This project was to see whether radio frequency (RF) energy, at the industrial scale, could be used to kill insect pests in stored wheat grains and canola seeds without damaging their product qualities. We found out this project was much more challenging than we thought in terms of amount of work, technical issues on electronics and electromagnetics, and budget. Nevertheless, we consider this project was successful in general.
We built a pilot scale or semi-industrial scale RF heating system (15 kW, 27 MHz, 50 ohm technology based) including an applicator with parallel arrangement of electrodes. We were able to match impedance with respect to basic electrical parameters (voltage standing wave ratio, inductance, capacitance, and frequency) for maximum power applied to the wheat and canola. We measured dielectric, thermal and physical properties of canola, and developed models that relate these properties to temperature, moisture content, and electromagnetic wave frequency. We determined heating uniformity by determining the temperature distributions in a packed-bed of wheat/canola during RF disinfestation at lab and pilot scales. Using 3D simulation software, COMSOL Multiphysics and a high-speed/RAM computer, we successfully simulated the selective heating of the insect pests in stored wheat and canola by 27 MHz RF waves and could explain the inherent non-uniform temperature distribution during standard RF heating. For the 3D computer simulation, electromagnetic energy transfer, heat transfer, and fluid flow were computed simultaneously with proper boundary conditions including surface-to-surface thermal radiation. The developed simulation program was used to check heating efficiency with two different applicator designs (parallel, and rod and cylinder) for with input variables. We found out the parallel arrangement was much more efficient than the other was. We tested the immediate and delayed mortalities of the red flour beetle infesting the stored canola seeds and rusty grain beetle in stored wheat grain during RF infestation. The effects of moisture content, the bulk volume, and the final temperature of the infested wheat and canola on the mortality of the insects were determined. It seemed that with higher heating rate (higher power), lower activation energy in the thermal death kinetics was observed. In general, the insect pests were killed at the grain/seed temperature of around 70°C for 5 min treatment for wheat and around 65°C for 3 min for canola. This means the average product temperature during disinfestation were 33-35°C as the product temperature increased almost linearly. The results of the physicochemical properties after RF disinfestation was very promising as no significant damages were observed for wheat (germination, color, baking and milling qualities) and canola (oil quality, color, and germination). For germination of higher moisture content wheat (15% and 18%), there were slight decrease in germination. However, their moisture contents are considered higher for normal storage condition. For some cases, even the germination rate slightly increased compared to control (untreated).
From this project, we can conclude that radio frequency disinfestation is very effective to control insect pests in stored wheat and canola at industrial scale. In order to kill insect pests in 32 kg of wheat or canola it takes 3 to 5 min at 7 kW (384 to 640 kg per hour). Thus, using a large-scale commercial unit at 700 kW, 38 to 64 ton of wheat or canola can be disinfested without chemicals and damage on product quality.