Radio Frequency Heating Process Research Articles

Developing radio frequency (RF) heating protocol in packed tofu processing by computer simulation

Packed tofu was produced by reheating the mixture of preheated soymilk and coagulant in a sealed container. This study aimed to replace the conventional heating method with RF heating during the reheating of soymilk for packed tofu production. In this study, dielectric properties (DPs), thermal properties (TPs), and rheological properties of soymilk were determined. A mathematical model was developed to simulate the RF heating process of soymilk to determine the appropriate packaging geometry. Water holding capacity (WHC), texture analysis, color measurement, and microstructure observation were performed to evaluate the quality of RF-heated packed tofu. Results showed that soymilk added with Glucono-Delta-Lactone (GDL) coagulated at the temperature above 60 °C, and the loss factor (ε″) was slightly reduced when soymilk was converted to tofu at coagulation temperature. Based on the simulation results, the cylindrical vessel (φ50 mm × 100 mm) was chosen as the soymilk container for desired heating rate (5.9 °C/min) and uniformity (λ = 0.0065, 0.0069, 0.0016 for top, middle, and bottom layers). The texture analysis revealed that the hardness and chewiness of packed tofu prepared by RF heating were enhanced (maximum 1.36 times and 1.21 times) compared with commercial packed tofu, while the springiness were not significantly changed. Furthermore, the denser network structure was observed inside RF-heated packed tofu by SEM. These results indicated that packed tofu prepared by RF heating was of higher gel strength and sensory quality. RF heating has the potential to be applied in packed tofu production.

Improving the shelf life stability of vacuum‐packed fresh‐cut peaches (Prunus persica L.) by radio frequency heating in water

SummaryThe present study aimed to investigate the influence of radio frequency (RF) heating at 70 and 80 °C, in comparison with the traditional conventional heating. The RF heating technology was applied for maintaining the colour and textural quality as well as inactivating the enzymatic activity of polyphenol oxidase (PPO) and reducing the Escherichia coli growth in packaged fresh‐cut peach samples. To evaluate this, 120 g of freshly cut peach fruit was placed in heat‐resistant plastic bags, and 50 mL of sugar solution (12°Brix) was added to each bag and vacuum‐packed for conventional heating and radio frequency (RF) heating. Our results demonstrated that the RF heating process could reduce the heating time by up to 83% at 70 and 80 °C as compared to conventional heating. Moreover, RF heating at 70 and 80 °C reduced the PPO enzymatic activity, which caused enzymatic browning, up to 85.46% and 93.5%, respectively, as compared to 40.86% reduction only for conventional heating. Furthermore, RF heating completely inhibited E. coli growth. Collectively, we demonstrated that RF heating is an emerging and promising technology for improving the quality of fresh‐cut fruits such as peach during various storage conditions. The present study provides relatively novel information on the effectiveness of RF heating in maintaining the quality of fresh‐cut peaches. In the future, we would like to investigate the effectiveness of RF heating on other agricultural products.

Characterization of 50-ohm radio frequency heating of bulk canola seeds (Brassica napus. L) in a tubular applicator with parallel electrodes and post-treatment quality

A pilot-scale 50-ohm (Ω) radio frequency (RF) heating system was used to determine the temperature distribution of bulk canola seeds (Brassica napus L.), 9% moisture content (MC) wet-basis (w.b.) in a tubular applicator with parallel electrodes. Non-uniformity of the temperature distribution of bulk canola was observed during the RF heating process of the canola seeds. The hottest spot was observed at the front side of the tubular cavity of the applicator adjacent to the hot electrode. Tavg of the canola seeds was 38 °C, 42.2 °C, and 40 °C at 3 kW, 5 kW, and 7 kW respectively. Temperature distribution was relatively uniform in the back zone (0.287, 0.433, and 0.278 for 3 kW, 5 kW, and 7 kW respectively). The physicochemical properties of canola seeds changed significantly after 50-ohm RF heating at various end temperatures and power levels. Industrial relevance textRadio frequency heating is an emerging technology and has immense potential in food grains, pulses, and oilseeds processing. The radio frequency heating can be used for disinfestation of insects, drying, and removal of anti-nutritional compounds in stored food grains, pulses, and oilseeds. Radio frequency heating is based on dielectric heating principle thus allowing selective and volumetric heating. The radio frequency waves have wavelengths ranging from 1 mm to about 100 km with a frequency of 3 kHz to about 300 GHz, the penetration depth is comparatively higher than microwaves, which has wavelengths ranging from 1 mm to about 1 m with frequencies between 300 MHz and 300 GHz. Thus, radio waves penetrate larger volume objects compared to microwaves. This research investigated a 50-ohm radio frequency heating system with a through field parallel plate type applicator to observe the temperature distribution throughout the bulk canola at different power levels. Understanding the characteristics of 50-ohm radio frequency heating of bulk canola seeds at different power levels can help design effective heating processes involved in canola seeds storage and handling.

