Radio Frequency Heating Uniformity Research Articles

Radio-frequency treatment for stabilization of wheat germ: Dielectric properties and heating uniformity

Abstract Wheat germ (WG) is a valuable by-product of wheat milling but is highly susceptible to lipid rancidity induced by lipase (LA) activity. Therefore a stabilization step is required to reduce LA activity. Radio-frequency (RF) heating is an alternative thermal technology suitable for processing food and has great potential for WG stabilization. However, in order to develop effective RF stabilization treatments, a knowledge of the dielectric properties (DPs) (i.e. dielectric constant (e″) and dielectric loss factor (e″)) of WG at frequencies used for heating is required. Furthermore, an understanding of the influence of RF processing conditions on heating uniformity and LA inactivation is needed. Findings in the present study show that e′ and e″of WG increased with increasing temperature and moisture content (MC). Furthermore, the relationship between DPs and temperature/MC can be described by quadratic order models. RF heating uniformity in WG gradually improved with decreasing heating rate from 19.6 to 6.3 °C/min and MC from 15.96 to 7.05% (wet basis, w.b.). Placing the WG on a moving conveyor at 4 m/h slightly improved RF heating uniformity in the interior layers of the WG. Placing a pair of rectangular polyetherimide plates (12 × 8 cm) above and below the container during RF heating at 6.3 °C/min further improved temperature distribution leading to a more uniform LA inactivation in WG. Overall, the results obtained from this study are useful in computer simulation and optimizing process parameters for WG stabilization by RF heating.

Radio frequency heating uniformity evaluation for mid-high moisture food treated with cylindrical electromagnetic wave conductors

Abstract Non-uniform heating is an important obstacle for applying radio frequency (RF) energy in food processing, especially for the material with high moisture content. To further extend wide applications of the RF heating uniformity improvement based on our previous study with cross electromagnetic wave conductor (EWC), a novel and effective method with cylindrical electromagnetic wave conductors and cylindrical containers was introduced in this study to improve the electromagnetic energy distribution inside the sample with mid-high moisture content. The associated computer simulation model with cylindrical EWC and container was also developed and validated based on RF experimental results to evaluate the heating uniformity. The results showed that the parameters of EWC (diameter and height) had a positive effect on the RF heating uniformity index. The sample treated with cylindrical EWC had better heating uniformity but lower temperature than that treated with cross EWC based on the comparison results. The improved target uniformity index (TUI) and the decreased heating time also indicated the positive effects of cylindrical EWC. A simplified structure for cylindrical EWC was developed and evaluated by computer simulation, which may provide potential applications of the cylindrical EWC to achieve the required RF heating uniformity in mid-high moisture food.

Experimental investigation of the combined translational and rotational movement on an inclined conveyor on radio frequency heating uniformity

Abstract The objective of this study was to explore the possibility to improve radio frequency (RF) heating uniformity by subjecting the food to RF energy on a moving inclined conveyor and horizontally rotating it about its central axis at the same time. To do this, wheat flour in a rectangular container was heated on a rotating turntable moving on an inclined conveyor using a radio frequency oven operating at 27.12 MHz and a maximum power of 2 kW. A fixed electrode gap setting of 165 mm was employed. Heating experiments were also conducted without rotating the sample both on inclined conveyor and on horizontal conveyor positioned at two different heights between the RF electrodes. Temperature at four different internal locations was continuously measured using fiber optic probes. Temperature at the top surface and the mid-layer was also measured using an infrared camera immediately after heating. The combined translational and rotational movement of sample was found to reduce temperature gradients within the rectangular volume allowing to reach the target temperature in a more controlled manner. The proposed approach can be easily implemented in the industry to improve uniformity of heating.

Pilot-scale radiofrequency blanching of potato cuboids: heating uniformity.

Traditional hot water blanching has a slow heat transfer rate, whereas radiofrequency (RF) heating has the advantages of a much faster heating rate and a higher penetration depth. In the present study, RF heating was applied to improve heating uniformity for subsequent blanching experiments involving potato cuboids. Potato cuboids were treated in a pilot-scale, RF heating system (27.12 MHz, 6 kW) under different operating conditions. The dielectric constant increased first and then decreased with temperature, whereas the loss factor increased as the temperature increased. The results of the present study reveal that the electrode gap, sample height and NaCl solution had significant effects (P < 0.05) on the temperature distribution and heating uniformity of the sample after RF heating. The optimum RF heating uniformity was obtained at an electrode gap of 120 mm, a sample height of 60 mm and when immersed in NaCl solution of 0.5 s m-1 . The central heating pattern was presented in a sample. Cold spots were located at the edge of the top surface of the sample. The present study shows the great potential of RF heating for the blanching of vegetables. Future studies should aim to determine changes in the texture and nutrient contents of vegetables during RF heating. © 2017 Society of Chemical Industry.

