Improving radio frequency heating uniformity using a novel rotator for microorganism control and its effect on physiochemical properties of raisins

In the present study, a novel material rotator was designed to evaluate the possibilities to improve radio frequency (RF) heating uniformity in raisins. The rotator provided a means to rotate materials when subjected to RF heating. The results showed that RF heating uniformity was significantly (p < 0.05) improved with the rotation of raisins. The relative heating uniformity parameter (λ) reduced to less than 15% at rotational speed of 40 and 60 rpm. No significant (p > 0.05) influence on heating rate was observed. Pasteurization experiments showed the total bacterial (4.88 log) count was reduced by 2.55 log CFU/g and the molds (4.83 log) were completely inactivated when raisins were heated to 80 °C and maintained for 10 min, which indicated a great disinfection efficiency of RF heating. Quality evaluation showed the physiochemical property changes of raisins were satisfied with acceptable color and nutrients loss. Therefore, the method used in this experiment could be expanded to develop an effective treatment protocol for controlling microorganisms in raisins.     

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.     

Dielectric properties,heating rate,and heating uniformity of wheat flour with added bran associated with radio frequency treatments

The purpose of the study was to determine influences of dielectric properties (DPs) on radio frequency (RF) heating rate and uniformity and provide essential information for developing effective pasteurization processes of wheat flour. DPs of the wheat flour as a function of frequency, moisture content (MC), wheat bran content (WBC) and temperature were determined using an impedance analyzer. A 27.12 MHz, 6 kW pilot-scale RF system at an electrode gap of 100 mm was used to evaluate the heating rate and temperature distribution in each wheat flour-bran mixture. The results showed that DPs of wheat flour increased with increasing temperature and MC but with decreasing WBC due to high fat content in wheat bran, and their relationship can be represented by cubic or quadratic models. The heating rate of wheat flour decreased first and then increased with increase in MC while decreased with decreasing WBC due to the increasing difference between dielectric constant (ε′) and loss factor (ε″). Decreased WBC also resulted in decreasing uniformity index (λ) value. Reducing MC and WBC in wheat flour-bran mixtures could help to improve RF heating uniformity.     

Effects of layer arrangement on heating uniformity and product quality after hot air assisted radio frequency drying of carrot

The conventional drying technology prolongs the shelf life of carrots but may cause the serious loss of nutrients. The aim of this study was to investigate the effects of intermittently rearranging layers on the heating uniformity, drying characteristics and quality of carrot slices under the hot air assisted radio frequency (RF) heating. The carrot slices were dried for 270 min using hot air (60 °C) assisted RF heating at the electrode gap of 100 mm firstly, and then followed hot air drying to achieve the final moisture level (0.11 kg/kg (d.b.)). The results showed that the selected hot air assisted RF drying protocol for the carrot slices reduced 30% of the duration compared to the single hot air drying. The carrot slices dried by the combined drying method had the highest total carotenoid value (P ≤ 0.05) except for possessing accepted color and rehydration. Therefore, the combined drying method could improve the drying rate and maintain heat sensitive substances in carrot slices.     

Heating uniformity in radio frequency treated walnut kernels with different size and density

Radio frequency (RF) treatment is one of the most potential methods for disinfestations and pasteurizations of postharvest agriculture products. Non-uniform heating is the main issue for RF treatment to be used in a large-scale industrial application. A 6 kW, 27 MHz polit-scale RF system was used to reveal the heating uniformity of walnut kernels associated with different size and density. The results showed that the thicker sample in the container indicated the higher heating rate, but the worse heating uniformity when the sample was heated by RF energy with the same weight but larger size. The heating rate of samples decreased with increasing electrode gap, which was beneficial for improving the RF heating uniformity. Meanwhile, the RF heating uniformity index of samples increased with increasing kernel size and decreased with increasing density when the samples were treated with the same volume and heating rate. The heating uniformity indexes were 0.124 for whole kernels, 0.115 for half kernels, 0.104 for quarter kernels, and 0.082 for cracking kernels after RF thermal treatment with the same volume and electrode gap of 98 mm. Furthermore, the heating uniformity of mixing kernels was improved by increasing percentages of cracking kernels to whole or half kernels. The results of this research may provide valuable information to improve the RF heating uniformity in walnut kernels for large-scale industrial applications.     