Uncertainty Quantification of an Explicitly Coupled Multiphysics Simulation of In-Situ Pyrolysis by Radio Frequency Heating in Oil Shale

SummaryIn-situ pyrolysis provides an enhanced oil recovery (EOR) technique for exploiting oil and gas from oil shale by converting in-place solid kerogen into liquid oil and gas. Radio-frequency (RF) heating of the in-place oil shale has previously been proposed as a method by which the electromagnetic energy gets converted to thermal energy, thereby heating in-situ kerogen so that it converts to oil and gas. In order to numerically model the RF heating of the in-situ oil shale, a novel explicitly coupled thermal, phase field, mechanical, and electromagnetic (TPME) framework is devised using the finite element method in a 2D domain. Contemporaneous efforts in the commercial development of oil shale by in-situ pyrolysis have largely focused on pilot methodologies intended to validate specific corporate or esoteric EOR strategies. This work focuses on addressing efficient epistemic uncertainty quantification (UQ) of select thermal, oil shale distribution, electromagnetic, and mechanical characteristics of oil shale in the RF heating process, comparing a spectral methodology to a Monte Carlo (MC) simulation for validation. Attempts were made to parameterize the stochastic simulation models using the characteristic properties of Green River oil shale. The geologic environment being investigated is devised as a kerogen-poor under- and overburden separated by a layer of heterogeneous yet kerogen-rich oil shale in a target formation. The objective of this work is the quantification of plausible oil shale conversion using TPME simulation under parametric uncertainty; this, while considering a referenced conversion timeline of 1.0 × 107 seconds. Nonintrusive polynomial chaos (NIPC) and MC simulation were used to evaluate complex stochastically driven TPME simulations of RF heating. The least angle regression (LAR) method was specifically used to determine a sparse set of polynomial chaos coefficients leading to the determination of summary statistics that describe the TPME results. Given the existing broad use of MC simulation methods for UQ in the oil and gas industry, the combined LAR and NIPC is suggested to provide a distinguishable performance improvement to UQ compared to MC methods.

Dielectric Properties and Heating Properties of Aniseed Powder with Radio Frequency Heating

Microbial contamination is currently a major safety issue in low moisture foods. Current sterilization and pasteurization practices, such as conventional thermal processing involving conductive heat transfer to inactivate pathogens, appear inadequate to address the contamination problem in low-moisture foods, which require long time heating, leading to quality deterioration with significant loss of colour and flavour. Radio Frequency (RF) heating has great potential in pasteurization of low moisture foods and offers the possibility to rapidly inactivate microorganisms while maintaining the food quality. Aniseed powder as one of typical low moisture foods possesses significant risk of microbial contamination. This study was to develop RF heating process to inactivate microorganisms in aniseed powder. The dielectric properties (i.e. dielectric constant (ε') and dielectric loss factor (ε'')) of aniseed powder, the influences of processing conditions on heating properties and the inactivation of microorganisms in aniseed powder were all studied in this research. Results showed that ε' and ε'' of aniseed powder decreased with an increase in frequency from 6.78 MHz to 47.46 MHz but then fluctuated between 300 and 2745 MHz. The overall change of ε' and ε'' ultimately exhibited a downward trend. Under a constant frequency treatment, ε' and ε'' of aniseed powder increased with an increase in moisture content, whereas the penetration depth decreased. Particle size showed no significant effects on the dielectric properties, heating rate and penetration depth. With an increase in RF processing time, the heating rate of the sample increased and adding the rolling-over operation step helped to improve the uniformity of RF heating. After processing for 100 s, the temperature of aniseed powder could increase to 60 °C, and the number of microorganism in aniseed powder was reduced by 6.18 logs when time extended to 150 s.

