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Microwave heating

Identification

Key words Microwave, weight, geometry, dielectric, electromagnetism, thermodynamics, temperature, surface temperature, speed, heating, heat transfer, calorimetry, cooling, thawing, drying
Latest version 2010/12/20
Completed by IRTA

How does it work?

Primary objective To heat food products.
Working principle Microwave energy is a form of electromagnetic radiation between 300 MHz and 300 GHz delivered in an enclosed cavity. When developing products for microwave processing it is important to recognize that microwave is an electromagnetic energy and not a form of heat. The heat is manifested by the interaction on the electromagnetic radiation and the material present in the cavity. In all materials, such energy interacts using two main mechanism dipole rotation and ionic polarization. Dipole rotation occurs on polar molecules such as water. Polar molecules are charged and asymmetric molecules which tend to align themselves with the incoming electric field. The oscillation of such molecules generates heat. For molecules, like fatty acids, with a polar area the oscillation is slower but generates more heat due to their volume and mass. Ionic polarization occurs in the free ions present in the material. These ions are charged particles and they are accelerated by the electric field. The kinetic energy is released in form of heat when collision occurs. Microwave energy is created by a magnetron and transport to the cavity by a waveguide. Because geometry is important different designs for a microwave system exist.
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Additional effects In microwave heating, heat is generated inside the food, but as for all wavelength, the distance of penetration of microwave is limited. Distance of penetration depends on the dielectric properties of the food products and of the frequency used. Typically a 2450 MHZ frequency microwaves have a distance of penetration of 1 to 4 cm, which implies that product with a thickness above 7 cm are unlikely to be processed with this frequency.
Important process parameters frequency, power, geometry of the products, composition of the products, dielectric constants, packaging material and geometry.

This leads to the fact that each application is unique and requires some study.

Important product parameters food geometry, package geometry and composition, food composition, food weight.

What can it be used for?

Products Raw food, food products, ready meals, as such or already in a package.
Operations Microwave is an alternative heating technology and all materials susceptible to be heated or defrosted can use this technology. The main applications in the food sector are cooking, pasteurization, drying, decontamination and defrosting (particularly for the 915 MHz frequency).
Solutions for short comings In different industrial application such as blanching, pasteurization or thawing, conventional methods are time consuming, work in mostly in a non-continuous mode and sometimes use a lot of water and energy. Microwave technology can provide solution to enhance this processing because is a adaptable technology that can be used in combination with other heating source, in a continuous or non-continuous mode, for liquids and solids food products.

What can it NOT be used for?

Products With highly conductive material (i.e metals) or insulating material (mostly plastic packaging or carton) with a low loss factor, the microwave technology does not work. In most products a certain water content is required for the cooking/pasteurization procedure. In dry products (moisture <40%) such as green coffee beans, microwave can still be used to extract more water.
Operations -
Other limitations
  • Uniformity of treatment may be limiting, dependent on product geometry
  • Thawing of food is challenging due to different absorption rates of water and ice
Risks or hazards The microwave delivers a powerful electromagnetic field and microwave leakage needs to comply with the regulation.

Implementation

Maturity Since the first patent in the sixties, microwave is a mature technology for domestic cooking and reheating, but also for industrial applications since the eighties.
Modularity /Implementation Technology can be inserted in an existing production line replacing the whole or a portion of the production line. Microwave generators are small and can be inserted in an existing line if they comply with the microwave leakage regulation.
Consumer aspects The technique is well accepted by customers, as the majority of the households is equipped with a microwave.
Legal aspects Equipment using microwave source have to comply with the ISM bands and with regulation EN 55011:2009 (Industrial, scientific and medical equipment. Radio-frequency disturbance characteristics. Limits and methods of measurement) and CE marking.
Environmental aspects In comparison with conventional technology, microwaves have an efficient electrical energy conversion, to generate heat in the product, thus resulting in energy saving.

Further Information

Institutes IRTA, DIL, SP, Nofima, Wageningen UR - FBR
Companies Petrie Technology, Sairem, Stalam, Eodiss Systems, Remak, MEAC
References 1. Aymerich T.,Picouet P., Monfort J.M.(2008). Decontamination technologies for meat products. Meat Science, Volume 78, Issues 1-2, Pages 114-129

2. Barringer S.A., Davis E.A., Gordon J., Ganapathy Ayappa K., Davis H.T. (1995). Microwave-Heating Temperature Profiles for Thin Slabs Compared to Maxwell and Lambert Law Predictions. Journal of Food Science. Vol. 60 (5); 1137-1142

3. Buffler (1993), “Dielectric Properties of Foods and Microwave Material” In Microwave Cooking and Processing (pp46-69) New York USA, Van Nostrand Reinhold.

4. Knoerzer K., Regier M. & Schubert H. (2008). A computational model for calculating temperature distributions in microwave food applications. Innovative Food Science & Emerging Technologies, Volume 9, Issue 3, July 2008, Pages 374-384

5. Lorence M.W. & Pesheck P.S. (2009) Development of packaging and products for use in microwave oven ISBN 978-1-84569-420-3.

6. Maranesi M., Bochicchio, D.,Montellato L., Zaghini A., Pagliuca G. & Badiani A. (2005).Effect of microwave cooking or broiling on selected nutrient contents, fatty acid patterns and true retention values in separable lean from lamb rib-loins, with emphasis on conjugated linoleic acid. Food Chemistry Vol. 69; 359–368

7. Picouet P., Fernández A., Serra X., Suñol J.J. & Arnau J. (2007). Microwave Heating of Cooked Pork Patties as a Function of Fat Content. Journal of Food Science,2007 Mar;72(2):E57-63

8. Ryynämen S. & Ohlsson T. (1996) Heating Uniformity of ready meals as affected by placement, composition and geometry. Journal of Food Science, Vol. 90: 205-218

9. Sosa-Morales M.E., Valerio-Junco L., López-Malo, A. & García H.S.(2010). Review: Dielectric properties of foods: Reported data in the 21st Century and their potential applications. LWT - Food Science and Technology 43 1169-1179

10. Schubert H. & Regier M. (2005) The Microwave processing of foods. ISBN 978-185573-964-2.
11. Swain, M.J, James S.L. & Swain M.V.L. (2008). Effect of power output reduction of domestic microwave ovens after continuous (intermittent) use on food temperature after reheating. Journal of Food Engineering,Vol 63 (3), 620- 624.

12. Zhang L., Brunton M., McKenna (2004). Dielectric and thermophysical properties of meat batters over a temperature range of 5–85 ºC Meat Science Vol 87 (1); 11-15

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Created by Hte irta on 9 February 2012, at 10:00