Radio frequency heating of corn flour: Heating rate and uniformity

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 relevanceThis 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.     

Radio frequency tempering multiple layers of frozen tilapia fillets: the temperature distribution,energy consumption,and quality

Tempering is an important step for the seafood processing industry. Selecting an appropriate tempering method can optimize the quality of tilapia. Volumetric radio frequency heating is able to compete tempering within minutes regardless product sizes. Twelve pieces of frozen tilapia fillets in four layers were used as target samples for a radio frequency tempering study. To improve the tempering uniformity, a thin layer of polyurethane foam was applied in-between layers. The present study explored the optimum radio frequency (RF) tempering processing parameters including three input powers (600, 800, 1000 W) and three electrode gaps (10, 12, 14 cm), batch and continuous mode, and compared it to water-tempering effects on the temperature distribution, drip loss, color, TBARS and texture of tilapia. Results demonstrated that the temperature distribution of tilapia with the optimum RF tempering condition (800 W, 12 cm) was more uniform than water tempering, and the TBARS value of samples tempered by RF is lower than that of water tempering. RF-tempering processes retained the L* value (58.76) and a* value (6.54) of tilapia samples, and also resulted in the highest hardness at 298.59 g. Most of the texture indexes of the tempered tilapia fish show no significant difference (p > 0.05) among all tempering conditions. Thus, RF shows a great potential in fish-tempering industry with its fast, uniform and controllable characteristics and a better retention of fish 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.     

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.     

Radio frequency heating as a disinfestation method against Corcyra cephalonica and its effect on properties of milled rice

Radio frequency (RF) heating has the potential to be developed as an alternative non-chemical disinfestation method. In contrast to microwave (MW), RF exhibited higher penetration depth, which helps RF to be a useful technique in disinfesting packaging foods. A 3 kW, 27.12 MHz RF system was used to validate the practical of radio frequency technology for rice moth (Corcyra cephalonica) control in milled rice. Rice samples were placed in the polystyrene bag and moved at a speed of 0.8 m/min, and heated in the RF system with intermittent mixing. Four electrode gaps and five sample thicknesses were chosen to confirm the optimal conditions of RF treatment. The results showed that the sample thickness of 15 mm and electrode gap of 40 mm could provide the optimum heating rate for rice. Mortality of each stage (adult, larva, egg) of C. cephalonica increased with increasing heating temperature and reached 100% while RF heated 180 s (45.8 °C), 300 s (56.9 °C), and 420 s (70 °C), respectively. No C. cephalonica was determined in the samples during 45 days storage incubation period at RF treatment to 70 °C. There were no significant differences between control and RF treated samples in quality parameters (moisture, protein, fat, gelatinization, and sensory attributes). Therefore, RF treatment may provide a practical and effective method for disinfesting milled rice without affecting product quality.     

Thermal behavior of CMC solutions under simulation of radio frequency pasteurization

