Developing A Rotation Device In Radio Frequency Systems For Improving The Heating Uniformity And Disinfestation Efficacy In Coix Seeds

Coix lachryma jobi L.known as the king of cereals because of its rich nutrients and widely used in food processing.The Coix lachryma jobi L.is the product of coix seed,which has been shelled,polished and dried.At present,chemical fumigation is the main method to control insect of coix seed and other grains.Quarantine treatments using the chemical fumigant method emerged many problems,including environmental pollution and pest resistance.Since the chemical residues on agricultural products may affect human health,the demand for nonchemical disinfestation of agricultural products is increasing.Radio frequency(RF)energy with the longer wavelength results in deeper penetration depth than microwave,and can be used to heat larger size samples.Compared with traditional heat convection,heat conduction and thermal radiation,RF energy can heat bulk agricultural products rapidly and volumetrically.RF heating has been widely applied in disinfestation.However,a major obstacle for industrial RF treatments is non-uniform and run-away heating.The research showed that rotation was an effective way to improve the RF heating uniformity.The RF heating uniformity was influenced by the rotating speed,volume ratio,moisture content and particle diameter of samples.Based on the research results of thermal lethal kinetics of the most heat-resistant insects(Rhyzopertha dominica)in coix seed,four different target temperatures and different holding times were selected to explore the effect on disinfestation efficacy of RF heating.The main research contents and results are as follows:(1)Developed a sample rotation device in RF heating system.The rotation device was completed from design to production,fabrication and measurement.Based on the rotation device,the driving part could stably control the rotation speed through the motor controller during the RF heating.The coupling connected the working part with the driving part,which ensured the interchangeability and reduced the cost of manufacture.Besides that,the friction wheel transmission achieved simple operation,safety and stability.(2)RF heating uniformity test with low moisture granular samples.The RF heating uniformity was influenced by the rotating speed,volume ratio,moisture content and particle diameter of samples.The results showed that the RF heating uniformity index λ decreased and then increased with increasing rotation speed,but increased and then decreased with increasing volume ratio.The RF heating uniformity index increased with increasing moisture contents.The best heating uniformity was obtained at the rotating speed of 16 r/min,volume ratio of75.0%,and in mung beans(8.15 ± 0.12% w.b.).The rotation device could be considered as an effective method to improve the RF heating uniformity for particle samples.(3)Exploration of RF disinfestation efficacy of coix seed based on the rotation device.According to the optimal process parameters for improving the RF heating uniformity,the effect of hot air on the heating rate of coix seed was verified,and the heating time was re-determined.Then,RF disinfestation treatment was carried out at different target temperatures and holding time with stationary or rotation conditions,respectively.The results showed that hot air assisted RF heating coix seed samples would raise the heating rate slightly to 2.3 ℃/min.The mortality rate of Rhyzopertha dominica after RF heating to target temperature and product quality under the given conditions was verified.In the process of RF disinfestation,the rotation could reduce the holding time required for 100% mortality of Rhyzopertha dominica and holding time of 0min at 55 °C was the best disinfestation protocol.