Effect Of Freezing-Repetitive Freezing And Thawing On The Quality Of Micropterussalmoides

In addition to fresh fish,the preservation of fish is extended by freezing.We explored the effects of different freezing methods on the quality of carp,and used freeze-thaw method to explore changes on the quality and odor of fish during frozen storage process.The main conclusions are as follows:1.Passed different freezing methods(-20?-SF;-20?-QF;-70?-QF;LN-DF;LN-IF)to explore changes in frozen meat loss,thawing loss,texture,water distribution,microstructure,protein structure and so on.It was found that the-70?-QF,LN-DF and LN-IF were not significantly different in the loss rate.Quick freezing methods were significantly better than the conventional freezing(p<0.05).LN-IF showed hardness,degree of adhesion,and recovery preferably.LN-DF has an advantage in chewing.while conventional-20?-SF is superior to quick freezing in terms of elasticity and cohesiveness.The combined water and the non-flowable water were sequentially enhanced in the freezing mode of-20?-SF,-20?-QF,-70?-QF,LN-DF and LN-IF,and the free water was weakened in turn.We can see quick-freezing muscle fibers perform Significant shrinkage,and neatly arranged by-70?-QF,LN-DF,LN-IF.The ?-helix content decrease and ?-fold increases in-20?-SF,-20?-QF,-70?-QF,LN-DF,and LN-IF.However,there was no significant change in ?-turn and random curl.The myofibrillar protein ?H and phase temperature decrease,the changes of LN-QF and LN-IF were smaller.The endogenous fluorescence intensity of LN-IF,LN-IF are closer to fresh sample.Low temperature quick freezing has less effect on fish meat.2.Using-20?-SF and LN-DF freezing treatment to study the changes of fish quality during freeze-thaw process,it was found that the color change and water loss of LN-DF treated fish were less than-20?-SF during freeze-thaw process.The denaturation temperature of LN-DF is less than-20?-SF freeze-thaw,and the turbidity of-20?-SF and LN-DF reaches 0.50 and 0.40,respectively.When the number of freeze-thaw cycles reaches the fifth time,the hydrophobicity increased by 3.8 times and 2.9 times compared with that of unfreeze-thaw.The particle size of the LN-DF group was not significant during the freeze-thaw process.The fluorescence intensity of the protein decreased.It indicates that the fish meat treated with liquid nitrogen before freezing can better control the quality change during the freezing process.The liquid nitrogen quick freezing can be used as a pretreatment before freezing to achieve better storage effect.3.Using the electronic nose and GC-MS detection method,the electronic nose can distinguish the number of freeze-thaw cycles,odor change between-20?-SF and LN-DF with the increase of the number of freeze-thaw cycles,the odor began to overlap slightly.GC-MS detected aldehydes,alcohols,esters,ketones,hydrocarbons,aromatics and amines.LN-DF treatment changed less than-20 ?-SF in freeze-thaw,and the amines appeared after-20?-SF.Though the first freeze-thaw,LN-DF showed amines during the third freeze-thaw,and content is much smaller than the-20?-SF group.