Pasteurization and enzyme-inactivation are an economical and convenient way to provide safety and shelf-life stability. However, long heating time and non-uniform temperature distribution are the main issues of traditional thermal processing for enzyme-inactivation. Because of the volumetric heating and high heating rate, radio frequency (RF) treatment is considered as one of the most promising physical enzyme-inactivating methods. A 6 kW, 27.12 MHz RF heating system was applied to explore the effect of cylindrical container dimension and solution concentration on the thermal behavior of high-viscosity liquid food. Carboxymethylcellulose (CMC) solution as a non-Newtonian fluid was selected as a model liquid food in this study. The target temperature for enzyme inactivation was 70 °C. The experimental results showed that the electrode gap, container length, and container diameter affected the heating rate and uniformity of the liquid food. Furthermore, when the concentration of CMC solution increased from 0.5% to 2.0%, the heating rate of the solution firstly increased and then decreased, while the heating uniformity index of the solution increased from 0.008 ± 0.001 to 0.056 ± 0.002. The simulation results showed that the higher electric field strength led to a higher heating rate, temperature, and velocity of natural convection of the solution at the ends of the container. Then, the recirculation zones from the ends to the center of the container were formed, which was diminished with the increasing concentration solution, resulting in the increased maximum temperature difference of solution. The results of this research may provide useful data for subsequent continuous flow studies and information on potential RF enzyme-inactivation in high-viscosity liquid foods.     

Pasteurization mechanism on the cellular level of radio frequency heating and its possible non-thermal effect

Radio frequency (RF) heating is a novel processing technology. Non-thermal effect of RF pasteurization has been debated in the last few years. This study aimed to explore the cellular pasteurization mechanism of RF heating and its possible non-thermal effect. Constant and consistent sample temperatures were achieved using a laboratory-developed device when heated by RF or hot water (HW). Results showed that the sub-lethal rates and death rates of RF and HW-treated microorganisms were not significantly (p > 0.05) different. Mechanism studies showed that cell structural damages after RF or HW treatment were similar. The α-helix content of protein showed similar increasing trend. However, a small number of differently expressed genes were detected between RF and HW-treated microorganisms. These results indicated that no non-thermal effect of RF was observed in cell lethality or structure changes. Only slight differences were observed in gene expression between RF and HW treated microorganisms.     

小颗粒（含粉料）物料射频干燥机设计与分析

针对小颗粒(含粉料)状物料干燥方式简单,热利用率低、获得的物料干制品质量得不到有效保障的问题,利用射频给热技术,设计了针对小颗粒(含粉料)物料中试射频干燥机。该设备由给热系统和输送系统两部分组成,给热系统主要包含射频发生器和翅片加热管,输送系统的主要传送部件为输送带。同时为保证射频干燥的均匀性,设计了物料厚度的限制装置,并以玉米颗粒为试验物料,对所设计的射频干燥机进行物料衡算及能耗分析。结果表明,干燥1 000kg玉米颗粒,平均耗时40.83min,平均干燥能耗4 241.438 9kJ/kg·H2O。该机可以较大幅度地提高热能利用率,保障物料干制品质量。     

Effects of combined radio frequency with hot water blanching on enzyme inactivation,color and texture of sweet potato

Sweet potato is an important food crop with rich nutritional value, and also a commonly used feed and industrial raw material. However, the quality of sweet potatoes may decline during storage and thermal processes because of enzymatic reaction induced by peroxidase (POD). Radio frequency (RF) blanching was thus proposed to reduce POD activities. Effects of electrode gaps and sample thicknesses on the RF heating rate and uniformity in sweet potato were studied. Influences of hot water blanching and combination of RF heating with hot water blanching on enzyme inactivation and physiochemical properties of sweet potatoes were also analyzed. Results showed that the optimum RF heating uniformity was obtained at an electrode gap of 90 mm and a sample thickness of 60 mm. Combined RF with hot water blanching effectively inactivated POD in sweet potatoes. Compared to hot water blanching, combined RF with hot water blanching gave better sample color, texture values, and lower weight loss when achieving the same level of enzyme inactivation (< 10%). Therefore, combined RF with hot water blanching is recommended as an effective, uniform and high-quality blanching technology for sweet potatoes.     

