40 MHz Radio Frequency Pasteurization
Radio Frequency Company Macrowave™ Heating Systems are ideal for pasteurization applications. In a radio frequency heating system the RF generator creates an alternating electric field between two electrodes. The material to be heated is conveyed between the electrodes where the alternating energy causes polar molecules in the material to continuously reorient themselves to face opposite poles much like the way bar magnets behave in an alternating magnetic field. The friction resulting from molecular movement causes the material to rapidly heat throughout its entire mass.
The illustration below depicts a radio frequency heating system with a product between the electrodes. Polar molecules within the product are represented by the spheres with + and - signs connected by bars.
The amount of heat generated in the product is determined by the frequency, the square of the applied voltage, dimensions of the product and the dielectric loss factor of the material which is essentially a measure of the ease with which the material can be heated by radio frequency waves.
The increase in temperature of a material due to dielectric heating can be calculated from (Nelson, 1996):
ΔT = (5.563 x 10-11 ƒ E2 ε"Δt)(ρCp)-1
where: ΔT is the temperature rise in the material in ºC, f is the frequency in Hz, E is the electric field intensity in V/m, ε" is the dielectric loss factor, Δt is the time duration in s, ρ is the density of the material in kg/m3, and Cp is the specific heat of the material in Jkg-1ºC-1. From this equation we can see that the rise in temperature is proportional to the material's dielectric loss factor, in addition to electric field intensity, frequency and treatment time.
Past efforts to use microwave energy to pasteurize bulk products such as nuts at 915 or 2450MHz have experienced difficulty in achieving uniform temperature distribution throughout the material. Wide temperature dispersion can result in some product not reaching a high enough temperature to ensure effective microbe kill and/or some product being overheated resulting in deleterious effects on product quality.
From the above formula, if we assume the dielectric loss factor, frequency, and treatment time were constant throughout the product being pasteurized then we can assume that the variability in heating experienced throughout the bulk nuts at microwave frequencies was attributable to variations in electric field intensity.
A major factor affecting electric field intensity is penetration depth, defined as the depth where the power is reduced to 1/e (e=2.718) of the power entering the surface. The penetration depth dp, in meters, of RF and microwave energy in a high loss material can be calculated by (vonHippel, 1954):
where: c is the speed of light in free space (3x108 m/s), ε' is referred to as the dielectric constant and represents stored energy when the material is exposed to an electric field, ε" is the dielectric loss factor, which influences energy absorption and attenuation, and ƒ is the frequency in Hz.
The penetration depths of almonds and walnuts have been calculated (Wang 2003) based on measured dielectric properties at 538 and 654cm at 27MHz versus only 2-3cm at 915 and 2450MHz. This limited penetration depth in nuts would suggest that large scale pasteurization at the microwave frequencies of 915 and 2450MHz is an impractical solution.
At first glance, pasteurization at 27MHz would appear promising. However, at this low frequency the voltage required to generate the power levels necessary to achieve the desired heating can result in arcing that can damage the product and cause production interruptions. To overcome these technology barriers Radio Frequency Company has developed a product line of Macrowave™ Heating Systems that operate at 40MHz. At 40 MHz the penetration depths in almonds and walnuts are approximately 530 and 644cm respectively, only slightly less than at 27MHz, while the voltage requirement is reduced by approximately 20%. This solution dramatically reduces the potential for arcing while still maintaining the high penetration depths necessary for large scale bulk pasteurization applications.
Radio Frequency Company has bulk pasteurization and deinfestation experience with nut meat, flour, tobacco, and fishmeal and we have extensive capabilities to conduct feasibility testing on customer specific applications.
For evaluating new applications, RFC has the industry's most advanced hybrid RF/convection heating system in its testing laboratory. The Macrowave™ OmniTherm™ Simulator was developed by RFC to conduct product trials and define the necessary parameters to heat or dry materials under production conditions. Through the use of this advanced hybrid RF heating system, you can accurately determine the scale-up requirements necessary to meet your production goals.
Macrowave™ OmniTherm™ RF Process Heating Simulator
The Macrowave™ OmniTherm™ Simulator can apply up to 30kW of output power at an operating frequency of 40MHz. This system is a fully instrumented conveyorized heater-dryer that can apply RF and convection heat to diverse materials in a wide variety of modes.
This system permits RFC's engineers to design Macrowave™ production heating systems based on tests that simulate the passage of the material through a full-scale production system that might consist of a number of applicators in tandem, each powered by its own RF generator. Each generator/applicator section is referred to as a "zone." It has exceptional versatility and can be configured to simulate virtually any production processing system that can utilize RF heating or drying.
OmniTherm™ Simulator tests can provide very useful scale-up data to determine the configuration of the Macrowave™ RF system that will be technically and economically best suited to meet full-scale production and quality goals.
Nelson S O (1996). Review and assessment of radio-frequency and microwave energy for stored-grain insect control. Transactions of the ASAE, 39, 1475-1484
von Hippel A R (1954). Dielectric Properties and Waves. John Wiley, New York
Wang S; Tang J; Johnson J A; Mitcham E; Hansen J D; Hallman G; Drake S R; Wang Y
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