Effects Of Water Retention Agents And Thawing Methods On The Quality Of Frozen Tilapia Fillets

Freezing mainly influences the quality of tilapia fillets, including moisture loss, myofibrillar protein denaturation and weight loss. Adding non-phosphorus water retention agent to tilapia could increase the water-holding capacity of meat, as well as improve the texture and delay protein denaturation. During thawing, foods were subjected to damage by chemical and physical changes and microorganism, it would occurred, including drip loss, thaw loss, flavour change and lipid oxidation. Therefore, optimum thawing procedures should be of concern to food technologists. This paper mainly focused on vacuum-steam thawing（VT） and ultrasonic thawing（UT）, in order to select the most suitable thawing method of tilapia fillets. The main research contents and conclusions are as follows.In order to obtain the optimal concentration of trehalose, potassium bicarbonate and chitosan, experiment was monitored by the soak weight gain rate, the coating weight gain rate, water holding capacity, thawing loss, pH value, Ca²⁺-ATPase activity, and texture, as well as low field nuclear magnetic resonance（LF-NMR） analysis, which could efficiently evaluate the influence of three additives on tilapia quality during 3 months storage. The result showed that the best supplemented concentration of them is 5⁸% trehalose, 1.0% potassium bicarbonate and 0.5% chitosan respectively. Based on L8（27） Taguchi experiment. The optimal level combination of water-retaining agents was 4% trehalose+1.2% potassium bicarbonate+0.2% chitosan, with comprehensive consideration the influence of three kinds of water-retaining agents and their interactions on the quality of frozen tilapia fillets.The influence of different vacuum-steam thawing methods on the quality of tilapia fillets and the optimum process condition of VT were investigated in this study. The thawing time, thawing loss, the 2-thiobarbituric acid（TBA）, the K value and pH were carried out on tilapia fillets, which were thawed from-20 ℃ to 4 ℃ in different vacuum conditions（15、12、9、6、3 and 0.8 kPa）. The results obtained were compared with air thawing statistically, VT was faster. Moreover, the thawing time shortened signifcantly with the concomitant increase in vacuum degree（p<0.05）. The samples were thawed totally within 30 min under 9 kPa, thawing loss, TBA and K value of thawed fillets decreased significantly（p<0.05） when compared to other tests: thawing loss was-0.04%, TBA was 0.097 mg/100 g, K value was 7.15%. However, pH with 6.60 increased pronouncedly（p<0.05）. The results indicated that VT under 9 kPa can maintain the freshness of tilapia fillets to the utmost extent as a rapid thawing method.Low frequency ultrasonic thawing was produced by custom design of JMP10, which was studied with ultrasound power and temperature. The optimum thawing conditions was determined by comprehensive values of thawing time, pH and K value. Results showed that the thawing time and K value increased or decreased significantly of tilapia fillets as temperature grew continuously, the best ultrasonic temperature was about 20 ℃ when taken all consideration. In other work, a block of frozen tilapia fillets was exposed to different ultrasound powers, the quality of samples changed slightly. Interaction effects between power and temperature had no influence on physical-chemical quality on meat. The maximize intend of prediction profiler of ultrasonic thawing showed that tilapia own higher quality and freshness when ultrasonic power was 500 W and temperature at 21 ℃.A study to assess the effect of different thawing methods, including chiller thawing in 4 ℃ refrigerator（CT）, air thawing in ambient temperature（AT）, VT and UT, on the physicochemical and micro-structure of frozen tilapia fillets was carried out. The experiment was monitored by thawing time, thawing loss, the TBA, the K value, the pH value and the muscle tissue in the transverse section by scanning electron microscope（SEM）. Results indicated that the thawing time shortened（p<0.05） significantly by VT and UT, thawing loss, TBA and K value of thawed fillets decreased significantly and pH value increased significantly（p<0.05） as well, when compared with CT and AT. SEM showed that four thawing methods destroyed the microstructure of muscle. However, VT method had minimal impact on the fish microstructure, which was similar to fresh fish to the largest extent. the muscle fiber bundles of the CT and AT were damaged to some extent, which of UT group were damaged more seriously. Therefore, it was shown that VT is an appropriate way to defrost tilapia fillets with reduced the thawing time and deterioration of meat quality.Compared with blank processing and natural air thawed fish fillet, novel non-phosphorus water retention agent combined with VT could improve the physicochemical quality of tilapia filllets significantly. The quality parameters that were evaluated are Ca²⁺-ATPase activity, texture and LF-NMR analysis were reference indexes. The results have showed that excellent water retention agent formula,with 4% trehalose+1.2% potassium bicarbonate+0.2% chitosan, could not only reduced the protein denaturation degree, but maintained high water-holding ability of tilapia fillets. What’s more, VT under 9 kPa could further improved the quality of tilapia fillets, which also gave useful insights and instruction to further studies on this subject.