Design And Research Of Auxiliary Device For Radio Frequency Sterilization Of Spices

China is the largest spice condiments producer and consumer.However,spice condiments are easily contaminated by microorganisms in the process of production,processing,transportation and storage,which cause great economic losses and even cause food safety problems.Research has shown that radio frequency(RF)sterilization technology is an efficient,environmentally friendly,and safe sterilization technology with great potential for application in the field of solid seasoning sterilization,but uneven heating is the main obstacle for the industrial application of this technology.Therefore,this study took spice condiment as the research object and a numerical simulation model for continuous rotational heating of spice condiment in the RF field has been established by the finite element software COMSOL.The influence of parameters such as inclination angle,rotation speed and sample size on the RF heating uniformity of condiment was analyzed through the model.Then,an inclined rotary RF auxiliary heating device was designed,and the effects of changes in sample bulk density,shape,and dielectric parameters on the heating uniformity during the rotary heating process were further studied using this device,to find the best scheme for improving the RF heating quality of spice condiments.The major research contents and results are as follows:(1)A numerical model of RF heating of red pepper powder(RPP)with continuous rotating was established by a finite element software COMSOL,and the model was verified by RF heating experiments.The verification results showed that under the condition of horizontal rotation,the temperature distribution and heating rate of RPP at each layer were similar,and the maximum average temperature difference on the surface was about 2.57°C,which showed that the simulation and experiment had a good agreement.The effects of rotation speed,inclination angle and sample size on heating uniformity were further analyzed by using the verified model.It was found that when the inclined angle was 0°,the change of rotation speed had no effect on the RF heating quality of RPP.When the inclined angle was greater than 0°,the heating uniformity index first decreased and then tended to be gentle with the increase of rotation speed.The minimum heating uniformity index could be obtained at a rotational speed of 0.64-1.28 rpm.This speed range was also applicable for different sample sizes.When the rotation speed was 0.64 rpm,the heating uniformity index of RPP first decreased and then increased with the increase of the inclination angle.For samples with heights of 23 mm,33-53 mm and 63 mm,the heating uniformity was best at inclination angles of 20°,25°,and 30°,respectively.(2)An inclined rotary device suitable for RF heating system was designed.The parts of the device inside the top and bottom electrodes were made of non-metallic materials and would not be heated during RF heating.The device could achieve stable rotation of materials on the inclined surface,with convenient adjustment of rotational speed.The device was applied to the inclined rotating heating experiment of RPP(20°,0.64 rpm),and compared with the samples heated with horizontal rotation.The results showed that the application of the inclined rotation device increased the heating rate of the sample by 68.8%,and the maximum temperature difference was reduced from 21.9°C to 10.2°C.This improved the heating efficiency of the material while also improving the heating uniformity,proving that the device can be further used for subsequent experimental research.(3)RF heating experiments were conducted on samples with different dielectric parameters,bulk density,filling ratio,and rotation speed using an inclined rotation device.The results showed that the heating uniformity and heating rate of samples with inclined rotation were better than those with horizontal rotation under different dielectric parameters.When the bulk density changed from 340 kg/m~3 to 420 kg/m~3,the heating uniformity and heating rate of the sample could not change significantly(P>0.05).Decreased the filling rate(FR)or increased the rotational speed would affect the RF heating effect of the material with inclined rotation.For the sample with 80%filling rate,the heating uniformity index of the top and bottom layers of the sample at high rotating speed(rotating speed:125 rpm)increased by23.75%and 41.27%,respectively,compared with the sample at low rotating speed(rotating speed:1 rpm).In summary,this study provides a new method for improving RF heating uniformity and lays a solid theoretical foundation for the industrial promotion and application of RF heating technology.