Developing radio frequency heating treatments to control insects in mung beans

Radio frequency (RF) treatments are often used for disinfestations while maintaining the quality of postharvest agricultural products. The purpose of this research was to determine the efficacy of RF treatments for disinfesting mung beans. A pilot-scale, 27 MHz, 6 kW RF unit was used to study the heating uniformity of 6.0 kg mung beans in three different packaging methods heated from 25 °C to 54 °C, and the quality of RF treated mung beans was evaluated. The results showed that the best treatment was achieved by using an eight-layer polypropylene packaging method, an electrode gap of 12.5 cm for heating 5 min, 54 °C surface forced air heating, and 6 min 54 °C forced air holding due to the low heating uniformity index. Then a single layer of forced room air was used for cooling to avoid quality degradation. There was no significant difference in the moisture content, water activity, protein, starch, germination rate, and color values of mung beans before and after RF treatments. Therefore, RF treatments may hold the potential for the effective disinfestations of mung beans.     

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

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 relevanceAlthough 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.     

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

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 relevanceRadio 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.     

Analysis of radio frequency (RF) power distribution in dry food materials

The objectives of this research were to investigate the influence of various factors on radio frequency (RF) power distribution in dry food materials, placed in a 12 kW, 27.12 MHz parallel plate RF system, using a validated finite element computer model. The factors investigated were sample size, shape, relative position between the RF electrodes, and dielectric properties (DPs) of the sample and the surrounding medium. Effects of electrode gap and top electrode configuration on the RF power distribution behavior of the sample were also studied. The RF power uniformity in the samples was compared using RF power density uniformity index (PUI). Simulated results showed that the RF power uniformity in cuboid shaped samples, placed on the bottom electrode, first decreased and then increased with the increase in sample size. The sample shape and its vertical position between the fixed gap parallel plate electrodes also affected the RF power distribution and uniformity. A cuboid sample had higher RF power densities at the edges, while an ellipsoid had higher power densities in the center parts. Simulated results showed that the smaller values of DPs resulted in better RF power uniformities in the samples. Reducing the electrode gap improved the RF power uniformity of the sample. While studying the influence of the top electrode configuration on the RF power distribution and uniformity, the results showed that optimum RF power uniformity in a particular sample size could be achieved with a particular top electrode bending position and angle. The results are useful in understanding complex RF heating, designing and scaling up of efficient RF systems.     

Evaluating performances of a small-scale 50 Ω radio frequency heating system designed for home applications

Industrial-scale radio frequency (RF) heating systems have been extensively used in processing agricultural products and foods. However, small-scale RF heating systems for household applications are yet to be realized. The purpose of this study was to develop small-scale 50 Ω RF heating systems and evaluate its heating performances with cooked rice (moisture content (MC) of 62.5 ± 0.5% wet basis (w.b.)). Two RF systems (13.56 and 27.12 MHz) were developed, and the heating rate, heating uniformity, and energy efficiency of cooked rice samples were compared between the developed RF units and a 2450 MHz microwave (MW) oven under static conditions. Results showed that the RF heating systems had smaller temperature differences and better heating uniformity than the MW oven under similar overall heating rate and the same input power (700 W) levels. The 13.56 MHz RF system performed better than the 27.12 MHz one. These findings may provide some clear performance indicators and guidelines for domestic application of small-scale 50 Ω RF heating systems.     

Effect of water,fat, and salt contents on heating uniformity and color of ground beef subjected to radio frequency thawing process

The comminuted meat products are popular and important in the global market. Radio frequency (RF) heating has been proposed as a new alternative method for thawing bulk frozen comminuted meat products with the main advantages of rapid and volumetric heating. However, the temperature non-uniformity is the main obstacle for its industrial applications. In the present study, effects of the level of added water (15–25%), fat (15–25%) and salt (0.1–1.0%) on the heating uniformity and color of ground beef were investigated using a 6 kW 27.12 MHz RF system during thawing process. The results showed that added water had a negative impact on the RF heating uniformity, and a positive impact on beef color. Fat had a major negative effect on the RF heating uniformity and a positive impact on the lightness of the beef sample. The high salt content had a positive influence on the RF heating uniformity, and a significant negative effect on L* value of beef samples. The results may provide some potential methods to improve the RF heating uniformity and quality of food during RF thawing processing.