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.     

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.     

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.     

Storage stability of pistachios as influenced by radio frequency treatments for postharvest disinfestations

There has been an increased interest in developing alternative physical methods for disinfesting postharvest nuts under growing international pressures to replace chemical fumigation due to its adverse effects on human health and environment. The present research explored the possibility of using radio frequency (RF) heating as a non-chemical treatment for disinfestations of pistachios. A pilot-scale, 27 MHz, 6 kW RF unit was used to study RF heating uniformity, develop a treatment protocol, and evaluate quality attributes and storage stability in treated samples. Only 5.6 and 5.5 min were needed to raise the centre temperature of 1.8 kg in-shell and 2.0 kg shelled pistachios to reach 55 °C using RF energy, respectively, compared to about 82 and 117 min when using hot air heating at 55 °C. RF heating uniformity in both types of pistachios was improved by adding forced hot air, sample movements on the conveyor and a single mixing in the middle of the treatment time. The final average temperatures on the surface and in the interior of both types of pistachio kernels exceeded 52 °C, following a holding step at 55 °C for 2 min using hot air. This provided a conservative and 100% mortality of fifth-instar Indianmeal moth (Plodia interpunctella [Hübner]). RF treated samples were not significantly different from control samples in weight loss, peroxide values, fatty acid values, fatty acid composition and kernel colour. RF treatments can, therefore, potentially provide an effective and rapid protocol against stored product pests in pistachios as an alternative to chemical fumigation.Industrial relevanceA pilot-scale 6 kW RF system with conveyor belt was used to determine the heating uniformity and quality changes of pistachios. For a combination with hot air surface heating and mixing, an effective and continuous RF disinfestation method could be developed for pistachios. The RF heating technology has been successfully demonstrated for disinfesting walnuts in California, USA. We tried to expand the industrial applications of RF heating for disinfesting pistachios, to replace the chemical fumigation. This research may provide potential industrial applications of RF treatments for disinfestations based on fast and uniform heating.     

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.     

Developing treatment protocols for disinfesting pine wood product using radio frequency energy

Numerous non-native insect pests are moved around the world via international trade and have caused major forestry losses, especially for pine trees. The purpose of this research was to study a potential application of radio frequency (RF) heating for disinfesting pine wood products to reduce forestry damage caused by exotic pests. A pilot-scale 6 kW, 27.12 MHz RF system was used to develop a treatment protocol for disinfesting pine wood products based on the heating uniformity studies. The results showed that the heating time needed was only 6.7 min to heat the wood blocks (250?×?400?×?60 mm³) with moisture content of 13.3% d.b. from 25 to 60?°C using RF energy, but 320 min to reach 56.2?°C using hot air at 60?°C with 1.6 m/s. Based on the heating uniformity study, the best condition occurred when the lower wood moisture content was used and the wood width approached the RF electrode one. The RF treatment protocol was finally developed to combine RF energy with forced hot air at 60?°C, movement of conveyor at 7.4 m/h and holding at 60?°C hot air for 1 min. Wood quality was not affected significantly by the RF treatments because quality parameters of treated wood samples were similar to those of controls. The RF treatment may provide a rapid, effective and environmentally friendly method to disinfest wood products.     

Application of radio frequency pasteurization to corn (Zea mays L.): Heating uniformity improvement and quality stability evaluation

Mildews caused grain losses and serious outbreaks have been becoming increasing concerns in domestic and international corn processing industry. This study intended to explore the possibility of using radio frequency (RF) heating as an effective treatment to eliminate the mold contamination and reduce the damage to corn quality. A pilot-scale, 27 MHz, 6 kW RF unit was used to study the heating uniformity in corn samples with five moisture contents (MC) and using three plastic material containers, and develop a treatment protocol for a corn sample with the MC of 15.0% w.b. and evaluate quality attributes and storage stability of treated samples. The results showed that only 7.5 min was needed to raise the central temperature of 3.0 kg corn samples from 25 °C to 70 °C using the RF energy, but 749 min for samples to reach 68.6 °C using hot air at 70 °C. The RF heating uniformity was improved by adding forced hot air, moving samples on the conveyor, and mixing during the treatment. An effective RF treatment protocol was finally developed to combine 0.8 kW RF power with a forced hot air at 70 °C, conveyor movement at 6.6 m/h, two mixings, and holding at 70 °C hot air for 14 min, followed by forced room air cooling through thin-layer (2 cm) samples. Corn quality was not affected by RF treatments since quality parameters of RF treated samples were better than or similar to those of untreated controls after the accelerated shelf life test. RF treatments may hold great potential as a pasteurization method to control molds in corns without causing a substantial loss of product quality.     

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.     

