| Pork is the main body of meat consumption in China,in which frozen pork is not only the main form of consumption and circulation,but also an important part of meat reserves in our country.Frozen pork must be thawed before further processing or consumption,and the thawing process has an important impact on meat quality.Compared to traditional thawing methods with slow thawing rate and high risk of pollution,radio frequency(RF)thawing technology has attracted wide attention in recent years because of its advantages of fast thawing rate and deep penetration.The poor uniformity heating is the main obstacle in the commercial application of RF tempering technology.The uniform temperature of RF tempering is related to the composition,shape and thickness of the products to be tempered.If we can reveal the influence of the characteristics of the products to be thawed(including component distribution,shape and thickness,etc.)on the uniform temperature of RF tempering,it will help to speed up the commercial application of RF tempering technology.To sum up,this study taken frozen pork as the research object.Firstly,the dielectric and thermal properties of frozen pork are measured and analyzed,and then from the form of component distribution,the effects of thickness changes and irregular shapes at different positions in the sample on the temperature uniformity of RF tempering were studied.The specific research contents were as follows:(1)The dielectric properties and thermal properties of lean meat,fat meat and fat-lean pork were measured in the temperature range of-20～20°C and-20～20°C at the frequency of 2～302MHz.The effects of frequency and temperature on dielectric properties(dielectric constant and loss factor)and the effects of temperature on thermal properties(thermal conductivity and specific heat capacity)were analyzed and explained respectively.(2)Taking 8×8×8 cm~3pork blocks with equal volume of fat and lean meat as an example,numerical simulation was carried out by COMSOL Multiphysics(?)to study the effect of 12 different combinations of lean meat and lean meat on RF homogenization.(3)Taking a sample with a single component(pure lean)and a size of 14×10×2 cm~3as an example,the COMSOL Multiphysics(?)finite element simulation software was used to study the effect of varying degrees of thickness at different positions(edges,corners and middle)on the temperature uniformity of RF tempering.(4)Based on the research content of the previous chapter,the RF tempering temperature distribution of samples with complex shape and uneven thickness was studied by means of three-dimensional scanning technology and numerical simulation.Finally,the following conclusions were drawn:(1)The dielectric properties of pork are affected by frequency,temperature and moisture content of components.At the same temperature,the dielectric properties decreased with the increase of frequency,and the decrease was the fastest between 2～50 MHz.At the frequency of 27.12 MHz,the dielectric properties increased slowly from-20℃to-10℃,and no obvious difference among the three samples with different compositions.Between-10℃and 0℃,the dielectric constant increased rapidly with the increased of temperature,and the higher the moisture content was,the higher the water content was.After the temperature reaches 0℃,the growth rate of dielectric constant slowed down(basically tends to be stable),while the dielectric loss factor continues to increase at a higher rate and the difference among the three was increasing.Except that the specific heat capacity of the three types of meat increased rapidly at first and then decreased rapidly between-5℃and 5℃,the specific heat capacity of the three types of meat remained unchanged in other temperature ranges.On the other hand,the thermal conductivity decreased sharply at0℃,and the basic temperature was basically unchanged at other temperatures.(2)Under the condition that the degree of aggregation of the components is equal,when the fat meat of the component was located in the upper part of the sample or when the lean meat was mixed in the middle of the sample,a better thawing temperature uniformity can be obtained.When the component aggregation degree is equal but the distribution direction is not consistent,the component parallel electric field direction can improve the tempering temperature uniformity(up to 43%);reducing the component aggregation degree was helpful to improve the tempering temperature uniformity.(3)Under the condition of equal thickness,the corner position has the greatest influence on the hot spot temperature of the sample(=45.6℃),and the influence of the middle part was the least(ΔT<0.1℃).Under the thickness change of±25%,the middle part has the greatest influence on the cold point temperature,and there was basically no difference among the three different division forms of the middle part.Under the thickness change of±50%,except that the change of the corner position has a great influence on the cold point temperature,the cold point temperature under the other position change was basically the same as the cold point temperature obtained under the thickness change of±25%.Under the condition of the same thickness,the edge position has the greatest influence on the overall temperature uniformity of the sample,followed by the edge position,and the middle part has the least influence(ΔTUI<0.005).(4)The change of the thickness in the middle position of the sample has little effect on the surface temperature of the sample(ΔT<5℃),while the increase of the thickness of the corner position of the sample is easy to cause overheating and the overheating degree of the bottom position of the sample is larger.The geometric profile files of irregular samples are obtained by three-dimensional scanning technology,which can be used to accurately predict the surface temperature distribution of RF thawed samples. |
PDF Full Text Request