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Low Moisure Food Safety Low Moisure Food Safety

Low Moisture Food Safety FAQ

 

  • What is Radio Frequency (RF)?
    • Radio frequency waves are electromagnetic (EM) waves, similar to microwaves. They travel at the speed of light in free space (3.0×108 m/s). RF waves typically oscillate in the frequency range of 3 kHz-300 MHz, while microwaves cover the frequency range from 300 MHz to 300 GHz.
      Electromagnetic Spectrum
      Courtesy of NASA
      For more detailed information regarding the electromagnetic spectrum please visit this very informative NASA Mission: Science web site.

 

  • How does RF heat food?
    • Electrode DiagramFor RF heating, food products are placed between two electrodes which generate an alternating electrical field. Two major mechanisms are responsible for the interaction between food and the electrical field. One of them is the dipole rotation of polar molecules, and the other is ionic conduction caused by charged ion movement.
      Polar molecules in food such as water have an electrical dipole moment, and the positive charge center and negative charge center in these polar molecules do not coincide. When subjected to an electrical field, polar molecules will rotate to align themselves to the field. As a result, the negative and positive charges move to different ends and lead to polarization. This phenomenon is known as dipole rotation (Marra et al., 2009). If placed in an alternating electrical field, the polar molecules will rotate in an opposite direction and back again to follow the changing fields. During this process, the friction among surrounding molecules converts the electromagnetic energy to heat and gives rise to an increasing temperature of the treated materials.
      In addition, dissociated ions in foods also move accordingly with the applied alternating electrical field. The direction of motion of the molecules varies with time due to the changing electrical fields. Heat will also be generated within the material by the friction among molecules caused by the forward and backward oscillation of the ions  in the material (Buffler, 1993). This mechanism is called ionic conduction. Ionic conduction is the dominant mechanism for RF heating (Ryynänen, 1995).

 

  • What are the available frequencies for food industry applications?
    • Both RF and microwave waves lie in the radar range of the EM spectrum and can interfere with communication systems, such as commonly used in cell phones. As a result, only a few selected frequencies are allowed to be used for industrial, scientific and medical (ISM) applications. For RF waves, the frequencies of 13.56, 27.12 and 40.68 MHz are permitted to be used in the United States for ISM applications.Table 1. Frequencies assigned by the FCC for industrial, scientific and medical (ISM) use.

      ISM Frequency

      RF

      13.56 MHz ± 6.68 kHz
      27.12 MHz ± 160.00 kHz
      40.68 MHz ± 20.00 kHz
      Microwave915 MHz ± 13 MHz
      2450 MHz ± 50 MHz
      5800 MHz ± 75 MHz
      24125 MHz ± 125 MHz

 

  • What are the advantages of RF heating for food compared with conventional heating methods?
    • The primary advantage of RF heating for food products is rapid and volumetric heating. The fast heating can reduce the come-up time for thermal processing when compared to conventional heating. This is particularly true for solid and semi-solid foods because of their low heat conduction rate which would result in a slow heat transfer rate during conventional heating. Therefore, RF heating holds the benefits of high-temperature-short-time (HTST) processing by killing the target bacterial, while thermal degradation of food quality is reduced.
      With RF’s unique volumetric heating, a relatively more uniform heating profile can be achieved. Compared to conventional heating with conduction as the transfer mechanism the heating rates are low and the process times are relatively long. That leads to surface overheating and non-uniformity in the temperature distribution.
      Another advantage of RF heating is that it can be turned on or off instantly, and the RF system is more energy efficient compared to conventional heating.

 

