| Tilapia is one of the most internationally competitive aquaculture species in China.However,at present,tilapia is only sold in the form of primary products,the product is relatively simple,and there are few deep-processed products.The export and domestic sales of tilapia are both challenged.Therefore,the development of high-quality and diversified tilapia processed products is of great significance for improving the economic value of tilapia and rationally utilizing its resources.Salting is one of the ways in which fish products are processed.With the emphasis on healthy living,light salted products are becoming more and more popular.The shelf life of light salted products can be extended effectively by freezing.However,there are few studies on the frozen stability of light salted tilapia fillets.In this study,the tilapia(Oreochromis Niloticus)meat was taken as the experimental object.Firstly,the effects of salting conditions on the physicochemical properties and quality of tilapia fillets were explored.Secondly,the effect of frozen storage on the quality of light salting tilapia fillets was investigated.Finally,the mechanism of the effect of the freeze-thaw stability of light salting tilapia fillets was elucidated.The results of the study are as follows:(1)The NaCl and water content,the NaCl and water weight,the content of released proteins,the compositions of released proteins,the yield,p H,color,textural properties,and TBA value and microstructure of the tilapia fillets obtained by brining(1-10 h)with different concentrations of NaCl solution(3%,6%,9%,12%,and 15%)were analyzed to explore its physicochemical and quality changes.The results showed that brined with 3%NaCl solution for more than 4 hours and 6% NaCl solution for 10 hours,the fish fillets were water-absorbing.The fish fillets dehydrated when they were brined by 9%,12% and15% NaCl solutions.With the extension of brining time and increase of the NaCl concentration,the NaCl weight and the content of released proteins increased,which were mainly water-soluble,myosin,and actin proteins,as shown in SDS-PAGE profile.The p H of fish meat showed a downward trend during the brining process.Salting could improve the brightness of tilapia fillets,but when the NaCl content of the fish was too high,the lipid oxidation was promoted.The muscle fibers became smoother and plump due to moderate brining,which contributed to the improved springiness and cohesiveness of the fish fillets.However,when the fish was brining with 12% and 15% NaCl solutions for more than 6 h,the connective tissue of fish meat would degrade,and its springiness and cohesiveness decreased.(2)The quality changes of lightly salted tilapia fillets(S1 6%-2 h,S2 6%-4 h,S39%-1 h,and S4 12%-0.5 h)and the underlying mechanisms were investigated during frozen storage.The amount of ice crystals in tissues(salt content-1.5%)decreased in the order S1>S2=S3>S4 due to different water content and freezing points(p < 0.05).The improved tissue recoverability and less degradation of titin contributed to the increased water-holding capacity,springiness,and cohesiveness for the S2,S3,and S4 groups.The water-binding capacity of the salted groups was 14.69%-18.62% higher than that of the unsalted counterparts after 28 d(p < 0.05).The content of sulfhydryl groups decreased gradually,which might accelerate the aggregation of the unfolded proteins,resulting in a declined solubility(p < 0.05).Light salting under proper conditions(S3)contributed to the enhanced water-holding capacity and texture by modifying the physicochemical and microstructural properties.(3)The thawing loss,centrifuging loss,water-bolding capacity,textural properties,color and other indicators were analyzed to understand the quality changes of light salting(9%-1 h)tilapia fillets after thawing.And the mechanism of its quality change was studied from the aspects of water distribution,lipid oxidation,protein properties,microstructure and ice crystal morphology.The results show that after freeze-thaw cycles,the free water peak of group S samples disappeared.Intracellular ellipsoidal ice crystals were observed in the lightly salted samples but not extracellular large icicles in the control samples.Due to the increase of bound water ratio,the improvement of ice crystal morphology,and the high recovery ability of tissue structure,the light salting samples still had higher springiness and water-holding capacity even after 7 freeze-thaw cycles.However,the freeze-thaw cycle aggravated lipid oxidation in the light salted samples due to the reduction of catalase activity.However,the content of catalase in the light salting samples decreased and the degree of lipid oxidation increased during repeated freeze-thaw cycles. |
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