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Oatmeal Boils Over

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On one of my other blogs, A Year From Scratch, I put together a simple make your own microwavable oatmeal packets article. After reading this, Julie K. asked via Twitter,


@thefoodgeek: Why does oatmeal boil over so often in microwave? (I eat it with Cayenne)

Hi, Julie,

It is not the cayenne that causes your boil over, though that sounds like a fantastic addition to a morning treat. Even if you had just water and oatmeal, your bowl is likely to boil over. The reason is in the starch.

Starches are great for lots of things, especially thickening sauces. Flour and cornstarch are two popular starches used for thickening liquids. When you heat starches in water, the starch acts like the earbuds for your iPod or phone. Properly coiled, the cord for the earbuds take up no space and sit neatly in your drawer. But if you just toss them into your pocket with your keys and some change, soon you will not be able to remove a quarter without discovering that everything in your pocket is all tied together with the earbud cord. The starch molecules expand into ropes when heated in water and get in between the water molecules and whatever else you are thickening. This is handy when you do want to make a béchamel, pudding, or gravy, but not necessarily what you want in other circumstances.

It is popular, when making pasta, to either boil the pasta in a very large pot with a lot of extra water, or to put some oil in the pot with the pasta and water. The reason you do this is because if you use a smaller amount of water, you draw out enough starch from the flour in the pasta to start thickening the water.  Because your pasta is always boiling, it forces bubbles into the starchy liquid, and that is able to build fantastic structures of starch and water (aka foam) that can overflow the pasta pot.

The reason why you would put oil into the pasta pot (though I don’t necessarily recommend it) is because the oil keeps the bubble structures from forming properly. It won’t keep the starch from thickening your liquid, but it insinuates itself in between the starch molecules and the water, or between different starch molecules, and makes the prevents the complex structures from forming. Consequently, you don’t get boil-over. (Of course, the drawback to this method is that you can end up with slippery pasta that doesn’t adhere properly to its sauce–so best to stick with a large pot of plenty of water.)

As you may have guessed, the same thing is going on in the oatmeal bowl. There is lots of starch in the rolled oats, and you only use enough water to cook the oatmeal properly, unlike pasta where you drain it after you are finished cooking. Part of the creamy texture of the oatmeal comes from the starches thickening it up when you cook. But, when it’s in a microwave, you have little control over what happens inside, and you are usually using a bowl that’s just about the size of what you need it to be for the oatmeal. Hence the boil-overs.

Your options are to use larger bowls, which will contain the boiling a bit better, or to put some butter on the top of the oatmeal before you cook it. The latter has the added benefit of being delicious, and it’s a really good excuse for why you need to add some butter to the oatmeal.



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