Microwaves Revisited

We received the following information in an email from a Chronicle subscriber regarding microwaves:

Please do not take this email as a criticism of the author. I think Ms. Mefford is doing a great job and should be encouraged. I would like to address what I think is an inaccurate statement in her column, “How does a microwave cook food,” in the Dec. 30 issue. She states, “Microwave ovens cook food from the inside out.” This is incorrect. As with any other heating source, a microwave oven cooks food in the same way as any other heat source. It does not cook food any differently than, say, a gas oven. The difference is, it heats the water molecules to an excited state, causing them to vibrate, which produces heat.

The reason it cooks faster is that it causes the molecules to react quicker, i.e., excite them faster than conventional heat does. With most foods, you are heating the water contained in the food, which causes it to heat very quickly. You can prove this by placing a roast in the microwave oven, turning it on, and then testing the heat temperature in the center of the meat as opposed to the outer temperature. The outer temperature will be higher, because the waves have not penetrated as deeply, thus heating the water molecules that are closer to the surface at a faster rate.

                                            Stephen Gainer, Attleboro, MA

In the sprit of science, I investigated and found that I had made a mistake. I stated that microwaves cook food from the inside out. This is not a completely accurate statement. Let’s start briefly with what happens inside a microwave oven.

The microwave oven is basically a “metal box.” The metal mesh is what keeps the microwave radiation from escaping. A special type of vacuum tube emits microwave radiation into the metal mesh, where it bounces around until it is absorbed by your food and converted to heat.

I often heat food in the microwave. When I hear the “beep,” I get my food. I get situated at the table, take a big bite of my food, and “yuck,” it is still cold in the middle. So back it goes into the microwave until I’m sure it has been heated through.

While microwave radiation does penetrate the surface of food and start to heat the inside at roughly the same time as the surface, it is not necessarily accurate to say the food is cooked from the inside out. Microwaves heat food by being absorbed by primarily liquid water molecules. If the food is uniform in consistency and contains a lot of water, the water molecules near the surface of the food heat up first since there is less material between the surface and the microwaves. In this way, the food really heats from the outside in, like a traditional oven.

If the surface of the food is drier than the center, like a baked potato, the center will heat up faster. You can see this in microwave foods with a dry crust and moist filling, like a McDonald’s apple pie. If you cook it for 20 seconds in the microwave, the crust would be cool to warm to the touch, while the filling can be quite hot.

Because of this characteristic, microwave cooking instructions for many foods give you a “standing time,” after removing the food from the microwave. This allows the heat to diffuse throughout the entire food, so that a uniform temperature is reached before serving.