A microwave is the ultimate culinary convenience — but most people don't realize that it can go from being helpful to harmful.
In London recently, a 9-year-old girl attempting to re-heat a boiled egg in the microwave received a serious eye injury. The girl placed the egg in the microwave for 40 seconds at full power, then removed it and put it in a bowl. About 30 seconds later, the egg exploded, sending parts of the shell shooting into her right eye — causing serious damage to her eye, according to a letter of warning sent by British ophthalmologists to the British Medical Journal.
At first, the girl could only see hand movements with her right eye, but her vision was restored after a series of operations and the insertion of a plastic lens in her cornea.
Microwave manufacturers warn against heating eggs with a shell, and recommend multiple piercings before cooking or heating eggs in general. Advocates, including the British ophthalmologists, say such warnings should be displayed on the microwave itself.
But it's not just eggs. Several years ago, an Illinois woman scalded her face and her corneas after a bowl of water she heated in the microwave exploded in her face.
No Visible Bubbles
Professor Louis Bloomfield, a University of Virginia physicist, says there's a simple scientific explanation for how these explosions can happen.
"You're used to having water or liquid boil when you heat it above a certain temperature, but there are occasions, and they're more frequent than you'd expect, in a microwave oven when the water goes to or above boiling without any bubbles forming," Bloomfield said. "And that's a phenomenon known as superheating."
Water that has gone beyond boiling is very dangerous, he said.
"Well, it's almost like a bomb once you've got it superheated adequately, because anything that triggers the boiling, once you've reached that temperature, will cause catastrophic, very sudden flash boiling," Bloomfield said.
Water Exploded ‘Like a Bullet’
Patty Long of Naperville, Ill., found out about the dangers of just boiling water the hard way in the spring of 2000. After heating a container of water for three minutes in the microwave, Long sensed something was wrong, because there were no bubbles.
"So I opened it up and brought the container out, and I just kind of brought it down," Long said. "I was going to touch it with my right hand, and before I got my right hand there, I heard this like whoosh, and I got hit with all that water into my face."
Long had first- and second-degree burns all over her face. Even worse, the corneas of her eyes had been scalded. It would take six months of treatment to fully recover her sight.
The water in the container basically exploded "like a bullet," she said.
"It just hit the ceiling, too," Long said. "There was no water left in the cup."
Tips for Microwave Users
Despite this danger, Bloomfield says a microwave is "actually one of the safest devices in your kitchen when used correctly." Here are some tips to prevent superheating and other microwave problems.
Put your teabag or instant coffee in the water prior to heating the water in the microwave. Always stir liquids before heating. Don't put twist ties in the microwave. They are like a wire with too much electricity running in it, and the charges at either end can make sparks. Don't cook things too long. According to the Association of Home Appliance Manufacturers, you should not heat liquid for more than two minutes.
Stir food midway through cooking to distribute heat and homogenize the temperature. Unlike a conventional oven, a microwave heats food from the inside out, rather than from the outside in. So the container may be cool but the contents beyond boiling temperature. Allow standing time before touching. "A decent rule of thumb is to wait maybe a minute or two for every cup of water you've got," says Bloomfield, "to give it time for the evaporative process to cool down the liquid and bring it back toward the boiling temperature from above." Sample food before giving it to a child or test the temperature and several places. "Until you stir it and then feel it carefully," says Bloomfield, "you don't know what the temperature of that next spoonful is." Do not let plastic wrap touch foods during microwave use, and be sure to use a brand of plastic wrap that is microwave-safe. Loosen the plastic wrap on one corner. Don't microwave in Styrofoam. Do not dry herbs in the microwave, as they can ignite. Lift a lid or plastic wrap away from your face. Use a turntable inside the microwave to keep the liquid moving and to promote more even heating.