Radio Frequency Heating Uniformity Research Articles

Application of radio frequency energy in processing of fruit and vegetable products.

Thermal processing is commonly employed to ensure the quality and extend the shelf-life of fruits and vegetables. Radio frequency (RF) heating has been used as a promising alternative treatment to replace conventional thermal processing methods with advantages of rapid, volumetric, and deep penetration heating characteristics. This article provides comprehensive information regarding RF heating uniformity and applications in processing of fruit and vegetable products, including disinfestation, blanching, drying, and pasteurization. The dielectric properties of fruits and vegetables and their products have also been summarized. In addition, recommendations for future research on RF heating are proposed to enhance practical applications for fruits and vegetables processing in future.

Effect of hot air-assisted radio frequency rotation heating system on improving heating uniformity of dried black fungus (Auricularia auricula)

A large-capacity radio frequency (RF) rotation heating system was employed in this study to heat dried black fungus, and hot air was used for assistance treatment. The effects of electrode gap, rotation rate and hot air on the uniformity of RF heating of dried black fungus were explored. The optimal heating process parameters were obtained, and the sterilization effect was validated. The results showed that the rotation system could significantly enhance the RF heating uniformity of black fungus (P > 0.05). The optimal heating process for heating a large-capacity (480 g) black fungus was to utilize RF heating assisted by 70 °C hot air with an electrode gap of 175 mm and a rotation rate of 60 rpm, followed by 30 min of heat preservation at 70 °C hot air with a rotation rate of 40 rpm. After the entire sterilization treatment, the heating uniformity index λ of black fungus was reduced to 0.027 ± 0.004, the total plate count decreased by 3 log CFU/g, and the moisture content dropped to below 13 %. The ideal RF heating uniformity and sterilization effect of black fungus were obtained in this study through the simultaneous use of hot air and rotation, providing a theoretical foundation for the application of RF technology in the processing of black fungus.

Extended applications of the small-scale radio frequency system for improving the heating performance of multi-component rice

With the changing lifestyles and the rapid development of the prepared food industry, there are many different multi-component rice-based foods on the market today. Further, manufacturers of multi-component foods prepare different types of rice products and provide them to markets and restaurants. In this study, the multi-component rice was heated by a self-built small 50 Ω, 13.56 MHz radio frequency (RF) heating system to explore the effective scheme for improving heating uniformity. The dielectric properties (DPs) of the single-component material (cooked rice, carrot, and sausage) were first measured and the heating rates were recorded at a selected input power level of 500 W and electrode gap of 80 mm. Based on the above results and the occurrence of edge effect, three types (I, II, and III) of the multi-component test samples were designed to compare the heating performances and the quality changes after RF treatment with three different electrode gaps (70, 80, and 90 mm). Results demonstrated that the Type-I sample arrangement resulted in a relatively shorter treatment time and a lower heating uniformity index as compared to the other two sample arrangements (Types of II and III). It also resulted in a more uniform temperature distribution and better-quality retention than that was possible with the single-component treatment. Thus, the layered scheme (Type-I) based on the DPs of the material could effectively improve the RF heating uniformity of multi-component food.

Improving radio frequency heating uniformity in peanuts: Effects of packaging geometry, electrode gap, particle size and interlayer displacement process

This study aimed to assess the impact of various processing parameters including electrode gap, packaging geometry, and particle size on the heating efficiency of radio frequency (RF) processing for peanut thermal decontamination. In addition, interlayer displacements were presented to evaluate the effect of the mixing on heating uniformity.Experimental studies were carried out using 3D-printed polylactic acid (PLA) containers (square, rectangular, and cylindrical) to place the peanut samples with particle sizes ranging from 0.71 to 2.00 mm at 100–140 mm electrode gaps. Temperature changes within the samples were monitored using fiber optic sensors and surface temperature distributions were determined with an infrared camera. Heating uniformity index (λ) was then calculated to assess the effects of RF processing conditions on RF heating uniformity.Results showed that the cylindrical container exhibited a more consistent and uniform heating profile (λ = 0.12) with 1.58 °C min−1 heating rate compared to other containers, and a single interlayer displacement between top and bottom layers further improved temperature distribution uniformity. Smaller particle sizes also led to faster heating rates from 1.71 to 1.55 °C min−1 with more uniform heating due to the reduced porosity within the particles with increased thermal conduction and electromagnetic energy absorption. This study presented valuable insights for developing an effective RF process as an alternative to conventional thermal treatments for decontamination of low-moisture, high-fat particulate foods. Industrial significanceThis research presented an innovative approach applying RF heating for a possible decontamination process for peanut samples. The research tackled the growing worldwide consumer demand for food products to satisfy food safety regulations while peanuts were selected to demonstrate the process effect due to the recent food safety issues in peanut butter considering that a pre-process for decontamination would lead to a decrease in the resulting safety issues.

