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Microwave ovens

            A microwave oven cooks food using electromagnetic waves quickly by penetrating the food and cooking it from the inside as well as from the outside without heating the oven and the container of the food. Microwaves does not pass through metal but passes through plates and dishes made of China, plastic or glass. The magnetron tube emits these high frequency microwaves to a waveguide inside the oven. It travels down a metal duct at the frequency of 2500 MHz and a wavelength of 12 cm and are scattered around by a metal fan. This metal stirrer directs the waves throughout the oven while the metal reflects the waves off the metal making it bounce off the walls and direct it towards the food. However it has limitations on foods wrapped with aluminum foil. It will cause the foil to spark called as arcing.  Putting a metal dish and equipment inside the oven will reflect the electromagnetic waves directed towards it causing an electric spark.  The electric spark between the metal dish and the oven’s wall may damage the magnetron tube and cause an uneven cooking. The possibility of a radiation leakage is also highly feasible.

The microwaves produced by the magnetron reflected by the metal are being absorbed by the food. The wire mesh at the door serves to reflect the electromagnetic waves back inside to prevent it from going out. Microwaves are considered as potentially dangerous source of radiation which is the main reason for the design of its strong doors and metal lined case. The paper, glass and some plastics used to contain the food transmit the electromagnetic wave which is absorbed by the water, fat and sugar present in the food (Bonnet et al 2003).  Microwaves don’t brown food unless they have features like grills and browning plates for it. Microwave oven cooks food quickly and uses less electricity reheating a whole meal while retaining its taste like it’s just been cooked.

Microwave is used for defrosting, reheating and primary cooking. The principle behind electromagnetic cooking is naturally dielectric heating. Dielectric heating uses insulating materials and operates at very high frequencies with a multitude of applicator like the walls of the microwave oven (Scott & Smith 2004). The microwaves are being discharged in random configuration using the walls of the applicator which is obviously the walls of the oven to cause random reflections of the wave. Note that this does not heat the circulating air inside the oven, the oven walls nor the vessel holding the food. This is because dielectric heating materials are nonconductors of electricity but have the important characteristic of becoming electrically polarized. The concept of becoming electrically polarized is best manifested in its ability to excite molecules or its ability to influence rearrangement of charge on a molecular level if placed in an electromagnetic field.

The waves are being absorbed by water molecules in the food which make them vibrate causing them to rub against each other producing friction. The friction produces heat which heats up the water and in the process cooks the food. It is evident that the wave thermal penetration reverses the polarity of the water and other liquid molecules which make the food molecules vibrate causing enough friction to heat it and cook the foodstuff. One major disadvantage of this method is that the microwave energy might be concentrated at various points resulting to localized heating. The turntable or the metal stirrer function is to alter the electromagnetic field resonance pattern and helps cook the food evenly. This is the main reason why food with large proportion of water cooks quickly.

Food shape, volume and food composition are critical factors which affects the amount and spatial pattern of absorbed electromagnetic energy. The dielectric properties of food relates on its ability to store microwave energy. They greatly affect the wave’s depth of penetration and dissipation of heat. Heating occurs simultaneously throughout the whole foodstuff. Speed is the name of the game in microwave cooking which puts the product on the edge over the conventional ovens used in traditional kitchens. Optimization of microwave heating requires proper balance of its transmission, reflection and absorption which greatly affects the microwave heating behavior (Hui 2006). It is best to read the directions in the product first and not to assume anything on how it works.

References

Bonnet, R. et al (2003). Giant book of winning science fair projects. New York, NY: Sterling

Publishing Company, Inc.

Hui, Y. H. (2006). Handbook of food, science, technology and engineering. Boca Raton, FL:

            CRC Press.

Scott, J. & Smith, Y.H.H. (2004). Food processing: Principles and application. Ames, Iowa:

            Blackwell Publishing Professional

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