Modeling of radio frequency heating of egg white powder continuously moving on a conveyor belt

The radio frequency (RF) heating of low moisture foods continuously moving on a conveyor belt is a promising approach for preheating to reduce come-up time and/or pasteurization to ensure food safety. Three-dimensional multiphysics-based models were developed to simulate the RF heating process using two approaches (moving mesh and discrete moving steps) to incorporate the movement of the food product. The models were validated by RF heating of a package of egg white powder (EWP) continuously moving on a conveyor belt for 47 min through the system. The dielectric properties of the porous product were determined by using four mixture equations and their effects on model prediction accuracy were evaluated. The model using dielectric properties calculated by the Complex Refractive Index mixture equation showed best prediction accuracy. The simulated point temperatures from the model using moving mesh approach matched best with the experiment at eight locations inside the food product, with a smallest average Root-Mean-Square-Error value of 3.9 °C. The heating uniformity of the moving EWP was not improved or deteriorated when compared to the stationary product which was heated at the center of the system after 47 min heating.

Industrial Applications of Selected JFS Articles

Industrial Applications of Selected <i>JFS</i> Articles

ANALYTICAL SOLUTION OF HYPERBOLIC BIOHEAT EQUATION IN SPHERICAL COORDINATES APPLIED IN RADIOFREQUENCY HEATING

This paper presents an analytical solution for the hyperbolic bioheat equation in spherical coordinates under nonuniform distributed heat source term, which is theoretically modeling the Radiofrequency Heating (RFH) technique. In RFH technique a spherical electrode is inserted into the diseased tissue which leads to the heat generation there due to the imposed electromagnetic field. Because of the non-Fourier behavior of biological tissues, the hyperbolic Penne’s equation is adopted as perfused thermal model. The equation has been solved using Eigenvalue method and the closed form solutions are introduced. The temperature profiles are determined for a sample RFH procedure in cornea and liver and the effect of perfusion term in Penne’s bioheat equation is shown. The effective parameters in the RFH process such as electrode radius, power and different material are studied analytically too. The derived solution can play a role of verification basis of other numerical ones in this area.

Development of radio-frequency heating-assisted nanoimprint with PETG solution for nanostructure-based biosensors

We present radio-frequency (RF) heating-assisted nanoimprint lithography (NIL) for the rapid fabrication of nanostructured biochips. The chips were spin-coated using a RF-sensitive polymer film, namely polyethylene terephthalate glycol-modified (PETG). The RF heating process takes advantage of rapid temperature increases and cooling for NIL. Using a patterned nickel film as the mold, various nanostructures, such as nanowire, nanorod, and nanogrid arrays, were successfully fabricated within several seconds. For surface plasmon biosensing chips, the silver-coated nanowire arrays achieved a linewidth of 6.01 nm and wavelength sensitivity of 550 nm per refractive index unit. The functionality of the sensor was verified by observing the label-free antigen–antibody interactions.

Modeling radio frequency heating of food moving on a conveyor belt

A multiphysics model that coupled electric fields and heat transfer was developed to simulate the radio frequency (RF) heating of food moving on a conveyor belt. A discrete moving step approach was used in the simulation. The total power absorption increased considerably during the entry of the sample into the RF system, remained stable when the sample was fully covered by the top and bottom electrodes, and decreased when the sample moved out of the system. Edge and corner heating was observed from the power absorption distribution and electric field distribution. The model was validated by heating a rectangular container of wheat kernels moving on a conveyor belt in a RF system (27.12MHz, 6kW). The predicted spatial temperatures in top, middle, and bottom layers showed less than 3.5°C lower prediction than the experimental result. The measured anode current showed a good linear correlation with the predicted total power absorption. The optimum number of discrete moving steps was determined to be nine for accurate temperature prediction with total conveyor belt movement distance of 1.13m which is equivalent to optimize step size of 0.1256m/step (0.3m for entry and exit, respectively, and 0.53m for fully covered by electrodes) at speed of 14.23mh−1. The movement of food product could help improve the heating uniformity of RF heating process.