Developing a combined rotating and air-circulating system for improving radio frequency heating uniformity and controlling Aspergillus in almond kernels

It is important to improve radio frequency (RF) heating uniformity for developing effective pathogen control protocols before storage of almonds. The purpose of this study was to design a combined rotating and air-circulating system (CRACS) in the RF cavity for improving sample temperature uniformity during pasteurization, drying, and cooling of almond kernels. The results showed that the optimal rotating speed of 15 r/min and loading volume ratio of 60% of CRACS were determined to achieve the required RF pasteurization level. The pasteurization protocol was developed based on the above selected parameters of CRACS, RF heating/drying, and forced air cooling. Compared to the five-layer container (five single layers stacked vertically) used for RF pasteurization of almond kernels, the RF heating uniformity in almond kernels using CRACS was clearly improved, resulting in moisture content reduction of only 0.70% wet basis (w.b.). The pasteurization validation curve of almond kernels in CRACS was closer to the predicted survival one based on the cumulative thermal lethal time model, suggesting that CRACS was suitable for the effective RF pasteurization of almond kernels. The moisture content and color values of almond kernels after RF pasteurization were beneficial for maintaining their long-term storage and stable quality. This CRACS in the RF system may provide a practical and effective technology for pasteurizing almond kernels.

Developing a spouted bed for combined radio frequency-hot air treatments to improve the heating uniformity in high-moisture carrot cubes

Uneven and thermal runaway heating is the biggest obstacle to industrial applications of radio frequency (RF) technology, while the existing research mainly focuses on the RF heating uniformity of low-moisture foods with fewer reports about the useful solutions to avoid heterogeneous heating of high-moisture foods. Thus, the combined RF-hot air treatment in a newly designed spouted bed (RFHASB) was developed in this study to improve the RF heating uniformity of fresh carrot cubes selected as the high-moisture model sample. This RFHASB was used to evaluate effects of four processing parameters (cone angle, nozzle diameter, inlet air speed and electrode gap) on the RF heating rate and uniformity of carrot cubes with comparison of those between the RFHASB and the hot air treatment alone in the spouted bed (HASB). The results showed that the RFHASB effectively alleviated the non-uniform heating in the treated high-moisture carrot cubes through mixing and stirring particles. Both heating rate and uniformity index in RFHASB increased with decreasing cone angles and electrode gaps but decreased when nozzle diameters and inlet air speeds were reduced. The RFHASB had better heating performances than the HASB. These findings may provide the theoretical basis for the effective drying of high-moisture fresh fruits and vegetables in RFHASB.

Effect of BaTiO3 as a filling material with adjustable dielectric properties on improving the radio frequency heating uniformity in red jujubes

