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Purpose The aim of this study was to prove that radio-frequency (RF) energy with 13.56 MHz can be used for heating building structures in a controlled manner exploiting the advantage that homogeneous heating with sufficient penetration depths can be achieved. Design/methodology/approach Because parallel electrodes on both sides of the heated structure cannot be used in many practical applications, two special electrode designs have been developed by modeling the field distribution and energy absorption and by carrying out test experiments to validate the simulation results. Findings One solution is based on a two-dimensional surface capacitor providing certain penetration depths and being especially suitable for treating thin structures such as wooden parquet floor. Such an arrangement can be particularly used for pest control even when sensitive surfaces have to be protected. The other solution uses a capacitive coupling between the grounded shielding and an electrode or an equivalent structure (e.g. moist soil) at the other side of the masonry to establish a sufficiently strong electrical field between a “hot” electrode on the side of the shielding and the coupled rear electrode. Originality/value Both solutions significantly enhance the application potential of RF heating.

Foodborne illnesses occur due to contamination by pathogenic microorganisms. Therefore, decontaminating food is vital before marketing and circulation. Radio frequency (RF) heating stands out in several branches of industry, mainly food processing, as an alternative method to conventional pasteurization which takes long process times and overheating. RF heating functions without relying on heat conduction. It generates internal heat by inducing the rotation of polar molecules and the motion of ions. The advantages of dielectric heating with greater wave penetration include rapid, uniform and volumetric heating, presenting high energy efficiency. Furthermore, it is an effective, validated method for eliminating pathogens in agricultural products and is free from chemical residues. Although many reviews have discussed this technology, few reviews have covered the research trends in this field in the recent years, during which the number of studies discussing RF treatment of foods have increased. Therefore, this review focuses on the RF applications in the food industry for pest control, microbial and enzymatic inactivation of solid, liquid, and powdered foods in the last five years. Besides covering the fundamental aspects of RF technology, we also examine its benefits and drawbacks, address the challenges it presents, and explore future prospects Graphical Abstract

We describe a new technique for the efficient generation of high-energy ions with electromagnetic ion cyclotron waves in multi-ion plasmas. The discussed ‘three-ion’ scenarios are especially suited for strong wave absorption by a very low number of resonant ions. To observe this effect, the plasma composition has to be properly adjusted, as prescribed by theory. We demonstrate the potential of the method on the world-largest plasma magnetic confinement device, JET (Joint European Torus, Culham, UK), and the high-magnetic-field tokamak Alcator C-Mod (Cambridge, USA). The obtained results demonstrate efficient acceleration of 3 He ions to high energies in dedicated hydrogen–deuterium mixtures. Simultaneously, effective plasma heating is observed, as a result of the slowing-down of the fast 3 He ions. The developed technique is not only limited to laboratory plasmas, but can also be applied to explain observations of energetic ions in space-plasma environments, in particular, 3 He-rich solar flares. Triggering and sustaining fusion reactions — with the goal of overall energy production — in a tokamak plasma requires efficient heating. Radio-frequency heating of a three-ion plasma is now experimentally shown to be a potentially viable technique.

The aim of this study was to optimize sequential ultrasound-radio frequency–assisted extraction (URAE) of pectin from pomelo peel. Effects of sonication power and time, radio frequency (RF) heating temperature, and time on the pectin yield (PY) were evaluated. Based upon optimized URAE parameters, the yield, physicochemical, and structure properties of pectin recovered from sequential radio frequency-ultrasound–assisted extraction (RUAE), ultrasound-assisted extraction (UAE), and RF-assisted extraction (RFAE) were also compared. A maximal PY of 28.36 ± 0.85% was attained at the optimized URAE conditions including solvent pH of 1.5 (citric acid), sonication at 183 W for 24 min, and RF heating at 87 °C for 23 min. Although all four samples had a high degree of esterification more than 50%, URAE was the lowest. No significant changes were observed in the types of monosaccharides among different samples. Furthermore, all four samples (6.6–10.3 mg GAE/g) showed significantly higher total phenolic content than those of commercial citrus pectin (1.2 mg GAE/g), and among them, RFAE was the highest with the best antioxidant capacity. The water and oil holding capacities of the four samples were between 3.5 to 4.0 and 2.6 to 3.0 g/g, respectively, but there was no significant difference ( p  > 0.05) between each other. Structure properties indicated that there were no significant differences in the main chemical structures among the four pectin samples. Morphology analysis of URAE showed a more compact, smoother, and flatter surface than that of RUAE and RFAE. The results observed in this paper suggest that sequential URAE is an efficient strategy for the recovery of high-quality pectins.

