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.     

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.     

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

Because of its fast and volumetric nature, radio frequency (RF) heating has been looked upon as a way to overcome the problems associated with conventional heating methods used for disinfestation of fruits. But non-uniform heating within fruits is a major obstacle in adaptation of this technology. In this study, RF heating patterns influenced by dielectric properties (DPs) of fruits were investigated both experimentally and mathematically. A computer simulation model was developed using FEMLAB 3.4, a commercial software for solving Maxwell’s electromagnetic and Fourier’s heat transfer equations. Orange, apple, grapefruit, peach, and avocado fruits, selected for these studies were subjected to RF heating in a water filled container equipped with a mechanism to keep fruits rotating and moving during RF heating in a 27.12 MHz, 12 kW parallel plate RF unit. DPs of constitutional parts of the selected fruits were measured by open-ended coaxial probe method. The study showed that dissimilarity in peel and pulp DPs greatly influenced the RF heating behavior of the fruits. Core heating was prominent in apple, peeled orange and grapefruit; whereas subsurface/peripheral heating in whole oranges and grapefruit, and avocado. The computer model was an effective tool in characterizing and explaining the heating patterns in the fruits based on DPs. The study helped in better understanding the complex RF heating characteristics of fruits, which may be useful in assessing the design feasibility of product specific RF energy based treatment protocol.     

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

Using a 2 kW, 27.12 MHz RF heater, studies were conducted to evaluate the effectiveness of RF heating in inactivating surrogates of both Listeria and Escherichia coli cells in milk under continuous flow conditions. Depending on product residence time and RF power level, RF heating was found to be capable of inactivating both Listeria and E. coli in milk, with E. coli being the more heat sensitive of the two. For a total residence time of 55.5 s (i.e., 29.5 and 26 s in the applicator and holding tube, respectively), up to 5- and 7-log reductions were found for heating Listeria and E. coli, respectively at 1200 W, and an applicator tube exit temperature of approximately 65 °C. This study demonstrates that RF heating could be used to effectively pasteurize milk by manipulating incident power levels and flow rate. While these studies have been conducted under mild fluid flow (laminar) conditions, further studies are necessary to justify its industrial application using more realistic flow conditions.

Industrial relevance

Radio frequency (RF) heating can provide rapid heating. Compared to the microwave, RF has the added advantage of higher penetration depth. These advantages could be harnessed for pasteurizing large volume liquid foods and, most importantly, RF heating could potentially replace traditional heat exchangers that are easily fouled by products such as milk. This study demonstrates the effectiveness of using RF heating under mild flow conditions to inactivate microorganisms that could contaminate milk.     

Recent developments in applications of radio frequency heating for improving safety and quality of food grains and their products: A review

Abstract

Food grains constitute a vital part of the daily diet of the population worldwide, and are generally considered as safe products with high storage stability due to their low moisture contents. However, post-harvest losses (PHL) caused by insects, fungi, food-borne pathogens, and undesirable enzymes remain a major concern for the grain industry. Thermal treatments are commonly used to reduce the PHL of grains and their products without any chemical residues. Among which, radio frequency (RF) technology has been regarded as a promising alternative to traditional heating methods for improving safety and quality of food grains due to its fast, volumetric, and deep penetration heating characteristics. This review provided comprehensive information about principles of RF technology and its main applications including disinfestation, pasteurization, enzyme inactivation, drying, and roasting for processing food grains and their products. The methods to improve the RF heating uniformity and effects of RF heating on product quality were also reviewed. Finally, the current problems and recommendations for future work related to RF processing of grains and their products were discussed. This review would improve the understanding of RF heating for food grains and their products and promote the application of RF technology in the food grain industry.     

Radio-frequency heating of heterogeneous food - Meat lasagna

This research studied feasibility of using radio-frequency (RF) energy to thermally process highly heterogeneous foods in large containers as shelf-stable products. Meat lasagna was selected as the food for the study. Dielectric properties of beef, mozzarella cheese, noodles, and sauce were determined between 1 and 1800 MHz and from 20 to 121 °C. A 6-kW 27-MHz RF system was used to sterilize packaged meat lasagna in large polymeric containers (295 × 253 × 42 mm) during which the temperature of different components were monitored. Computer simulations were also conducted to evaluate the influence of the dielectric properties of each food component on the electric field distribution and heating pattern during RF heating. The measurements indicated small temperature differences in beef meatballs, mozzarella cheese and sauce when they were properly distributed between layers of noodles. Simulation results suggested that in spite of large differences in electric field intensities in different food components, adequate heat transfer reduced differential heating. Thus, RF heating can be used to process pre-packaged heterogeneous foods and retain product quality.     

