# Microwaves

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Physics
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Assignment
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High School
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1632

Microwaves and its Applications in Cooking

Introduction

The discovery of existence of electromagnetic spectrum gave light to its modern day applications. The electromagnetic spectrum comprises of different radiations categorized based on the magnitude of frequency and wavelength. These energy radiations which are in form of a wave travel at speeds equal to that of light and comprises of both the magnetic and electric fields oscillating perpendicular to each other (Gupta & Eugene, 2007). Among these radiations of different frequencies making up the electromagnetic spectrum is Microwaves. Microwaves are radiations with frequencies ranging from 300 Megahertz to 300 Gigahertz or wavelengths ranging from 1 Millimetre to 1 Metre. The theory of microwaves first found its way in the academic disciplines in 1931 (Gupta & Eugene, 2007). Initially, microwaves were regarded as those radiations whose wavelengths were around 0.5 m. However, further research enabled better classification. They are further classified based on frequencies within that range. The three sub-categories are the ultra high frequency band (UHF), the super high frequency band (SHF) and the Extremely high frequency band (EHF). The UHF band have frequencies ranging from 300MHZ to 3 GHZ, the SHF have frequencies ranging from 3 GHZ to 30 GHZ and the EHF have frequencies ranging from 30 GHZ to 300GHZ (Gupta & Eugene, 2007). Microwaves were initially used in communication before finding its way into cooking.

Application of Microwaves in Cooking
A microwave oven is a common appliance in modern day homes. It is normally used to cook, thaw and reheat foods conveniently. The unique aspect of microwave oven is its ability to cook food only without heating any other thing and it cooks within a relatively short time. This is due to volumetric heating where the food particles absorb radiant energy and converts it to heat (Lee & Cho, 2000). Therefore, food is heated both internally and externally (from inside out) as opposed to conventional methods where heat must diffuse first hence the outer section of food is heated first. Moreover, food is only heated because most objects which are used for containing food inside the oven such as ceramics, glass and plastics do not absorb radiations of these frequencies. Therefore, they do not get heated up inside the microwave oven (Lee, 2000; FEHD, 2005). Contrary to this, metals reflect these radiations which may in turn cause sparking inside the microwave oven (Lee, 2000). It is the reason why it is not advisable to use metal ware when cooking food using a microwave oven. The discovery of application of microwaves in heating occurred in 1951 after world war II where scientists found out that birds became well cooked and sizzled after colliding with radar posts (Hill & ILSI, 1998; FEHD, 2005). The idea of microwave ovens emerged from this discovery (Hill & ILSI, 1998).

Microwave radiations of frequencies of about 2.5 GHZ are used in microwave ovens to heat food. This implies that the electromagnetic fields causing microwaves change direction 2.5 billion times in every second (Lee, 2000). This high frequency radiation carries huge amounts of energy. This energy is transferred to food which in turn excites the atoms in food molecules. Therefore, food heating is as a result of atomic excitation as opposed to heat conduction (Lee, 2000). Food molecules are made up of dipoles having both positive and negative charge of opposite sides (Lee, 2000). When they interact with electromagnetic fields, they are rearranged. The positive charge is realigned to negative pole while the negative charge is realigned to the positive pole (Lee, 2000).

Microwave Oven

A microwave oven consists of various parts. The basic components of a microwave oven are; the power supply and control, the magnetron, waveguide, stirrer, turntable, cooking cavity and door and choke.

Basic Structure of a Microwave Oven

Source: (FEHD, 2005)

Magnetron

It is the source of microwave energy which is form of electrons. The magnetron acts as a converter of electric power from the power supply and control unit into electromagnetic waves. They were first designed in 1921 but improved designs were introduced in 1940’s (Vollmer, 2004). Domestic microwave ovens are designed such that the magnetron produces an electromagnetic wave oscillating at a frequency of 2.45 GHZ. A magnetron has an anode and a cathode.

The part labelled 1 and 2 represents a cathode and anode respectively. The heated cathode produces electrons which are projected in the direction of the arrow labelled 3 (Vollmer, 2004). An anode of a magnetron has resonant cavities labelled 4. Electrons generated by the heated cathode are projected through a magnetic and electric fields. Since they are electrically charged, electrons experience a force which causes them to move in a circular path labelled 5. Electrons move forcefully through the cavities. The cavities resonate the energy in the electrons which ensures that microwaves are produced as opposed to sound waves labelled 6. The part labelled 7 is the waveguide and it helps in channelling the microwaves produced.

