Effects Of Movements And Rearrangements Of Milled Rice On Radio Frequency Heating Uniformity And Disinfestation

China is the largest milled rice producer and consumer in the worl.However,the infestation by various pests causes significant quality and economic losses and even food safety problems.At present,chemical fumigation has been the most widely used treatment for insect control,but brings environmental pollution concerns,adverse effects of chemical fumigation on increased insect resistance,and chemical residues in agricultural products are harmful to human health,and therefore people have the increased demand of non-chemical alternative disinfestation methods for milled rice.In recent years,radio frequency(RF)heating has been extensively used for postharvest disinfesting agricultural products due to its volumetric and fast heating,selective heating,without chemical residue,and self-balance of moisture content.However,the heating uniformity is one of the obstacles for the industrial application of RF heating technology.Therefore,a 6 k W,27.12 MHz parallel plate RF heating system was used to investigate the effects of various directional movements/rearrangements of milled rice on RF heating uniformity.Based on the research results of thermal lethal kinetics of the most heat-resistant pests(Rhyzopertha dominica[Fabricius])in milled rice,three moving modes were selected to explore the effect on disinfestation efficacy of RF heating.The main research contents and results are as follows:(1)Effects of various directional movements of milled rice on RF heating uniformity.The sample size of milled rice in a container was 270×185×42 mm~3,weight was 1.5 kg,and the target temperature was 54℃.The electrode gap was determined to be 95 mm with the heating rate of 4.5℃/min,so the heating time was determined to be 6.5 min.During the test,the milled rice was moved in two horizontal directions and a vertical direction,and the horizontal movement included moving from the inside and outside of the electrode plate.The results of experiments showed that the electric field between the top and ground electrodes of the RF heating system was relatively uniform,and there was no significant improvement in sample movement inside the electrodes.The RF heating uniformity of rice samples was improved by moving in and out of the electric field.The vertical movement also improves the heating uniformity and reduces the temperature distribution difference among the three layers.(2)Effects of various directional rearrangements of milled rice on RF heating uniformity.The RF processing parameters were same as before,and three types of rectangular plastic containers with the same size were verified to have no significant influence on the RF heating uniformity.At the two-time steps where the heating time was divided into three parts,the rice sample was rearranged in two horizontal directions and a vertical direction.The same method was used for mixing test.The results of experiments showed that the horizontal direction rearrangement could reduce the overheating in the corner and edge of the sample and improve the RF heating uniformity.The vertical layer rearrangement also improved the uniformity of RF heating and reduced the temperature and uniformity difference among three layers.Mixing was the best way to improve the uniformity of RF heating,and by considering the effectiveness and simplicity of heating process,the treatment with mixing two times was the best protocol for the milled rice samples in this test.(3)The effects of movement,layer rearrangement and mixing two times of milled rice on disinfestation rate of RF heating.Firstly,the effect of hot air on the heating rate of milled rice was verified,and the heating time was re-determined.Then,the RF treatment protocols in disinfestations were carried out under the conditions of stationary,moving,layer rearrangement and mixing two times,respectively.The results of experiments showed that adding hot air during RF heating of milled rice samples would raise the heating rate slightly to 5.1℃/min,and the RF heating time with hot air was determined to be 6 min.RF heating with hot air improved the uniformity in rice samples,especially at the top layer of the sample.When sample was heated to 54℃,and holding at hot air for 13 min,100%mortality of pests was obtained.The uniformity of the sample became better with the extension of holding time.In the process of RF heating,the holding time for 100%mortality of Rhyzopertha dominica could be reduced by movement,layer rearrangement or mixing,which were 9,11 or 7 min,respectively.Therefore,this research provides a theoretical support for improving RF heating uniformity,optimizing the parameters of RF treatment protocols,and promoting industrial-scale implementations in disinfestations of agricultural products.