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Prevent Oatmeal from Boiling Over

By Cook's Illustrated Published March 2013

Does adding a pat of butter to oatmeal when cooked in the microwave prevent boilovers?

Update: July 2013

After this article was originally published, recommending the use of a large bowl when microwaving oatmeal to avoid boilovers, some of our readers have asked: "Why not just cook the oatmeal at a lower power?"

Oatmeal tends to overflow when the water boils and bubbles appear, while at the same time, the starches in the cereal swell and form a gel. This viscous gel makes it difficult for the bubbles to escape, causing the oatmeal to rise up and eventually spill over. Previously, we found that adding fat (we used butter) will coat the starches and prevent a strong gel from forming, but for the method to be successful, you have to use a lot of butter. We also tested laying chopsticks horizontally across the bowl, which didn’t work at all. Some readers suggested adding dried fruits or nuts to the oatmeal, but we found that the gel had no trouble forming around these add-ins, so spillovers still occurred.

The suggestion of lowering the power (and cooking longer) did actually work: The mixture boiled less vigorously, fewer bubbles formed, and the oatmeal never achieved the same height as the batch cooked at high power. But “low” power can vary significantly from microwave to microwave and might not be a guarantee that fewer bubbles will form. Therefore, we’re sticking with our previous recommendation: Simply use a larger bowl, which gives the oatmeal more space to expand.

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Here’s why oatmeal boils over: As the oatmeal cooks, the water boils and bubbles appear. At the same time, the starches in the cereal swell and uncoil to form a mesh. As this mesh thickens, it becomes more difficult for the escaping bubbles from the boiling water to burst through it. Eventually, so many bubbles form that the oatmeal rises up and spills over the sides of the bowl. This problem can also occur when oatmeal is cooked in a pot.

Hypothetically, fat works to prevent boilovers by coating some of the oatmeal starches, weakening the mesh so that air bubbles can escape. That said, we found that we needed a hefty 2½ tablespoons of butter to prevent a boilover in a single serving of oatmeal cooked in the microwave—and 10 tablespoons of butter when we cooked four servings in a pot. Not much of a remedy, in our opinion. Another recommended solution—laying a chopstick across the bowl in the microwave or a wooden spoon across the pot on the stove—didn’t work at all.

The best prevention when cooking oatmeal in a pot is what we recommend in our recipes: Stir the oatmeal a few times as it cooks, which breaks up the bubbles. The solution to boilovers in a microwave is even simpler—just use a larger, wider bowl.