  • What are the similarities and differences between RF heating and microwave heating?
    • Similarities between RF and microwave heating: Governed by Maxwell’s Equations and producing only thermal effects. Both RF and microwave waves are EM waves and transfer to treated products by radiation. The quantity of heat absorbed by the products depends on the dielectric properties of the material while the temperature changes inside the material depend on the thermal properties of the material. RF and microwave heating take place due to the polarization effect of the EM field radiation, and the governing equations for RF and microwave heating are same.
    • Rapid and volumetric heating: Both RF and microwave heating are rapid and exhibit volumetric heating. Dielectric properties play a very important role in both RF and microwave heating. RF and microwave are known as dielectric heating, and are usually applied to dielectric materials. Most food materials belong to this category and can be treated by RF or microwave heating.
    • High energy efficiency: RF and microwave systems are capable of instantaneously applying or removing the heat source, and the system heating efficiency is much higher than conventional hot air or hot water heating. RF and microwave systems have been recognized to have 50–70% heating efficient in comparison to 10% efficiency with conventional ovens (Memelstein, 1997).
    • Heating uniformity: The main challenge in RF and microwave heating is reducing the temperature difference between the hot and cold spots in the food allowing for a more uniform heating. The cold and hot spots during dielectric heating exist due to the non-uniformity of the electrical field distribution and product properties. Generally, the dielectric loss factor would keep increasing with the temperature of the material; the already hot area will receive more energy than the cold area. This phenomenon is known as thermal runway which would give rise to significant non-uniform heating.
    • Differences between RF and microwave heating
      • Different frequency/wavelength and penetration depth: The frequency ranges for RF and microwave are different. RF uses much lower frequencies than microwaves. Therefore, RF waves have longer penetration depth than microwaves. Penetration depth limits the size and shape of the treated food. Thus, RF heating can be used to treat bulk material with relatively larger dimensions than microwave heating. Another difference is the limitation in the shape of the product. Microwave treatment has no limitation in shape while products need to be of a regular, simple shape when using RF heating.
      • Different system design/cost: The source and system design for RF and microwave heating are different. In RF heating, the electrical field is generated in a directional manner between a pair of electrode plates, but the electrical field could approach the product from all directions in microwave heating depending on different mode design; the microwaves are transmitted by waveguides which the RF system does not have. The system configuration is also totally different for the RF and microwave systems. RF systems are generally simpler to construct than microwave systems, and the cost is less than that of microwave systems.Comparison of system configuration of RF oscillator circuits and 50Ω technology
      • Different heating uniformity: With concern to heating pattern, RF heating can provide relatively uniform field distribution as compared to the complex non-uniform standing wave patterns in a microwave heating system. As a result, a better heating pattern for RF processing could be obtained compared to microwave processing. RF heating system efficiency is a little higher than microwave system (~70% vs. ~50%).
  • Is food processed by RF heating safe to consume?
    • RF waves, similar to microwave, are non-ionizing radiation since the frequencies in the RF range cannot produce sufficient energy to ionize molecules in foods such as water, unlike higher levels of energy such as X rays and Gamma rays which can change molecular structures. RF is therefore regarded as a safe treatment for edible food products. RF and microwave treatments also meet organic labeling standards which are more acceptable to consumers (Mitcham et al., 2004).

 

  • What applications of RF heating are currently in use in the food processing area?
    • RF Dryer
      150 KW Combination RF Convection Dryer (From PSC company, Cleveland, Ohio).

       

      Because of the unique heating character of RF heating compared with conventional heating methods, such as rapid and volumetric heating, RF heating has been successfully applied in, several industry sectors, such as the textile and wood industry. In the food processing area, RF heating has already been successfully applied for drying of food materials, post-baking of biscuits & crackers and other food products, and thawing/tempering of frozen meat. Recently, new applications of RF heating are emerging, such as pasteurization/sterilization of foods and disinfestations of agricultural products.

      RF Tempering System
      50 KW RF Tempering System for Seafood (From PSC company, Cleveland, Ohio)

       

       

      RF Microwave
      Radio Frequency Company’s New Macrowave™ Pasteurization Systems-40 MHz.

       

       

      RF Disinfestation System
      “RF/G 85 kW” equipment for the disinfestation of 5 ton/h of organic rice (From Stalam company, Italy)

       

 

  • What are the main challenges for applying RF energy in the food industry?
    • The main challenge for RF heating is non-uniform heating due to non-uniform electric field distribution and non-homogenous food products. Therefore, improvement of the RF heating uniformity to ensure a better food quality is always the first concern when developing RF treatment protocols. Another challenge is to reduce the energy cost and initial equipment investment of RF systems, which would make RF technology more acceptable by industry applications.

 

  • What is the difference between traditional oscillator RF systems and 50 ohm RF systems?
    • Oscillator Based RF System Chart
      Oscillator based RF systems vs. 50Ω RF systems

       

      Comparison of system configuration of RF oscillator circuits and 50Ω technology
      Comparison of system configuration of RF oscillator circuits and 50Ω technology (Source: Ben Wilson, PSC, inc. presented at the 45th IMPI meeting).

 

References

  • Buffler C.R. (1993). Microwave cooking and processing: engineering fundamentals for the food scientist. Van Nostrand Reinhold, New York.
  • FDA Document. Kinetics of Microbial Inactivation for Alternative Food Processing Technologies: Microwave and Radio Frequency Processing.
  • Marra F., Zhang L. and Lyng J.G. (2009). Review: Radio frequency treatment of foods: Review of recent advances. Journal of Food Engineering, 91, 497–508.
  • Mermelstein N.H. (1997). Interest in radiofrequency heating heats up. Food Technology, 51(10), 94–795.
  • Mitcham E.J., Veltman R.H., Feng X., de Castro E., Johnson J.A., Simpson T.L., Biasi W.V., Wang S., and Tang J. (2004). Application of radio frequency treatments to control insects in in-shell walnuts. Postharvest Biology and Technology, 33, 93–100.
  • Ryynänen S. (1995). The electromagnetic properties of food materials: a review of basic principles. Journal of Food Engineering, 26, 409–429.
  • http://www.stalam.it/en/disinfestation-and-sanitisation-equipment.html
  • http://www.radiofrequency.com/products/index.html
  • http://www.pscrfheat.com/