| The purpose of this study is to contribute to the promotion of fish dielectric properties,additives detection and RF tempering technology.Dielectric properties describes the ability of matter in electromagnetic field to interact with electromagnetic wave.The research on the dielectric properties of food,especially its radio frequency(electromagnetic wave of 300 KHz-300 MHz)and microwave(electromagnetic wave of 300 MHz-300 GHz),has led to a different method from the traditional heating methods of convection,conduction and radiation :dielectric heating at 1940 s.Subsequently,the dielectric properties of food and the development of microwave heating equipment has become the focus of research.The research on the dielectric properties of food is helpful to understand the dielectric behavior of food in the electric field and provides the basis for making full use of energy and developing more economical dielectric heating equipment.The dielectric properties of food are related to the compositions,such as water,salt,fat and the structure of food.Therefore,the dielectric properties can also be used in the detection and the processing of food,providing reference for the research in the field of dielectric properties in China.RF heating,as a kind of dielectric heating,is becoming a promising food processing technology due to the advantages of fast heating speed,uniform heating,strong penetration and low energy consumption.RF heating can not only be used for food heating and drying,but also has a good performance in food thawing.This paper includes the basic concept,influencing factors and related applications of dielectric properties,the application of RF tempering technology in fish products and the future prospect of RF tempering.Dielectric heating has been applied in the fishing industry for thawing and cooking.However,the difference in dielectric properties for various species of fish can greatly influence the heating rate and uniformity.In this study,the dielectric properties of saltwater fish(Pomfret and Hairtail)and freshwater fish(Bream and Crucian Carp)were determined within the frequency range of 1-2500 MHz at-20 °C to 100 °C.Results showed that the dielectric constant(ε’)and dielectric loss factor(ε ")of the four different species decrease with increasing frequency.The dielectric constants at 27.12 and 40.68 MHz increase with increasing temperature,and those at 915 and 2450 MHz increase rapidly and then decrease.The ε’ of Hairtail showed the highest increase,followed by Bream,Crucian Carp,and Pomfret;and the ε" of Hairtail showed the highest increase,followed by Pomfret,Bream,and Crucian Carp due to the composition difference between the species.Thus,the dielectric heating process for various species of fish can be considerably different at different frequencies.Furthermore,the most significant difference in dielectric properties among all species of fish was observed at 27.12 MHz,which indicated that this frequency could be the most suitable frequency for the detection of fish species based on their dielectric properties.Dielectric properties of food are affected by its components and measuring frequency.Thus,dielectric properties can be used to predict additives and their contents in original foods.In this paper,silver carp surimi and surimi products are taken as research objects,cryoprotectant,corn starch and soybean protein isolate(SPI)were used as additives,the effects of cryoprotectant(0.3 %),SPI(0 %,3 %,6 %,9 %,12 %)and starch content(0 %,5 %,10 %,15 %,20 %,25 %,30 %)on dielectric properties of surimi and surimi products were studied in a certain frequency range(1-2500 MHz).The results showed that the dielectric constant(ε’)and dielectric loss factor(ε")of silver carp surimi decrease with the increase of frequency in the measured frequency range.The ε’ of silver carp surimi with cryoprotectant was higher than that without cryoprotectant at 27.12,40.68 and 915 MHz,and there was no significant difference at2450 MHz.The ε" of silver carp surimi with cryoprotectant was higher than that without cryoprotectant in the frequency range.The ε’ of silver carp surimi decreases with the increase of SPI content at 27.12 and 40.68 MHz,and increases with the increase of SPI content at 915 and 2450 MHz,while the ε" decreases with the increase of SPI content.The ε’ and ε" of silver carp surimi showed the same trend with the increase of starch content as with the increase of SPI content.The ε’ and ε" of surimi products have the same trend as that of surimi with the increase of starch content.However,the ε’ of surimi products at low frequencies(27.12 and 40.68 MHz)is higher than that of surimi,at high frequencies(915 and 2450 MHz),the ε’ of surimi products is higher in the range of 0-20 % starch content than surimi,and vice versa in the range of 25-30 %;The ε" of surimi products were lower than surimi in the range of measured frequencies.The polynomial fitting equation of ε’ or ε" of silver carp surimi varying with SPI or corn starch content was established at a specific frequency,which can be used to predict the content of silver carp surimi additives and to develop nondestructive testing equipment for the content of silver carp surimi additives.Different systems using air,water,vacuum heat,high pressure,microwave and infra-red have so far been applied for thawing frozen fish,but there are some problems,such as slow rate,high weight loss,microbiological spoilage,chemical spoilage,overheating and high cost.RF tempering is a new type of dielectric heating technology,because the constant change of electric field always causes the ions to move in the direction of opposite charge,which makes the ions constantly collide with each other and generate heat,resulting in the direct conversion of electromagnetic energy into heat energy,which can better maintain the quality and avoid undesirable changes in the frozen fish,while obtaining a fast defrosting rate.The dielectric properties,shape,and size of the foods,the location and distance between RF parallel electrode plates and input power are important factors contributing to the temperature uniformity in RFtreated foods.In order to explore the advantages of RF thawing in thawing aquatic products compared with traditional hydrostatic thawing,this paper studies the thawing characteristics and quality of RF thawed tilapia fillet under different input power(600W,800 W,1000 W)and different distance between RF parallel electrode plates(10 cm,12 cm,14 cm),then compared with the water tempering(with or without package),the following conclusions are drawn: Compared with the water tempering,the RF tempering has a smaller thawing loss rate and a more uniform surface temperature distribution.In terms of cooking loss rate,p H value,water holding capacity and other indicators,the quality of tilapia fillets can be well maintained by both water and RF tempering;When the conveyor belt is opened,the temperature uniformity of the sample surface after thawing is slightly better than that after static thawing,which has no significant effect on the quality of fish. |
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