Radio Frequency Heating Rate Research Articles

Salt content dependent dielectric properties of pistachios relevant to radio-frequency pasteurization

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

Effects of temperature, moisture, and metal salt content on dielectric properties of rice bran associated with radio frequency heating

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

Dielectric properties, heating rate, and heating uniformity of various seasoning spices and their mixtures with radio frequency heating

Low moisture foods, including seasoning spices, have been associated with a number of multi-state outbreaks of salmonellosis in the past decade. The long-term objective of this study was to develop an effective in-package pasteurization treatment for seasoning mixtures based on radio frequency (RF) heating. Seasoning spices obtained from grocery stores included red, white, and black pepper; cumin; curry powder; and garlic powder with moisture contents ranging from 3.1 to 12.3% (wet basis). The dielectric properties (DP) of the seasoning spices and their mixtures as influenced by frequency, moisture, temperature, mixing fraction and salt content were determined using a precision LCR meter and liquid test fixture at frequency ranging from 1 to 30 MHz. The RF heating rates of each spice and their mixtures were evaluated using a 27.12-MHz, 6-kW pilot scale RF system with 105 mm gap between electrodes. To evaluate the effect of mixing on heating uniformity, a sample (50 g) was placed into a polystyrene plastic cylindrical container and heated to 70 °C, and surface images were taken by an infrared camera. The results showed that the relationship among moisture content, temperature and DP of white pepper can be explained by a second-order model at 13.56 and 27.12 MHz. The DP and heating rates of spice mixtures ranged between the highest and lowest values of their respective individual spices. Increase in salt content resulted in a decrease in heating rate, which resulted a better heating uniformity with smaller uniformity index (UI). The RF heating rate of samples ranged from 2.97 to 23.61 (°C min−1). The highest heating rate in samples was in a correspondence to the worst heating uniformity, and highest average temperature on the sample surface. The most uniform heat distrubition on top surface was obtained for garlic powder as 0.012 (UI) at 70 °C. The information obtained from this study is important to develop an effective RF heating strategy for pathogen control in seasoning mixture.

Inactivation kinetics for Salmonella typhimurium in red pepper powders treated by radio frequency heating

Radio frequency (RF) dielectric heating has been investigated to inactivate pathogens in some low-moisture foods. This study was aimed to evaluate RF inactivation effects on Salmonella typhimurium in red pepper powders, by focusing on the influence of sample initial water activity (aw) and applying Weibull model to describe the inactivation curves. The experimental results showed that RF heating rate increased when aw was in the range of 0.57–0.71, but decreased after aw reached to 0.71. During the come-up time of RF heating, 2–3 log reduction of the pathogen was achieved depending on aw levels. Increasing initial aw could first increased log reductions and then decreased the log reductions, optimum aw level was 0.71 for RF inactivation of Salmonella in red pepper powders. For red pepper powders with aw of 0.71, RF heating to 70 °C (come-up time was 110 s) with holding time over 60 s could achieve >5 log reduction of S. typhimurium. Weibull model well fitted the survival curves of the pathogen with goodness of fit (R2 > 0.93, RMSE<0.29). Scale factor (b) of the model increased with treatment temperature increasing, while the shape factor (n) was independent on temperature. This study provided basic guideline for using RF heating to inactivate Salmonella in red pepper powders.

Radio frequency heating of corn flour: Heating rate and uniformity

Abstract 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 relevance This 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.

The influence of cell and nanoparticle properties on heating and cell death in a radiofrequency field.

The ability of nanoparticles to either direct heating or increase susceptibility of cancer cells to radiofrequency (RF) energy remains controversial, as is the impact of cell attributes on susceptibility of cells to RF-induced cell death. This manuscript examines the impact of nanoparticle charge, size, and cellular localization on RF-induced cell death and the influence of nanoparticles on the heating rates of water and biologically-relevant media. Susceptibility of immortalized or primary cells to RF energy and the impact of cell size are also examined. The ability to selectively modulate RF heating rates in specific biological locations or in specific cell populations would enhance the therapeutic potential of RF therapy.

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.

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

Dielectric properties of selected vegetables powders as influenced by moisture content (MC), compaction density and temperature were determined using a precision LCR meter and liquid test fixture at radio frequency (RF) ranging from 1 to 30 MHz. The RF heating rate of samples was evaluated using a 27.12-MHz, 6-kW RF oven.The results showed that dielectric properties, namely, the dielectric constant, ε′, and loss factor, ε′′, of vegetable powders were influenced by MC, compaction density and temperature of the samples and the RF frequency. Both the dielectric constant and loss factor increased with increasing MC and temperature, but decreased with increasing frequency. Additionally, dielectric properties of samples increased with compaction density to a peak, then decreased. The relationship between MC, temperature and dielectric properties of broccoli powder at 13.56 and 27.12 MHz can be described by quadratic models with high correlation coefficients (R2 > 0.96). The RF heating rate in samples increased linearly with dielectric loss factor and MC. The information provided in this study is useful to develop an effective RF heating strategy to pasteurize dried vegetable powders.