Dielectric properties of salmon (Oncorhynchus keta) and sturgeon (Acipenser transmontanus) caviar at radio frequency (RF) and microwave (MW) pasteurization frequencies

Radio frequency (RF) and microwave (MW) heating provide an important advantage of more rapid heat penetration in pasteurization processes for heat labile high value foods, which to date, have only been pasteurized by conductive heating. The objectives of this work were to determine the dielectric constant, loss factor and power penetration depth for salmon (0.8% and 2.3% total salt) and sturgeon (0.20 and 3.3% salt) caviars at RF frequency of 27 MHz and MW frequency of 915 MHz (20–80 °C). The dielectric constant (ε′) and dielectric loss factor (ε″) for salmon and sturgeon caviar increased markedly with increasing temperature at 27 MHz but not at 915 MHz. Power penetration depth was higher at 27 MHz compared to 915 MHz, and in unsalted compared to salted roe. Power penetration depth tended to decrease as temperature increased.     

Disinfestation of insect pests in stored agricultural materials using microwave and radio frequency heating: A review

This article provides a comprehensive review of the disinfestation of insects in stored agricultural materials using radio frequency (RF) and microwave (MW) heating. It covers RF and MW disinfestation principles; measurement methods and effects of the operating parameters on dielectric properties; comparison between RF and MW and their effect on the mortality of different insects, quality of the host materials after disinfestation, and heating uniformity; and disinfestation economics. This review shows that RF and MW disinfestation are useful without affecting the quality of the host materials. However, RF heating is being preferred over MW heating for its higher selective heating characteristic.     

Development of a computer simulation model for processing food in a microwave assisted thermal sterilization (MATS) system

The microwave assisted thermal sterilization computer simulation model (MATS-CSM) was developed to improve the previous computer simulation model for the microwave assisted thermal sterilization (MATS) system. Development of the new MATS-CSM included determination of optimum heating time step, evaluation of electromagnetic field distribution and the resulting heating pattern in food, and experimental validation of heating patterns. It was determined that the minimum number of discretization that would not cause immediate divergence of the EM-heat transfer solution was 32 steps corresponding to 97 mm and 5.6 s of displacement and heating time for every step, respectively. Furthermore, this study successfully demonstrated the symmetrical electromagnetic field distribution between top and bottom microwave entry ports and a staggered electric field pattern from one cavity of the MATS to the next. In addition, MATS-CSM confirmed that incorporating heat diffusion in the simulation model reduces the difference in hot spot and cold spot temperature by 65%. It also confirmed that water circulation reduces the edge heating effect, as observed in experiments. The heating pattern generated by MATS-CSM was verified experimentally through a chemical marker method. Based on the percent areal cross section of the weighted average temperature, there were no noticeable differences between the heating zones generated by the MATS-CSM and by the chemical marker method. The percent areal cross section of the cold area 1, cold area 2, and hot area by MATS-CSM were 35%, 25%, and 40%, respectively, and the cold area 1, cold area 2, and hot area by chemical marker method were 35%, 30%, and 35%, respectively.     

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.     

Using whey protein gel as a model food to study dielectric heating properties of salmon (Oncorhynchus gorbuscha) fillets

It is desirable to develop rapid commercial microwave and radio frequency sterilization processes to produce high quality shelf stable muscle foods, particularly aquatic foods. Whey protein gels containing d-ribose and salt were studied as a model food to determine heating patterns in salmon fillets during high temperature microwave sterilization processes. Dielectric constant (?′) and loss factor (?″) of whey protein gels with d-ribose (0.5 g/100 g, 1 g/100 g, and 1.5 g/100 g) at different salt contents (0, 0.1 g/100 g, 0.2 g/100 g, 0.3 g/100 g, 0.4 g/100 g, and 0.5 g/100 g) and frozen and thawed pink salmon (Oncorhynchus gorbuscha) fillets were determined over the frequency range of 27-1800 MHz at temperatures ranging from 20 to 120 °C. The dielectric properties of whey protein gels containing 1 g/100 g d-ribose and 0.2 g/100 g or 0.3 g/100 g salt closely matched the dielectric behavior of salmon fillets in both radio frequency (RF, 27 MHz) and microwave (MW, 915-1800 MHz) ranges. Altering the salt content had a greater impact on dielectric constant and loss factors at lower frequencies. These results suggest that whey protein gel may be a good model food for microwave sterilization process development, particularly for determining the locations of cold and hot spots in complex muscle foods.     

