| Temperature is of great importance on the quality changes of food during storageand transportation. However, although food labeling system based on time length inChina can indicate shelf life according to the degree of food spoilage, it can not reflecttemperature history during actual storage and transportation process directly. Hence,this labeling system can not instruct the freshness of food accurately. Besides, for thehigh-end seafood needing uncooked consumption, or for food needing high levelfreshness requirements, it is quite important to judge the freshness of food simply andaccurately. Therefore, there should be an effective and intuitive way to indicatetime-temperature process food experienced. Time tempetature indicator (TTI) is asimple, cost-effective method in instructing time-temperature process foodexperienced. The research on TTI now is still in its infancy in China and theoreticalstudy has not form a complete system. Consequently, TTI products are rarely used.In this thesis, tyrosine and tyrosinase were chosen to develop a new kind ofsolid-state TTI based on enzymatic reaction principle. Through theoretical study offeasibility analysis, optimizating instruction time and dynamical analysis, a TTIproduct had been made to indicate the freshness of food, and the product had beenpreliminarily applied to instruct the quality of turbot sashimis. The main contents ofthe thesis were as follows:1. The thesis analyzed the browning trend differences between the reaction oftyrosine and tyrosinase in liquid system under different time and temperatureconditions, illustrating the feasibility of developing TTI products based on enzymaticreaction principle. To further confirm the feasibility of developing solid-state TTIproducts, single factor experiment had been made to study the release rate ofimmobilized substrate, the results showed that: immobilized tyrosine could be slowlyreleased from polyvinyl alcohol gel film and reacted with tyrosinase. When the 0.6mg/mL, the film forming method was repeated freeze-thaw cycle method, then therelease rate of the solid phase system could reach the minimum. The instruction timeof the system under these conditions could be about18h.2. Tyrosine and tyrosinase were immobilized in polyvinyl alcohol gel-releasefilm separately to develop a new kind of solid-state time-temperature indicator basedon enzymatic browning principle. To prolong the instruction time of TTI, preparationconditions were optimized. The overall color change ΔE was used to determine TTI’sinstruction time. When the value of ΔE was no longer change (ΔE=about38.21±2.00), it showed that TTI’s instruction time had been reached. A mathematical modelhad been established successfully according to response surface experiments. Thedependent variable of the model was the instruction time of TTI, the independentvariables of the model were tyrosine content, tyrosinase concentration and polyvinylalcohol concentration. The formula was t=50.40+0.25A+0.50B+1.50C-10.33A2-7.83B2-8.83C2-3.75AB+0.75AC+1.25BC, where t means instructiontime, A means tyrosine content, B means tyrosinase content and C means polyvinylalcohol concentration. The value of each independent variable could be determinedaccording to the forecast of the model when TTI achieved the longest instruction time.The values were: tyrosine30.08mg, tyrosinase0.30mL and polyvinyl alcohol15.18%.The longest instruction time of TTI could reach50.48h under these conditions.3. The optimized conditions above were selected to prepare a new solid-state TTIand its dynamics were analyzed. The analysis could provide a theoretical basis for theselection of food products and the application of TTI product. The results showed thatthe activation energy Ea of TTI was0.409kJ/mol, the pre-exponential factor A was2.91. SAS software was chosen to analyze the mathematical model of TTI and it wasshowed that a binary quadratic function formula was met among t(time),T(temperature) and F(X)(color response value). The formula wasF(X)=-0.00586T2-0.24002T+0.00902T×t-0.02060t-1.73930, R2of the formula was0.9661. This showed that the formula fitted the model well. The model established therelationship among color changes of TTI, time and temperature, laying the foundationfor the application of the indicator. 4. Turbot sashimi was selected as the product that TTI applied to because of itshigh freshness requirements and high value-added. The freshness changes of turbotsashimi were studied by measuring the changes of total number of colonies, TVBN,pH and weight loss rate at different temperatures (4,12,18℃) from the initial to48hwhen the turbot died. The results could clarify the freshness changes of turbot sashimiduring the time period (0-48h). Experimental results showed that total number ofcolonies varied significantly within48h, and the total number of colonies exceededomophagia standards when turbot sashimi stored at4℃for about48h. While thechanges of TVBN, pH and weight loss were not significant. Therefore, the totalnumber of colonies was selected to indicate the freshness of turbot sashimi. By thedynamic analysis of the freshness changes, the activation energy for the freshnesscould be calculated and that was0.009kJ/mol. The activation energy of turbot sashimiwas close to the activation energy of TTI calculated before (0.409kJ/mol), which metthe standards TTI applied to products (Standards required that the difference ofactivation energy between TTI and its application products should be less than40kJ/mol). It manifested that the TTI developed in the thesis could be applied toinstruct the freshness changes of turbot sashimi. Meanwhile, a device had beendesigned. The device could adhere TTI to the packaging surface of turbot sashimi.The device was easy to activate, small in size and good protection. Besides, the devicewas designed for transparent material and it was facilitated for consumers to observe.Studies show that the new kind of solid-state TTI developed in the thesis had aclear color change and its reaction system was rather simple, and it could be appliedto judge the freshness of highly perishable foods which need cold chains, such asturbot sashimi. It is also meaningful for consumers to judge the freshness of such kindof food with the aid of TTI. |
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