| The introduction of tilapia in China was originally from Thailand and Vietnam.The time can be traced back to the 1950s.The provinces of Guangdong,Hainan,Guangxi and Fujian are the main areas of tilapia culture in China.After nearly two decades of development,with the changes in the world fishery economy,market development and consumer demand,the export volume and economic benefits of tilapia in China have declined significantly in recent years,and the tilapia industry has faced severe situation.In this thesis,tilapia fillets were selected as the research object.The tilapia fillets were selected and partition research was carried out according to the quality characteristics of fish meat.The fractal dimension(FD)were introduced to evaluate and predict the quality changes of frozen fillets.The optimization and improvement of the processing technology of frozen tilapia fillets and multi-dimensional quality evaluation system were established which provide technical support for the promotion of tilapia export processing industry.The research contents and results are as follows:(1)Selection of standard tilapia filletsThrough in-depth analysis of tilapia raw materials and different breeding bases of tilapia production enterprises,the common production process of tilapia fillets in China is:raw material holding,color development(no coloration),bloodletting,descaling,opening,peeling or peeling,trimming,quick freezing and other processing procedures.Comparing the chromogenic and non chromogenic tilapia fillets from different breeding bases,according to the quality requirements of the production enterprises,and choosing fillets with good surface gloss,clear lines,uniform color and good integrity as standard fillets.The quality is the closest to the standard of fish freshness and superior quality.It can be used as a comparison standard to analyze and predict the degree of quality deterioration of fillets under different freezing processing and storage conditions.(2)Determination of quality indicators of standard tilapia filletsComparing the differences in surface color and texture distribution between standard chromogenic and non chromogenic tilapia fillets,the fillets are divided into three regions:the back,the abdomen and the tail.Among the three specifications of the main 3/5 ounces(85-142g),5/7 ounces(142-198g)and 7/9 ounces(198-255g),three standard fillets are selected according to the requirements of standard fillets,and the differences between different specifications and partition in pH,color,electrical conductivity,TVB-N content and texture parameters of standard tilapia fillets were determined.The result showed that there were significant differences in color and electrical conductivity between the two types of standard fish fillets.The differences in pH,TVB-N content and texture parameters were small.Based on this,the quality parameters and variation range of pH,color,conductivity,TVB-N content and texture parameters of standard tilapia fillets were determined.The study also determined that principal component analysis and fractal dimension can be used as indicators for the quality of fillets.(3)Application of fractal dimension to the quality evaluation of tilapia filletsThe transverse and longitudinal sections of the standard tilapia fillets were prepared,and the fractal features were obtained by image analysis,and the FD was calculated.The transverse and longitudinal sections FD of the standard chromogenic fillets were 1.9283±0.0021 and 1.8706±,respectively.The transverse and longitudinal sections FD of the non chromogenic fillets were 1.9234±0.0019 and 1.8348±0.0048,respectively.After comparison,it was found that there was no significant difference between the transverse sections of the two types of fillets,and there were differences in the longitudinal sections.The fractal dimension and variation characteristics of the fillets treated with ice temperature and different temperature frozen storage conditions were studied.The results showed that the fractal dimension of the transverse slices of the fillets showed a continuous decline with the extension of time under the two frozen storage methods.The transverse slice FD of the ice temperature storage method decreased from 1.9271±0.0019 at the initial storage period to 1.7408±0.0061?1.7588±0.0004 at the end stage;the transverse slice FD of the frozen storage method decreased from 1.9271±0.0019 at the initial stage of storage to 1.7695±0.0282?1.7979±0.0020 in the final stage.the FD values of the fillets in the two storage methods are significantly lower than that of the standard fillets,but the decline in the frozen storage method is significantly smaller than the ice temperature storage method,indicating a big difference of the two storage conditions.The fractal dimension of the longitudinal section under the two storage methods is fluctuating,and the correlation with the quality of the fillet is poor.(4)Establishment of correlation model of profile fractal dimension and quality change of frozen tilapia filletsThe correlation model between the comprehensive quality of frozen tilapia fillets and the profile fractal dimension was obtained by principal component analysis.Under the condition of ice temperature storage,the quadratic function correlation model has the highest fitting degree,and the fitting coefficient is between 0.841 and 0.962,which can better express the correlation between the fractal dimension of the section and the quality change of frozen tilapia.The fitting coefficient of the longitudinal section correlation model is less than 0.300.Under the frozen storage conditions,the quadratic function model under the transverse section of-18? and-24? and the linear function model under the-30? frozen storage condition have higher degree of fit.The fit coefficients are 0.979,0.961,and 0.940,respectively,while the longitudinal section correlation model has a fit factor of less than 0.400.The study concluded that the degree of quality change of frozen tilapia fillets can be evaluated and predicted by the correlation model of fractal dimension and comprehensive quality of transverse section.(5)Establishment of multi-dimensional evaluation method for the quality of frozen tilapia fillet.In this experiment,the quality of fillets was evaluated by common evaluation methods firstly.The quality parameters such as pH,color,electrical conductivity,TVB-N content and texture parameters of frozen tilapia fillets were determined.According to these quality parameters,the quality changes of fillets were analyzed.The reduction of storage temperature have a significant slowing effect on inhibiting the rate of change of fillet quality indicators.Based on the conventional evaluation method,the principal component analysis method was used to integrate many quality indicators into one comprehensive quality index.According to the comprehensive quality index,the comprehensive quality change of the fillets was obtained.Under the ice storage condition,the comprehensive quality score of the fillets decreased from 3.7426 to-3.4030?-5.1729.Under the frozen storage condition,the comprehensive quality score of the fillets was from 4.0729 dropped to-1.6205?-3.0082.During the same storage period,the quality deterioration of the ice storage fillets was significantly higher than that of the frozen fillets;the study also established a correlation model between the profile fractal dimension and the comprehensive quality of the fillets.The quality of the fillets can be evaluated by the profile fractal dimension(PFD),which established and further improved the multi-dimensional quality evaluation methods of the fillets. |
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