Non-uniform heating of RF energy is the main obstacle for its large-scale application in blanching. In this study, we proposed a new blanching method: steam (ST)-assisted radio frequency (RF) blanching of fresh-cut stem lettuce cuboids. The relative peroxidase (POD) activity, heating uniformity, and physicochemical properties of samples treated by steam-assisted RF blanching at different conditions (RF-50 °C + ST-2.5 min, RF-65 °C + ST-2 min, and RF-80 °C + ST-1 min) were evaluated in comparison with single RF heating and steam blanching. Both the temperature distribution images and the heating uniformity index indicated that steam-assisted RF blanching overcame the defect of local overheating of single blanching mode and achieved more uniform temperature distribution. Besides steam-assisted RF, blanched reduced the thermal damage to cells, which can be demonstrated by lower relative electrolyte leakage rate, thereby maintained better texture, color, and higher vitamin C retention at the same relative POD activity of 5%. Among the three steam-assisted RF blanching treatments, RF-80 °C + ST-1 min achieved optimum heating uniformity and quality of samples. This study suggested that steam-assisted RF blanching can be used as a potential blanching method to improve inactivation efficiency of enzyme and quality of fruits and vegetables.

Conventional thermal treatment in food processing relies on the transfer of heat by conduction and convection. One alternative to this conventional thermal treatment is radio-frequency (RF) heating in which electromagnetic energy is transferred directly to the heated product. The longer wavelengths of RF compared with microwaves are able to penetrate further into the food products resulting in more even heating. A review of RF heating for the food processing industry is presented here with an emphasis on scientific principles and the advantages and applications of RF. Applications of RF heating include blanching, thawing, drying, and processing of foods. RF heating represents considerable potential for additional research and the transfer of technology to the food processing industry. Computer simulation can be used to improve RF heating uniformity. Moreover, the heating uniformity in the rotated eggs is greater than in the static eggs. RF has also been used to blanch vegetables to increase ascorbic acid content to achieve the highest vitamin C levels. The use of the thawing technology has resulted in better quality of treated food. There has been increased interest in the RF-drying method due to the homogeneity of heating, greater penetration depth, and more stable control of the product temperature. RF-treated meat had improved quality and coagulation with acceptable taste and appearance. In addition, RF heating is used in pasteurization of yogurt and destruction of microorganisms in liquid and solid foods.

Heat-induced gel properties are important indicators for evaluating meat quality. Conventional water bath (WB) heating is a common method for processing gelatin meat products, but the longer heating times have some adverse effects on their network structures. Therefore, this study aimed to explore a novel approach by combining radio frequency (RF) processing with WB heating to improve the gel properties of minced chicken breast. Effects of electrode gaps (110, 115, and 120 mm) and sample diameters (42, 49, and 56 mm) on RF heating rate and uniformity were investigated under a fixed sample height of 80 mm. The heating time and gel quality changes after RF + WB (55, 60, 65, 70 ℃) at different transition temperature, single RF, and sole WB treatments were also compared. The results showed that when the electrode gap was 110 mm, the sample with a diameter of 49 mm and a height of 80 mm provided the optimized heating uniformity and required processing time. The combination of RF and WB processes reduced cooking time by 17 to 44% compared to single WB heating (34.8 min). Furthermore, the RF + WB (65 ℃) treatment exhibited the better gel quality or texture, and smoother microstructure of minced chicken breast compared to the other applications, the water holding capacity (WHC) value of the sample reached the maximum (78%), and the gel strength increased by 64.32%. This study demonstrated that RF + WB (65 ℃) heating might be presented as an efficient method to improve the gel properties of minced chicken breast with its environmental-friendly features in the view of process sustainability.

