David explores
how microwave
ovens work,
and how they
interact with
foods and
packaging
materials.
What is the science behind microwave cooking?
Microwaves are a form of electromagnetic energy, like light waves or radio waves, and occupy a part of the electromagnetic spectrum. Microwaves are used to relay long-distance telephone signals, television programs and computer information across the earth or to a satellite in space. They are even used for a type of medical treatment called diathermy.
The microwave is most familiar as the energy source for cooking food. Every microwave oven contains a magnetron, which generates microwaves at just the right frequency to interact with the molecules in food and heat it directly.
All wave energy changes polarity from positive to negative with each cycle of the wave. In microwaves, these polarity changes happen millions of times every second. Food molecules - especially the molecules of water - have a positive and negative end, in the same way a bar magnet has a north and a south pole. When microwaves at the right frequency bombard food, they cause the polar molecules to rotate at the same frequency, millions of times a second. All this agitation on the molecular level creates friction, which heats up the food. Because microwaves don't interact with molecules of glass, plastic or paper, only the food is heated.
Metal containers can produce dangerous arcing in a microwave oven. However, many food packages actually contain thin films of metal that speed the cooking process. For example, new packaging techniques use metalized polyethylene terephthalate (PET) film laminated to paperboard as a heat susceptor. This surface absorbs microwaves, and becomes a miniature "frying pan" to brown or fry the foods in the package.
Radiation--The spreading of energy by electromagnetic waves.
Conduction--The transfer of heat by molecular motion from a source of high temperature to a region of lower temperature, tending toward a result of equalized temperatures.
Convection--The mechanical transfer of heated molecules of a gas or liquid from a source to another area, as when a room is warmed by the movement of air molecules heated by as radiator.
Molecules--The smallest particle of any compound that has the chemical properties of that compound. Materials are considered to be made of molecules held together by attractive forces.
Susceptor--A material, usually a thin, metalized film inside the packaging of microwave foods, which converts microwave energy into heat energy and promotes the browning of foods.
Cooking By the Numbers!
See the differences in how microwave ovens and conventional ovens heat different substances.
Main Activity
Although both microwaves and conventional ovens ultimately use thermal energy to cook food, the molecules in the food respond differently when exposed to an energy source.
Materials
1. Pour 250 milliliters of water into one of the microwaveable containers. Measure and record the temperature of the water. 2. Pour 250 milliliters of sand into a similar container. Measure and record the temperature of the sand. 3. Place the container of water in the microwave oven and run the oven at its highest power setting for two minutes. Remove the container of water. Measure and record the new water temperature. 4. Place the container of sand in the microwave oven and run the oven at its highest power setting for two minutes. Remove the container of sand. Measure and record the new temperature of the sand. 5. Preheat the conventional oven to about 100 degrees Celsius or 200 degrees Fahrenheit. 6. Pour 250 milliliters water into one of the oven-safe containers. Measure and record the temperature of the water. Make sure that the temperature of this water is the same temperature that you recorded for the water that you put in the microwave oven. 7. Pour 250 milliliters of sand into one of the conventional oven containers. Measure and record the temperature of the sand. The sand, too, should be the same temperature as the sand used in the microwave oven. 8. Place both containers in the preheated oven. Heat the two pans for about 10 minutes. Remove both containers from the oven. Quickly measure and record the temperature of both the water and the sand. 9. Develop a table that lists your results.
Questions
1. What was the temperature change of the water and the temperature change of the sand in each oven? Which material had the greatest increase in temperature for each type of oven? Why do you think these results happened? 2. How would your results change if the water was left in the conventional longer? Try it! 3. What kinds of food cook best in a microwave oven? In a conventional oven? What are you basing your choices on?
Many terms are used in describing electromagnetic waves. Investigate the
following terms: wavelength, amplitude, cycle and frequency. How can these
terms be used when describing a stone dropped into a tray of water?
Different waves affect the human body. Ask your school nurse about waves
used in the study of human physiology. Invite a radiologist to your class.
Ask him/her about brain waves and X-rays. Investigate biorhythms. How are
they measured?
Purchase several microwaveable products and dismantle the packaging. Compare
the packages. How have theories of energy absorption been combined with
cooking? Do any packages look like they might release adhesives into the
product at high temperatures? Do any use heat susceptors?
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Educational materials developed with the National Science Teachers Association.