Study On Comprehensive Quality Evaluation And Shelf-life Prediction Model Of Frozen Tilapia Fillets

Tilapia（Oreochromis niloticus）is one of the most important varieties in freshwater aquaculture industry with high yield.Tilapia is widely favored by consumers because of its fewer intramuscular spines and nutritious flavor.Fish are prone to spoilage after death due to the combined action of exogenous microorganisms and endogenous enzymes.Because frozen storage could maximize the quality of aquatic products and extend the shelf-life,tilapia is mainly circulated in the form of frozen tilapia fillets.However,the quality deterioration of frozen products will still occur in the process of frozen storage,such ascolor,texture and structure morphology.The degree of quality deterioration is mainly related to internal and external factors such as the freshness of raw materials,freezing rate,frozen storage temperature and time,thawing method,ice crystal growth and protein degeneration.Therefore,it is crucial to explore the quality changes in the process of pre-freezing,frozen storage and thawing,and further predict the shelf-life for consumers and tilapia processing industry.At present,there have been some relevant studies on the quality change on the quality change and control technology of tilapia fillets during storage,preservation and processing at home and abroad.However,there is still a lack of research on the tissue structure,ice crystal morphology and protein degeneration of frozen tilapia fillets.Comprehensive quality evaluation,mechanism analysis and quality prediction of frozen tilapia fillets in terms of ice crystal morphology and protein denaturation during frozen storage have not been reported.Therefore,in this study,frozen tilapia fillets were taken as the research object.The water-holding capacity,microstructure and protein degeneration of tilapia fillets during long-term frozen storage at different temperatures,and determined the sensitive index of quality deterioration of tilapia fillets during frozen storage were evaluated in this study.At the same time,the differential proteins were further identified and analyzed with the help of proteomics technology to explore the mechanism of the protein changes during frozen storage.Secondly,the Arrhenius model and ANN model were used to predict the quality of frozen tilapia fillets by selecting ice crystal morphology,protein denaturation and freshness index.Then,based on the characteristic mechanism of proteindenaturation,a simple process of non-exogenous addition was used to regulate the water-holding capacity of frozen products,and the effect of different postmortem chilling time on the qualityof frozen tilapia fillets was further explored.Finally,considering that tilapia must be thawed before consumption and future processing,the effect of different thawing methods on the quality of tilapiafillets during frozen storage was also studied.The main research conclusionswere as follows:（1）Study on quality change of tilapia fillets during frozen storage The changes of water-holding capacity,color,texture,flavor,ice crystal morphology and protein denaturation of tilapia fillets during frozen storage at-20℃,-40℃and-80℃were studied.The results showed that compared with-20℃frozen storage group,the thawing loss of fillets stored for 12 M at-40℃and-80℃decreased by 45.23%and56.06%,respectively.The hardness of-20℃frozen storage group was 0.74 and 0.63times that of-40℃and-80℃respectively.And the equivalent diameter of ice crystals in-20℃frozen storage group was 1.41 and 1.80 times that of-40℃and-80℃respectively.There was little difference in the quality of frozen tilapia fillets between-40℃and-80℃.After the comprehensive evaluation of the quality of frozen tilapia fillets,several main evaluation indexes that could objectively reflect the quality of frozen tilapia fillets were determined:protein denaturation indicators(Ca²⁺-ATPase activity,sulfhydryl,endogenous fluorescence intensity),ice crystal morphology（cross-sectional area,equivalent diameter）,texture properties（hardness,springiness,chewiness）and water-holding capacity（thawing loss）.