Computer simulation for improving radio frequency (RF) heating uniformity of food products: A review

ABSTRACTRadio frequency (RF) heating has great potential for achieving rapid and volumetric heating in foods, providing safe and high-quality food products due to deep penetration depth, moisture self-balance effects, and leaving no chemical residues. However, the nonuniform heating problem (usually resulting in hot and cold spots in the heated product) needs to be resolved. The inhomogeneous temperature distribution not only affects the quality of the food but also raises the issue of food safety when the microorganisms or insects may not be controlled in the cold spots. The mathematical modeling for RF heating processes has been extensively studied in a wide variety of agricultural products recently. This paper presents a comprehensive review of recent progresses in computer simulation for RF heating uniformity improvement and the offered solutions to reduce the heating nonuniformity. It provides a brief introduction on the basic principle of RF heating technology, analyzes the applications of numerical simulation, and discusses the factors influencing the RF heating uniformity and the possible methods to improve heating uniformity. Mathematical modeling improves the understanding of RF heating of food and is essential to optimize the RF treatment protocol for pasteurization and disinfestation applications. Recommendations for future research have been proposed to further improve the accuracy of numerical models, by covering both heat and mass transfers in the model, validating these models with sample movement and mixing, and identifying the important model parameters by sensitivity analysis.

Radio frequency heating of corn flour: Heating rate and uniformity

Abstract Non-uniform heating is a major challenge for using radio frequency (RF) heat treatment in pasteurization of low moisture food products. The objective of this study was to evaluate the effect of different electrode gaps, moisture content (MC), bulk density and surrounding materials on RF heating uniformity and rate in corn flour. Additionally, the dielectric and thermal properties of corn flour were determined as affected by MC, temperature (°C), and frequency (MHz). Changes in MC, water activity (a w ) and color in the sample after RF heating were measured to evaluate treatment effect on food quality. A precision LCR meter and a liquid test fixture were used to study DP of the sample at RF frequency ranging from 1 to 30 MHz. The RF heating uniformity and temperature profiles of corn flour as exposed to RF heating were obtained with an infrared camera and a data logger connected to a fiber optic sensor. The DP values increased with increasing MC and temperature, but decreased with frequency. The heating rate increased from 3.5 to 6.8 °C min − 1 with increasing MC (from 10.4 to 16.7%), but decreased from 12.7 to 5.2 °C min − 1 with increasing electron gap (from 11 to 15 cm). The corner and edge heating were observed at all layers of the samples for all the distances, and the hottest and the most uniform layer were determined as the middle layer at an electrode gap of 15 cm. Glass petri dish provided better uniformity than those of polyester plastic petri dish. Covering by foam led to more uniform RF heating uniformity in corn flour, and better moisture and a w distribution. This study provided useful information to develop an effective RF process as an alternative of conventional thermal treatments for pasteurization of low-moisture products. Industrial relevance This paper describes a novel methodology based on Radio Frequency heating to pasteurize food powder while maintaining the quality. The study addresses the ever-increasing global demand from consumers for safe food products.

A novel method to improve heating uniformity in mid-high moisture potato starch with radio frequency assisted treatment

An alternative method to improve treatment efficiency for industrial applications of mid-high moisture potato starch was explored based on radio frequency (RF) heating, but the large cold spot area during RF treatment was the biggest obstacle for technology development. The purpose of this study was to develop a new method to reduce the cold spot area and improve RF heating uniformity for mid-high moisture potato starch by modifying the electromagnetic field distribution in the sample with a Electromagnetic Wave Conductor (EWC). The simulation model of RF heating was established to study the temperature distribution using commercial COMSOL software. Dielectric properties of potato starch with moisture content of 31.55% w.b. were measured for computer simulation, and the associated RF experiments were conducted as the validation of the EWC method. The results showed that the parameters of EWC (width and height) had a positive correlation with the RF heating uniformity. The improved target uniformity index (TUI) and the decreased heating time indicated significant effects of EWC. Finally, optimizing EWC parameter equations were developed and could be used for improving RF heating uniformity in future applications.

