Effect Of Immersion Freezing On The Quality And Myofibrillar Protein Denaturation Of Sciaennops Ocellatus During Frozen Storage

Frozen storage is the most important preservation method for aquatic products,while the frozen method is one of the key factors affecting the quality of frozen products.Immersion freezing(cryogenic liquid freezing)is an efficient freezing technology that can accelerate the freezing rate based on the heat and mass transfer between the freezing medium and materials.It has the advantages of fast freezing rate,high quality of frozen products and low energy consumption,but there are still some technical problems such as quality decline after continuous use of freezing medium,solute penetration and cracking of frozen products.It has become an important development trend of immersion freezing to develop new multi-compound freezing medium for different aquatic products and to improve the pretreatment method of freezing.Consequently,this paper takes Sciaennops ocellatus as the research object,which is an important mariculture fish in China.Firstly,four kinds of multi-compound freezing medium were screened and the effect of freezing medium on the quality of Sciaennops ocellatus fillets during frozen storage was systematically evaluated.Secondly,the advantages and disadvantages of air freezing(AF),blast freezing(BF),liquid nitrogen freezing(LNF)and immersion freezing were compared,which aimed to maintain the frozen quality of fillets.Finally,the effect of immersion freezing with packing(IF-P)on the frozen quality of fillets was studied.It provides a reference and basis for the application of immersion freezing in aquatic product freezing process and the quality control of frozen product.Main findings of this study were as follows:1.Taking AF as the reference,the effect of four kinds of multi-compound freezing medium(20%ethanol,30%propylene glycol,10%sodium chloride aqueous solution,IF-1;20%ethanol,20%propylene glycol,10%glycerol,10%sodium chloride aqueous solution,IF-2;20%ethanol,20%propylene glycol,10%polyethylene glycol,10%sodium chloride aqueous solution,IF-3and 20%ethanol,20%propylene glycol,5%glycerol,5%polyethylene glycol,10%sodium chloride aqueous solution,IF-4)immersion freezing on the freezing rate,quality of frozen storage and myofibrillar protein(MP)denaturation of Sciaennops ocellatus fillets were studied.The results showed that the freezing rate of four IF groups were 6.13,4.69,4.03 and 2.94 times than that of AF group,and IF significantly shortened the time cross the formation zone of maximum ice crystal.Compared with AF group,the ice crystals of the IF group were smaller and more uniform during frozen storage,the microstructure damaged less,the myofibrillar fragmentation index(MFI)decreased less,the texture characteristics(hardness and elasticity)remained better,thawing loss and water migration reduced significantly,and the water holding capacity(WHC)of IF-1 and IF-2 samples higher than other groups.In addition,the MP of each group had different degrees of denaturation during frozen storage.Among them,the MP of IF group had higher maximum transition temperature(T_max1and T_max2),the toal sulfhydryl content,Ca²⁺-ATPase activity and relative?-helix content than those in AF group.In conclusion,IF could significantly increase the freezing rate of Sciaennops ocellatus fillets,and slow down quality deterioration and denaturation of MP during frozen storage.IF-2 group was more suitable for the immersion freezing of mariculture fish.2.The effect of different freezing methods(AF,BF,LNF and IF)on the quality and physicochemical properties of MP from Sciaennops ocellatus during frozen storage was compared.The results showed that the freezing rate of different freezing methods was affected by medium temperature and heat transfer coefficient,and the freezing rate of BF,LNF and IF was 2.93,59.52 and 6.33 times higher than that of AF group.The cryogenic freezing process of BF,LNF and IF groups could slow down the damage of muscle fiber structure,reduce the size of ice crystal,the loss of thawing and cooking,delay water migration and texture softening.In addition,cryogenic freezing performed better in delaying the process of fat oxidation,inhibiting color deterioration,slowing down the increase of TBA contents,TVB-N accumulation,the degradation of ATP-related compounds and changes of p H values,which was beneficial to maintaining the freshness of fillets.The TBA values of the BF,LNF,and IF groups were 0.95,0.66,and 0.83 mg MDA/kg,which were significantly lower than 1.41 mg MDA/kg of AF group(P<0.05)after 90 days of frozen storage.Under the same freezing temperature of BF and IF groups,cryogenic liquid freezing of IF had faster heat transfer,which was more effective in maintaining the quality of frozen fillets.The total sulfhydryl content,Ca²⁺-ATPase activity,thermal stability,?-helix relative content of MP with cryogenic liquid freezing of LNF and IF groups were higher than those in AF and BF groups,and better performed in retaining the stability of protein structure.In conclusion,compared with the AF group,the treatments of BF,LNF and IF could slow down quality deterioration and decrease the changes of physicochemical properties of MP from Sciaennops ocellatus during frozen storage.Under same freezing temperature,IF heat transfer coefficient was higher than BF,and more conducive to maintaining the quality of frozen products.3.Immersion freezing with packaging(IF-P)and immersion freezing with contacting(IF-C)were compared to study the effect of prepackaging on the quality and physicochemical properties of MP from Sciaennops ocellatus during frozen storage.The results showed that the freezing rate of IF-C group was higher than that of IF-P due to direct immersion heat and mass transfer.The damage of muscle fiber and distribution of ice crystal were similar under different freezing methods,but IF-C caused more water migration.The thawing loss of the IF-C group was 2.10%,which was significantly higher than that in IF-P group at 1.46%(P<0.05)after 90 days of frozen storage.IF-P treatment isolated the contact of air showed advantages in delaying the color deterioration,TVB-N accumulation,fat oxidation,ATP-related compounds degradation and changes of p H values,which was beneficial to maintaining the freshness of fillets.After 90 days of frozen storage,the TBA value of IF-P group was 0.83 mg MDA/kg,which was significantly lower than 2.58 mg MDA/kg of IF-C group(P<0.05).The K value of IF-P group was 12.24%,which was significantly lower than 23.60%of IF-C group(P<0.05).The K value indicated that the fillets of the IF-C group had decreased to the second degree of freshness after 90 days of frozen storage.In addition,IF-P treatment delayed the increase of carbonyl content,?-helix relative content and thermal stability of MP were higher than those of IF-C group.In conclusion,IF-P could slow down quality deterioration and decrease the changes of physicochemical properties of MP from Sciaennops ocellatus during frozen storage.It was feasible to improve immersion freezing technology and the quality control of frozen products by prepackaging.4.In summary,IF could significantly increase the freezing rate of Sciaennops Ocellatus fillets,and slow down quality deterioration and denaturation of MP during frozen storage.Compared with the AF group,the treatments of BF,LNF and IF could slow down quality deterioration and decrease the changes of physicochemical properties of MP during frozen storage.Under same freezing temperature,IF heat transfer coefficient was higher than BF,which was more conducive to maintaining the quality of frozen products.Compared with the IF-C group,IF-P could slow down quality deterioration and decrease the changes of physicochemical properties of MP from Sciaennops ocellatus during frozen storage.