Tag: microwave ovens

Don’t microwaves change the molecular structure and composition of food, by ejecting some electrons from atoms and forming cancer-causing free radicals? If I should stand away from a microwave to avoid possible leakage, why would I eat microwaved food?

Microwaves don’t affect the molecular structure of the food, except through the thermal effects we associate with normal cooking (e.g., denaturing of proteins with heat and caramelizing of sugars). That’s because, like all electromagnetic waves, microwaves are emitted and absorbed as particles called “photons.” The energy in a microwave photon is so tiny that it can’t cause any chemical rearrangement in a molecule. Instead, it can only add a tiny amount of heat to a water molecule. During the microwave cooking process, microwave photons stream into the food and heat it up. But millions of them would have to work together in order to cause non-thermal chemical changes in the food molecules and they don’t normally do that. The photons can only work together if there is a conducting material, such as a metal wire, inside the oven. In that case, the photons can accelerate mobile electric charges along the conducting paths and create sparks. Such sparks can cause chemical damage, but nothing worse than the chemical damage caused by scorching food with a flame or broiler. Even if your microwave is full of sparks for some reason, I doubt that the food will be any worse for you than it would be if you cooked it over an open flame or barbecue.

How does radar absorbent materials work. How effective is stealth technology? — DP, Scottsbluff, NE

I believe that most radar absorbing materials are partially conducting plastic composites. As a microwave from the radar transmitter penetrates these composites, the electric field in that wave drives charges back and forth through the composites. Since the composites don’t conductor electricity well, they turn the wave’s energy into thermal energy and thereby absorb it. A similar effect occurs for light waves when you shine them on a pile of powdered charcoal. (According to David Ingham, some radar absorbing materials include lossy magnetic materials—materials such as ferrite and carbonyl iron that respond to the magnetic field in a microwave.) Because there is always some reflection whenever an electromagnetic wave enters a material that slows the wave down, stealth aircraft are also careful to deflect the reflected wave away from the radar transmitter so that its receiver won’t detect the return wave. In fact, these materials can be corrugated so that any microwaves hitting them reflect into the corrugations and have many opportunities to be absorbed. As I understand it, the microwaves that return to the radar receiver from a stealth plane are remarkably weak. I wouldn’t be surprised if a whole stealth plane reflected less microwaves back at the radar unit than would reflect from a foil chewing gum wrapper.

I recently acquired a microwave that “doesn’t cook as fast as it used to.” Does this sound right? What type of service might need to be performed? – W

It is possible for a microwave to lose cooking speed. If the microwave source isn’t able to produce as intense microwaves as before or if it doesn’t turn on reliably and steadily, it won’t cook as fast. For the source to produce less intense microwaves, the high voltage power supply would probably have to be weak. Its storage capacitor could have failed or one or more of its high voltage diodes could have burned out. According to a reader, the most likely cause of weak cooking in a microwave oven is a failed capacitor—with no ability to store separated charge in its capacitor, the oven produces pulsing rather than steady microwaves and delivers less average power. I suppose that the magnetron itself could be dying, with the most common failure (according to that same reader) being shorting out, the result of electromigration of the filament material. For the source to not turn on reliably, it would probably have to have a bad connection to the power line. One good possibility is that the relay that turns on power to the high voltage power supply is not making good contact.

Listen to the microwave as it operates on a medium setting. It should cycle on and off every five or ten seconds. You should hear it hum softly during the on half of the cycle and then stop humming during the off half of the cycle. Different power levels simply vary the fractions of on time and off time. If you don’t hear the hum or the hum is intermittent, then something is probably wrong with the power relay or with something else in the high voltage power supply. If the relay is flaky, a little cleaning of its contacts may cure the problem. Be careful of the high voltage capacitor, which can store a lethal charge even when the unit is unplugged.

In a microwave oven, does food cook from the inside out or outside in? — KS, Essex, England

If the piece of food isn’t too large, it all cooks at once. The microwaves that heat the food pass deep into it and they deposit energy in every part of the food simultaneously. Only if the piece of food is so large that an appreciable amount of microwaves are absorbed before they reach the center will the center cook more slowly than the outside. I doubt that this shielding of the center is a problem with foods small enough to fit inside a normal microwave oven. However, the microwaves in a microwave oven aren’t perfectly uniform, so that some parts of a meal will cook a bit faster than others. That’s why it’s important to move the food about during cooking to achieve uniform heating throughout.