Improvement of radio frequency (RF) heating uniformity for peanuts with a new strategy using computational modeling

Radio frequency (RF) heating has been considered as an alternative method to traditional thermal treatments for postharvest pasteurization and disinfestations. However, the non-uniform heating in food samples is the main obstacle to be urgently solved. In this study, mica plates were placed on top of the samples with polypropylene blocks placed in between the samples as a new strategy to improve the RF heating uniformity. A computer simulation model was developed based on the commercial software COMSOL to evaluate the temperature distributions. Experiments with peanut samples placed in three dimensions of containers were conducted to validate the simulation model. The results showed that adding mica plates with the similar dimension to the cold spot of samples and increasing the plate thickness effectively improved the RF heating uniformity. Adding polypropylene blocks raised sample average temperatures and finally optimized the RF heating uniformity.

Performance comparison between the free running oscillator and 50Ω radio frequency systems

Abstract Two common kinds of radio frequency (RF) systems (free running oscillator and 50 Ω) are widely applied in industry and research. However, their unclear performances based on different operational mechanisms influence effective implementations of the two systems. The purpose of this research was to explore advantages and disadvantages of the two systems by experimental comparison. The results showed that heating rates and energy efficiency of the free running oscillator RF system were higher than those of the 50 Ω RF system under the same conditions (e.g. powers, electrode gaps, sample sizes and moisture contents), but control accuracy of the 50 Ω RF system was better than that of the free running oscillator RF system since its RF power could be adjusted under the same electrode gap. The RF heating uniformity and stability of the 50 Ω RF system were also better than those of the free running oscillator RF system. These comparison results may provide detailed performance information of the two RF systems and help to promote further practical applications of the two RF systems in food processing industry.

Computer simulation model development and validation of radio frequency heating for bulk chestnuts based on single particle approach

A computer simulation model was developed using finite element-based commercial software, COMSOL, to simulate temperature distributions of single particle chestnuts packed in a rectangular plastic container and treated in a 6kW, 27.12MHz radio frequency (RF) system. The developed model was validated by temperature distributions of three horizontal layers and temperature profiles at three representative positions in the container without mixing. Both simulated and experimental results showed similar heating patterns in RF treated chestnuts under same conditions, in which corners and edges were overheated and highest temperatures were located in sample contact points of top and middle layers at four corners in the container. A heating uniformity index (HUI) was used to evaluate effects of processing conditions on RF heating uniformity. The simulated and experimental results showed that the HUI was reduced when three layers chestnuts were separated with two plastic sheets. The better heating uniformity in chestnuts was obtained when they were treated with a single layer, or under mixing conditions. The developed model can help to explore the RF heating patterns on a single particle chestnut and RF heating uniformity in chestnuts of the container, and provide valuable methods to improve the RF heating uniformity for future industrial applications.

Application of radio frequency pasteurization to corn (Zea mays L.): Heating uniformity improvement and quality stability evaluation

Mildews caused grain losses and serious outbreaks have been becoming increasing concerns in domestic and international corn processing industry. This study intended to explore the possibility of using radio frequency (RF) heating as an effective treatment to eliminate the mold contamination and reduce the damage to corn quality. A pilot-scale, 27 MHz, 6 kW RF unit was used to study the heating uniformity in corn samples with five moisture contents (MC) and using three plastic material containers, and develop a treatment protocol for a corn sample with the MC of 15.0% w.b. and evaluate quality attributes and storage stability of treated samples. The results showed that only 7.5 min was needed to raise the central temperature of 3.0 kg corn samples from 25 °C to 70 °C using the RF energy, but 749 min for samples to reach 68.6 °C using hot air at 70 °C. The RF heating uniformity was improved by adding forced hot air, moving samples on the conveyor, and mixing during the treatment. An effective RF treatment protocol was finally developed to combine 0.8 kW RF power with a forced hot air at 70 °C, conveyor movement at 6.6 m/h, two mixings, and holding at 70 °C hot air for 14 min, followed by forced room air cooling through thin-layer (2 cm) samples. Corn quality was not affected by RF treatments since quality parameters of RF treated samples were better than or similar to those of untreated controls after the accelerated shelf life test. RF treatments may hold great potential as a pasteurization method to control molds in corns without causing a substantial loss of product quality.