The non-uniformity of radio frequency (RF) heating is a major obstacle to develop successful large-scale disinfestation processes for agricultural products with large particles. To effectively improve the RF heating uniformity in red jujubes, the chemical material of BaTiO3 (BT) with different volume percentages was added into food-grade silicone (FGS) to form FGS/BT composite particles with various dielectric properties. The various FGS/BT composite particles were filled inside the rectangular container of red jujubes and used to evaluate the effect of matching degree of their dielectric constants on the RF heating uniformity. The results showed that the dielectric constant of the composite increased from 3.41 ± 0.14 to 8.31 ± 0.15 as the BT volume percentage increased from 0% to 20% at 25 °C and 27.12 MHz. Compared with FGS, the filled FGS/BT composite with a BT volume percentage of 20% not only improved the heat conduction but also optimized the temperature distributions among red jujube particles, which reduced the heating uniformity index in the top, middle, and bottom layers by 40%, 32%, and 27%, respectively. As the electrode gap increased from 160 mm to 180 mm, the heating rate and uniformity index in FGS/BT filled samples decreased significantly (p < 0.05). There was no significant (p > 0.05) difference in color and textural properties between the treated and control samples. This study provided the effective filling materials with matched dielectric constant in the RF treatment of red jujubes and a feasible method to further improve the RF heating uniformity in large-size agricultural products.

Evaluation of mechanical properties, heating rate and radio frequency explosive puffing (RFEP) quality of purple sweet potato under different moisture contents and moisture equilibrium process

The effects of different initial moisture contents (15–55%) and moisture equilibrium process (MEP) on the mechanical properties of the gas-blocking surface layer of purple sweet potato chips and the radio frequency (RF) heating rate, uniformity, and radio frequency explosion puffing (RFEP) quality were discussed in this article. Low-field nuclear magnetic resonance indicated that hot-air pre-drying reduced the amount of free water and caused the transfer of immovable water to bound water. This process was accompanied by an increase in the thickness and elastic modulus of the gas-blocking surface layer, resulting in a significant increase in the RF heating rate and uniformity. Prolonged RF heating time led to a decrease in anthocyanin content. The results demonstrated that the puffing rate initially increased and then decreased with increasing initial moisture content. The puffed sample with an initial moisture content of 45% exhibited the highest puffing rate, crispiness, and porosity. X-ray micro-tomography results reveal that MEP improved the morphology of puffed products, ensuring even distribution of small pores and reducing the presence of hard edges. In conclusion, considering the heating time and anthocyanin retention rate, the initial moisture content ranges from 35% to 45% after MEP was the best pre-treatment.

Performance evaluation of an experimental radio frequency heating system designed for studying the solid-liquid extraction

Radio frequency (RF) heating has been extensively used in food processing. However, most RF equipment used for food processing studies is for general-purpose on a pilot scale, rather than being designed on specific processes. This study aimed to develop an experimental 50 Ω RF system equipped with vertical parallel plate electrodes to facilitate the liquid heating in the solid-liquid extraction (SLE) studies, and its heating performances and electromagnetic (EM) leakage were also evaluated. Results showed that compared with the cylindrical container, rectangular one had better heating rate and uniformity, and was more suitable for the vertical parallel plate system. For solid-liquid mixture, RF heating rate increased with increasing particle size within a certain range, but the heating uniformity of the sample surface decreased. Both RF heating rate and uniformity increased with increasing liquid-solid ratio. Furthermore, air bubble stirring at a flow rate of 0.8 L/min resulted in a larger heating rate with better heating uniformity. Shielding using 4 mm stainless steel plate in the self-designed metallic enclosure ensured that the EM leakage was much smaller than the limiting value required by International Commission on Non-Ionizing Radiation Protection. The results from this study would provide some basic information and guidelines for application of vertical parallel plate RF system in the study of SLE of bioactive compounds from agri-food by-products.

Sensitivity analysis of container properties to heating uniformity of soybeans during radio frequency treatment

Radio frequency (RF) treatment has been proved to be effective in disinfestation and pasteurization. However, inhomogeneous heating is still the main issue for RF treatment to be applied in large-scale processing industry. In this study, a finite element model was built to analyze the sensitivity of container properties (dielectric constant, loss factor, thermal conductivity, density, heat capacity, and resistivity) to the RF heating uniformity of soybeans in pilot-scale systems and the established model was validated by the experiments, which were conducted by a 6 kW, 27.12 MHz RF system. Temperature profiles of the sample predicted from the model matched the experimented results well because the maximum relative percent error was less than 10 %. The sensitivity indexes of dielectric constant, loss factor, thermal conductivity, density, heat capacity, and resistivity changed from 0.254, 0, 0.059, 0.015, 0.015, and 0, to 0.292, 0, 0.004, 0.151, 0.151, and 0, respectively, when the thickness of the container increased from 2 mm to 5 mm. Since the dielectric constant determines the electric field distribution in sample during RF treatment, it was the most sensitive property in affecting RF heating uniformity of soybeans, followed by the thermal conductivity, density, and heat capacity of the specific container. Loss factor and resistivity of the container had less effect on the RF heating uniformity of sample. These results may provide a theoretical basis for the selection of appropriate container materials to improve RF heating uniformity and promote its applications in large-scale processing industry.

