Research On Radio Frequency Thawing Technology Of Pollack Surimi

Surimi is one kind of raw material for the deep processing of aquatic products which is produced through meat separation,rinse,refined filtration,dehydration,repackaging and freezing process.Aquatic products have the characteristics of seasonal and perishability,and sugar and polyphosphates were added to Inhibition of frozen denaturation of protein in order to meet the needs of the Industrialization and standardization.Frozen surimi has high nutritional value and delicate taste as well as long storage time,which could be used in large-scale industrial food production to improve the economic value and meet the needs of food beverage and family dishes.However,it is necessary to thaw the frozen surimi before producing gefillte fish.While different thawing methods will affect the quality of the products.How to have rapid thawing the raw materials and maintain their quality is not only a difficult problem in the production of surimi-based foods,but also an urgent need to improve of the food processing.Radio frequency is a kind of electromagnetic wave range from 3 KHz to 300MHz.High frequency alternating electromagnetic generated from radio frequency can stimulate the food vibration and water molecules polarity rotation resulting in heating.In addition,radio frequency carrys low energy,which does not has the potential risk of molecular ionization.Radio frequency heating has the characteristics of whole heating,which can heat up both inside and outside of the material without heat conduction.It can keep a high tempering speed,reduce the heating time and energy consumption,increase the uniformity of tempering,and thus achieve the effect of improving the thawing efficiency and maintaining the quality of frozen food.In this paper,the dielectric properties of Pollack surimi were determined by open ended coaxial-line probe method.The regression equation of the dielectric constant and dielectric loss was obtained.Based on the dielectric property,a numerical simulation of the temperature distribution during radio frequency tempering of Pollack surimi was carried out by application of COMSOL Multiphysics software,to obtain the physical process of radio frequency tempering for dynamic numerical simulation,the finite element method was used to solve the electromagnetic wave dissipation and heat transfer coupling equation,and was experimentally verified by conducting radio frequency tempering on Pollack surimi.Finally,the effects of natural thawing,running water thawing,direct heating and intermittent heating of radio frequency on thawing efficiency and quality of frozen Pollock surimi were compared.The main conclusions of this study were as follows:1.The dielectric constant of Pollack surimi decreased with the increase of the frequency.While with the increase of temperature,the dielectric constant changed quadrially:first increased and then decreased.The loss factor decreased with the increase of frequency.The loss factor of frequency at 27.1MHz was much higher than that at 915 MHz.The penetration depth of frequency at 27.1 MHz was much higher than that at 915 MHz.So microwave has the limitation of low penetration.Radio frequency is more suitable than microwave to temper large materials.2.The temperature distribution of the upper surface,middle layer and lower surface of the surimi block was the same.The numerical results agreed with the experimental results.The cold temperature point of each layer was always located in the center of the surimi,while the hot spot was always located in the corner area of the surimi.The temperature of the middle layer of the surimi was significantly lower than the upper and lower surfaces,and the upper surface temperature of the surimi was higher than the lower surface.By comparing the experimental value with the measured,it was shown that the electromagnetic wave dissipation and heat transfer coupling equation obtained by COMSOL Multiphysics software could accurately simulate the temperature change of the radio frequency heating process,which could be used to predict the temperature distribution of the material.3.In terms of the rate of tempering,the monitoring point 5 and point 6 were the slowest,and the monitoring point 8 and point 3 were the fastest.Most of the measured temperature values of 9 monitoring points in the surimi sample fell on the simulated temperature change curve.It could be seen that the measured value was basically in agreement with the simulated value.This mathematical model could better predict the temperature change of the radio frequency heating process of the surimi.4.From the orthogonal test results,it could be seen that the order of the influence of temperature distribution was that RF power（A）>Electrode gaps（B）>Intermittent time（C）;The order of the effects on the time required for the thawing was Electrode gaps（B）>RF power（A）>Intermittent time（C）.According to the comprehensive balance method,the best thawing parameter of the surimi was A₁B₃C₃.The test results were in line with the optimized results.5.It is only 8.2 min for thawing that the central temperature of Pollack surimi could be reached to-3℃by radio frequency heating,while It requires 195 min to reach over-3℃the central temperature of Pollack surimi by the air thawing.The radio frequency heating has the characteristics of the whole heating,and the heat transfer process can make the surimi inside and outside at the same time,and the heating is rapid.It can shorten the thawing time to 1/20 of the traditional method.However,the temperature distribution of Pollack surimi by the traditional air thawing and the water thawing was more uniform,and the local superheat appears in the corner of the Pollack surimi during the radio frequency thawing,and the intermittent heating of radio waves could effectively delay the excessive increase of the surface temperature of the surimi.6.There was no significant difference between the water holding capacity and whiteness of Pollock surimi thawed by radio frequency compared with natural thawing and running water thawing.The water holding capacity of Pollack surimi after radio frequency intermittent heating was 78.8%and the whiteness value was 61.7.The radio frequency thawing could effectively inhibit the growth and reproduction of the surimi microorganism,and the total number of colony was only 4.9×10³ CFU/g.There was no significant difference in the distance of the surimi depression by four thawing methods.Air thawing had the largest gel strength which could reach 492.72 g?cm.While the gel strength of surimi treated by radio frequency intermittent heating was 445.62 g?cm.In addition,there was no significant difference in the salt-soluble protein content in surimi by water thawing and radio frequency heating treatment,and the highest concentration of salt-soluble protein content in Pollock surimi with air thawing was 1.87 mg?mL^-1,There was no significant difference on the ATPase activity of the surimi by the four thawing methods.The ATPase activity of radio frequency intermittent heating was slightly better than that of radio frequency continuous heating.The ATPase activity of the two thawing methods was 5856 U?L^-11 and5841 U?L^-1.RF heating treatment would not significantly reduce ATP activity and maintain the salt-soluble protein content.Radio frequency intermittent heating treatment could better maintain the gel strength and salt-soluble protein content.Regarded as a potential thawing method,radio frequency intermittent heating has a high efficiency of tempering which could maintain the quality of frozen food.