If microwave cookers are so energy-efficient, why can’t similar machines be used as hot water heaters or in central heating systems? – GB

Microwave ovens transfer about 50% of the electric energy they receive from the electric company to the food. Conventional ovens transfer only something like 10%. Cooking just isn’t a very energy efficient process because you’re trying to get heat into an object from outside that object. In contrast, an electric space heater transfers 100% of the electric power it receives to the room around it. Home heating is much more energy efficient because you’re getting heat into an object from inside that object. In effect, your microwave oven is also 100% efficient at heating your room—every bit of electric energy it consumes eventually enters your room as heat. But it’s an expensive sort of “space heater” and you do better just to use conventional heating systems.

How does a magnetron work? — MM, Czech Republic

A magnetron has a ring of resonant electromagnetic cavities around a hot central filament. Each resonant cavity acts like an electromagnetic “tuning fork”—electric charges and electromagnetic waves swing back and forth inside a resonant cavity at a particular frequency; the cavity’s resonant frequency. As electrons are “boiled” off the hot filament, a high voltage attracts them toward the walls of the resonant cavities. The resonant cavities tend to have at least small amounts of electric charge “sloshing” back and forth in them at their resonant frequencies and the electrons from the filament are attracted more strongly to the cavities’ positively charged walls than to their negatively charged walls.

However, there is also a magnetic field present in the magnetron and this field deflects the streams of electrons so that they hit the wrong walls of the resonant cavities. Instead of canceling the charge sloshing in the walls of the resonant cavities, the newly arrived electrons add to it. As electrons flow to the resonant cavities, more and more charge sloshes in the resonant cavities and these cavities accumulate huge amounts of energy. Some of this energy is tapped by a small wire loop and a microwave antenna. This antenna radiates some of the energy from the cavities into a metal channel that leads away from the magnetron. In a microwave oven, this channel leads to the cooking chamber so that energy from the resonant cavities is delivered to the food in the oven. Energy is extracted from the magnetron slowly enough that the filament and high voltage power supply can replace it and the operation continues indefinitely.

What does the inside of a microwave oven look like? Please show illustrations. — Dade County, FL

A microwave oven contains (1) a magnetron that produces the microwaves, (2) a high voltage direct current power supply (a high voltage transformer, a set of rectifiers, and a capacitor) that provides power to the magnetron, and (3) a computerized control system that turns the power supply and magnetron on and off. A metal pipe connects the magnetron to the cooking chamber of the oven. While there are photographs and drawings of the insides of a microwave oven in my book, I can’t reproduce them here because of copyright issues.

I have heard that microwaving can destroy certain nutrient molecules in food, such as vitamins. Is this true? — D, Boulder, CO

A microwave oven heats the food it cooks; nothing more. If it damages nutrients, then it’s by overheating those nutrients. Such overheating could happen in a microwave oven if you don’t move the food about during cooking. That’s because the microwaves aren’t uniformly distributed in the cooking chamber and some parts of the food heat faster than others. Some parts of the food could become hotter than you intend and this overheating could damage sensitive molecules. However, I think that microwave cooking is probably less injurious to the food than conventional cooking. It’s pretty hard to burn food in a microwave!

Can plastic melt in a microwave oven? How does this process work? Can plastic burn in a microwave oven? – HD

Most plastics are unaffected by microwaves and do nothing at all in a microwave oven. For them to absorb energy from the microwaves, the plastics must either conduct electricity or their molecules must undergo the twisting motions that water molecules experience in the microwave oven. There are a few conducting plastics and these may melt or burn in a microwave as the microwave electric fields propel electric currents through them. There are also some plastics that trap water molecules and these may also melt or burn as the water molecules gather energy from the microwaves. I suppose that there are also a few plastics that have polar molecules in them that respond to the microwaves the way water does. However, most plastics do none of these and only melt or burn if they accidentally come in contact with very hot food or pieces of metal that happen to be in the microwave oven.

Does cooking in a microwave oven destroy the nutritional value of foods? Are microwaves radioactive? Does radiation “leak” from the oven? – DL

Microwaves are essentially high frequency radio waves. They heat food by twisting its water molecules back and forth so that those water molecules rub against one another. Like all electromagnetic waves, microwaves are absorbed and emitted as particles or “photons,” but the photons of microwaves have so little energy that they are unable to cause chemical changes in the molecules they encounter. They simply heat food; they don’t “irradiate” it. The only way a microwave oven damages the nutritional value of foods is if it overheats. Microwaves are not radioactive—radioactivity is the spontaneous fragmentation of the nuclei of atoms and is usually associated with the emission of high-energy particles; particles that can induce chemical changes in the molecules they encounter. Finally, if a microwave oven was properly constructed and hasn’t been damaged, virtually no microwaves leak from it. A small amount of microwaves won’t hurt you anyway—they are present all around us already because of satellite transmissions, cellular telephones, and even the thermal radiation from our surroundings.