Computer simulation of radio-frequency heating applied to block-shaped foods: Analysis on the role of geometrical parameters

During radio frequency (RF) processing, geometrical factors such as distance between electrodes and projection of top electrode on the samples’ exposed surface area (area that electric field passes through the sample) have a certain effect on power absorption, temperature distribution, heating rates and heating uniformity in the samples. Knowing the role played by these factors can be helpful for further studies in optimization of design parameters of an RF heating process. Therefore, to evaluate effect due to distance between electrodes and projection area during RF heating process, samples of same volume but exhibiting different projection areas were used to determine the variability in their power absorption, temperature change and heating rates. RF electrodes in this system were positioned with a fixed gap between samples’ top–bottom surfaces and the electrodes and, case by case, at different distance between electrodes. The results indicated that both parameters have strong effect on power absorption, temperature distribution, heating rates and heating uniformity in the samples, which is further amplified when it is combined with the change of dielectric properties with temperature. In the investigated range, wider projection area and shorter distance between electrodes led to faster heating, in terms of average temperature, but less uniform temperature distribution, at least in the early stage of the RF heating process, before the changes in dielectric properties of the heated load started modifying the trend of the loss tangent and penetration depth. The results of this study are expected to be used in planning geometrical configuration and design parameters of RF systems and sample dimensions for further optimization studies.

OPTIMIZATION OF RADIOFREQUENCY HEATING OF IN-SHELL EGGS THROUGH FINITE ELEMENT MODELING AND EXPERIMENTAL TRIALS

Considering Radio Frequency (RF) heating as a viable alternative for the in-shell heating of eggs, Finite Element Modeling and simulation of RF heating of in-shell eggs at 27.12MHz were carried out to assess the feasibility and heating uniformity of the process. According to the recommendations of USDA-FSIS for the pasteurization of eggs, egg white must be heated up to 57.5 - C, and the egg yolk has to be heated up to 61.1 - C for 2min. The objective of the simulation was to determine the location of hot and cold spots generated due to non-uniform heating. A parallel plate setup for Radio Frequency heating was simulated for difierent electric fleld strength levels and orientations of the egg (long axis parallel and long axis perpendicular to the plates). The simulation results were experimentally verifled and the simulation procedure was validated using a laboratory parallel plate RF setup. A coaxial cavity design was simulated with a similar approach. Results indicated that both the parallel and coaxial cavity designs were suitable for in-shell pasteurization of eggs provided that the eggs were rotated to maintain the uniformity in heating. After the simulation of RF heating process, the process optimization was carried out to determine the most efiective procedure for the process. The varying parameters obtained by using difierent modeling techniques for radiofrequency heating of in-shell eggs, were optimized using MATLAB. Laboratory scale experimental trials were conducted to test the validity and efiectiveness of the optimized parameters. The optimal parameters set forth were found to be more e-cient in terms of heating time and uniformity.

Computer simulation model development and validation for radio frequency (RF) heating of dry food materials

Radio frequency (RF) heat treatment has been identified as a novel pasteurization and non-chemical quarantine method for dry food materials. But the major obstacle of this treatment is non-uniform heating in these food materials. The objective of this study was to help understand RF heating process by developing a computer simulation model. A finite element based commercial software FEMLAB was used to develop the computer model for a 12 kW, 27.12 MHz parallel plate RF system. Wheat flour was selected as a model food to represent dry food materials. Dielectric properties of wheat flour were measured using an open-ended coaxial probe connected with impedance analyzer, whereas thermal properties were determined using a duel needle probe method. Simulated and experimented temperature profiles (°C) of wheat flour were compared in four different horizontal layers after 3 min of RF heating, with a fixed electrode gap of 155 mm. Both, simulated and experimental results showed that temperature values were higher at the mid layers followed by top and bottom layers. Corners were more heated than centers in each layer. Sensitivity analysis showed that temperature uniformity in the sample was most affected by top electrode voltage and sample dielectric properties. The developed model can further be used to study the effect of some important parameters such as sample size, position, shape, and dielectric properties on RF heating of dry food materials.