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Microwaves heating

General Information


The development of the microwaves technology for the food industry can capitalize on the current development of microwave technologies in telecommunications, radio frequencies and electricity (Cambridge University Press 2015; Mukherjee 2015). The boom of microwave ovens (house level) and fast food preference enabled the market of cheap magnetrons (main component of the ovens). (Scaman, Durance, Drummond & Sun 2014)

Heat and mass transfer data of the food products has important potential for the expansion of this technology (Ozkoc, Sumnu & Sahin 2014)


  • Rapid processing. The operation temperature is reached faster than in conventional processes. Combination with conventional heating can enhance the heating homogeneity (Muredzi, 2012).
  • Potential in software use for tailored microwaves profile applications.
  • More controllable processes.

(Scaman, Durance, Drummond & Sun 2014)


  • The effective heating process vary with the dialectic uniformity of material. This means that some parts of the food may reach a too high temperature when others just reach the needed one (Muredzi, 2012).
  • High sensitivity to process parameters (potential deviations). Extensive experimentation to correct deviations is also needed.
  • Metallic materials are not suitable to be processed.
  • Complete reprocessing is needed to handle under processed material.

(Muredzi, 2012)


  • Use of electromagnetic waves to produce heat in a material (2450 MHz and 915 MHz typically for industry) (Muredzi, 2012). Microwaves are electromagnetic radiation from 1mm to 1m wavelength and frequencies from 300 MHz to 300 GHz. (Scaman, Durance, Drummond & Sun 2014).
  • Dipolar rotation and ionic conduction mechanisms allows the transformation of electromagnetic energy to heat. (Ozkoc, Sumnu & Sahin 2014).
  • Dielectric and ionic properties of food are crucial parameters to identify the potential of microwave heating for a specific product. Water, as a polar molecule, is affected by the microwaves generating an oscillation movement that it is the main source of heat in food matter due to the abundance and the polarity compared with organic compounds. (Muredzi, 2012)
  • A dielectric constant shows the level of a material to store microwave energy and a dielectric loss factor measures the ability of the material to dissipate the microwave as heat. (Ozkoc, Sumnu & Sahin 2014). Dielectric constants of food decreases as the content of moisture decrease and with temperature itself unless the material is below the freezing point of water. (Scaman, Durance, Drummond & Sun 2014)
  • Also the volumetric heating or the capacity of the food to absorb the heat is highly relevant as a base of the technology. (Ozkoc, Sumnu & Sahin 2014)
  • Heat and mass transfer phenomena: the change in temperature is governed by the convective heat gain or loss, the radiative heat transfer and the evaporative heat loss. The surface of the material is colder than inside due to surface phenomena as evaporation, contrary to conventional heating in which the higher temperature is in the contact with the external hot surface source of the heat (Ozkoc, Sumnu & Sahin 2014).
  • The role of water transfer inside and outside the material are critical process regarding the quality of the food (Muredzi, 2012). The relaxation time is the time taken to a molecule to reach a disordered state after the microwave field that it was alienated with is removed. This occurs in the molecules of water bound in a Hydrogen bounded network. Water bound to proteins or carbohydrates or in ice form is hindered form this relaxation (Scaman, Durance, Drummond & Sun 2014).
  • The capacity of reflection of microwaves in some materials is also important. (Scaman, Durance, Drummond & Sun 2014)

Description of techniques

There are several processing factors to be taken into account for the application of the technique. One of the most important is the location of the coldest point in the material depending on composition (ionic content, moisture, density, and specific heat). The shape and size of the food are also relevant. The frequency of the microwaves and the applicator oven design are important factors too. Processing time is also a factor, as the temperature rises, the location of the coldest point may shift (magnitude time temperature) (Muredzi, 2012).

There are some solutions to avoid not uniform heating issues of the technology: combined microwave heating, controlling food size and geometry, the use of pulsed microwave heating, process and food specific microwave oven design, the use of lower microwave power in solely and combined applications. Variable frequency microwave processing oven are possible and can enable phase control microwave processing.


Figure Cross-section of a Magnetron (Ozkoc, Sumnu & Sahin 2014, p.430)

Changes in process (Operation Unit Applications)

Cooking and boiling

  • The technology enables short heating times. The air around the processing food is not heated, the condensation of vaporized water is easier as the air is easier to saturate enabling liquid water into the food avoiding crisping reactions.
  • Combination with infrared radiation and other thermal techniques represent a great synergy. It reduce the processing time while retaining the level of quality compared with conventional processes. Promising research regarding Cake and bread cooking has been made. Products should be baked at the lowest possible microwave power and the highest halogen lamp power (limited by change in surface color and quality level affects) in order to have conventional levels of quality or better in the shortest period of time.
  • Depending of the type of food, there are important quality problems that can appear regarding firm and tough texture, rapid staling, lack of color and crust formation and a dry product.
  • Browning is an issue for microwave cooking but with jet Impregnation (high speed convention heating) combined with the technology provide brown color and crisp in microwave baked products.

