Category: microwave ovens

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.

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.

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.

Are microwaves distributed unevenly in the oven? Why do manufacturers claim that microwaves with turntables are more effective than microwaves without turntables?

As the microwaves bounce around the inside of the cooking chamber, they tend to interfere with one another. There are usually regions in which the waves that follow various paths almost cancel one another and regions in which the waves reinforce one another. These regions don’t cook food equally well. If the microwaves are canceled in one region, cooking will be slow there. If the microwaves reinforce one another in another region, cooking will be fast there. If you simply leave food in one place and try to cook it in the microwaves, the cooking will be uneven. However, if the food is rotated continuously, these good and bad cooking regions will be blurred away so that the food will all cook at about the same speed.

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.

Are microwaves harmful to you? Is eating microwaved food harmful?

Microwaves can heat your body by adding thermal energy to the water molecules in you. This heating can be damaging if it’s not controlled. Most of your body is protected from slow heating because your blood carries heat away from any local hot spots so that you warm evenly. However there are a few places that aren’t cooled by your circulation and can heat up locally enough to denature the protein molecules and cause biological injury. The cornea of your eye is a good example. It can be heated and damaged because it’s not cooled well. That’s why you must be careful not to look into a strong beam of microwaves. As for microwaved food, the only effect of cooking with microwaves is hot food. There is no “radiation damage” or “radioactivity,” as there might be with x-ray or gamma radiation. Some foods should not be cooked in a microwave only because the uneven heating may allow certain parts to become too hot. Those parts may burn you when you eat them or they may suffer thermal damage that diminishes their nutritional value.

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.

Why does water react in a violent and dangerous way when overheated in a microwave oven? CA

Water doesn’t always boil when it is heated above its normal boiling temperature (100 °C or 212 °F). The only thing that is certain is that above that temperature, a steam bubble that forms inside the body of the liquid will be able to withstand the crushing effects of atmospheric pressure. If no bubbles form, then boiling will simply remain a possibility, not a reality. Something has to trigger the formation of steam bubbles, a process known as “nucleation.” If there is no nucleation of steam bubbles, there will be no boiling and therefore no effective limit to how hot the water can become.

Nucleation usually occurs at hot spots during stovetop cooking or at defects in the surfaces of cooking vessels. Glass containers have few or no such defects. When you cook water in a smooth glass container, using a microwave oven, it is quite possible that there will be no nucleation on the walls of the container and the water will superheat. This situation becomes even worse if the top surface of the water is “sealed” by a thin layer of oil or fat so that evaporation can’t occur, either. Superheated water is extremely dangerous and people have been severely injured by such water. All it takes is some trigger to create the first bubble-a fork or spoon opening up the inner surface of the water or striking the bottom of the container-and an explosion follows. I recently filmed such explosions in my own microwave (low-quality movie (749KB), medium-quality movie (5.5MB)), or high-quality movie (16.2MB)). As you’ll hear in my flustered remarks after “Experiment 13,” I was a bit shaken up by the ferocity of the explosion I had triggered, despite every expectation that it would occur. After that surprise, you’ll notice that I became much more concerned about yanking my hand out of the oven before the fork reached the water. I recommend against trying this dangerous experiment, but if you must, be extremely careful and don’t superheat more than a few ounces of water. You can easily get burned or worse. For a reader’s story about a burn he received from superheated water in a microwave, touch here.

Here is a sequence of images from the movie of my experiment, taken 1/30th of a second apart: