More than 90 percent of U.S. households own microwave ovens. But they still confuse and worry many people, especially because of warnings circulating on the Internet.
How does a microwave oven cook?
Microwaves are a form of electromagnetic energy that causes water molecules in food to vibrate and rapidly produce heat. Contrary to popular belief, microwaves cook the outside first, and then heat is conducted to the interior. Though it's often called "nuking," microwaving has nothing to do with nuclear energy and does not make food radioactive.
Can microwaves leak from the oven and harm you?
Microwave ovens must meet federal safety standards that include features that shut down production of microwaves as soon as the door latch is released. Only a tiny amount of leakage is allowed, far below the level known to cause harm. And as you move away from the oven, even a few inches, the level falls substantially. As long as the door hinges, latch and seals are not damaged, don't worry.
Does microwaving destroy nutrients?
All cooking destroys nutrients to some degree. But because cooking time with a microwave is relatively short, and because you use little if any water, microwaving preserves nutrients better than other methods, such as boiling (where nutrients are lost in the water) and baking (with long cooking times). It is an especially good way to prepare vegetables—their high water content makes them cook fast.
What materials are safe to use?
You should use microwave-safe glass or ceramics. You can also use plastic containers labeled for microwave use—but avoid tubs from margarine, yogurt and the like, as well as plastic takeout containers and plastic grocery and storage bags, which can warp, melt, leach chemicals and/or burn you. Trays from frozen microwavable foods are meant for single use only. Some paper is also safe: parchment paper, wax paper, paper plates, white paper towels and oven cooking bags—but not newspaper, brown bags or recycled paper.
Can you use metal in a microwave oven?
Metal dishes and aluminum foil are generally not recommended because the microwaves bounce off them, so the food does not cook properly. And they may damage the oven and even cause fires. Thin metals in particularas in glazes and gold-rimmed dishescan disrupt the microwaves and create charged (ionized) air, which can cause sparking (called "arcing"). Some metals may be safe in your microwave, however—check your manual.
What about plastic wrap?
There has been concern that plastic wraps can leach small amounts of harmful chemicals, called plasticizers, into heated food. But most plastic wraps sold for home use no longer contain these chemicals. Polyethylene wraps (such as Saran and Glad Cling Wrap), for example, contain no plasticizers. Still, you don't want any plastic melting into your food, so choose a wrap that is labeled microwave-safe and place it so that it doesn't touch the food. Alternatively, cover food with a paper towel or a microwave-safe lid or plate.
How can you tell if a dish is microwave-safe?
If it's not labeled, put it in the microwave oven empty along with a glass container holding about a cup of water. Microwave on high for one minute. If the dish stays cool, it's okay to use. Keep in mind that even if it’s microwave-safe, the dish will eventually absorb heat from the contents and may become very hot.
Do microwave ovens cook foods to safe temperatures?
Yes, but you must be careful. Microwaves tend to cook unevenly, leaving undercooked "cold" spots where bacteria can multiply. To minimize risk, cut food in uniform pieces, arrange them evenly in a dish and then cover it. Rotate the dish during cooking (most microwaves have a turntable feature) and stir halfway through. Also, don't skimp on the "standing time" that the recipe or package directions call for after cooking, since this helps with even heat distribution and actually increases the temperature of the food a few degrees. When cooking meat (boneless is best), check with a meat thermometer in several places.
Is it true that microwaved water can explode if overheated?
Yes, though it's rare. Called "superheating," this occurs when a liquid is microwaved above its boiling point but doesn't actually boil. The surface looks smooth when you take the bowl or cup out of the microwave, but when the liquid is disturbed—as when you stir it or even just jiggle the cup—it can erupt and spray boiling liquid onto your hands or face. To lessen the risk, place a wooden spoon or stick in the cup while heating and don't exceed the recommended heating time.
Can carrots really catch fire in a microwave? Can foods explode?
Some foods, like carrots and hot dogs, may cause sparking, similar to what happens with metallic material. According to the USDA, it may occur in foods that contain minerals. Anything in a tight skin or shell can explode because the water inside can expand and burst through. Before microwaving, prick the skin of potatoes, cut winter squash and similar vegetables in half, take eggs out of shells (and pierce the yolk) and poke holes in the plastic wrap on frozen packaged foods.
Does microwaving create dangerous compounds in food?
No. Microwaved food is as safe as conventionally cooked foods. Microwaving doesn't do anything to food molecules that regular cooking doesn't do. You'll find such rumors on some websites selling products that they claim are "proven" to safeguard you from noxious chemicals and other detrimental effects from all kinds of radiation, including microwaves. This is nonsense—microwaving is simply heating.
Are microwave ovens energy-efficient?
Though many factors are involved in comparing cooking methods, microwaves generally use less energy than conventional ovens and stoves—as much as 80 percent less by one estimate—because they cook faster and primarily heat the food, not the air. Though the differences may be small for heating, say, a cup of water, the energy savings add up for foods that need longer cooking times.
Source: Originally published in The University of California, Berkeley Wellness Letter (November 2011)