Validation of radio frequency treatments as alternative non-chemical methods for disinfesting chestnuts

Chemical fumigation has been widely used to control insects in postharvest chestnuts but is inherent dangers when using fumigants. The purpose of this study was to validate application of radio frequency (RF) treatments for disinfesting chestnuts as an alternative to chemical fumigation. A practical process protocol was developed to control insect pests in chestnuts using a 27.12 MHz free-running oscillator RF system. Fifth-instar yellow peach moth, Conogethes punctiferalis, more heat tolerant than chestnut weevil, Curculio elephas, under three temperature and time combinations using a heating block system, was selected as the targeted insect to validate the RF treatment protocol. Mortality of fifth-instar C. punctiferalis increased with increasing holding time at 55 °C using RF heating and reached 100% while holding in hot air for at least 5 min. Furthermore, there was no significant quality difference in color, fat, firmness, moisture content, protein, and soluble sugar content of chestnuts observed between RF treatments and controls. RF treatment methods hold potential to scale up for industrial applications of disinfesting chestnuts.     

Investigation of the Feasibility of Radio Frequency Energy for Controlling Insects in Milled Rice

Many studies showed that radio frequency (RF) holds great potential to control insects in grains, while a few more technical issues need to be addressed to further make this technology ready for industrial application. Therefore, the effect of RF heating rate on rice fissure ratio and broken rate, RF heating lethality on immature rice weevils, and RF disinfestation treatment on cooking and eating quality of milled rice was investigated in this study. Results indicated that RF heating rate had no significant influence on rice broken rate, while fast RF heating rate (>7.2 °C/min) had adverse effects on rice fissure ratio, and mild RF heating rate (<5.8 °C/min) had no significant influence on fissure ratio of milled rice. RF treatment (50 °C, 5 min) could completely control immature rice weevils in rough, brown, and milled rice. Even though RF disinfestation treatment (50 °C, 5 min) influenced the cooking quality (water absorption, adhesive strength) of milled rice, it had no significant effect on overall sensory quality. This study provided valuable information for considering the application of RF energy in milled rice process to control insects.     

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.     

Postharvest treatments with radio frequency for 10 and 20 kg batches of black beans (Phaseolus vulgaris L.)

Radio frequency (RF) heating has been considered an alternative to conventional postharvest disinfestation methods in stored agricultural products. RF treatments at pilot scale (12 kW, 27.12 MHz) were applied in black beans (batches of 10 and 20 kg), until reach the target temperature for weevil mortality (50 °C). Temperature was held by 1 min, and then beans were cooled by forced air. During the protocol, the temperatures were followed by thermal images. For the 10 kg batch, the average heating time was 2.72 min, while for the 20 kg batch, only 1.29 min were required to reach the target temperature of 50 °C. This reduction in heating time is due to dielectric heating provides rapid and volumetric heating. Average cooling time was 14.75 and 26.75 min for batches of 10 and 20 kg respectively. The heating uniformity index values (λ) were close to zero (0.025 for 10 kg batch and 0.023 for 20 kg), indicating even heating. Water absorption capacity, density and viability of beans were not affected by the RF treatments (P > 0.05). Due to volumetric heating, better results were obtained for 20 kg batch, demonstrating the feasibility of RF technology for disinfestation purposes in relatively large batches of black beans.     

Radio frequency heating pasteurization of pine wood nematode (Bursaphelenchus xylophilus) infected wood

The research was aimed to establish whether dielectric heating at radio frequencies (RF)??an alternative method to ozone-depleting methyl bromide fumigation and to convective heating??could bring green lodgepole pine boards infested with pinewood nematodes (PWN) to pasteurization requirements. Artificially infected specimens were treated using a laboratory RF oven and post-treatment nematode survival levels were assessed. Both tested combinations of?56°C/30?min and 60°C/15?min were effective in eradicating PWN. A?power density of 50?kW/m³ maintained for 75?minutes fully pasteurized all the tested boards and proved to be the right combination if no temperature sensors are used. The data gathered supports the idea that pasteurization by RF heating is a reliable and swift alternative treatment to fumigation and convective heat treatment.     

A Comparison of Conventional and Radio Frequency Thawing of Beef Meats: Effects on Product Temperature Distribution

This study examined the use of pilot-scale radio frequency (RF) heating for thawing beef meat blends (lean, 50:50 lean/fat and fat). The aim was to thaw blocks (4 kg) to within a target temperature range of −1 to +5°C. Post-thawing temperature distribution in the blocks was compared to that of blocks thawed by conventional air thawing. The optimum RF conditions for thawing lean meat was 35 min of RF heating delivered in a noncontinuous fashion (20 min on, 10 min off, and followed by 15 min on) at 400 W, which gave a mean temperature of 0.2°C (SD 1.8). By comparison, conventional thawing was achieved in 50 h 20 min which represented an 85-fold difference in thawing time. Comparable uniformity of temperature distribution was obtained by each method. For the lean/fat mixture and 100% fat, the target range could not be achieved due to problems of runaway heating. The latter phenomenon relates to the manner in which the absorbed energy is transferred throughout the material as influenced by the thermophysical properties of the product.     

