The Influence of Radiofrequency Heating on the Quality and Shelf of Life of Pepper

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

Characterization of radio frequency heating of fresh fruits influenced by dielectric properties

Dielectric heating including microwave (MW) and radio frequency (RF) energy has been regarded as alternative thermal treatments for food processing. To develop effective rice bran (RB) stabilization treatments based on RF and MW heating, dielectric properties (DPs) with dielectric constant (ε′) and loss factor (ε″) of RB samples at frequencies (10–3000 MHz), temperatures (25–100 °C), moisture content (MC, 10.36–24.69% w.b.) and three metal salt levels (0.05–2.00%) were determined by an open-ended coaxial probe and impedance analyzer. Results indicated that both ε′ and ε″ of RB samples increased with increasing temperature and MC. The increase rate was greater at higher temperature and moisture levels than at lower levels, especially at frequencies lower than 300 MHz. Cubic order models were developed to best fit the relationship between DPs of RB samples and temperature/MC at five frequencies with R2 greater than 0.994. Both ε″ and RF heating rate of RB samples increased significantly with added NaCl (2%), KCl (1%) and Na6O18P6 (2%). The obtained data are useful in developing computer models and simulating dielectric heating for RB stabilization and may also provide theoretical basis for synergistic stabilization of RB under combined dielectric heating with metal salts.

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

Current quarantine method, fumigation with methyl bromide, for fresh fruit is under international pressure to phase out because methyl bromide damages the ozone layer. To develop alternative methods, many researchers have explored use of non-chemical treatment methods. But all these methods have their own limitations. For example, conventional hot water treatment takes long time which results in undesirable quality changes in treated fruit. Radio frequency (RF) heating has been looked upon as a means to shorten thermal treatment time. In this study, RF heating patterns in orange, apple, grapefruit, peach, and avocado were investigated. The selected fruits were placed in a fruit mover and RF heated in a 12 kW parallel plate RF unit operating at 27.12 MHz. Dielectric properties of each fruit constitutional part were measured by open-ended coaxial cable probe method. The study showed that peel dielectric properties along with peel thickness greatly influenced the RF heating behavior of an orange and a grapefruit. Core heating was prominent in apple, peeled orange and grapefruit; whereas in whole oranges the subsurface was at higher temperature than the core. The study helped in better understanding of the complex RF heating characteristics of fruits for designing practical product specific treatment protocol.

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.

Several recent studies have found that measuring the dielectric permittivity of soil can be used to determine the level of environmental pollution. However, there is limited research on the measurement of dielectric properties in soil contaminated with biodiesel-diesel blends from Thailand. This paper presents to monitor the dielectric properties of soil contaminated with biodiesel-diesel blends. Specifically, we use the commercial grade diesel B7 to contaminate a sample of sand soil. We also study the measurement of dielectric properties in contaminated soil with the diesel B7 using a dual electrode plate-based radio frequency (RF) heating system. This allows us to observe the behavior of the contaminated soil before and after RF heating treatment. The experimental result showed that the dielectric properties of uncontaminated and contaminated soil were different. In addition, the RF heating system utilizing the electric field intensities of 450 kV/m resulted in the dielectric properties of the contaminated soil becoming similar to those of uncontaminated soil. These findings suggest that using RF heating on contaminated soil samples improve the air in the pore space compared to unheated contaminated soil. This approach may be effective for the treatment of soil in Thailand using an RF heating system with dual electrode plates.

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

Radio frequency (RF) heating is a promising technology for food applications because of the associated rapid and uniform heat distribution, large penetration depth and lower energy consumption. Radio frequency heating has been successfully applied for drying, baking and thawing of frozen meat and in meat processing. However, its use in continuous pasteurization and sterilization of foods is rather limited. During RF heating, heat is generated within the product due to molecular friction resulting from oscillating molecules and ions caused by the applied alternating electric field. RF heating is influenced principally by the dielectric properties of the product when other conditions are kept constant. This review deals with the current status of RF heating applications in food processing, as well as product and system specific factors that influence the RF heating. It is evident that frequency level, temperature and properties of food, such as viscosity, water content and chemical composition affect the dielectric properties and thus the RF heating of foods. Therefore, these parameters should be taken into account when designing a radio frequency heating system for foods.

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.

Inactivation of Escherichia coli K-12 and Listeria innocua in milk using radio frequency (RF) heating

