Radio frequency (RF) assisted disinfestation of red flour beetle (Tribolium castaneum) in stored canola seeds (Brassica napus L.)

To control the insect pests in stored grains, conventional methods (fumigation, controlled atmosphere, hot and cold air, and ionizing radiation) have been applied, however those methods have major drawbacks (remaining toxic chemicals, inefficient heating, long treatment time, etc). One of the alternative methods for the disinfestation is radio frequency (RF) heating based on electromagnetic radiation. The main advantage of the RF heating over thermal gradient driven conventional heating methods is volumetric and selective heating. It is governed by the characteristics of the electromagnetic energy, and the electrical, thermal, and physical properties of materials. Dielectric property of bulk canola seed (Brassica napus L.) and red flour beetle (Tribolium castaneum, Herbst, 1797) were measured at different moisture contents (5%, 7%, 9%, and 11%), temperatures (30 to 80°C) and frequencies (5 to 30 MHz) using a precision LCR (L : inductance, C : capacitance, and R : resistance) meter and a dielectric test fixture. The dielectric properties decreased with increasing frequency regardless of temperature or moisture content (MC), but increased in proportion to increasing temperature or MC. Other properties (specific heat, thermal conductivity and thermal diffusivity) of the bulk canola seeds increased with temperature or MC. On the other hand, density and emissivity decreased with temperature or MC. Properties (specific heat, thermal conductivity and dielectric properties) of the red flour beetle increased in proportional to increasing temperature, which the excepting of a density, which decreased with increasing temperature. Temperature distribution in the bulk canola seed at various volumes (small – 196.3 cm3, medium – 1,766 cm3, and large – 6,503 cm3) and MCs (5%, 7%, 9%, and 11%) was determined during RF heating (1.5 kW, 27.12 MHz). The temperature of the small and the large volumes (having relatively larger top and bottom surface areas than the electrode) of seeds decreased from the center toward the wall region due to heat losses from the outer boundary of the seed. The contrast, in the case of the medium volume of seed, a relatively higher temperature at the region between the middle and outer boundary was observed due to the edge effect. A 2% agar gel was used to demonstrate the effect of sample volume on the formation of the electric field affecting the RF heating pattern. The temperature distribution of the 2% agar gel was in good agreement with that of the bulk canola seed. The mortalities of the red flour beetle infesting canola seeds of the small and medium volumes during RF heating were determined at different end temperatures (30 to 80°C) and initial seed MCs (5%, 7%, 9%, and 11%). Complete mortalities of the adult insects were achieved at 80ºC for the small volume of the seeds at all MCs, at 60ºC and 9% and 11% MCs, and at 70ºC and 5% and 7% MCs for the medium volume. Complete mortalities at the other life stage (pupa, larva, egg) of the insect were achieved at 55ºC at all MCs and volumes. Over 95% mortality of the adult insects was achieved at over 60ºC temperature at any MCs and volumes. Germination, major and minor axes, roundness and colour of the seeds and quality of the seed oil were not affected significantly by RF treatment temperature at all MCs. There is potential to realize complete mortality at all life stages of the insect infesting the seed at 60 ºC by RF heating, regardless of the volume of the seed sample, with proper design to achieve more uniform temperature distribution and shorter RF heating time. The thermal death kinetics of the adult red flour beetles infesting the stored canola seed at various MCs (5%, 7%, 9%, and 11%) and volumes (small and medium) of seed were characterised based on the dynamic temperature increment of the seed and experimental data of the insect mortalities during RF heating. The first order reaction model was the most satisfactory to describe the thermal death kinetics of the adult red flour beetle. The determined activation energy (100 kJ/mol) for the thermal death of the adult red flour beetles was lower than that for some other insects (codling moth, Indianmeal moth and navel orangeworm) determined from experiments with a heating block system, which indicated that the adult red flour beetle was more susceptible to thermal death of RF heating than the other insects heated using the heating block. Selective heating of the red flour beetle in two different volumes of bulk canola seed at 5%, 7%, 9% and 11% MC using the RF heating unit was simulated using the finite element method based commercial simulation package (COMSOL Multi-physics). The simulated temperatures of the seed and insects were compared with measured temperatures at the various MCs and all volumes. Non-uniform RF heating of the seed was observed regardless of the MC or volume. The RF selective heating of the insect was most effective for the small volume of seed at 11% MC. The temperature of the insects was a maximum of 14.6°C higher than the temperature of the seed. As an application of the developed simulation, the effect of voltage (tested with half and double the estimated voltages from the previous simulation) on RF heating uniformity and selective RF heating was determined by comparison of simulated temperature distributions of the seed at 11% MC and the insects. The RF heating uniformity in the seeds improved and the selective RF heating of the insects was enhanced with increasing voltage, regardless of the volume of the seed. To improve RF heating uniformity, mixing the seed with a rotating auger during the RF heating was considered. From further simulations, uniform RF heating of the seeds was observed near the outlet by help of mixing effect