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The Microwave Demystified

Microwaves may be a fixture in most kitchens, but there’s a lot about them we don’t understand. How do they work? Are they safe? We asked scientist and author Robert L. Wolke to answer the hard questions about this mysterious appliance.

Fine Cooking Issue 94
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How do microwave ovens heat food?

In a microwave oven, an electronic device generates a form of electromagnetic radiation called microwaves, which are very similar to radio waves but with a shorter wavelength and higher energy. When you put food in a microwave oven, water molecules in the food absorb the microwaves and start flip-flopping around and getting hot. This hot water and steam, in turn, heat the rest of the food.

Why do microwaves cook so much faster than regular ovens?

A regular oven heats the air inside the oven, and this hot air gradually transfers its heat energy to the food. This type of heat transfer is a slow, inefficient process. Microwaves, on the other hand, efficiently deposit their energy directly into the food, agitating the molecules and quickly creating widespread heat within the food.

Why does food in the center of a container take longer to heat up? And why does the container get hotter than the food?

The food in the center of a container takes longer to heat because microwaves don’t reach it there. Microwaves penetrate food to a depth of about 1/2 inch. This outer region heats up rapidly, creating a layer of very hot food that surrounds a cool interior. Heat energy transfers from this hot outer region to the container, making it very hot. Heat also transfers to adjacent food molecules in the cooler center. But as in a regular oven, it takes time for heat from the hot exterior of the food to work its way deep into the food.

In a microwave you can speed things along by stopping and stirring the food, which distributes the heat and moves cool food out toward the walls of the container where the microwaves can reach it when you continue the heating.

What foods can’t be cooked in a microwave?

Microwaves are absorbed mainly by water and to some extent fats, so dry, fat-free foods won’t heat up.

Honestly, how safe are microwave ovens?

They’re safe. Period. For at least three reasons:

1. The microwaves bounce back and forth off the walls of their steel-box enclosure and remain imprisoned. (If, however, your oven is beat up and the door doesn’t close tightly, you’d be wise to replace it.)

2. Microwaves have a wavelength of an inch or two and simply can’t fit through the holes in that perforated metal screen in the door. So go ahead—stand in front of the door and watch the fun going on inside for as long as you wish.

3. The device that generates the microwaves turns off instantly when the door opens. That said, there are a couple of hazards to watch out for. Hot containers, for one. And when heating a mug of water for tea or coffee, be careful. Even before the water appears to boil vigorously, pockets of water in the cup may actually become “superheated” (i.e., hotter than water’s boiling point, 212°F). Then, if you disturb the water by grabbing the cup, the water may boil explosively and scald you. To prevent this, I put a fork in the cup to “defuse” any superheated water before removing the cup from the oven.

What does it mean when something is labeled microwave safe?

The microwave-safe symbol (right) or the words “microwave safe” mean three things:

1. That an object won’t absorb microwaves and get hot as a result. (But of course, any container can get hot from contact with its hot contents.) So in that sense, all plastics, glass, cloth, and paper are “safe.” But metals, in general, are not, because they will not only overheat but may actually spark.

2. That an object will not melt or deform in the microwave. Plastic film and some plastic containers are not labeled microwave safe because they may warp or even melt when they come in contact with hot food, possibly leading to spillage. If you must microwave foods in a plastic-wrap-covered container, leave an air space between the food and the plastic so that the wrap doesn’t melt from contact with the hot food. Use a container large enough to allow an inch of space between the wrap and the food and turn back a corner or cut vents in the wrap to allow steam to escape.

3. That the object is chemically and toxicologically harmless according to the FDA’s or the manufacturer’s own tests. Although the United States Food and Drug Administration certifies various plastics used in packaging as safe for contact with food, this does not mean that all those plastics have been deemed safe for hot food. There is some concern that the heat from hot food (not the microwaves) could cause some plastics to leach harmful chemicals. So before declaring a plastic microwave safe, the FDA performs rigorous leaching tests under different conditions of food type, temperature, contact time, and area. A plastic is certified as microwave safe only if the amount of leached chemical is hundreds or thousands of times less (per pound of body weight) than what has been found to harm laboratory animals after a lifetime of use. (Manufacturers do not have to submit their products to the FDA for testing.) Recently, there has been growing concern about a chemical called bisphenol A, or BPA, leaching from polycarbonate bottles, especially baby bottles. Fetuses, infants, and children are most susceptible to its effects, which mimic those of sex-related hormones. (Polycarbonate containers are rigid and transparent, with the recycling code 7 on the bottom.) To play it safe until more is known about this hazard, the National Toxicology Program recommends that you use glass or soft-plastic baby bottles and that you do not microwave food in polycarbonate containers.

Microwave dos and don’ts

  • Do use containers that are designed for use in the microwave and are labeled microwave safe.
  • Do vent the lid.
  • Do prevent splatters by covering food with waxed paper, parchment, or white paper towels.
  • Don’t use plastic containers that are designed for cold storage, such as margarine tubs, yogurt containers, water bottles, etc.
  • Don’t let plastic wrap touch the food that’s being heated.
  • Don’t put plastic bags from the grocery store in the microwave. They will melt.

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