How a Microwave Works

A simple description of how a microwave works.

1. FOSC 613 INDUSTRIAL FOOD PROCESSING TECHNOLOGIES MICROWAVE PROCESSING Presentation by Emmanuel Berko (10386416) Cecille Wendy Aboagye (10387989) Dinah Adomako (10600065)

2. Introduction Microwaves refer to the electromagnetic waves in the frequency range of 300 to 300,000 mega hertz (million cycles per second). Once microwave energy is absorbed, polar molecules and ions inside the food will rotate or collide according to the alternating electromagnetic field and heat is subsequently generated for cooking. Microwaves can pass through materials like glass, paper, plastic and ceramic. Can be absorbed by foods and water; but they are reflected by metals.

3. Heating Process Generally speaking, the alternating electromagnetic field generated inside the microwave oven would lead to excitation, rotation/collision of polar molecules and ions inside the food. These molecular frictions would generate heat and subsequently lead to temperature rise.

4. The Mechanisms There are two major mechanisms namely; dipolar interaction ionic interactions and these explain how heat is generated inside food.

5. Dipolar interaction Once microwave energy is absorbed, polar molecules such as water molecules inside the food will rotate according to the alternating electromagnetic field. The water molecule is a “dipole” with one positively charged end and one negatively charged end. Similar to the action of magnet, these “dipoles” will orient themselves when they are subject to electromagnetic field. The rotation of water molecules would generate heat for cooking. ( Buffler , 1993; Hill, 1998 and Ohlsson , 1993)

6. Ionic interaction Ionic compounds (i.e. dissolved salts) in food can also be accelerated by the electromagnetic field and collided with other molecules to produce heat. As the concentration of ions (e.g. dissolved salts) increase, the rate of heating also increases because of the ionic interaction with microwaves.

7. Basic Structure of a Microwave Oven

8. Structure of a microwave oven power supply and control : it controls the power to be fed to the magnetron as well as the cooking time. magnetron : it is a vacuum tube in which electrical energy is converted to an oscillating electromagnetic field. Frequency of 2450 MHz has been set aside for microwave oven for home use. waveguide : it is a rectangular metal tube which directs the microwaves generated from the magnetron to the cooking cavity. It helps prevent direct exposure of the magnetron to any spattered food which would interfere with function of the magnetron.

9. Contd. stirrer : it is commonly used to distribute microwaves from the waveguide and allow more uniform heating of food. turntable : it rotates the food products through the fixed hot and cold spots inside the cooking cavity and allows the food products to be evenly exposed to microwaves. cooking cavity : it is a space inside which the food is heated when exposed to microwaves. door and choke : it allows the access of food to the cooking cavity. The door and choke are specially engineered that they prevent microwaves from leaking through the gap between the door and the cooking cavity.

10. How it works Electrical energy is transformed and converted to direct current. The magnetron then uses the direct current to generate microwaves with a frequency of 2450MHz. The microwaves produced are directed by an antenna at the top of the magnetron into a waveguide. The wave guide channels the microwaves to a fanlike device called a stirrer which disperses them into the oven cavity.

11. How it works (contd.) The microwaves then reflect off the metal walls of the oven’s interior and are absorbed by the molecules in the food. The negative and positive components of the of the wave causes the food molecules to bump into each other. The heating system in a microwave is by volumetric heating. Here the material (molecules) absorbs microwave energy directly and converts it to heat.

12. Short Video on Principle

13. Factors that affect the rate of heating in a microwave Shape of the material Size of the material Dielectric constant Nature of the microwave equipment used. Hence the composition of a food will affect how it will be heated up inside the microwave oven.

14. Challenge There is no uniform temperature for heating. This is due to inherently non-uniform electric field commonly found in multimode applicators

15. Safety Even though microwave radiation is a non-ionizing radiation, it is necessary to protect one from any possible hazard. This can be achieved by ensuring all joints and doors are functioning properly

16. Conclusion The use of microwave oven provides a convenient way to thaw, cook or reheat foods nowadays. Many studies have been conducted to assess the safety as well as possible nutrient loss associated with microwave cooking.

17. References Buffler , CR. Microwave cooking and processing: engineering fundamentals for the food scientist. New York: Van Nostrand Reinhold; 1993. Hill, A and ILSI Europe Microwave Oven Task Force. Microwave Ovens. Brussels: ILSI Europe; 1998. Ohlsson , T. Domestic use of microwave ovens. In: Macrae R, Robinson, RK and Sadler, MJ, editors. Encyclopaedia of food science food technology and nutrition. Vol. 2. London: Academic Press; 1993. p. 1232-1237.