（2）Study on muscle proteomics during frozen storage of tilapia fillets Theassociations between tilapiafillets quality characteristics（p H,color,texture,ice crystal morphology and protein denaturation）and proteome profiles during a 12-month frozen storage period was studied by tandem mass tag technology（TMT）,based on the quantitative proteomics strategy.The quality changes of tilapia fillets during frozen storage were further explained from the mechanism.A total number of 175 proteins were found as differential abundance proteins（DAPs）in tilapia meat from two frozen storage temperatures（-20°C and-40°C）compared with the control group（0 day）,of whichcorrelation analysis indicated that 80 DAPs were related with the quality traits.At the same time,bioinformatics analysis revealed that 49,9 and 2DAPs were involved in binding proteins,metabolic enzyme and protein turnover,respectively,which were mainly closely associated with texture properties and protein denaturation compared to other quality traits.（3）Prediction of shelf-life model of tilapia fillets during frozen storage The main quality changes(ice crystal morphology,Ca²⁺-ATPase activity,total sulfhydryl[SH]content,intrinsic fluorescence intensity[IFI],and K value[freshness determination])of tilapiafillets were studied during 16 weeks of frozen storage at four accelerated storage temperatures（265 K,259 K,253 K and 233 K）,and the rate of quality deterioration and shelf-life prediction model were established.There was a dramatic increase in cross-section area and equivalent diameter and a sharp decrease in Ca²⁺-ATPase activity and SH content during the first 4 weeks（p<0.05）.IFI_λmaxdecreased by 43.95%,29.77%,28.97%and 18.58%after 16 weeks at 265 K,259 K,253 K,and 233 K.The quality deterioration rate（P）of frozen tilapia fillets based on factor analysis and logistic regression method was determined:P=（?）,where F1 is the main factor.The ANN model was superior to Arrhenius models and performed better for all indicators.The kinetic model established by IFI_λmaxcould be accurately described the quality changes during storage at 233～265 K.However,the prediction accuracy established by other indices decreased at later stages（14～16 weeks）.Finally,considering the operability and simplicity of the actual production,the shelf-life prediction model（t_i）of frozen tilapia fillet based on thawing loss was established:（?）,where A0 is the initial thawing loss of tilapia fillets at a specific frozen storage temperature,and A_i is the thawing loss at frozen storage time tat this temperature.（4）Effect of postmortem chilling time on quality of tilapia fillets during frozen storage The effects of postmortem chilling time（4℃）and storage time（-20℃）on sensory,texture,protein denaturation and microstructure of tilapia fillets were studied.The results showed that the overall sensory acceptability,hardness,chewiness and shear force of P8 sample were the highest.During the process of frozen storage,the texture properties（hardness,chewiness,shear force）,water-holding capacity,sensory score and protein degeneration of fish in all treatment groups gradually decreased,and the postmortem chilling time and frozen storage had no significant effect on the springiness and resilience of tilapia fillets.The sensory score of fish was within the acceptable rangewithin 3 months of frozen storage.P12 had the greatest influence on the physicochemical indicators and sensory scores of tilapia fillets.With the increased frozen storage time and postmortem chilling time,the tilapia tissue structure gradually destroyed,and the P12 samples were the most destructive.Both the postmortem chilling time and the frozen storage time could affect the quality of tilapia fillet,and the effect of frozen storage time was greater than thawing method.The quality of tilapia fish was significantly different after postmortem chilling time for 8 h and frozen for more than2 months.（5）Effects of thawing methods on quality of tilapia fillets during frozen storage The effects of three traditional thawing methods:25℃room temperature water thawing（WT）,25℃room temperature air thawing（AT）,4℃chilled air thawing（CAT）and-20℃frozen storage time on the quality of tilapia fillets were studied.The results indicated that the thawing time for WT,AT and CAT were approximately 7,83and 616 min,respectively.Thawing and cooking losses in the CAT samples were significantly lower than those in WT and AT samples（p<0.05）.The thawing and cooking losses of WT sample were the largest.The microstructural integrity of CAT samples was less compromised than that of WT and AT treated samples and had the highest fluorescenceduring frozenstorage.However,no significant difference（p<0.05）was observed in protein secondary structure changes,especiallyβ-sheets andβ-turns.The effect of frozen storage time on protein denaturation was stronger than thawing method.Ca²⁺-ATPase activity and SH content decreased with increasing storage time and the frozen storage time.Therefore,CAT is suggested to be an effective thawing method to minimize quality change of tilapia fillets during frozen storage.