Home  > Environment  > Polystyrene Fact & Fiction

Polystyrene Fact & Fiction

Microwaving

Microwave ovens help make life today more convenient, and because they are so efficient, they reduce our energy needs. The plastics packaging in which we receive much of our food and beverages provides convenience and energy savings. Because plastic packaging protects so well, it also reduces food waste.

There are plenty of stories surrounding microwaves, and some have attained "urban legend" status – "everyone" knows they’re true, even though there is no proof. Once the science behind microwaves is understood, it is clear that proper use of polystyrene plastic cups or containers is fine, as long as they are not overheated.

How does a microwave work?

Typically, microwaves agitate polar molecules (molecules that have magnetic properties), like water, into very rapid motion. The collisions between these rapidly moving molecules create frictional heat, first within the liquid water, which is then transferred to the entire food or beverage contents. Since polystyrene foam cups or containers themselves do not contain water within their molecular structure, they are "transparent" to the microwaves. The microwaves pass right through, and the container is unaffected. If the container’s temperature changes, it’s only because of the increasing heat of the liquid or solid they contain.

How does a microwave work compared to conventional methods?

With the conventional method of heating water, the container (a metal teakettle, for example) absorbs the stove’s heat, and then transfers it to the water it contains. By contrast, a microwave heats the water first, which then transfers its heat to the container.

What does a microwave oven do?

Interacts with water molecules in food or beverage. Through this interaction, cooks or reheats food or beverage directly (without the delay or increased energy needs of heating the container) and then the liquid or food transfers heat to container.

What does a microwave oven not do?

  1. Interact in any significant way with a plastic container used in the oven, because the container itself has no water molecules.
  2. Chemically change the food, other than the normal changes which take place with any means of cooking or heating.
  3. Irradiate the food, or destroy vitamins.

What about overheating?

  1. Most of our foods and beverages have a very high water content. Water's boiling point is 212°F at sea level and 1 atmosphere of pressure.
  2. Food with a high sugar or fat content (or both) can raise the boiling temp to over 212°F. If cooked or heated too long, some foods can soften the container, leading to a mess in the microwave oven and danger of burns from hot food or liquid.
  3. The much-publicized phenomenon of "superheating" water in a microwave, which can occasionally cause an explosive release of the contents, usualy occurs only in containers with an exceedingly smooth cellular structure, such as glass or glazed ceramics. Polystyrene foam cups or containers do not have that structure.

Microwave best practices

  1. Remember, the microwave oven heats the food or beverage first, and then the food or beverage heats the container.
  2. Don't use more heat or time in the microwave than you need. It only takes a few minutes (or less) to reheat or warm food that already has been cooked.
  3. Foods or beverages high in sugar and fat can get hotter than 212°F. Be careful not to heat too long, as you may melt your container and burn yourself on its contents.
  4. Melted polystyrene containers won't hurt you if you eat a bit of them, but they won't taste very good.
  5. Follow package directions, particularly for cooking.
  6. Use a microwave with a turntable so your food heats more evenly.

Additional information on Microwaves and Plastics in the Microwave is available at:

Sources:
Bloomfield, Louis A. How Things Work: The Physics of Everyday Life. New York: John Wiley & Sons, Inc., 1997.

Breder, Charles V., PhD. Common-Sense Approach to the Use and Reuse of Food-Contact Plastics to Heat & Reheat food in Microwave Ovens. Washington, DC: American Plastics Council. (www.plastics.org, 2002)

Meadows, Michelle. "Plastics and the Microwave." FDA Consumer Magazine. Washington, DC: U.S. Food and Drug Administration, November-December 2002. (http://www.fda.gov/fdac/features/2002/602_plastic.html)