Industrial-scale radio frequency treatments to control Sitophilus oryzae in rough, brown, and milled rice

Many previously studies have confirmed that radio frequency (RF) treatments have the potential to be developed as non-chemical alternative disinfestation methods. However, most of them are conducted in laboratory scale RF systems, and it is necessary to scale up the treatment protocol for industry scale applications. A pilot-scale, 27.12 MHz, 6 kW RF system was used to simulate the continuous industrial processing and finally estimate the heating efficiency and throughput for controlling adult rice weevils, Sitophilus oryzae (L.), in milled, brown, and rough rice. An electrode gap (11.5 cm) was chosen based on the appropriate heating rate (6–8 °C/min). RF heating uniformity in the three types of rice samples was improved by adding 50 °C forced hot air surface heating, sample movements on the conveyor belt at a speed of 12.5 m/h, and holding in 50 °C hot air for 6 min. The final industrial RF treatment achieved a complete mortality of adult S. oryzae and provided acceptable quality attributes in moisture content, water activity, color, protein, free fatty acid, and ash. The average heating efficiency and throughput of the RF treatments were 77.7, 76.3, and 74.3%, and 268.8, 247.3, and 224.8 kg/h for milled, brown, and rough rice, respectively. The industrial scale-up studies provide alternative physical methods for disinfesting milled, brown, and rough rice to replace chemical fumigation.

Simulation and validation of radio frequency heating with conveyor movement

Computer simulation is widely used as a valuable tool to systematically study radio frequency (RF) heating with time-saving and effective cost. A computer simulation model using finite element-based commercial software, COMSOL, was developed to simulate temperature distributions in wheat samples packed in a rectangular plastic container and treated in a 6 kW, 27.12 MHz RF system under stationary and moving conditions. Both simulated and experimental results showed similar heating patterns in RF-treated wheat samples, in which corners and edges were more heated and the temperatures were higher in the lower sections of the container. The simulation results demonstrated that the RF heating uniformity could be improved by conveyor movement and selecting the top electrode area similar to the sample size. The validated model can be further applied to optimize parameters of conveyor movement and top electrode configuration to improve RF heating uniformity.

A novel strategy for improving radio frequency heating uniformity of dry food products using computational modeling

Abstract This study attempted to quantify effects of dielectric properties (DPs) and densities of a surrounding container and treated food products on heating uniformity in a 6 kW, 27.12 MHz parallel plate radio frequency (RF) system. A computer simulation model was established with finite element-based commercial software, COMSOL Multiphysics®, and experiments with 1.5 kg soybean flour packed in a rectangular polystyrene container were performed to validate the developed model. Surface temperature distributions of soybean flour in three different horizontal layers were obtained with an infrared camera, and temperature–time histories at two representative locations inside the container were monitored with two optical fiber sensors. The uniformity index (UI) was used as a criterion to evaluate the RF heating uniformity within food products. Results showed that the RF heating uniformity in food samples was clearly influenced by DPs and density of the surrounding container. UI was the lowest when the surrounding container dielectric constant was in a comparable range of the sample's, with the loss factor values of surrounding container lying between 0.01–0.1% of the sample's. The optimum RF heating uniformity in food products could be achieved with a smaller density value of the surrounding container. The correlations of DPs and density between surrounding container and food products derived from the validated simulation model could provide valuable information and strategy to improve the RF heating uniformity in low moisture foods for insect or microbial control. Thus, the established strategy can further be used for developing effective industrial-scale RF treatment protocols after optimization of this process by the food industry. Industrial relevance Although the most important characteristic of radio frequency (RF) treatments is fast and volumetric heating generated by dipole rotation and ionic conduction, edge over-heating is still a major problem for foods heated in rectangular containers. The validated model was used to study the effects of dielectric properties and density of sample and surrounding container on sample uniformity index. Simulated results illustrated that the RF heating uniformity could be improved when the dielectric constant and density of surrounding container and sample were in accordance with the established relationships. The established strategy may provide valuable optimized methods to ensure RF heating uniformity in industrial applications.

