| Pacific white shrimp is one of the three excellent shrimp species with the highest yield in the world.It is favored by consumers because of its rich protein content,low fat content and mostly unsaturated fatty acids.The captured Pacific white shrimp is usually transported and sold in a frozen way.However,the temperature fluctuation in the process of transportation and sale not only causes mechanical damage to muscle caused by ice crystal growth,but also promotes the growth of microorganisms and the activation of enzyme systems.especially melanosis,reducing the edible and commercial value of shrimp.Freezing is an important way to keep shrimp fresh.Tempering is a necessary step before further processing and consumption of frozen shrimp.Inappropriate tempering methods will lead to the deterioration of shrimp quality.However,conventional tempering methods such as water tempering and roomtemperature tempering may lead to protein,fat oxidation and microbial growth because of their long tempering time.As a new tempering method,radio frequency tempering has a good application prospect in the tempering and preservation of aquatic products.Therefore,this study takes frozen Pacific white shrimp as the research object and uses crushed ice as the surrounding medium for radio frequency tempering shrimp to explore the optimal tempering process of radio frequency tempering of Pacific white shrimp.Study on the effect of freeze-thaw cycles on the melanosis and quality of Pacific white shrimp tempered by radio frequency.In the process of radio frequency tempering shrimp,the effect of crushed ice with different porosity on the tempering uniformity of shrimp was simulated,which provides a theoretical basis for improving the uniformity of multi-layer shrimp tempered by radio frequency.For the radio frequency tempering of the Pacific white shrimp with an irregularly shaped,the optimum tmpering process was explored.By measuring the dielectric properties of Pacific white shrimp,the penetration depth of Pacific white shrimp at27.12 MHz RF system was calculated,and the optimum thickness for RF tempering was determined to be 6 cm.The effects of different electrode gaps(14 cm,16 cm and18 cm),power(800 W and 1200 W)and final tempering temperature(-4 ℃,-2 ℃ and0 ℃)on the tempering uniformity and energy consumption of 6 layers shrimp tempered by radio frequency were investigated.The results showed that when the electrode gap is 16 cm,the power is 1000 W and the final tempering temperature is-2 ℃,the standard deviation range of the upper surface temperature distribution of each layer was 1.0 ～2.1 ℃,and the energy consumption of 0.92 k W·h / kg,6 layers shrimp has the optimum RF tempering uniformity and low energy consumption.In order to study the effects of repeated freeze-thaw and tempering methods on the melanosis and quality of Pacific white shrimp,frozen Pacific white shrimp was tempered for 5 times after three tempering methods(radio frequency tempering,10 ℃water bath tempering and 4 ℃ refrigerator tempering).After freeze-temper for 0,1,3and 5 times,the quality indexes including tempering loss,volatile basic nitrogen(TVBN),polyphenol oxidase activity,degree of melanosis,total sulfhydryl content,thermal properties and texture properties of myofibrillar protein were analyzed.The results showed that the central temperature of 6-layer shrimps reached-2 ℃ after RF tempering for 6 min,and crushed ice as a low-temperature surrounding medium improved the RF tempering uniformity of 6-layer shrimps.In terms of quality,the tempering loss and TVBN value of RF tempered shrimps were lower than those of water bath tempering and refrigerator tempering.The activity of polyphenol oxidase and the degree of melanosis increased with the increase of freeze-temper times.RF tempering effectively inhibited the melanosis degree of shrimp.After the third freeze-thaw,the enthalpy and sulfhydryl content(0.16 mmol/gprot)of RF tempered samples increased significantly.After each freeze-thaw cycle,radio frequency tempering effectively maintained the hardness and chewiness of shrimp.With the characteristics of fast and uniform heating,radio frequency tempering effectively inhibited the melanosis of Pacific white shrimp during freeze-thaw cycle and reduced protein oxidation.In the previous study,it was found that crushed ice with different porosities as surrounding medium affected the tempering uniformity of shrimp.In order to explore the influence of tempering uniformity of shrimps surrounded by crushed ice with different porosities,the mixed dielectric properties of crushed ice and air are calculated by Complex Refractive Index mixture equation(CRIME).The radio frequency tempering process of sample surrounded by natural stacked crushed ice(porosity =0.62),compacted crushed ice(porosity = 0.44)and a block of ice are simulated by COMSOL.The radio frequency tempering uniformity and electromagnetic field distribution of three-layer shrimps without crushed ice were compared.The results showed that after tempering 15 min,the temperature distribution of the head and meat of the three-layer shrimps without crushed ice was uniform.However,the highest temperature of the shrimp tail was 40 ℃.Reducing the porosity of crushed ice improves the dielectric properties of shrimp surrounding medium,which makes it closer to the dielectric properties of shrimp.The tempering uniformity of the three layers of shrimp surrounded by compacted crushed ice is better than that surrounded by natural stacked crushed ice and wrapped with whole ice.The temperature distribution of the tail,head and body of the first layer shrimp is uniform,and the temperature difference is about7 ℃.The experimental results are consistent with the simulation results.Therefore,reducing the porosity of crushed ice is beneficial to improve the uniformity of radio frequency tempering of shrimp. |
