The Design Of Double Screw Conveyor That Suitable For Radio Frequency Electromagnetic Field And Its Application Of Disinfestation Of Jujube

As a dielectric heating technology,radio frequency(RF)heating has been widely used in post-harvest processing for agricultural products due to its greater penetration depth,rapid and volumetric heating,selective heating,and non-chemical pollution.However,non-uniform heating is a major obstacle for its commercial applications in the food industry,especially in the continuous RF processing.Therefore,in this study,a double screw conveyor that was suitable for the RF electromagnetic field was designed and the heating uniformity was improved by continuously mixing and moving during the RF treatment.The influence of the operating parameters on the heating rate and uniformity was investigated and the quality parameters of samples under screw conveying with the best operating parameters were compared with under the stationary status in RF treatment.After that,the disinfestation of jujubes was regarded as a case and the heating uniformity,insect mortality,sample burning ratio,and heating efficiency during RF treatment were compared under moving and mixing and only moving condition.The characteristics of two motion forms of screw conveying were determined during RF treatment,which promoted the development of RF technology.The major contents and conclusions were as follows:(1)Jujubes were selected as the target samples and a double screw conveyor suitable for RF treatment was designed,which led the simultaneous conveying and mixing of samples during continuous RF treatment to improve the heating uniformity.The technical requirements of the equipment and the relevant technical parameters were listed.Combined the above requirements and the technological requirements of RF disinfestation of granular food,the overall plan and workflow were developed.The equipment adopted horizontal structure with double screw stems,and its screw configuration adopted meshing type.The screw conveyor was composed of frequency converter,drive motor,transmission device,screw stem and conveying groove.The power of equipment was provided by the driving motor and transmitted to the screw through the coupling.The stepless regulation of screw rotation speed was realized in the range of 0 to 4 rpm under the control of frequency converter.(2)A double screw conveyor was used to investigate effects of conveying direction,screw rotation speed,filling ratio,and electrode gap on the heating rate and uniformity of jujubes.The results showed that there was no significant difference between forward and backward conveying directions both in heating rate and uniformity.The screw rotation speed had no significant effect on heating rate,while the RF heating uniformity gradually improved with increased screw rotation speed.RF heating rate increased gradually with the increase of filling ratio or decrease of electrode gap.Except for the significantly higherλvalue found in the filling ratio of 80%,no significant changes were observed between 20and 60%.Furthermore,theλvalues also increased with decreasing electrode gap.Quality evaluation indicated that there were no significant differences in the color values,texture properties,ascorbic acid,and moisture content between control and samples treated by RF heating under the double screw conveying.(3)Dried jujubes at three grades were used as a model food for comparative RF disinfestation under moving and mixing and only moving conditions.The results showed that the heating rate increased with decreasing electrode gaps and increasing sample grade.The heating uniformity index showed a decreasing trend with the decrease of sample particle size,however,significant decreasing can only be observed in the only moving condition.Furthermore,under moving and mixing condition,the λ values decreased and then increased with increasing screw rotation speeds,while in only moving it increased continuously with increasing moving speeds.Significant burning status and lower insect mortalities can be observed under the only moving condition as compared to moving and mixing condition,even at the lowest conveying speed.Heating efficiency of RF disinfestation increased with increasing conveying speeds both under screw conveying modes.