Verification of radio frequency heating uniformity and Sitophilus oryzae control in rough, brown, and milled rice

Radio frequency (RF) heating is considered to be an alternative physical method for disinfesting postharvest agricultural products to replace chemical fumigation due to its inherent dangers on human health and environment. A pilot-scale, 27.12 MHz, 6 kW RF system was used to study the RF heating uniformity and validate the developed RF treatment protocols for disinfesting rough, brown, and milled rice. The results showed that the optimum RF heating uniformity in rough and brown rice was obtained by an appropriate electrode gap of 11 cm with a forced hot air heating at 50 °C, movement of the conveyor with a speed of 12.4 m/h, two mixings, and holding at 50 °C hot air for 5 min. Mortality of adult rice weevils, Sitophilus oryzae (L.), increased with increasing heating temperature and holding time, and reached 100% while RF heating at 50 °C holding in hot air for at least 6 min. There were no significant differences in quality parameters (moisture, protein, water activity, starch, free fatty acid, ash, and color) between RF treatments and untreated controls during storage (P > 0.05). The developed non-chemical alternative RF technology may hold potential for disinfesting rough, brown, and milled rice required by the growing organic market.

Computational modelling of the impact of polystyrene containers on radio frequency heating uniformity improvement for dried soybeans

Abstract Radio frequency (RF) heating has been explored as a new method for postharvest pasteurization and disinfestations, but the non-uniform heating problem needs to be solved. In this study, an experimentally validated simulation model was developed to investigate effects of applying a surrounding polystyrene container on RF heating uniformity improvement in dried soybeans. The computer model was built for soybeans with both low and high moisture contents placed in the rectangular shaped polypropylene and polystyrene containers and heated in a parallel plate RF system. Results showed that the temperature uniformity was greatly improved by placing soybean samples in the polystyrene container other than the polypropylene one. The maximum temperature difference and uniformity index (UI) were reduced. The inner corner radius of 8 cm combined with container thickness of 8 cm could provide the best heating uniformity for soybeans. This newly developed technique can be easily implemented to improve the RF heating uniformity of other low-moisture legumes for industrial applications. Industrial relevance Radio frequency (RF) heating has been explored as a new method for postharvest pasteurization and disinfestations of low-moisture granular legumes, but the non-uniform heating problem needs to be solved before being applied in industry. A validated simulation model was used to demonstrate the better container material and design for ensuring good temperature distributions in practical RF treatments. The optimal parameters from computer simulation may save time and reduce the cost to facilitate the design and scaling-up of the RF treatment protocols.

Storage stability of pistachios as influenced by radio frequency treatments for postharvest disinfestations

Abstract There has been an increased interest in developing alternative physical methods for disinfesting postharvest nuts under growing international pressures to replace chemical fumigation due to its adverse effects on human health and environment. The present research explored the possibility of using radio frequency (RF) heating as a non-chemical treatment for disinfestations of pistachios. A pilot-scale, 27 MHz, 6 kW RF unit was used to study RF heating uniformity, develop a treatment protocol, and evaluate quality attributes and storage stability in treated samples. Only 5.6 and 5.5 min were needed to raise the centre temperature of 1.8 kg in-shell and 2.0 kg shelled pistachios to reach 55 °C using RF energy, respectively, compared to about 82 and 117 min when using hot air heating at 55 °C. RF heating uniformity in both types of pistachios was improved by adding forced hot air, sample movements on the conveyor and a single mixing in the middle of the treatment time. The final average temperatures on the surface and in the interior of both types of pistachio kernels exceeded 52 °C, following a holding step at 55 °C for 2 min using hot air. This provided a conservative and 100% mortality of fifth-instar Indianmeal moth (Plodia interpunctella [Hubner]). RF treated samples were not significantly different from control samples in weight loss, peroxide values, fatty acid values, fatty acid composition and kernel colour. RF treatments can, therefore, potentially provide an effective and rapid protocol against stored product pests in pistachios as an alternative to chemical fumigation. Industrial relevance A pilot-scale 6 kW RF system with conveyor belt was used to determine the heating uniformity and quality changes of pistachios. For a combination with hot air surface heating and mixing, an effective and continuous RF disinfestation method could be developed for pistachios. The RF heating technology has been successfully demonstrated for disinfesting walnuts in California, USA. We tried to expand the industrial applications of RF heating for disinfesting pistachios, to replace the chemical fumigation. This research may provide potential industrial applications of RF treatments for disinfestations based on fast and uniform heating.