(Ozkoc, Sumnu & Sahin 2014)


  • The technology enables an improved uniformity of heating for in-package sterilization. A microwave power profile optimized for the package is possible.
  • One of the most promising techniques is rotating and oscillating product surrounded by an absorption medium.
  • There is a promising 915 MHz single mode sterilization for processing packaged food. The food is immersed in pressurised hot water simultaneously heated by microwaves. 5-8 min processing for safe and high quality food. Especially good for not homogeneous food. (Ozkoc, Sumnu & Sahin 2014).
  • Major issues: enhanced edge heating may happen. The complex, expensive, non-uniformity of heating. Unfavorable economics when compared with frozen food processing in the USA.

(Muredzi, 2012)


  • Pasteurization processes with microwave technology has a special relevance for solid and semi-solid materials in terms of pasteurization and sterilization avoiding high levels of degradation. (Muredzi, 2012)
  • The technology has been on and off for over 30 years, mainly in the industries of yoghurt and milk. (Muredzi, 2012)
  • It is effective in the destruction of microorganisms or inactivation of enzymes through its thermal effect (Ozkoc, Sumnu & Sahin 2014)
  • Effects on microorganisms: Temperature inactivation (similar to conventional thermal processing), electroporation, cell membrane rupture and cell lysis due to electromagnetic field coupling. (Muredzi, 2012)
  • The not thermal effects of microwaves have been reported to add to the inactivation opening possibilities for cold processing. Longer recovery of injured microorganism due to the damage of RNA caused by the microwaves. Reported microorganism kills at sub lethal temperatures due to potential magnification of thermal effects. (Muredzi, 2012)
  • Synergic effects with conventional heating is expected to be more than the sum of the separated effects due to emerging non thermal phenomena. (Muredzi, 2012)
  • There are resistant organism to the technology as Bacillus Cereous, Listeria Monocytogenes, among others. (Muredzi, 2012)
  • Continuous flow microwave pasteurization is used for apple cider and packaged acidified vegetables. (Ozkoc, Sumnu & Sahin 2014)


Listeria monocytogenes inactivation kinetics under microwave and conventional thermal processing in a kiwifruit puree

Innovative Food Science & Emerging Technologies, Volume 22, April 2014, Pages 131-136

María Benlloch-Tinoco, María Consuelo Pina-Pérez, Nuria Martínez-Navarrete, Dolores Rodrigo


  • The technology enables faster processing and better quality avoiding addition of water and enabling higher nutritional value.
  • Potential off flavor in Peanuts processing, the problem of high internal temperature degradation and the enabling of undesired chemical reactions are important issues.

(Ozkoc, Sumnu & Sahin 2014)


Comparison study of conventional hot-water and microwave blanching on quality of green beans

Innovative Food Science & Emerging Technologies, Volume 20, October 2013, Pages 191-197

Luis M. Ruiz-Ojeda, Francisco J. Peñas


  • The technology enables rapid heating of the solvent and sample, reducing the solvent use and the processing time. A higher extraction rate becomes possible. This is based on a volume expansion at micro scale leading to explosions at cellular level, making easier the diffusion processes or mass transfer.
  • A wider range of solvent can be used (less reliance on chemical affinity)
  • Extraction of targeted compounds becomes a possibility. Solvent free micro wave extraction has been developed by the combination with dry distillation at atmospheric pressure. It was used in essential oils extraction from aromatic herbs. A more simply, economically, quickly way than conventional method. Promising synergies with ultrasonic technology.

(Ozkoc, Sumnu & Sahin 2014)

Recent Case:

Contribution of microwaves or ultrasonics on carvone and limonene recovery from dill fruits (Anethum graveolens L.)

Innovative Food Science & Emerging Technologies, Volume 17, January 2013, Pages 114-119

