Comparative Studies on Continuous Radio Frequency Treatment of Granular Foods Under Belt and Double Screw Conveying: A Case Study of Disinfesting Dried Jujubes

To investigate the influences of conveying modes and sample particle sizes on the RF heating characteristics of granular foods, dried jujubes at three grades were used as a model food for RF disinfestation under screw and belt conveying. The heating rate and uniformity index (λ), insect mortality, product quality (burning ratio), and heating efficiency were compared under the equivalent conditions of these two conveying modes. The results showed that the heating rate increased with decreasing electrode gaps and increasing sample particle sizes. The λ values showed a decreasing trend with the decrease of sample particle size, however, significant decreasing can only be observed in the belt conveying. Furthermore, under screw conveying, the λ values decreased and then increased with increasing screw rotation speeds, while in belt conveying, they increased continuously with increasing belt speeds. Significant burning status and lower insect mortalities can be observed in belt conveying as compared to screw conveying under equivalent conditions, even at the lowest conveying speed. Heating efficiency of RF disinfestation increased with increasing conveying speeds both under screw and belt conveying modes, but no significant differences were observed between the two conveying modes except at the lowest conveying speed. The results of this study may provide valuable information in guiding the industrial scale RF treatment under screw and belt conveying modes to obtain better product quality with higher throughput.