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.     

ZnO nanoparticles combined radio frequency heating: A novel method to control microorganism and improve product quality of prepared carrots

Effects of ZnO nanoparticles combined radio frequency (RF) heating on the sterilization and product quality attributes (hardness, color, carotenoids and microstructure) of prepared carrots were investigated comparing to ZnO nanoparticles or RF heating treatment alone. The results showed that the combined sterilization effect of ZnO nanoparticles with radio frequency treatments was superior to ZnO nanoparticles or RF heating treatment alone and extended the shelf life of prepared carrots up to 60 days. ZnO nanoparticles combining RF heating 20 mm/20 min (plate spacing/RF heating time) reduced the loss of hardness, color difference value (ΔE), and carotenoids of prepared carrots. The Scanning electron micrographs (SEM) showed that the cell microstructure of prepared carrots that treated by RF treatments within 20 min was most closely integrated with each other. Therefore, the experimental results of ZnO nanoparticles combined with radio frequency heating could be helpful for food processing industries to produce high quality sterilized prepared carrots.Industrial relevanceZnO nanoparticles combined with radio frequency heating achieved logarithmic reduction of bacterial population and met China's national standard while preserving the color, texture and nutrition better. This information can be helpful for food processing industries to produce high quality sterilized prepared carrots and this same approach can be applied to many other prepared food formulations.     

Improvement of radio frequency (RF) heating uniformity for peanuts with a new strategy using computational modeling

Radio frequency (RF) heating has been considered as an alternative method to traditional thermal treatments for postharvest pasteurization and disinfestations. However, the non-uniform heating in food samples is the main obstacle to be urgently solved. In this study, mica plates were placed on top of the samples with polypropylene blocks placed in between the samples as a new strategy to improve the RF heating uniformity. A computer simulation model was developed based on the commercial software COMSOL to evaluate the temperature distributions. Experiments with peanut samples placed in three dimensions of containers were conducted to validate the simulation model. The results showed that adding mica plates with the similar dimension to the cold spot of samples and increasing the plate thickness effectively improved the RF heating uniformity. Adding polypropylene blocks raised sample average temperatures and finally optimized the RF heating uniformity.     

食品射频灭菌过程加热均匀性调控研究进展

作为一种新型物理灭菌技术,射频(radio frequency, RF)加热具有穿透深度大、传热效率高、无化学残留、适合批量处理等特点,可有效杀灭食品中的致病菌。然而,受物料本身特性参数、射频系统结构、物料周围介质等因素影响,非均匀加热一直是射频灭菌规模化应用面临的主要挑战之一。为进一步明确食品射频灭菌过程均匀性调控的最新研究进展,本文基于射频杀菌的机制及特点,对调控射频加热均匀性的具体措施进行了梳理,包括调控物料介电参数、改变环境介质、优化射频加热系统等,为加热均匀性调控的进一步研究及射频灭菌技术的进一步应用奠定基础。     

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.     

Computational investigation of the effect of orientation and rotation of shell egg on radio frequency heating rate and uniformity

The objective of this work was to computationally investigate radio frequency (RF) heating rate and uniformity of shell egg as affected by orientation and rotation. To do this, a multiphysics model was developed using COMSOL to simulate RF heating of a vertically oriented non-rotating shell egg. This model was experimentally validated against temperature measurements taken at three different internal locations within the egg. Simulations were then performed using the validated model with both non-rotating and rotating egg for different orientations (vertical and horizontal). Rotation of egg was incorporated into the model by rotating the RF system around the stationary egg instead of rotating the egg. Temperature-dependent material properties of the yolk, albumen and shell within a temperature range of 20 to 70 °C were used in the model. Simulated temperature uniformity index (STUI) values were calculated to evaluate the effect of orientation and rotation on heating uniformity. The results showed that heating was more uniform when shell egg was oriented horizontally rather than vertically. However, rotation did not further improve heating uniformity of the horizontally-oriented egg. Although heating uniformity was improved when the vertically-oriented egg was rotated, concentrated heating around the air cell could not be avoided.     