This work shows that radio‐frequency (RF) fields can simultaneously align carbon nanotubes (CNTs) dispersed in a resin and induce Joule heating to cure the resin. The timescales of alignment and curing using RF heating are numerically computed and compared at different field strengths in order to determine a temperature where alignment happens before the matrix crosslinks. Composites are experimentally fabricated at the desired target temperature and are optically analyzed and quantified; the CNT network is successfully aligned in the direction of the applied electric field. This methodology can be used to create composites where the local alignment can be varied across the sample. Composites fabricated using RF fields have higher electrical conductivity in the direction of the aligned CNTs than an oven‐cured, randomly aligned sample. Also, RF‐cured nanocomposites exhibit higher tensile strength and modulus in the direction of alignment compared to an oven‐cured sample. Finally, it is further demonstrated how this methodology can be coupled with a direct ink writing additive manufacturing process to induce alignment in any desired direction, even orthogonal to the shear forces in the extrusion direction. Radio‐frequency fields induce two competing effects in carbon nanotube (CNT)‐filled resins: alignment of CNTs due to electric field‐induced torque, and Joule heating which results in curing. This work addresses the potential to use these competing effects to rapidly heat and cure the CNT‐filled resin to fabricate nanocomposites with aligned CNT networks.

The effects of radio frequency (RF) heating treatments with different final temperatures (70, 80, and 90 °C) and electrode gaps (120, 160, and 200 mm) on the structural characteristics of soy protein isolate (SPI) dispersion were investigated. The results showed that RF heating significantly influenced free sulfhydryl groups and surface hydrophobicity of SPI. Free sulfhydryl groups increased with the increase of final temperature. The hydrophobicity of the RF-heated sample was higher than the original SPI without RF treatment. The highest hydrophobicity of the RF-heated SPI was found with electrode gap of 200 mm at 90 °C. RF heating treatment resulted in the reduction of ultraviolet absorption of SPI indicating the change of three-dimensional positions of soy protein but did not modify the protein primary structure of SPI. The Fourier transform infrared spectroscopy showed that hydration of SPI was decreased by RF heating. The self-reassembly from random coil structure to β-sheet structure suggested that RF heating treatment can change the secondary structure of soy protein to be more orderly. Graphical Abstract ᅟ

Dehydration reduces water activity and extends shelf life of perishable agricultural products. The purpose of this research was to study the application of radio frequency (RF) energy in dehydration of in-shell Macadamia nuts and shorten the lengthy process times needed in conventional hot air drying operations. A pilot scale 27.12-MHz and 6- kW RF system was used to determine the operational parameters, the drying curve, and the quality attributes of the processed nuts. The results showed that an electrode gap of 15.5 cm and a hot air temperature of 50 °C provided an acceptable heating rate and stable sample temperatures, and were used for further drying tests. The drying curves showed an exponential decay and required 750 and 360 min to achieve the final moisture content of 0.030 kg water/kg dry solid (3.0 % dry basis) in whole nuts in hot air drying and RF heating/hot air combined drying, respectively. The drying kinetics of the nuts were described well by the Page model for hot air drying, but a logarithmic model was more suited for RF/hot air drying. Peroxide value and free fatty acid increased with the drying time both for hot air and RF drying but remained within acceptable range required by the nut industry. The RF process shows potential to provide rapid, uniform, and quality-acceptable drying technology for the nut industry.