Smain Chemat, Erik D.C. Esveld


  • The technology enables reduced drying time and minor product degradation. It is suitable for high moisture content products.
  • Drying rate can be controllable by ration of microwave power and the water content in food. The drying process has 3 main regions: Increasing rate region (reaching boiling point), constant rate region (water driven free off) and falling rate region (remove water)
  • Better retention of flavor, colors, nutrients or other biologically active chemicals than conventionally is possible.
  • Dehydration cost a function of costs, labor, energy cost and efficiency. Relevant enhanced benefits combined with thermal method. (Scaman, Durance, Drummond & Sun 2014)
  • Capacity to create new products or products with unique characteristics.
  • Main issues: high initial capital investment is needed, textural damage due to rapid mass transport is possible. Changes in dialect properties with temperature hinder the extensive commercial application. (Ozkoc, Sumnu & Sahin 2014)
  • Rate of power form 800 w to 425 W. Limitation by the microwave power density (W/kg) (too much leads to arching or plasma discharge, >10,000 W/Kg) and the microwave penetration level.
  • Vacuum operation: Converting electromagnetic energy into heat inside the material using vacuum to keep a low temperature leading to an improved product quality. Higher re hydration rates are also possible.
  • Freeze drying, time reduction by 75%. Main factors: electric field strength, chamber pressure (++) and sample size. (Scaman, Durance, Drummond & Sun 2014)
  • Main issues: high initial capital investment is needed, textural damage due to rapid mass transport is possible. Changes in dialect properties with temperature hinder the extensive commercial application. (Ozkoc, Sumnu & Sahin 2014)
  • The technology combined with fluidized bed solve the problem of heating homogeneity with even better color quality plus up to 90% drying time reduction. Spouted bed combination shows good performance at 3.5 W/g and air temperature of 50°C. (Ozkoc, Sumnu & Sahin 2014)
  • Microwave heated air in agricultural crops is a promising application. Vacuum impregnation and osmotic dehydration prior to hot air microwave in fruit preservation is becoming widespread for reducing energy consumption, improving the quality and extending the shelf life.

(Ozkoc, Sumnu & Sahin 2014)

Recent cases:

Microwave-drying of sliced mushroom. Analysis of temperature control and pressure

Innovative Food Science & Emerging Technologies, Volume 11, Issue 4, October 2010, Pages 652-660

J.I. Lombraña, R. Rodríguez, U. Ruiz

Modelling of dehydration-rehydration of orange slices in combined microwave/air drying

Innovative Food Science & Emerging Technologies, Volume 4, Issue 2, June 2003, Pages 203-209

G.Ruı́z Dı́az, J. Martı́nez-Monzó, P. Fito, A. Chiralt


  • The technology enables a minimization of microbial growth and accelerated the process.
  • Some issues: Chemical deterioration, excessive drip loss and dehydration by reducing the process time. There are also issues of uneven or runaway heating (some parts cooked, some still frozen).
  • There are successful cases for Sauces.
  • Mathematical models improvements are promising.

(Ozkoc, Sumnu & Sahin 2014)

Recent Case:

Factors affecting the thawing characteristics and energy consumption of frozen pork tenderloin meat using high-voltage electrostatic

Innovative Food Science & Emerging Technologies, Volume 22, April 2014, Pages 110-115

Xiangli He, Rui Liu, Eizo Tatsumi, Satoru Nirasawa, Haijie Liu

Cooling, chilling and cold stabilization

  • The mechanical and biochemical stress caused by the ice leads to irreversible tissue damage. The application of electric and magnetic effects has been identified as a possible means to reduce the size of ice crystals during freezing of biological tissues.
  • The technology enables a reduction on the size of the crystals (60%) leading to a better conservation quality of the food.
  • Results show that the size of the formed ice crystals can be significantly reduced during microwave assisted freezing leading to a lower damage on the microstructure of meat.

(Xanthakis, Le-Bail, Ramaswamy, 2014)

Energy Savings

  • Lower energy use due to the minimizing of processing time (Muredzi, 2012)
  • Vacuum impregnation and osmotic dehydration prior to hot air microwave in fruit preservation become widespread for reducing energy consumption, improving the quality and extending the shelf life. (Ozkoc, Sumnu & Sahin 2014)

Change in Energy Distribution

  • Change thermal energy for electricity.
  • More electric power generations, enhanced variety of options for electric power sources of energy.


  • Muredzi, P. (2012) 'Chapter 8: Microwaves and radiofrequency processing', in Aleman, M. (ed.) Emerging Non-thermal Food Processing Technologies. USA: CBH books, pp. 167-194.
  • Ozkoc S., Sumnu G., Sahin S. (2014) 'Part IV: Alternative thermal processing: Chapter 20 Recent Development in Microwave Heating', in Sun, D. (ed.)Emerging Tehcnologies for Food Processing. UK: Academic Press, pp. 361-377.
  • Scaman C., Durance, T., Drummond, L., Sun D. (2014) 'Part IV: Alternative thermal processing: Chapter 23 Combined Microwave Vacumm Drying, in Sun, D. (ed.)Emerging Tehcnologies for Food Processing. UK: Academic Press, pp. 427-441.
  • Cambridge University Press (2015) International Journal of Microwave and Wireless Technologies, Available at: http://journals.cambridge.org/action/displayJournal?jid=MRF (Accessed: 14th March 2015).
  • Mukherjee, S. (2015) Microwaves—a journey in reshaping world technology, Available at: http://www.ieeeghn.org/wiki/images/6/6b/Mukherjee.pdf (Accessed: 13th March 2015).
  • Xanthakis, A., Le-Bail, A., Ramaswamy, H. (2014) 'Development of an innovative microwave assisted food freezing process ', Innovative Food Science & Emerging Technologies, 26(December), pp. 176-181.