Effect of radio frequency heating on the mortality of Rhizopertha dominica (F.) and its impact on grain quality

The use of radio frequency (RF) heating to reach temperatures lethal to stored product pests has significant potential. We examined the lethal effects of RF on the stored grain pest, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), under four different RFs, three types of grain, and different moisture contents of rice. The quality of two rice cultivars, after RF heating, was also tested. When heated to 58 °C by RF, R. dominica adult mortality reached 100%. The mortality of R. dominica adults reached 100%. The fastest mortality rate occurred at 50–52 °C. With an increase in RF, the time needed to attain a lethal temperature decreased. The heating time to reach 100% mortality differed among the three types of grain; for example, the heating time of wheat was faster than corn or rice. Kernel size and chemical composition was the main factor influencing the RF lethal rate in the different types of grain under the same experimental conditions. There was a significant linear relationship between moisture content (m.c.) and heating time (Y = −38.05 X + 15.501, R = 0.9803) in rice. RF heating of rice to 58 °C did not significantly affect milling quality, storage quality, or seed quality. Thus, heating time, types of grain, and the m.c. of the grains are three important factors that should be considered in RF heating to control stored product pests. These results indicate that RF heating could be an effective physical method for killing stored grain pests.     

Hot air-assisted radio frequency heating effects on wheat and corn seeds: Quality change and fungi inhibition

Hot air-assisted radio frequency (RF) heating has been proposed and studied as an alternative disinfestation method for grain seeds. In this study, hot air-assisted RF heating effects on fungi inhibition and its influence on physiological (seed vigor, germination rate) and biochemical (color, enzyme activity) qualities of wheat and corn seeds were further investigated. Results showed that high intensity hot air-assisted RF treatment (70 °C, 10 min) reduced seed vigor and germination rate for both wheat and corn seeds, but relatively mild intensity treatment (65 °C, 10 min) showed no significant influence on color and germination rate, and even enhanced enzyme activity (Superoxide dismutase-SOD, catalase-CAT and peroxidase-POD). Moreover, hot air-assisted RF treatment (65 °C, 10 min) reduced Aspergillus flavus by 2 and 3 log-scale (CUF/g) in wheat and corn seeds with the moisture content of 12.0%, and 3 and 4 log-scale for wheat and corn seeds when moisture content increased to 15.0%, respectively. These results indicated higher moisture content (a_w) of grain seeds would benefit fungi inhibition for hot air-assisted RF treatment, and with proper control of treatment conditions, hot air-assisted RF heating has the potential to inhibit fungi and maintain physiological and biochemical quality of grain seeds.     

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. T_avg 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.     

Pasteurization process development for controlling Salmonella in in-shell almonds using radio frequency energy

Radio frequency (RF) treatment holds potential as a pasteurization method to control Salmonella in almonds without causing a substantial loss of product quality. Thermal resistance of Salmonella can be reduced by increasing water activity, thus a soaking process was designed prior to RF treatments. A pilot-scale 27 MHz, 6 kW RF heating system was used to rapidly heat 1.7 kg washed in-shell almonds with hot air heating at 55 °C. To achieve appropriate heating rate, constant drying temperature and short time cooling, the RF treatment protocol was obtained using an electrode gap of 13 cm for heating, 14 cm for drying, and followed by forced room air cooling of 5-cm thick samples. The results showed that almond temperatures above 75 °C at 23% moisture contents for 2-4 min RF heating could meet the requirements to achieve 5-log reduction of Salmonella. The RF treatment process for 20 min reduced the moisture content to 5.7% w.b. Peroxide value, fatty acids values and kernel colors of the RF treated almonds met good quality standard used by nut industry.     

Radio frequency reheating behavior in a heterogeneous food: A case study of pizza

Radio frequency (RF) energy could rapidly raise the material bulk temperature with deep power penetration and high energy efficiency. Although many studies on RF treatments have been carried out on different agricultural commodities or food, application of RF energy for studying heating behavior of heterogeneous food matrix is limited. Pizza is typically a heterogeneous food model with a base, cheese matrix and added toppings, and has been rarely evaluated for RF reheating. This study was aimed at using computer simulation for evaluating the RF reheating uniformity of a typical model pizza after baking based on measured thermal and dielectric properties (DPs) of its components (pizza base, cheese, salami and onion). The validation experiments were carried out up to a maximum reheating temperature of 85 °C in a 6 kW, 27.12 MHz RF system. The results showed that both dielectric constant and loss factor of pizza components increased with temperature, and at elevated temperatures, DP values for salami were the highest cheese, onion and pizza base. Both computer simulation and experiment provided similar temperature-time histories (with the maximum RMSE of 0.034 °C) in pizza base, cheese, salami and onion after RF reheating for 7.33 min. The fast RF reheating rate was observed in cooked salami both from the temperature-time history and predicted temperature distribution due to closed dielectric loss to constant value. The high sample temperature resulted in low moisture content. The RF heating uniformity could be improved by moving or rotating pizza and specially locating components in the further studies.     

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.