What Einstein Told His Cook (31 page)

The solution is to keep the food moving, so that it averages out nonuniformities in microwave intensity. Most ovens these days have an automatic turntable, but if yours doesn’t, many recipes and defrosting instructions on frozen foods will remind you to rotate the food halfway through the heating time.

Why mustn’t one put metal into a microwave oven?

Light bounces off mirrors; microwaves bounce off metal. (Radar is a kind of microwave that bounces off your speeding car and cooks your goose.) If what you put in the oven reflects too many microwaves back instead of absorbing them, the magnetron tube can be damaged. There must always be something in the oven to absorb microwaves. That’s why you shouldn’t ever run it empty.

Metals in microwave ovens can behave unpredictably unless you have a degree in electrical engineering. Microwaves set up electrical currents in metals, and if the metal object is too thin it may not be able to support the current and will turn red hot and melt, as in the blowing of an overloaded fuse. And if it has sharp points, it may even act like a lightning rod and concentrate so much microwave energy at the points that it will send off lightning-like sparks. (Those paper-covered wire twist ties are notorious because they are both thin and pointed, so beware.)

On the other hand, the engineers who design microwave ovens can devise safe sizes and shapes of metal that won’t cause trouble, and some ovens actually do contain metal trays or racks.

Because it’s so hard to predict which sizes and shapes of metal are safe and which may cause fireworks, the best advice is never to put anything metallic in a microwave oven. And that goes for fancy dishes that have gold or other metallic trim.

Can the microwaves leak out of the box and cook the cook?

An old, beat-up oven with a warped door may indeed let enough microwaves out through the cracks to be a hazard, but there is extremely little leakage from today’s carefully designed ovens. Moreover, the instant the door is opened, the magnetron shuts off and the microwaves disappear like the light when you turn off a lamp.

What about the glass door itself? Microwaves can penetrate glass but not metal, so the glass door is covered with a perforated metal panel that lets light come through so you can see inside, but that the microwaves can’t get through because their wavelength (4¾ inches) is simply too big to fit through the holes in the metal panel. There is no basis for the belief that it is hazardous to stand closer than several feet from an operating microwave oven.

What makes a container “microwave safe”?

In principle, the answer is simple: Containers whose molecules aren’t dipoles and will not absorb microwaves. Such molecules will not be jerked around by the microwaves and will not get hot. But in practice, the answer isn’t quite so simple.

Surprisingly, in what many people perceive as our overregulated society, there appears to be no government, industry, or trade definition of the term “microwave safe.” I have attempted to extract a definition from the FDA, the Federal Trade Commission, and the Consumer Product Safety Commission, all to no avail. Nor could I get any manufacturers of “microwave-safe” products to tell me why they make that claim. (Lawsuits! Lawsuits!)

So it appears that we’re on our own. But here are some guiding principles.

Metals
: I’ve already explained why metals are to be avoided in microwave ovens.

Glass and paper
: Glass (that is, standard kitchen glass, not fancy crystal), paper and parchment are always safe; they don’t absorb microwaves at all. So-called crystal, which is glass with a high lead content, does absorb microwaves to some extent and may therefore get warm. In a thick piece, the heat might set up stresses that may lead to cracking. Best not to take the chance with that expensive stuff.

Plastics
: Plastics don’t absorb microwaves either. But microwaved food can get pretty hot, and any container, no matter what it’s made of, will then get hot from the food. Some wimpy plastics, such as thin plastic storage bags, margarine tubs, and styrene foam “doggie boxes” from restaurants, may even melt from the food’s heat. Certain kinds of plastic refrigerator containers may get distorted out of shape. You just have to learn this from experience.

Ceramics
: Ceramic cups and dishes are usually okay, but some may contain minerals that absorb microwave energy and get hot. If in doubt, test the suspect by heating it empty in the oven along with some water in a glass measuring cup. If the test object gets hot, it’s not microwave safe. (The water is in there to absorb microwaves and avoid the empty-oven problem that I referred to earlier.)

