| Tilapia is one of the world's three largest freshwater fish species which has strong ability to adapt to the environment,fast growth and reproduction,fresh meat and rich nutrition.It has been developed rapidly nearly more than 10 years in the global scope.China is the largest producer and exporter of tilapia whose one of mainly export products is frozen tilapia fillets.But in the recent two years,China frozen tilapia exports and benefit appeared continuous decline,because of the European Union and the United States changed their requirements of the tilapia processing product quality,together with tilapia's export volume in Indonesia,Vietnam,Thailand,Columbia,Honduras,Taiwan and other countries and regions of similar products have increased.China is facing a competition situation which became more and more seriously.The ice crystal content of frozen tilapia fillets is one of the most important quality indexes,but there is no real-time detection and control methods for the quality change of frozen tilapia fillets.This topic started from the physical science perspective to study the relevance of physical parameters such as electric conductivity,water activity,aberration with ice crystal content of tilapia in the freezing process,establish the relevant mathematical model and a new ice crystal content detection method in the frozen tilapia fillets based on the relevance of physical parameter and evaluate its quality and provide reference to improve the quality of tilapia fillets.The results are as follows:1.The change of electric conductivity of tilapia fillets under different freezing conditionsThe electric conductivity and temperature of tilapia fillets were measured on-8,-18 and-28 centigrade respectively.The test results showed that the electric conductivity decreased with the temperature decreasing,it demonstrated a basic downward three linear rule,and the influence of temperature on the conductivity of the fillets was significantly(P<0.01).The electric conductivity dropped rapidly at the same temperature in the three freezing conditions and thus determined the freezing point of tilapia fish was-2±0.2 centigrade.Nonlinear simulation of the change of the electric conductivity after the freezing point was made,the obtained correlation coefficients were 0.98693,0.90405,0.99340.2.The change of apparent water activity of tilapia fillets under different freezing conditions.The value of water activity of tilapia was measured on line using the method of water activity meter.And the concept of apparent water activity was put forward because there was difference between fresh tilapia and frozen tilapia which can be used to characterize quantitatively the interaction of ice crystal and unfrozen water in frozen status.The results showed that the value of apparent water activity was significantly reduced with the temperature of tilapia fillets decreasing,which has a line change before reaching the freezing point and has a non-linear decline after reaching the freezing point(P<0.01).At different temperature(-8?,-18?,-28?),the apparent water activity values were non-linearly simulated with the change of the fillets temperature after the freezing point,and the correlation coefficients were 0.98261,0.99108 and 0.99792 respectively,this proves that the regularity of its'existence is obvious.3.The change of color of tilapia fillets under different freezing conditionsIn the measure of color of tilapia fillets,the color aberration and temperature also reflect a good correlation.The value of AE*ab,L,and b*were significantly increased in different degree with temperature of tilapia fillets decreasing(P<0.01),but a*was reduced,and the lower freezing temperature,the bigger change of color aberration.4.The detection of ice crystal content in the frozen tilapia filletsThe ice crystal content of tilapia fillets was measured by DSC,Hsieh formula and TDR.The results showed the ice crystal content of fish slices was reduced with the temperature decreasing and the amount of ice crystal is mainly concentrated from-3? to-10?;the results in three measures were consistency,so the function relation of ice crystal content and tilapia fillets' temperature was determined.5.Fitting model of ice crystal content correlation modelBy measuring the change of physical parameters during freezing process,such as the electric conductivity value,apparent water activity value and color aberration,the relationship with tilapia fillets' temperature was obtained;DSC,Hsieh formula and TDR were used to determine the change of ice crystal content during the freezing process,thus the relationship between tilapia fillets' temperature and ice crystal content was confirmed;Based on the tilapia fillets' temperature,the relationship between the ice crystal content results and the physical parameters was found and the relevant mathematical model was established(the correlation coefficients were greater than 0.93).Based on the physical parameters,such as electric conductivity,apparent water activity and color aberration,a method for the determination of ice crystal content was established.Among them,the ice crystal content results determined by the DSC was closest with the mathematical models constructed by physical parameters,those models' correlation coefficients are greater than 0.96.6.Research on frozen tilapia fillets' quality evaluation by ice crystal contentThe tilapia fillets' ice crystal contents were determined by the model of established with apparent water activity and electric conductivity in different freezing conditions,taking the hardness,elasticity,cohesiveness and chewiness as indexes,texture analysis method(TPA)was used to observe the changes of the quality parameters of the tilapia fillets and compare the quality of frozen tilapia fillets by the results of different ice content with fresh fillets during 90 days storage.The results showed that the ice crystal content between 95.38%and 97.44%,1.53?1.58 ?S/cm of the conductivity and 0.713?0.731 of apparent water activity is more conducive to maintain the texture quality of frozen tilapia fillets. |
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