Investigation of radio frequency heating uniformity of wheat kernels by using the developed computer simulation model

To explore the potential of using radio frequency (RF) energy to control insects or microorganisms in low-moisture granular food crops, wheat kernels were selected as a sample product, and a computer simulation model was developed to study the influence of sample size (wheat kernels filled in plastic container in cuboid shape) and vertical position on RF heating uniformity and to determine the cold and hot spots in the samples. The simulated temperature profile of RF heating was compared with the experimental results to validate the computer model. Temperature uniformity index (TUI) was selected as a criterion to evaluate RF heating uniformity. Results suggested that temperature uniformity of sample depended on sample size and location, where for small sample size, sample placed on either bottom electrode or closed to top electrode would achieve better RF heating uniformity; while for sample in large size, placed in the middle or slightly lower than middle position between the two parallel electrodes achieved better temperature uniformity. Also, a larger sample size had better RF heating uniformity, especially when sample size equaled to the size of the top electrode, indicating that RF energy is more suitable for treating bulk materials. Corner heating was also observed where hot spots located at the corner of samples in cuboid shape, and cold spots depended on sample vertical position. In this study, cold spots were at the center of the top and bottom layers when the sample was placed at the middle position between the two parallel electrodes, at the center of top layer when sample was placed on the bottom electrode, and at the center of bottom layer when sample was placed close to top electrode. Information generated from this study would be essential for developing RF treatment protocol to control insects or microorganisms in low-moisture granular agricultural products.     

Improvement of radio frequency heating uniformity for millets by changing shape and adding polypropylene blocks

Radio frequency (RF) treatment is a burgeoning method in food processes. RF heating non-uniformity is a main obstacle for industrial implementations and influenced by the geometry of treated materials. In this study, the effect of convex-concavo shapes of the sample top surface and step shapes at the bottom of the rectangular container by adding polypropylene (PP) blocks on the heating uniformity was investigated using a 6 kW, 27.12 MHz free-running oscillator RF system. The results showed that the RF heating uniformity index values of millets decreased with increasing convex degree and decreasing concave degree of the sample top shape. Moreover, adding PP blocks at the bottom of the sample container was a better choice of improving RF heating uniformity owing to the wider adjustable range of sample sizes and better temperature distribution. This study provided a new method to improve the RF heating uniformity by simply changing the shapes of samples in either the top or bottom surface.     

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.     

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.     

Developing a rotation device in radio frequency systems for improving the heating uniformity in granular foods

A major obstacle for industrial radio frequency (RF) treatments is non-uniform and thermal runaway heating. This study aimed to improve the RF heating uniformity in granular products using a rotation device with sample mixing. After the run-out error of the rotation device met the allowable tolerance range, the RF heating uniformity was evaluated for mung bean, coix seed, peanut kernel and almond kernel under different rotating speeds, volume ratios and moisture contents (MCs). The results showed that the RF heating uniformity index decreased and then increased with increasing rotation speed, whereas the volume ratio presented the opposite effect. The RF heating uniformity index raised with increasing MCs. The heating uniformity in mung beans was the best because of their smallest particle size among the four selected samples. Therefore, the laboratory-scale rotation device applied in the RF system could provide a solid foundation for further large-scale industrial applications.     

射频解冻过程中食品加热均匀性影响因素的研究进展

射频加热(radio frequency, RF)技术作为一种新兴加热技术，是一种高效、低能耗的可替代传统加热的热处理方法。该方法在提高工业效率的同时又可保证食品的质量安全性及品质，可满足食品工业中的解冻以及其他加工需求，在食品工业中具有越来越大的应用潜力和发展前景。但解冻过程中存在的“边缘效应”等加热不均匀问题仍是制约该技术的一个瓶颈。本研究对射频解冻及其原理进行了介绍，并总结了影响解冻过程中加热均匀性等问题的因素，包括电磁场、射频功率、电极电压及间隙、食品的介电特性等，以期为射频解冻技术的研究及改善加热均匀性等未来研究发展方向提供参考。     

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.     

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.     

Computer simulation for improving radio frequency (RF) heating uniformity of food products: A review

Radio frequency (RF) heating has great potential for achieving rapid and volumetric heating in foods, providing safe and high-quality food products due to deep penetration depth, moisture self-balance effects, and leaving no chemical residues. However, the nonuniform heating problem (usually resulting in hot and cold spots in the heated product) needs to be resolved. The inhomogeneous temperature distribution not only affects the quality of the food but also raises the issue of food safety when the microorganisms or insects may not be controlled in the cold spots. The mathematical modeling for RF heating processes has been extensively studied in a wide variety of agricultural products recently. This paper presents a comprehensive review of recent progresses in computer simulation for RF heating uniformity improvement and the offered solutions to reduce the heating nonuniformity. It provides a brief introduction on the basic principle of RF heating technology, analyzes the applications of numerical simulation, and discusses the factors influencing the RF heating uniformity and the possible methods to improve heating uniformity. Mathematical modeling improves the understanding of RF heating of food and is essential to optimize the RF treatment protocol for pasteurization and disinfestation applications. Recommendations for future research have been proposed to further improve the accuracy of numerical models, by covering both heat and mass transfers in the model, validating these models with sample movement and mixing, and identifying the important model parameters by sensitivity analysis.     

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.     

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.