Dielectric properties of ground almond shells in the development of radio frequency and microwave pasteurization

To develop pasteurization treatments based on radio frequency (RF) or microwave energy, dielectric properties of almond shells were determined using an open-ended coaxial-probe with an impedance analyzer over a frequency range of 10–1800 MHz. Both the dielectric constant and loss factor of almond shells decreased with increasing frequency, but increased with increasing temperature and moisture content. The absolute value of the slopes of log–log plots between loss factor and frequency increased with increasing temperature at low frequencies, especially at high temperatures and moisture contents. The effective electrical conductivity of shell samples was close to zero at the lowest moisture content (6% w.b.) and 3–9 times larger at 90 °C than 20 °C for the highest moisture content (36% w.b.). A good linear relationship was observed between permittivity and density at 1800 MHz. The power penetration depths at RF range (27 and 40 MHz) were about 6–24 times as deep as those for microwave frequencies (915 and 1800 MHz) at each corresponding temperature and moisture content. It is likely that RF energy may provide uniform heating and high throughput treatments for controlling Salmonella in in-shell almonds after washing.     

Thermal death kinetics and heating rate effects for fifth-instar Cydia pomonella (L.) (Lepidoptera: Tortricidae)

Thermal death kinetic parameters of fifth-instar codling moths (Cydia pomonella (L.)) and the effect of three heating rates (1°C min⁻¹, 10°C min⁻¹, and 18°C min⁻¹) on larval mortality were determined by a heating block system. The insects were heated to four temperatures (46°C, 48°C, 50°C, and 52°C) held for predetermined periods followed by 24 h storage at 4°C before mortality evaluation. Thermal death kinetics for fifth-instar codling moths followed a 0.5th order of kinetic reaction. Minimum time required to achieve 100% mortality of a given population decreased with temperature in a semi-logarithmic manner. No larval survival was observed in samples of 600 insects after exposure to 46°C, 48°C, 50°C, and 52°C for 50, 15, 5, and 2 min, respectively. Activation energy for thermal kill of fifth-instar codling moths at the heating rate of 18°C min⁻¹ was estimated to be about 472 kJ mol⁻¹. The lethal time accumulated during the ramp period was about 1.8, 0.2, and 0.1 min for the heating rates of 1°C min⁻¹, 10°C min⁻¹, and 18°C min⁻¹, respectively.     

Effect of radio frequency (RF) heating on the texture,colour and sensory properties of a comminuted pork meat product

Radio frequency (RF) cooking is a form of dielectric heating in which products are heated by subjecting them to an alternating electromagnetic field between two parallel electrodes. Although similar in some respects to Microwave heating, RF has been proposed to be more suitable for industrial heating of meats because of the greater penetration depths possible with this technology. In this study an RF cooking protocol was developed and its effect on selected quality attributes of pork based white pudding was examined. Whilst cooking of the product in air proved unfeasible due to arcing, use of a polyethylene cell with circulating hot water (80 °C) facilitated successful heating of the product. Application of RF using an optimised cooking protocol (RF power = 450 W, cell volume = 500 ml and continuous circulation) resulted in a mean end-point temperature of 73 °C after 7 min 40 s. Similar mean end-point temperatures in water bath and steam oven heated products were achieved after 29 and 33 min, respectively. A factorial experiment was conducted to assess selected quality attributes of the cooked puddings. Results show that RF heated puddings were not significantly different (P > 0.05) from water bath and steam oven heated products with regard to instrumental colour, instrumental texture (Kramer shear and texture profile analysis) and expressible fluid. Furthermore, results of a sensory similarity test involving 60 panellists indicated that panellists were not able to detect differences between puddings cooked by RF and conventional methods. Overall this suggests that RF heating technology could have potential in pasteurisation of meat products though further work is needed to verify this.     

基于URIF模型的圆柱型物料冷冻干燥过程的二维模拟及验证

通过冰界面均匀退却模型(URIF模型)的研究,建立了圆柱型物料的二维质热传递数学模型并进行数值模拟.采用变时间步长法对所建立的模型进行求解,获得物料内部的温度分布及水分升华速率.最后通过对胡萝卜的实验表明,实验结果与理论计算值能很好的吻合,从而验证了模型的正确性.这些工作比目前模型简单、适用范围窄的研究更进了一步,对工程实践具有一定的指导作用.     