ABSTRACT: Currently, radio frequency (RF) heating is emerging as an alternative to steam thermal recovery methods, owing to its numerous advantages including cleanliness, environmental friendliness, and cost-effectiveness. Achieving maximum heating distance and temperature during RF heating hinges on understanding the interaction between RF electromagnetic waves and rocks. To address this, the present study elucidates the RF heating mechanism, establishes geometric and mathematical models accounting for rock properties, and validates the mathematical model using laboratory experiments. The calculations reveal that lower specific heat, thermal conductivity, relative permittivity, and density of rocks, coupled with higher rock electrical conductivity, contribute to higher maximum reservoir temperatures. Additionally, the maximum heating distance increases with decreasing specific heat and density of rocks. Optimal values of thermal conductivity, relative permittivity, and electrical conductivity exist, maximizing the heating distance. These findings provide crucial guidance for optimizing rock properties, enhancing maximum heating temperature, and extending heating distances before implementing RF heating technology. 1. INTRODUCTION Heavy oil resources are abundant around the world and have broad development and utilization prospects. However, the characteristics of heavy oil with high viscosity and poor fluidity greatly increase the difficulty of extraction. Currently, the steam recovery technologies are widely used for heavy oil extraction, but it has gradually exposed many shortcomings such as high energy consumption, high carbon emissions, and difficulty in effectively exploiting low permeability, deep and thin layers of heavy oil. So many oil companies have no option but to develop new heavy oil mining technologies. A novel heavy oil extraction technology combined with electromagnetic power appears and is collectively referred to as "RF heating" by some oil companies and scholars. Some literatures (Godard and Rey-Bethbeder; Hu and Li et al.; Saeedfar and Lawton et al.; Bera and Babadagli, 2015; Ghannadi and Irani et al., 2016) have introduced its advantages, such as being environmentally friendly, highly heating efficiency, extracting resource in deep oil-bearing formation, avoiding extra heat loss and so on. In the early days, some scholars done some important researches of RF heating for improving heavy oil recovery. In recent years, there were some latest studies on the development of RF heating. world-renowned companies have conducted several researches on RF heating technology. The most representative one is a project called "ESEIEH" carried out in the Steepbank mine in Canada in 2012. Several antennas that emit RF electromagnetic waves are mainly used to radiate electromagnetic waves to reduce the viscosity of heavy oil (Rassenfoss, 2012). In this project, the first phase of field testing has been successfully completed. Besides, Bientinesi et al. conducted an RF heating experiment and proved that RF heating can effectively reduce the heavy oil viscosity (Bientinesi and Petarca et al., 2013). In 2017, Bera and Babadagli experimentally confirmed that Ni and Fe nanoparticles added to the heavy mixture can improve the heating efficiency of RF electromagnetic waves (Bera and Babadagli, 2017). Harris Company developed the "Heatwave" technology to exploit the abundant heavy oil and oil sand resources in Canada, and the field test was successful. In the test, an antenna was uses to radiate electromagnetic waves with the frequency of 6.78MHz into the reservoir, and the maximum heating distance reaches 12.5m (Wise and Patterson, 2016). Wang et al. studied the RF heating mode based on the antenna arrays in 2018 (Wang and Gao et al., 2018; Wang and Gao et al., 2018). The simulation results showed that the antenna array configuration can greatly increase the heating range of heavy oil reservoirs (Wang and Gao et al., 2019; Wang and Gao et al., 2020). However, based on the current research progress, there is a lack of research on the influence of the rock properties on the maximum temperature distribution and furthest heating distance, making it difficult to accurately predict the heating range and heating temperature.

This study was conducted to investigate the effect of salt content during radio-frequency (RF) heating on rate of temperature increase, dielectric properties (DPs), and reduction of pathogens in pistachios. Also, the effect of RF heating on pistachio quality of varying salt content was determined. Pistachios of different salt content (0, 100, and 330 mg sodium/serving) were inoculated with Salmonella enterica and treated in a 27.12-MHz RF heater. The RF heating rate increased when salt content was in the range of 0–100 mg sodium/serving, but there were no significant (P > 0.05) differences in the rate of temperature rise after salt content reached to 100 mg sodium/serving. Both dielectric constant and dielectric loss factor of pistachios increased with rising salt content. Along with increased salt content, RF treatment time required to reduce this pathogen by 4 log CFU/g decreased first and then remained the same above an upper limit of salt content corresponding to the peak heating rate. RF treatment did not significantly (P > 0.05) cause changes in the color and level of lipid oxidation of pistachios. The results of the current study imply that RF heating may be a potential intervention for inactivating foodborne pathogens in pistachios and that the effect of pasteurization is influenced by dielectric loss factor relative to salt content.

Abstract. Radio frequency (RF) heating has been extensively studied as a novel disinfestation method for dry agricultural products. A major difficulty in using this method is that different heating rates at the corners and edges of materials may cause negative effects on product quality. A systematic analysis of factors that influence the RF heating rate is desirable to help in designing effective treatment protocols. A finite element model using COMSOL Multiphysics software was developed and experimentally validated with 3 kg of mung beans in a 6 kW, 27.12 MHz free-running oscillator RF system to study the influence of sample moisture content, density, specific heat capacity, thermal conductivity, dielectric properties, top electrode voltage, and electrode gap on RF heating rate. Simulation results demonstrated that the variation in sample density and specific heat capacity, especially thermal conductivity, had a relatively slight effect on RF heating rate. The RF heating rate was significantly influenced by electrode gap, top electrode voltage, and the dielectric properties and moisture content of the sample. These heating rate distributions might be valuable in guiding and optimizing RF treatment conditions, which are helpful to improve RF heating uniformity for disinfecting dry products.

Dielectric properties of dried vegetable powders and their temperature profile during radio frequency heating