Waveguide

It links the food chamber and the magnetron. Microwaves generated by the magnetron passes through the waveguide to the cooking chamber. Waveguide protects the magnetron from direct contact with splashed food which can cause it to malfunction (FEHD, 2005).

Turntable

The turntable continuously rotates when the oven is being used. It rotation allows the food to be heated evenly.

To ensure more even heating, a fanlike stirrer is connected to the waveguide end. Stirrer helps in distributing the microwaves inside the cooking cavity.

Cooking cavity

This is the space where food is put to cook. The walls of this cavity are made of metal which help in reflecting the microwaves to be absorbed by food molecules (Lee & Cho, 2000). The reflected waves form a standing wave inside the cooking cavity. One side of the cooking cavity is a door which is used to access it. To prevent leakage of microwaves outside the cavity, the door has been coated with a metal grid. The grid holes are designed to be smaller than the wavelength of microwaves (Vollmer, 2004). Hence they prevent leakage of microwaves. The gaps between the door and the cavity are coated with metal to reflect the microwaves. To further enhance the security over microwaves leakage, modern ovens have an automatic switch which stops the functioning of the magnetron when the door is opened. Nonetheless, despite several measures that have been put in place to ensure safety, it is important for users to be keen and ensure that joints and door are in place (Bradshaw et al., 1998). Microwave detectors can be used to detect any leaking radiation. Microwaves are known to be non-ionising but can still pose considerable safety hazards.

How a Microwave Oven Works

As earlier mentioned, polar molecules such as the water molecules contained in food are in form of a dipole. Food absorbs microwave energy and this causes them to realign and rotate in accordance to the alternating electromagnetic field (Lee, 2000). The process of realignment and rotation results to collisions and friction between the molecules (FEHD, 2005). Therefore heat is generated in that process which helps in heating the food molecules. At the same time, foods contain ionic salts which are accelerated by the electromagnetic field causing more collisions (FEHD, 2005).More collisions result in heat generation. Microwave oven uses the heat generated from inside the food molecules. It is for this reason that food cooked by use of a microwave oven cook from inside out as opposed to the conventional methods where heating start from outside. It is important to note that foods with higher moisture content and dissolved salts are therefore heated up as compared to foods with lesser amounts of moisture and salts. Despite having lesser amount of moisture, foods with higher fat content cook faster because of higher specific heat capacity of fats as compared to that of water (Singh & Heldman, 1993). The time taken by a microwave oven to cook food will depend on a number of factors including; the thickness , density, component and the quantity of food molecules (FEHD, 2005). Foods with high absorption power are cooked faster than those with low absorption. Moreover, presence of salts and moisture increases the heating effect.

There has been some doubts about the safety of foods cooked using a microwave oven. Research studies have proven that microwave cooked foods are safe. In addition to this, there has been no manifestation to show that using microwaves leads to production of carcinogenic substances which has been a major doubt in the recent past (FEHD, 2005). As a matter of fact, these oven cook well some types of foods. A major risk on microwave oven cooked is that of those foods with low absorption. Such foods may not cook well beyond certain thickness.

References

Bradshaw, S. M., Van wyk, E. J., & Swardt, J. B. (1998). Microwave heating Principles and the Application to the Regeneration of Granular Activated Carbon. The Journal of the South African Institute of Mining and Metallurgy, 201-212.

Food and Public Health Branch of the Food and Environmental Hygiene Department (FEHD). (2005).Microwave Cooking and Food Safety. Risk Assessment Studies, Report No. 19. Hong Kong: Food and Environmental Hygiene Department.

Gupta, M. & Eugene, W. (2007). Introduction to Microwaves In M. Gupta & W. Eugene (eds), Microwaves and Metals. New York: John Wiley & Sons (Asia) Pte Ltd, pp.1- 23.

Hill, A & ILSI (1998). Europe Microwave Oven Task Force. Microwave Ovens. Brussels: ILSI Europe.

Lee, B. & Cho, Y. (2000). The Principal of Microwave Oven and Microwave Heating. Yonsei University, Department of Electrical and Electronic Engineering.

Lee, S. (2000). Principles of Microwave Oven. Yonsei University, Department of Electrical and Electronic Engineering.

Singh, R. P. & Heldman, D. R. (1993). Introduction to Food Engineering. San Diego: Academic Press, Inc.

Vollmer, M. (2004). Physics of the microwave oven. Physics Education, 39(1): 74-81.