To further complicate our lives, some earthenware mugs and cups, even though made of purely innocent, non-energy-absorbing clays, can crack in the microwave oven. If the glaze has become chipped or cracked with age, water can seep into pores or air holes in the clay beneath the glaze, perhaps during dishwashing. Then, when microwaved, the trapped water will boil and its steam pressure can crack the cup. While that’s a rare occurrence, it’s best not to use your chipped or crazed heirloom cups in the microwave oven.

Why do some “microwave safe” containers still get hot in the oven?

“Microwave safe” means only that the container won’t get hot from the direct absorption of microwaves. But the food it contains does absorb microwave energy and therefore gets hot, and as I pointed out earlier, much of that heat is transmitted to its dish. How hot the dish gets depends on how efficiently it absorbs heat from the food, and different materials—even different “microwave-safe” materials—can vary quite a bit in that respect. Always use potholders when removing microwaved containers from the oven. And when opening the container, beware of pent-up steam, which can be very hot.

Is it dangerous to heat water in a microwave oven?

No and yes. No, it’s unlikely that anything serious will happen, but yes, you should be careful. Microwave-heated water that hasn’t yet come to a full, vigorous boil can indeed be a booby trap.

Because microwave energy is absorbed only by the outer inch or so of the water in a cup, the resulting heat must then diffuse into the interior portions before all the water can uniformly reach its boiling point. This diffusion of heat is a slow process, and some of the outer-portion water can get very hot indeed before the whole cup is observed to boil. Parts of the outer water can, in fact, get even hotter than the boiling point without boiling; it is then said to be superheated. Water—indeed, any liquid—may not boil even though hot enough, because in order to boil the molecules need a convenient place to congregate until there are enough of them in one spot to make a bubble of vapor. (Techspeak: They need nucleation sites.) A nucleation site can be a speck of dust or an impurity in the water, a tiny air bubble, or even a microscopic imperfection on the wall of the cup.

Now suppose that you have some clean, pure water in a clean, smooth, and blemish-free cup, so that there are virtually no nucleation sites at all. You put it in the microwave oven and, because you are of course in a hurry, you turn it on full blast, which heats the exterior portions of the water intensely. Under these conditions it is possible that you will produce some pockets of superheated water that are just dying to boil furiously if only given the chance. Then, when you open the oven door and grab the cup, you give them that chance by jostling the water. Because of the jostling, some of the excess “super heat” finds its way into a slightly cooler, not-quite-boiling portion, making it boil suddenly. This disturbance in turn makes the superheated portions also boil suddenly. The result is an eruption of unexpected bubbling that can spatter out hot liquid.

The reason that this delayed bubbling never happens in stove-heated water is that the heat at the bottom of the kettle continually creates tiny bubbles of air and water vapor that serve as nucleation sites, so superheating never gets a chance to develop. Also, the bottom-heated water is continually rising and circulating, which prevents too much heat from piling up in any one place.

Play it safe by never removing the cup from the microwave oven as soon as you see some bubbling going on, because there could still be some not-quite-boiling portions that could start boiling unexpectedly. Watch the water through the oven’s window and allow it to boil vigorously for several seconds before you turn off the oven and take it out. Then you’ll know that all the water is well mixed to a uniform boiling temperature.

Even so, always be careful when removing any hot liquid from the oven; it may still bubble up unexpectedly and scald you with its splatter. I have developed the habit of plunging a fork into the cup to “set off” any superheated spots before removing it from the oven.

When you subsequently add your tea bag or (ugh!) instant coffee to microwave-heated water, you will see some fizzing, but it’s not boiling and it’s not violent; it’s mostly air bubbles. The solids are providing new nucleation sites that didn’t exist before, and these sites liberate air that was dissolved in the original cold water but that didn’t have time to come out during the few minutes of heating.