Why do microwave ovens cook so rapidly?

When you put solid food (a potato, not soup) into a conventional oven, the heat flows slowly into the center of that food. This heat must work its way into the food via thermal conduction, in which adjacent atoms and molecules transfer their motional energies in a long bucket-brigade process. The last part of a potato to become hot is its center. However, in a microwave oven, the microwaves travel well into the solid food and deposit their energy everywhere. The potato cooks throughout at a relatively even rate. The actual amount of heat and energy involved in conventional and microwave cooking is about the same. However, the microwaves can heat the food throughout without having to wait for the slow process of conduction to carry it inward from the food’s surface.

What happens if you start the microwave oven with nothing inside?

The magnetron creates microwaves that travel into the cooking chamber and should be absorbed there. If there is no food (or rather no water-containing food), those microwaves will not be absorbed and will eventually find their way back to the magnetron. Eventually the magnetron will absorb as many microwaves as it emits. This situation is hard on the magnetron, which works best when it has very little radiation returning to it. That’s why you should never run a microwave empty for more than a second or two.

What exactly goes on when you’re cooking a potato in the microwave and it explodes?

A microwave oven heats food by depositing energy in its water. If you cook the food long enough, that water can begin to boil. If the food has a hard outer shell (e.g. a potato or a corn kernel), the boiling water can create enough pressure in the food to make it explode. That is what pops the corn in microwave popcorn and why the potato explodes if you don’t pierce it so that steam can escape.

What containers are not safe to use in a microwave? I am particularly concerned about Styrofoam containers as I use them to make TV dinners for my family. Is it OK to heat directly in these containers?

The two critical issues with containers in a microwave are (1) that they do not absorb or reflect microwaves and (2) that they tolerate high temperatures. Concerning the first issue, a container that absorbs microwaves will become extremely hot and may be damaged or destroyed. Most plastics (including Styrofoam) don’t absorb microwaves and are fine. Glazed water-free ceramics and glasses are usually also fine, as long as they don’t have any metallic trim. Metal dishes are a poor choice because they reflect microwaves and lead to uneven heating. Unglazed ceramics absorb water and will overheat.

Concerning the second issue, many plastics melt or soften below the temperature of boiling water. Polystyrene, the plastic from which Styrofoam is made, has a glass transition temperature of almost exactly 212° Fahrenheit (100° Celsius). That means that it will begin to soften at just about the temperature of boiling water. While pure water will boil without much problem in Styrofoam, water containing dissolved solids such as sugar or salt will boil at a higher temperature and may melt the Styrofoam. You’ll know when this happens…it’s not really a health issue, just a potential for a messy oven. I’ve only encountered the problem once myself, when a Polystyrene gravy separator melted in the microwave and let the gravy spill.

On the subject of defrosting frozen food in a microwave oven, you must refer to the old BTU formula which states “It takes one BTU to raise the temperature of 1 pound of water 1° (Fahrenheit), but when water is changing state from a solid (ice) to a liquid (water), it must absorb 144 BTUs (per pound).” – George R.

This observation accounts for much of difficulty with defrosting food in general and defrosting food in a microwave oven in particular. It often takes more heat to melt ice in the food than it does to actually cook the food once the ice has melted. Since ice doesn’t absorb microwaves well, heating frozen foods in a microwave oven is a tricky business. Any region of food that melts early will absorb microwaves strongly and overheat while any region of food that remains frozen won’t absorb microwaves well and won’t receive the enormous amounts of heat it needs just to melt. The result is typically a food item with some frozen parts and some boiling hot parts. To avoid this problem, microwave oven defrost cycles let the food sit in between bursts of microwave heating. That way, there is time for heat to flow through the food and keep the internal temperatures relatively uniform. Parts of the food that heat well have time to transfer heat to parts that don’t heat well and the whole item thaws and heats together.

Inside the microwave oven, what is it that heats the food? How does the heat come out; where did it come from?

The food is heated by the microwaves themselves and these microwaves are piped into the cooking chamber from the magnetron. The magnetron has electric charge sloshing back and forth in its tines. A small antenna uses that sloshing charge to emit microwave radiation. The water molecules in the food absorb this microwave radiation and turn its energy into heat. The usual rules of heat transfer don’t apply in the heating process—the energy arrives at the food as microwaves, not heat.

In microwaves – you heat up food really fast. Is it true that microwaved food will cool down faster than oven heated food? Someone told me “if it heats fast, it will then cool fast.”

No. Microwaves cook the food in a very different manner than normal thermal heating, but microwaved food has the same thermal energy that it would have if it had been warmed by more traditional methods. Microwaves heat food by exerting torques on the individual water molecules in the food. These molecules jiggle back and forth and sliding friction between them heats the food. This peculiar route to energy addition explains why frozen portions of the food don’t heat well: the water molecules are rigidly oriented and can’t jiggle back and forth in order to become hot. But despite the fancy heating scheme, the food retains no memory of how it was heated. Once it is uniformly hot, it cools at a rate that depends only on how heat is transported out of it. Microwaved food cools just as slowly as normally cooked food.

If a radio station operated at 2.45 gigahertz, could you pick it up when your microwave was turned on and attached speakers?

If some radio station were to operate at 2.45 gigahertz, the main effect would be very poor reception of that channel on your radio. The oven isn’t a transmitter for microwaves; it just makes them like crazy. Most of the microwaves never leave the cooking chamber and there are strict regulations on any leakage. But it would only take a few thousandths of a watt of leaking microwave power to cause trouble in your reception of the radio station. Your radio wouldn’t be able to distinguish that station’s transmission from microwaves leaking out of your oven. The radio would struggle to pick up the signal and you would probably hear lots of noise in the background.

If a microwave does not melt ice, how does the “Defrost” setting on the microwave work?

I’ve already noted the issues of warming frozen food. However, the “defrost” setting is an interesting issue. If you’ve ever watched a microwave trying to defrost food, you’ve probably noticed that it heats the food briefly and then waits. It repeats this process many times. What it is doing is depositing energy (via the microwaves) into whatever water molecules are able to absorb microwaves. It then waits for this energy to flow as heat into the nearby food. Once the heat has been distributed rather evenly, the oven adds some more energy by turning the magnetron back on. This cycle of heating and waiting allows the food to defrost fairly evenly. Still, microwaves are likely to create hot and cold regions in the food so that some parts of the food will cook rather than defrost while some parts remain frozen.

I’d heard that if I cook in the microwave oven, there will be a possible formation of free radicals. Is it true? If yes, how? — Angela I.

It’s doubtful that microwave cooking forms free radicals in food. The microwaves in a microwave oven cook by exerting torques on the water molecules and gradually increasing the water molecules’ thermal energies through friction-like effects. There is never enough energy present in a single molecule at one time to shatter that molecule and form a free radical. While ultraviolet light, such as that found in sunlight, carries enough energy per photon (particle of light) to split a molecule and form a free radical, microwave radiation carries very little energy per photon. That’s why microwave photons can’t do chemical damage the way ultraviolet photons can. However, even if microwave radiation could form free radicals in food, that wouldn’t necessarily cause you trouble when you eat that food. So much happens to the food before it enters your blood stream that a free radical probably won’t survive. The more harmful free radicals are ones that are actually created inside your body, where they can immediately attack important molecules in your cells.