Improving radio frequency heating uniformity in cauliflower by changing density in different zones

Radio frequency (RF) treatment is one of the most promising approaches for disinfesting, drying, and pasteurizing agricultural products. However, the inhomogeneous heating of RF treatment is the primary barrier for practical applications. To improve the RF heating uniformity, a 27.12 MHz, 6 kW pilot-scale RF system was applied to heat the cauliflower by changing its density in different zones of a rectangular container. The results proved that the RF heating rate and the average mass change rate of the cauliflower decreased with increasing electrode gap and bulk density of the cauliflower. The RF heating uniformity of the cauliflower was improved by increasing the electrode gap and the bulk density of the cauliflower during RF heating. With increasing of the bulk density of sample at the edges in the container, the RF heating rate and heating uniformity index of the sample firstly decreased and then increased when the sample density at the center and the corners were 405.5 and 516.8 kg/m3, respectively. The mass change rate of the sample presented an inverse relationship with the heating rate and heating uniformity index during RF treatment. The best heating uniformity and highest mass change rate were obtained when the bulk density of the cauliflowers at the center, edges, and corners in the container was 405.5, 461.1, and 516.8 kg/m3, respectively. The results of this study can offer effective method for improving the RF heating uniformity in high moisture products.

Experimental investigation of radio frequency heating uniformity of wheat flour surrounded by deionized water and NaCl solution

The present study proposes an approach to overcome the problem of corner/edge overheating of foods undergoing radio frequency heating. For testing, a container with a central rectangular compartment surrounded by a frame of four compartments was used. Wheat flour, a representative of low-moisture foods, was packed inside the rectangular compartment to provide a density of 800 kg/m3. Afterwards, in the surrounding compartments, air, deionized water and NaCl solution were used for independent radio frequency heating tests. Two different electrode gaps (135 and 145 mm) were employed. The temperature of the sample was measured continuously using fibre optic probes at four different internal locations. The difference between the maximum and the minimum temperatures for three different cases (a: air, b: deionized water and c: NaCl solution) were as follows for electrode gaps of 135 and 145 mm, respectively: (a) 27.3 and 26.1 °C, (b) 2.5 and 4.8 °C and (c) 3.7 and 1.8 °C. Uniformity index values were also calculated. The results showed that temperature variation within the sample was greatly reduced when the sample was surrounded by deionized water or NaCl solution. PRACTICAL APPLICATION The project findings will provide processors with a practical means to achieve more uniform radio frequency heating of low-moisture foods.

Developing a square container with ideal dielectric constant for improving radio frequency heating uniformity in low-moisture agricultural products

Developing a square container with ideal dielectric constant for improving radio frequency heating uniformity in low-moisture agricultural products

Developing cold air assisted radio frequency tempering protocol based on heating rate, uniformity, and quality of frozen chicken breast

Radio frequency (RF) heating as an emerging technology has been widely used for tempering and thawing of frozen meat products, while the RF heating non-uniformity is still the main problem before industrial applications. The objective of this study was to provide a new method for improving the RF heating uniformity by adding cold air (−5 °C, 1.5 m/s) around the frozen chicken breast in a rectangular polypropylene container. The effects of cold air assisted RF tempering (CARFT), RF tempering (RFT), air tempering (AT), water tempering (WT), and refrigerator tempering (RT) on heating behaviors and quality of chicken breast were also compared. The results showed that the cold air slightly reduced the RF heating rate but significantly ( P < 0.05) improved the heating uniformity of frozen chicken breast during RFT. Meanwhile, the optimal tempering method was CARFT at the electrode gap of 110 mm, which could provide a larger heating rate than AT, WT and RT and a better heating uniformity than AT, WT and RFT. Additionally, the quality, such as color, drip loss (1.42%), pH (6.38), and total volatile base nitrogen (10.37 mg/100 g), of CARFT treated chicken breast were better than those with other methods. Therefore, CARFT can further be expanded to optimize industrial-scale RF tempering protocols in frozen chicken breast. • RF heating non-uniformity is still the main problem of industrial tempering applications. • Added cold air improved the RF heating uniformity of frozen chicken breast. • Optimal tempering method was cold air assisted RF tempering with electrode gap of 110 mm. • Quality of CARFT treated chicken breast was better than that with other methods.

