How do I stop a pan boiling over?

Boil-overs are a foam problem, not a water problem — why starch, milk and fat cause them and the several things that stop it happening.

Difficulty: beginner
Time: 5 min
Read: 4 min
Safety: caution

Short answer

Water alone rarely boils over; starch, protein and fat make a foam that traps the steam. Use a much bigger pan than you think, keep the level below halfway, lower the heat as soon as it reaches the boil, and lay a wooden spoon across the rim or leave the lid slightly ajar. A splash of oil breaks the foam immediately.

The reason this happens with pasta, rice, potatoes, milk and jam and not with a pan of plain water is worth knowing, because it points straight at the fixes. Plain water boils and the bubbles burst. Starch, protein and fat make the bubble walls strong enough to hold, so the foam climbs.

Safety

A pan boiling over onto a gas burner can extinguish the flame and leave gas escaping, and onto a ceramic hob's touch controls it can make the hob behave unpredictably. Boiling starch or fat spilling down the outside of a pan also drips onto the hob and scalds anyone reaching for the handle. Never lean over a pan that is rising.

Step by step

Understand the mechanism.Starch dissolved from pasta, rice or potatoes, protein from milk and pulses, and fat all stabilise the bubble walls. Instead of bursting at the surface, the bubbles hold together and pile up into a foam that rises far above the liquid and goes over the rim, taking the liquid with it.
Use a bigger pan and fill it less.The simplest and most effective fix. Half full is a sensible maximum for anything starchy, and a third for milk and for jam, which foam far more violently. Most boil-overs are a pan that was the wrong size before the heat was even on.
Drop the heat the moment it reaches the boil.Full heat is needed to get to the boil and almost never after it. Once it is boiling, turn it down to the lowest setting that maintains the boil you need. This alone prevents most of them, and nothing cooks faster for being boiled harder.
Break the surface tension with oil.A teaspoon of oil or a knob of butter collapses the foam on contact — this is why it works and why it is the emergency fix when a pan is already rising. For pasta, purists object because oil coats the strands and stops sauce clinging; a very small amount does not.
Lay a wooden spoon across the rim.It genuinely works for moderate foaming. The dry wood is above the liquid temperature and the foam that touches it collapses. It will not stop a pan of milk that is properly running away, but it handles pasta and potatoes.
Leave the lid ajar or off.A sealed lid holds the steam and the foam together and directs everything at the rim. Off is best for pasta and potatoes; ajar is the compromise if you want to keep the heat in. For milk, always uncovered and always watched.
Stay with milk, jam and pulses.These are the three that go from calm to over the edge in seconds. Milk forms a skin that traps steam beneath it until it lifts the whole surface. Jam foams enormously as it reaches setting point. Dried pulses throw a heavy protein foam that should be skimmed off. None of them can be left.

Tips

Rinsing rice before cooking removes a lot of the loose surface starch, which reduces both the foaming and the stickiness.
Skim the grey foam off boiling pulses and stocks with a spoon. It is protein and impurities, it is what boils over, and removing it also gives a clearer result.
For milk, wet the inside of the pan with cold water before pouring the milk in. It reduces the film that sticks to the base and makes the whole thing less likely to catch and rise.
If it does go over, turn the heat off first and let it settle before touching the pan. On a gas hob check the flame is still lit; on a ceramic hob wipe the control panel dry.

If it doesn't work

Pasta water foams up the moment it boils

Cause: Too little water for the quantity of pasta and too much starch concentration — Fix: Use a bigger pan and more water, drop the heat to a steady rolling boil rather than a violent one, and add a small splash of oil if it is still climbing.

Milk boils over in seconds every time

Cause: The skin traps steam beneath it until it lifts — Fix: Heat milk over a lower heat, stir occasionally to break the skin, and never leave the pan. A wide pan with a lot of surface area is safer than a tall narrow one.

Potatoes boil over even in a large pan

Cause: Heavy starch release, especially from floury varieties — Fix: Start them in cold salted water, bring up to a gentle rather than a rolling boil, and cook them uncovered with the pan no more than half full.

Questions people ask

Does a wooden spoon across the pan really work?

For moderate foaming, yes — the foam touching dry wood above the liquid collapses. It is not a substitute for a big enough pan and it will not save a pan of milk.

Should I add oil to pasta water?

Only if it is boiling over. It does stop the foam, and it also coats the pasta slightly so sauce clings less well. A bigger pan and a lower heat solve the problem without the trade-off.

Why does the pan boil over when I put the lid on?

The lid keeps the heat in, so it boils harder, and it contains the foam until it forces its way out at the rim. Leave it ajar or take it off once boiling.