage were studied.The purpose of this study is to provide theoretical guidance for improving the quality of frozen prawn products.Firstly,the quality changes of frozen whole prawn,and headless prawn were compared and studied by analyzing the changes of texture,myofibril fragmentation index,myofibrillar apparent diameter,TCA-soluble peptide,?-amino nitrogen,total proteolytic activity and the growth of microorganisms.The results showed that the texture of shrimp meat deteriorated to different extents with the extension of freezing.The degradation rates of texture in frozen whole prawns were significantly higher than that in headless prawn.The shear force and hardness of frozen whole prawns decreased rapidly from 0 to 2 weeks,and then behaved slow downward trends from 2 to 6 weeks,and the texture deterioration of frozen whole prawns was the most significant.The water holding capacities of frozen whole prawns were consistent with the changes of texture,and decreased significantly throughout the frozen storage.In terms of protein changes,the protein degradation degrees of frozen whole prawns were significantly higher than the other two groups.At the same time,it was obvious that the internal structures of the myofibril in frozen whole prawns were destroyed through microstructure analysis after 8 weeks frozen storage.SDS-PAGE analysis showed that frozen storage had impact on degradation bands.Correlation analysis showed that there was a significant correlation between protein degradation and texture degradation of frozen whole prawns(p<0.05).Analysis of microbiological and total proteolytic activities showed that the change of total proteolytic activities were consistent with the rate of texture decrease.Compared with microorganism,the effects of protease on texture degradation were significant.Secondly,the effects of endogenous protease on texture deterioration of frozen whole prawns during frozen storage were studied.Protease inhibitor experiments and storage experiments were carried out to investigate the shear force,total proteolytic activity,TCA-soluble peptide,?-amino nitrogen and myofibril fragmentation index.The results showed that shearing forces of the three groups treated with protease inhibitors were higher than that of control group after 16 weeks of frozen storage.TCA-soluble peptide,?-amino nitrogen and MFI values of the three groups treated with protease inhibitors were lower than that of control group.As a result,endogenous proteases might play an important role in protein degradation and texture deterioration of prawns during frozen storage.Scanning electron microscope(SEM)analysis showed that the degree of muscle fiber deterioration in three groups treated with protease inhibitors were lower than that in control group.SDS-PAGE analysis showed that the intensity of the protein band in three groups treated with protease inhibitors were higher than that in control group.Finally,the activities and distributions of endogenous proteases in each subcellular structure(lysosome,mitochondrion,myofibrillar and sarcoplasm)of frozen whole prawns were investigated.The results showed that the activities of cathepsin B,L and H(for cathepsin D is in lysosome and mitochondria)in lysosome decreased obviously,while,the activities in myofibrillar and sarcoplasm increased rapidly in 2 weeks during frozen storage.Then,protease activities in each subcellular structure began to decline within 2~6 weeks,after 6 weeks,the protease activities increased.The activities of calpain in the sarcoplasm were much lower than cathepsin.The changes of calpain activities were similar with cathepsin.During frozen storage,cathepsin proteases were released into myofibrillar and sarcoplasm,resulting in increases in enzyme activities and rapidly degradation of protein and texture.