帘线钢丝干拉温度的计算机模拟及监控

对理论和实际生产数据进行分析,采用最小二乘法,运用M atlab建立帘线钢丝干式拉拔的温度数学模型,利用VB和Access开发帘线钢丝干式拉拔计算机跟踪模拟控制系统软件,对帘线钢丝干式拉拔的生产情况进行量的评估,并对模型与实际生产的符合性进行校验:当计算温度平均值分别为140,159,168,172,173,173,173,167,165℃时,实测温度平均值分别为135,158,167,170,171,168,169,171,165℃,结果显示,系统精度能够满足指导生产的要求。     

Comparison of heating methods and the use of different tissues for sensory assessment of abnormal odours (boar taint) in pig meat

Five heating methods (microwave, hotwire, boiling at 25 °C and 75 °C and melting) were used to generate cooking odours from backfat of entire male pigs and a 'composite' sample consisting of fat and muscle from the head along with salivary glands. The methods elicited significantly different scores for odours from 4 groups of 10 samples differing in their concentrations and ratios of skatole and androstenone. The odours (pork odour, abnormal odour, skatole odour and androstenone odour) were assessed by 3 experienced assessors. Correlations between skatole and androstenone concentrations and abnormal odour score in backfat were higher for skatole, suggesting it is the more important boar taint compound. In the composite sample, androstenone concentration was much higher than in backfat and androstenone was a more important contributor to boar taint. The microwave, hotwire and boiling (75 °C) methods produced the clearest separation between samples and the microwave method was considered the most suitable for on-line use.     

Effect of heating fatty fish: Baltic herring (Clupea harengus membras), European sprat (Sprattus sprattus) and rainbow trout (Oncorhynchus mykiss) on lipid oxidation and contents of eicosapentaenoic and docosahexaenoic acids

In this study, analyses were carried out to establish the impact of heating three species of fatty fish: trouts, herrings and sprats, on the lipids oxidation and on contents of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids. The comminuted fish tissue was heated at restricted access of oxygen at temperature of 60, 100 and 160 °C for 15–120 min. Lipids, extracted with the Bligh‐Dyer method, were determined for peroxide value (PV) and anisidine value (AV). The Fatty acid methyl esters were prepared directly from the tissue, whereas contents of EPA and DHA were determined with the GC/MS method. Depending on the temperature applied, 15‐min heating of fish meat tissue caused 30–80% decrease of PN and 20–40% decrease of AV, on average. Generally, the continued heat treatment caused successive decreases in both PV and AV and the rate of this process was observed to increase along with an increasing temperature. The heating of trout and sprats at temperature of 60, 100 and 160 °C even for 1–2 h did not evoke losses either in EPA or in DHA content. In turn, in the case of herring caught during the pre‐spawning season, ca. 90–120 min of heat treatment contributed to ca. 20–25% decrease in contents of these fatty acids.     

Modeling heat transfer for disinfestation and control of insects (larvae and eggs) in date fruits

A space-and-time dependent model describing the impact of heat transfer on insect survival rate was developed for analyzing the heat disinfestation process of dates. A 2D axial-symmetric domain was defined with five sub-domains: the bulk date pulp, a thin air layer and the date pit, a larva (located in the air layer) and eggs (on the date surface). The model included convective heating and microwave heating (915 MHz). Dielectric properties of the date pulp, egg and larvae were measured. The model predictions showed good agreement with experimental results in the case of the “biological model” (Ephestia kuehniella Zeller) in dates subjected to hot air treatment (70 °C). Sensitive analysis showed that the date size and water content greatly affected the heating time process, during hot air and microwave treatments. Moreover, regarding the eggs’ specific surface, it is advisable to combine convective hot air with microwave heating to induce egg mortality on the date surface.     

Effects of different food commodities on larval development and α-amylase activity of Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae)

The present work was undertaken to study the influence of four commodities (wheat flour, dates, sorghum and barley) on Plodia interpunctella post-embryonic development. Larval weight, larval mortality, pupation and adult emergence were recorded. The study also aimed to find out the effect of these commodities on protein and glycogen production as well as on α-amylase activity. Results indicated that the weight of fourth instar larvae placed on dates increased gradually. Percentage mortality was low. Pupation and adult emergence were delayed. In contrast, the weight of larvae placed on wheat flour, sorghum or barley remained low. Pupation and adult emergence occurred sooner than among those placed on dates and the percentage mortality was highest for larvae placed on barley. Results also showed that protein content and α-amylase activity were lower for larvae placed on dates than for those placed on other commodities. The biochemical composition of different commodities showed that dates are a rich source of glucose, while their protein and starch contents were very low as compared to the other commodities. In contrast, wheat flour, sorghum and barley contained large amounts of starch and protein and low amounts of glucose. Thus, the reduction in α-amylase activity was probably due to the high levels of glucose in dates.