Improving radio frequency heating uniformity in milled rice with different packaging shapes by changing temperature of forced air

Radio frequency (RF) treatment is a promising physical method for disinfesting and pasteurizing foods but the non-uniform RF heating is still a major obstacle for industrial applications. In this study, a three-dimensional model was established with COMSOL software to simulate temperature distributions in milled rice with different packaging shapes and experiments using a 6 kW, 27.12 MHz RF system were carried out to evaluate the improvement of heating uniform of sample by changing temperature of forced air. The established model was validated by temperature-time histories at three points in the sample and temperature distributions at the top and bottom surfaces. The results showed that the cold spot of the regular shape sample was found at the part with small thickness, and the RF heating uniformity was improved by increasing temperature of forced hot air. When the temperature of forced hot air increased from 40 to 60 °C, all the sample temperatures were above 50 °C, which might meet the required temperature of disinfestation, and the heating uniformity index decreased from 0.113 to 0.099. For the cuboid shape sample, the corner/edge effect was observed and the heating uniformity index firstly decreased and then increased with reduced temperature of forced cold air. When the temperature of forced air decreased to 10 °C, the best heating uniformity of the cuboid shape sample was obtained. The results of this research can help to select packaging shape and improve the heating uniformity of samples for RF treatment in large-scale applications.

Effect of different arrangements of globe particles on radio frequency heating uniformity: Using black pepper as an example

Different arrangements of stacked globe particles may result in various heating patterns in radio frequency heating. In this study, black pepper particles with an identical diameter (5 mm) and four different arrangements subjected to radio frequency heating were modeled and the heating rate and temperature distribution were compared. The four arrangements were simple cubic, body centered cubic, rhombohedral and face centered cubic. Within different arrangements, as the porosity increased from 0.25 to 0.47, the heating time increased from 500 to 780 s for reaching a target temperature of 70 °C. For heating rates comparison at the sample center and corners, rhombohedral model had the highest R2 (0.9953, 0.9975) and lowest RMSE (1.0709, 0.7642), respectively, among all arrangements. For temperature distribution on the top surface, the highest similarity of heating pattern between simulation and experiment was also found in rhombohedral model with an RMSE of 124.30. Therefore, the optimal prediction results was obtained in an arrangement with the closest porosity to randomly stacked globe particles in experiment. This study verified the importance of porosity and arrangement of particles in RF heating, and expanded the usage of RMSE for modeling accuracy evaluation.

Modeling the RF heating uniformity contributed by a rotating turntable

To explore the potential of using an angle-adjustable rotating device to improve the radio frequency (RF) heating uniformity of packaged food powder, a 3-D mathematical model was established using moving domain approach to predict effects of different rotation directions, speeds and package dimensions on temperature uniformity of red pepper powder (RPP) in a cylindrical container heated in a 6 kW, 27.12 MHz parallel plate RF system. The calculated end point temperatures from the model were in good agreement with those of the experiment at nine locations inside the sample, with a smallest maximum temperature difference of 3.26 °C at all 9 locations. Temperature distributions on the upper, middle and bottom surfaces and the central vertical section of RPP measured by an infrared camera were also matched well with the simulation results, which validated the mathematical model. Experimental and simulation results both showed that a best heating uniformity of RPP could be obtained with an inclined angle of 20° and rotation speed of 4 rad min−1 when RF heating to the pasteurizing temperature of 70 °C. The heating uniformity index (UI) decreased from 0.41 to 0.25 and the maximum temperature of packaged RPP was reduced from 105 °C to 88.7 °C, comparing with an inclined angle of 0° and rotation speed of 0 rad min−1. With the combined inclined angle and rotational movement of sample, the reduced temperature gradients showed that the heating uniformity was improved volumetrically. Therefore, the laboratory-scale rotating device could lay a substantial foundation for its large-scale applications in RF systems and make food products to reach the target temperature more controllable.