Finite Element Modeling and Analysis of Radio Frequency Heating Rate in Mung Beans

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

Effects of Dy3+ substitution on the structural and magnetic properties of Ni0.5Zn0.5Fe2O4 nanoparticles prepared by a sol-gel self-combustion method

A series of Ni-Zn ferrite nanoparticles with a nominal composition Ni0.5Zn0.5DyxFe2-xO4 (x=0.000–0.100 with steps of 0.025) was prepared by a simple sol-gel self-combustion method. The effects of composition and calcined temperatures on their morphology, specific area, magnetic properties and radio frequency heating rate are studied. It is found that all the Dy3+ substituted Ni-Zn ferrites exhibit typical spinel structure. Moreover, Dy3+ substitution can notably reduce the average crystal size and enhance the specific area of Ni-Zn ferrite. The value of specific area reaches 29.25m2/g when x is 0.1, about five times higher than that of Ni0.5Zn0.5Fe2O4 sample. With the increase of Dy3+ content, all the values of Ms, Mr and Hc increase first and then decrease, and they maximize to 67.801emu/g (Ms), 11.10 (Mr) and 79.89G (Hc), improving about 28%, 208% and 92%, respectively, compared with those of Ni0.5Zn0.5Fe2O4, when calcined at 1073K and x is 0.025. In addition, the heating rate of Dy-substituted samples are also enhanced markedly.

Preparation and Characterization of Wire-Liked NiFe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4 &lt;/sub&gt;Nanoparticles with High Specific Area and Excellent Magnetic Properties via a Template-Assembled Sol-Gel Method

A simple template assembled sol-gel method is used to prepare the wire-liked NiFe2O4 nano-particles with high specific surface area and excellent magnetic. Influence of template and calcined temperatures on the composition, microstructure, BET, magnetic properties as well as radio frequency(RF) heating rate are studied through XRD, TEM, BET, VSM and RF heat test. Results show that the template-assembled sol-gel method is a successful and easy method to obtain randomly oriented wire-liked NiFe2O4 nano-particles. The spinel NiFe2O4 was the main phase, impurity rhombohedral Fe2O3 phase is also detected in all samples. An increase in grain size is observed with the increased calcined temperatures as well as the concentration of sol, while the specific surface area decreases from 50.3 to 13.2 m2/g. As the sintering temperature equals to 700°C, the nano-particles can be observed with a diameter approximately 50 nm and a length approximately 700 nm. Ms increases with the increase of sintering temperature and slightly waves with the increase of sol concentrations, C-1-1000 exhibits higher Ms (73.0 emu/g) and lower Mr andHcvalues (13.5 emu/g, 79.3Oe ), respectively. The calculated hysteresis loss and the measured RF heating rate exhibits good agreement. The sample C-1-800 has the highest heating rate of 1.72 °C/s.

Effect of radio-frequency on heating characteristics of beef homogenate blends

The heating characteristics of different lean-to-fat ratios in ground beef (90/10, 85/15, 80/20, 73/27) was investigated using radio-frequency (RF) heating of ground beef homogenates. A homogenate model system was prepared with ground beef in phosphate buffer (pH 6.0). Homogenate blends were packaged in vacuum bags, immersed in pre-heated water at 10, 25 and 50 °C, and heated in the RF equipment to a target temperature of 60 °C. Beef homogenate blends with more lean and less fat (90/10 and 85/15) heated more rapidly, reaching the target temperature faster than the higher fat composition blends. The RF heating rates of the beef homogenate blends varied with water temperature; higher temperature resulted in rapid RF heating. The RF heating rate at 50 °C was the most rapid in heating all homogenate blends. The instrumental color characteristics for RF heated homogenate blends exhibited lower L* and a* values than uncooked homogenate blends. RF heating resulted in reduced redness and more oxidation of myoglobin (metmyoglobin formation) and lesser oxy- and deoxy-myoglobin redox forms in all beef homogenate blends independent of the initial water temperature. The results present scope to explore short time heating processes which will minimize quality deterioration in RF heated meat products.

Factors Affecting the Temperature Increasing Rate in Wood during Radio-Frequency Heating

Radio-frequency (RF) heating has been widely used in the wood industriesand it is essential to understand the factors affecting the heating rates. In this article, Chinese fir (Cunning hamialanceolata) was heated with RF energy intermittently at 2 min per cycle to study the effect of wood moisture content (MC), sample thickness, and RF power level on heating rates. The study revealed that wood MC significantly affected the RF heating rate (p < 0.01). The heating rate was the highest (5.0°C/cycle) at an MC around the wood fiber saturation point (FSP). The heating rates were lower with a lower MC of 15% (0.89°C/cycle) and a higher MC of 70% (1.2°C/cycle). Lumber thickness did not have a significant effect on the RF heating rate at the center of the lumber; however, the heating rates at various locations along the thickness direction were significantly different. The temperature at the lumber center was 20°C higher than the temperature at 10 mm below the surface during heating. The heating rate for lower RF power level was initially greater than the rate at higher power RF level and then rapidly decreased. However, the heating rate at the higher RF power level remained constant during the whole heating process. RF power levels had a significant effect on the internal heating rate of wood (p < 0.01).

Non-Newtonian Flow and Heat Transfer in an Obstructed Channel With Radio-Frequency Heating

The hydrodynamic and thermal conditions associated with the flow of a non-Newtonian fluid (polystyrene), which is heated volumetrically by electric radio-frequency excitation, is considered. The fluid is forced through a two-dimensional channel having local, stationary blocks, which restrict the flow and are maintained at a constant rms voltage potential. Detailed and overall system response is examined for a range of fluid velocities, restriction sizes, and radio-frequency heating rates. This heating reduces the pressure drop induced by the restriction and alters the flow structure, suggesting that it may be used to control and augment conventional injection molding operations.