Tag: microwave ovens

You claim that the metal walls of the cooking chamber in a microwave oven protect us from the microwaves. How can they protect us from microwaves when they aren’t even able to keep sound contained? You can hear popcorn popping through the walls. — RB, Beltsville, MD

The fact that sound waves can pass through the cooking chamber’s metal walls doesn’t mean that microwaves can. These two types of waves are very different and the chamber’s walls handle them very differently.

Any type of wave will partially reflect from a surface if passing through that surface causes the wave’s speed to change or, more generally, introduces a change in the “impedance” the wave experiences. Impedance is a quantity that relates various parts of a wave to one another—it relates pressure to velocity in sound and it relates the electric field to the magnetic field in a microwave. Since both sound waves and microwaves change speeds and impedances when they encounter the cooking chamber’s metal walls, they both partially reflect. The sound that you hear when popcorn pops inside the oven is slightly muffled because the sound is having some trouble escaping from the cooking chamber. However, the impedance change for the microwaves is so enormous that the reflection is complete. No microwaves at all escape from the cooking chamber! The same effect occurs when you hold a large mirror up in front of your face. You can hear what’s happening on the other side of the mirror because some sound can pass through the mirror. But light is completely reflected and you can’t see through the mirror at all.

Could microwave heating be used to treat sewage to wipe out disease organisms in it? — KO

While microwave heating could be used to sterilize sewage, it’s not the most energy efficient or inexpensive technique. Microwave heating is really only worthwhile in cases where you can’t reach the inside of an object directly—as is the case in most solid foods. Since sewage is essentially liquid, it can be heated quickly and efficiently by passing it close to a hot surface. Just about anything can be used to heat that surface—electricity, natural gas, coal, you name it.

But to be even more energy efficient, the sewage that was just sterilized a minute ago and is still hot can be used to heat the sewage that is about to be treated! A well designed thermal treatment facility could employ “counter-current exchange”—that is it could pass the hot, treated material through a heat exchanger to allow it to transfer most of its excess heat to the cooler, untreated material that is about to be sterilized. By recycling the heat in this manner, the facility could avoid having to burn so much fuel. The only drawback with this technique is that the heat exchanger must be leak-proof—it must keep the sterilized material from touching and being contaminated by the unsterilized material.

Does microwave cooking affect the nutritional value of food?

No more so than conventional heating does. Overheating some nutrients can damage them, so that microwave cooking does affect food’s nutritional value. But microwave cooking is far less likely to cause serious molecular damage to food than flame broiling or frying.

Is there a standard time that one should wait before eating food that has been heated in a microwave oven? – M

Apart from the usual precautions with hot food, there is nothing unsafe about food cooked in a microwave oven. You can eat it the instant the microwave oven turns off. The microwaves in the oven are absorbed so quickly that they vanish almost immediately after the oven stops producing them. By the time you get the oven door open, there is nothing hazardous left inside the cooking chamber or in the food. However, a microwave oven tends to heat foods unevenly, particularly if they were initially frozen. Thus you should be careful to stir the food or test its temperature at various places so that you don’t burn yourself. You should be particularly wary of solid foods, such as raisin biscuits, that are generally dry but have moist, microwave-absorbing objects inside them. Those moist objects can become dangerously hot and have been known to cause life-threatening burns in people who tried to swallow them without letting them cool off.

That said, a reader notes that the uneven cooking in a microwave oven can lead to bacterial safety problems—if parts of the food aren’t heated sufficiently to kill dangerous bacteria, then you could be exposing yourself to those bacteria. He suggests using the microwave oven for reheating only. He also notes that the lack of surface heating leaves the food relatively tasteless, as compared to more conventional cooking.

What about the effects of microwaves on the cellular structure of the item in the oven? I’ve heard that cells are ruptured violently by microwave radiation and that the ingestion of such materials affects the immune system. – AB

Just about any cooking damages the cells of the food being cooked, so microwave cooking is nothing unusual. Since our digestive systems destroy cells in the food we eat, cellular damage in cooking is inconsequential. As for the rumors about the unhealthiness of food cooked in a microwave oven, these are simply myths promulgated by people who don’t understand what microwaves are and fear them irrationally. The world was awash in microwaves from natural sources long before the developments of electricity and microwave ovens.

I recently place a green tomato in the microwave oven. I forgot to turn on the microwave and in the morning the tomato was ripe. Can you explain this? — KH

No. When a microwave oven is off, the cooking chamber contains nothing special at all—just some trapped air and perhaps a little light that enters through the window. Even when it is operating, a microwave oven never produces any ionizing (high energy) radiation so there are no long-term effects such as radioactivity present in the cooking chamber when the oven is off. The tomato was simply sitting in a sealed metal box overnight. Since some fruits ripen faster in sealed environments, perhaps that accounts for your observation.

When two identical items are cooked, one with a microwave oven and the other on the stove, which will cool faster? — CR

If the distributions of temperatures inside the items were the same after cooking, they would cool at the same rate. However, a microwave oven tends to cook relatively evenly throughout the food while the stove tends to cook from the outside of the food inward. That means that food cooked in a microwave oven tends to have more thermal energy near its center than food cooked on a stove, even when those foods contain the same total amount of thermal energy. Since foods lose heat through their surfaces, the extra thermal energy in the food cook by microwave will take longer to flow out to the surface of the food and from there to its surroundings. All else being equal, I would expect the food cooked in the microwave oven to cool slightly slower than the food cooked on the stovetop.

Assuming microwave ovens cook on the principle of “moist” heat cookery, what are the general effects of microwave cooking on various foods, including effects on chemical structure? — EJ, Sydney, Australia

Microwave ovens cook by depositing thermal energy in the water molecules, which isn’t the same as cooking food in moist hot air. Microwave cooking tends to heat food uniformly throughout where as more conventional “moist” heat cooking still heats food from the outside in. Nonetheless, the chemical effects on food are very similar for both types of cooking. Virtually all of these effects are caused by elevating the temperatures of the food. I’m not an expert on the chemistry of cooking, but elevated temperatures certainly denature proteins and caramelize sugars.

I know that microwaves only heat polar molecules but what about aluminum foil and graphitic carbon, which are both heated by microwaves even though they have no dipole moments? — EB

Aluminum foil and graphitic carbon are both conductors of electricity. When they’re exposed to microwaves, the electric fields in those microwaves causes currents to flow through them. If the aluminum were thick enough, it would be able to handle the currents without trouble. But aluminum is very thin and the current that flows through it may be more than it can tolerate, particularly if it’s only a narrow strip. It then becomes very hot. The effect is the same as would happen if you plugged the aluminum foil into an electric outlet and sent current through it that way. The same heating occurs in the carbon—the current that flows in it heats it up. In short, relatively poor conductors of electricity become hot in a microwave because they permit currents to flow in response to the microwave electric fields but then can’t tolerate those currents without becoming hot.

Do hand carried microwave heaters exist or must the microwaves always be enclosed, as they are in a microwave oven? — AL, Umea, Sweden

My understanding is that there are microwave heating systems that are not enclosed and that are used in medical therapies to provide deep warming to injured tissues in medical patients. But apart from such devices, I’ve never heard of unenclosed microwave heaters. That’s because such heaters would be dangerous, since a user would be exposed to the heating effects of the microwaves. To keep the microwave heating under control, microwave ovens always carefully enclose the microwaves in a metal cooking chamber from which they can’t escape.