Pasteurization of Salmonella spp. in black fungus (Auricularia auricula) powder by radio frequency heating.

Radio frequency (RF) heating has been studied to inactivate bacteria in some powder foods. In this study, a 6 kW, 27.12 MHz RF system was used to pasteurize Salmonella in black fungus (Auricularia auricula) powder. The effects of different conditions (initial aw, electrodes gaps, particle sizes) on RF heating rate and uniformity were investigated. The results showed that RF heating rate was significantly (p < 0.05) improved with decreasing electrodes gap and increasing initial aw, and the heating rate was the slowest when the particle size was 120-160 mesh. However, these factors had no significant (p > 0.05) influence on heating uniformity. RF pasteurization of Salmonella in black fungus powder was also studied. The results showed that, to inactivate Salmonella for 5 log reductions in the cold spot (the center of surface layer), the time needed and bacteria heat resistance at designated temperature (65, 75, 85 °C) decreased with increasing aw, and the first order kinetics and Weibull model could be used to fit inactivation curves of Salmonella with well goodness. Quality analysis results showed that although RF pasteurization had no significant (p > 0.05) effect on Auricularia auricula polysaccharide (AAP) and total polyphenols, obvious changes were found on color. Results suggested that RF pasteurization can be considered as an effective pasteurization method for black fungus powder.

Computer simulation analyses to improve radio frequency heating uniformity for watermelon seeds by inserting horizontal aluminum and polypropylene (PP) plates in a rectangular PP container

Uneven temperature distribution is a major barrier for applying radio frequency (RF) heating to replace traditional thermal treatments in foods or agricultural products processing, such as pasteurization of watermelon seeds. In this study, simulation models of watermelon seeds heated by RF energy were established using the commercial COMSOL software with or without inserting polypropylene (PP) or aluminum (AL) plate into a rectangular PP container and validated based on experimental results. The effect of inserting PP or AL plate at different horizontal positions on RF heating behavior of watermelon seeds was also analyzed by the validated simulation model. The results showed that inserting PP or AL plate caused different electric field distribution in the second layer of seeds. Meanwhile, when the insertion height increased from 5 to 35 mm, the volumetric RF heating uniformity index (0.033–0.118) for the second layer in seeds with inserting the AL plate was lower than that (0.127–0.165) of samples with inserting a PP plate. The optimal heating uniformity index could be obtained by inserting multiple AL plates with intervals of 10 mm among watermelon seeds. These findings can further be expanded to optimize the heating uniformity for developing an effective RF pasteurization and disinfestation protocol in foods or agricultural products without reducing their quality.

Effects of combined radio frequency heating with oven baking on product quality of sweet potato

As important primary food crops, sweet potatoes have rich nutritional values. Baking with a single oven is a common method for processing sweet potatoes. Since the single oven has some adverse effects on the product quality, radio frequency (RF) energy as a new heating technology was explored in combined baking with the traditional oven. Effects of single oven baking and combined RF and oven baking (RF-O) on baking time and product quality for cylindrical sweet potatoes were compared. Effects of electrode gap and sample diameter on RF heating rate and uniformity were also studied before comparing the baking characteristics between single oven and RF-O methods. Results showed that the best RF heating uniformity and acceptable heating time were obtained at an electrode gap of 90 mm and a sample diameter of 55 mm. The total baking time of 35.9 min for the single oven method was substantially reduced to 25.1 min for the RF-O method. Meanwhile, the sweet potato baked by RF-O had better color, or texture values, lower weight loss and lower acrylamide content than the single oven baking. Therefore, RF-O could be recommended as an effective, uniform, and high-quality baking method for sweet potatoes.