Do Microwaves Cook Food From The Inside Out?
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Posted by Eric Troy on 18 Feb 2014 16:21

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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 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 devise. Still, there had to be some science behind the myth.

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. Microwaves, when they hit water molecules 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.


Early Radarange by Raytheon, installed NS Savannah, Baltimore, Maryland, USA
image by Acroterion via wikipedia


Early Radarange by Raytheon, installed NS Savannah, Baltimore, Maryland, USA
image by Acroterion via wikipedia

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.

Invention the the Microwave Oven

Probably, the inside out myth started at the very beginning of the microwave oven industry, based on the observations of Dr. Percy L. Spencer of the Raytheon Corporation. The company had worked on Radar during World War II, but in 1946, Spencer, more or less by accident, hit on the idea of cooking food with microwave energy. He had been testing a magnetron, which is the device that emits the microwaves. Taking a snack break, he noticed a chocolate bar in his pocket had melted, even though the day was cool. Thinking that maybe it had to do with the magnetron, he aimed it at some pop corn kernels. They popped. Then he tried a raw egg. It exploded. Why did the egg explode? Steam pressure built up within its shell. If steam pressure could build up within the confined shell of an egg so quickly that it exploded, what does this mean? It must mean that the microwaves are cooking the inside part first.

The RadaRange

The Raytheon Corporation, after this discovery, developed the first microwave oven, the RadaRange.1 It was not very practical, by the way, being the size of a refrigerator and weighting over 700 pounds, but only being able to cook the same amount as modern microwaves ovens. But they soon improved on the design, i.e. the size and the boom was on. Later, the company found that microwaves pass through all sorts of materials. When the waves pass through, they don't do anything. They don't affect the material. But when the waves are absorbed, as in the case of water, we get heat.

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).

Microwave Uniform Cooking Myth

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.

Now, I mentioned that microwaves pass through many of the materials we cook foods in or on. Yet, if you are an experienced microwave user, you know that not all containers or dishes are microwave safe. First, certain plastics, like foam containers, simply melt easily from the heat of the food. You don't have to be a magnetron scientist to know that foam is not a good cooking vessel, though. You also know that you can't put metal in a microwave (the reason for that deserves a separate post). Many plastics do fine in the microwave. Many ceramics do as well. You know that most ceramics will eventually get hot from the food. But some ceramics seem to get scorching hot from the microwave itself. You've probably tried to heat water in a coffee mug, only to burn your hand on the red hot handle! Yet, in your Pyrex measuring cup, the handle doesn't get hot. I personally, have a couple of mugs that are so not microwave safe that the mug gets "red hot" while the water is barely warm. Not good. So, our mugs are not really microwave safe. It seems that those microwaves are not passing through the ceramic of that mug but instead are interacting with it and causing it to get hot.

There certainly isn't any water in ceramics to absorb microwave energy. However, there are some minerals that do absorb microwaves. If a ceramic has enough of these minerals, they will get very hot and so the container will get very hot. Since the microwaves are essentially only reaching the first few centimeters into a highly absorptive material, it is possible, in some cases, for the container to become hot enough to cause a serious skin burn before the water or food inside gets hot at all. Crystals may not be good in the microwave for this same reason, and they also might break from the heat stress. Also, if you have a glazed cup or other ceramic container with cracked glazing, you may want to avoid using it in the microwave, even if it had always been find before. Water from washing could have gotten into the ceramic through the cracks and this water will heat and turn to steam, possible causing the container to break. Microwaves pass through plastics, but of course, as before, some plastics will melt from the heat of the food. There are all sorts of other microwave cautions but, but since this post is about the basic science of the cooking from the inside out myth, I'll finish with only one related caution, which has to do with these so-called microwave safe dishes.

Beware of Microwave Safe Dishes

There is something you need to know about dishes which claim to be microwave safe. That is, the fact that a dish is microwave safe does not automatically mean you won't get burned from it. Microwave safe means that the microwaves themselves will not be absorbed by the material of the dish. They will pass right through it. So, for example, in a microwave safe mug, you can heat up your water for instant coffee or hot cocoa and the handle will probably stay cool long enough for you to grab it safely (although you need to be careful not to slosh yourself, and put in the instant coffee or cocoa slowly). However, if you touch the sides of this microwave safe coffee mug, instead of the handle, it will be hot. Perhaps very hot. You know better than to grasp it by the sides because the hot water has heated the sides. Well, this same thing goes for any microwave safe dish. It can still get hot from the food inside. The better the material is at absorbing and retaining heat in general, the quicker it will heat up. Any large ceramic dish filled with food, even if it has handles and is microwave safe, will need to be handled carefully with pot holders, as by the time the food is cooked, the dish, and probably the handles as well will be hot (through heat conduction). So, the basic warning is: Microwave safe dishes can still burn you.

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