Do Microwaves Cook Food From The Inside Out?

Posted by Eric Troy on 18 Feb 2014 16:21

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Ever since the invention of the microwave oven, there have been a number of myths regarding how microwaves cook and how the affect the food. One such myth really stands out because it seems so incredible. It is, in fact, one of the 'features' of microwave cooking that made people want to buy one when they first came on the market. Many, many people believe that microwaves cook food from the inside out. They have been told that the microwaves penetrate the food to the center, and beginning exciting the water molecules to produce heat, and then this heat moves outwards to cook the rest of the food. Or Something like that? Is it true? No, it is not even remotely true. Microwaves cook food from the outside in, just like a regular oven. In fact, most of the cooking on the inside of the food, depending on its thickness, is done by heat conduction from the outside surfaces inwards, as the microwaves do not actually penetrate that far into the food.

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There must be a reason for this myth that microwaves cook food from the inside out. First of all, if you really think about it, the idea of being able to cook anything starting with the inside first, seems a bit magical. Well, microwaves, when they first hit the market, did seem quite magical. After all, people were being told their food was cooked by invisible rays and that these waves passed right through containers. As well, the air in the microwave oven was not really heated (other than heat coming off the food itself). And besides all that, stuff that takes minutes to cook by conventional means takes only seconds in the microwave, and stuff that takes an hour takes only minutes. If you were one of the first customers to have bought a microwave oven, you would have thought it was proof that the world of the Jetsons was on its way, and would be willing to believe all sorts of amazing things about the device.

If you've ever cooked meat in the microwave, was it ever fully-cooked on the inside but still rare on the outside? I thought not! So what's the science?


The inside is well-done but the outside is rare!


The inside is well-done but the outside is rare!

Well, the science has to do with water, and how microwave energy interacts with it. If you put a porcelain or plastic container of water in a microwave, the waves pass right through the container and into the water. When microwaves hit water molecules they make them rotate more quickly (they already have some rotational energy). This is because microwave energy only interacts with molecules which have polarity, which means they have a negative and positive end. Water molecules are polar molecules. When microwaves pass by the molecules of water, they rotate a lot more quickly. This causes them to gain energy and the byproduct is heat. And a lot of it. So, the water boils. It seems, therefore, that only the "inside" of the container is being heated. And essentially, that's true.

And microwaves pass through more than just porcelain or plastic. They pass through glass. You've probably seen that at work. And paper. If you've cooked a microwave pot pie or a Hot Pocket, you've seen that at work. Microwaves also interact with fats to some extent, but it is the water that is the crucial part of the microwave cooking process. The more water there is in the mix, the more microwaves are absorbed by the water, the molecules gain energy, and heat is produced. So, essentially, the more water present in a food, the more molecules spinning around, and the more heat produced.

Once you realize that energy is not passing through water, but being absorbed by it, you are on to something. For, every centimeter of moist food that the microwaves contact, there is less energy for the next centimeter. The energy doesn't go very far at all before it's all absorbed. Only the very outer layers of the food actually get heated by the interaction with the microwaves. The rest of the cooking is actually due to heat conduction from the outside to the inside. It is pretty easy to test the myth directly for yourself. Especially if you have a meat thermometer. Get a nice cold roast, place it in the microwave, and begin cooking it on high. Cook for about five minutes. Compare the temperature of the outside layer of the roast to the center. The outside layer will be warm, if not hot. The center will still be cold. Myth busted (the Mythbusters, by the way, did test this myth, and they indeed busted it).

When you cook a pot pie in the microwave, you will notice that, indeed, the inside part get's hot a lot faster than the outside pastry. It seems cold on the outside but you stick your finger in and it's pretty hot! The pastry has very little water compared to the filling inside the pie. So the inside is going to heat up faster. However, lest you think that pie is cooking from the center out, stick you finger in a little further, and you'll notice it's still ice cold.

The idea that microwaves cook from the inside out, then, is based on an observation, but a faulty one. To form the myth, we went from observation, to hypothesis, to "scientific fact" without anyone really setting up any controlled experiments to test the hypothesis. At least nobody who did a good job informing the rest of the world. The myth, after all, has stuck around even until today, when only a relative few seem to know it is not true.

Microwaves Cook Foods More Uniformly?

A related myth to the myth that microwaves cook from the inside out, is the myth that microwaves cook foods more uniformly. In other words, this myth sidesteps the obvious magical thinking of cooking from the inside out, by replacing it with something that sounds more proofy, the word uniform. That is a nice, professional sounding word, ain't it? Too bad it's even more magical than inside out. Cooking a food uniformly implies that all parts of the food gain heat at the same rate. Since you've read thus far, you can see that this is as impossible as the former myth.

The uniform cooking myth seems to be touted by armchair science geeks who have never actually cooked in a microwave. Because if you've ever cooked in a microwave, you know that it does NOT heat food uniformly. Even once you've thoroughly microwaved something, there are cooler and hotter spots.

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