Improvement of radio frequency (RF) heating uniformity on low moisture foods with Polyetherimide (PEI) blocks

Radio frequency (RF) heating is rapid, volumetric, and can penetrate most food packaging material. Thus, it is suited for in-packaged food pasteurization applications. However, the non-uniform heating problem needs to be resolved. In this study, a method of adding Polyetherimide (PEI) cylindrical blocks on top of and at the bottom of peanut butter samples in a cylindrical jar (d=10cm, h=5cm) was evaluated to improve RF heating uniformity. A computer simulation model built with COMSOL Multiphysics® was used for heating pattern prediction, and a new temperature uniformity index was proposed to suitably evaluate pasteurization process heating uniformity. Results showed that a pair of PEI blocks with a diameter of 8cm among all five diameters (2, 4, 6, 8, and 10cm) added to the cold spots of a given peanut butter sample could make the sample reach the best heating uniformity. Furthermore, the best height of PEI blocks with a diameter of 8cm that allows the sample to be heated most uniformly was found to be 1.3cm after sweeping from 0.1 to 2.3cm with a step of 0.1cm. Simulation results also showed that the combination of PEI surrounding and the addition of 8cm diameter PEI blocks could further control the temperature distribution range in peanut butter within 7.1°C when the peanut butter was heated from 23 to 70°C. The newly developed temperature uniformity index provided a more reasonable evaluation on heating uniformity of pasteurization process than the traditional uniformity index.

Developing radio frequency technology for postharvest insect control in milled rice

Since methyl bromide fumigation has an adverse effect on human health and environment, it is urgently needed for developing a non-chemical method to replace chemical fumigation for disinfesting milled rice. The purpose of this research was to study possible applications of radio frequency (RF) energy for disinfesting milled rice without affecting product quality. A pilot-scale, 27.12 MHz, 6 kW RF system was used to study RF heating uniformity and develop a treatment protocol for achieving 100% insect mortality and finally evaluating quality attributes in RF treated milled rice during storage. The results showed that the heating time needed only 4.3 min to heat the 3.9 kg milled rice from 25 °C to 50 °C using RF energy, but 480 min for milled rice to reach 48 °C using hot air at 50 °C. After comparing three selected electrode gaps, an appropriate gap of 11 cm was obtained to achieve the heating rate of 5.8 °C/min for further heating uniformity tests. An RF treatment protocol was finally developed to combine 1.0 kW RF power with a forced hot air heating at 50 °C, movement of the conveyor with the speed of 12.4 m/h, two mixings, and holding at 50 °C hot air for 5 min, followed by forced room air cooling through single-layer (2 cm thick) samples. There were no significant differences in quality parameters (moisture, protein, fat, starch, hardness, and color) between RF treatments and untreated controls during storage (P > 0.05). Therefore, RF treatments may provide a practical, effective and environmentally friendly method for disinfesting milled rice.

Investigation of radio frequency heating uniformity of wheat kernels by using the developed computer simulation model

To explore the potential of using radio frequency (RF) energy to control insects or microorganisms in low-moisture granular food crops, wheat kernels were selected as a sample product, and a computer simulation model was developed to study the influence of sample size (wheat kernels filled in plastic container in cuboid shape) and vertical position on RF heating uniformity and to determine the cold and hot spots in the samples. The simulated temperature profile of RF heating was compared with the experimental results to validate the computer model. Temperature uniformity index (TUI) was selected as a criterion to evaluate RF heating uniformity. Results suggested that temperature uniformity of sample depended on sample size and location, where for small sample size, sample placed on either bottom electrode or closed to top electrode would achieve better RF heating uniformity; while for sample in large size, placed in the middle or slightly lower than middle position between the two parallel electrodes achieved better temperature uniformity. Also, a larger sample size had better RF heating uniformity, especially when sample size equaled to the size of the top electrode, indicating that RF energy is more suitable for treating bulk materials. Corner heating was also observed where hot spots located at the corner of samples in cuboid shape, and cold spots depended on sample vertical position. In this study, cold spots were at the center of the top and bottom layers when the sample was placed at the middle position between the two parallel electrodes, at the center of top layer when sample was placed on the bottom electrode, and at the center of bottom layer when sample was placed close to top electrode. Information generated from this study would be essential for developing RF treatment protocol to control insects or microorganisms in low-moisture granular agricultural products.