Why a Pot of Pasta Boils Over When a Pot of Water Never Does

Put a pot of water on the stove and let it boil, and it will bubble away for as long as you like without making a mess. Drop in a handful of spaghetti, turn round to find the colander, and by the time you turn back a white foam is climbing up the side of the pot and spilling over onto the hob. It is the same water on the same flame. The only thing that changed is the pasta.

The first thing the pasta does is give the bubbles somewhere to start. Bubbles in boiling water do not appear just anywhere; they form on surfaces, on tiny scratches and specks where steam can gather. Tipping in a pile of pasta suddenly adds a great deal of new surface, so a lot of bubbles form all at once and the pot surges. That is why a pot often froths up the moment the pasta goes in.

The second thing the pasta does is more important, and it is all about starch. Pasta is made mostly of flour, which means mostly starch. In hot water the tiny grains of starch on the surface of the pasta soak up water, swell and burst, and some of that starch escapes into the cooking water. You can see it happening: the water turns slightly cloudy.

Starch in the water changes the bubbles. In plain water, a bubble rises, reaches the top and pops almost at once, letting its steam escape. With starch dissolved in it, the thin skin of each bubble becomes stretchier and tougher, so bubbles reach the surface and simply sit there instead of bursting. They pile up into a foam, a sort of blanket across the top of the pot.

Now the trouble begins. Steam is still being made at the bottom of the pot, and it has to get out. It pushes up under the foam, the foam cannot pop fast enough to let it through, and so the whole blanket is lifted higher and higher until it goes over the rim. A lid makes it worse, because it keeps more heat in and gives the steam less room to escape.

The cures all follow from the same idea. Leave the lid off, or turn the heat down to a gentle boil, so less steam pushes from below. Use a bigger pot, so the foam has further to climb. Or lay a wooden spoon across the top: the bubbles are unstable, and when a rising foam touches the dry spoon, the bubbles that meet it burst and the foam sinks back. Salt, sadly, does nothing to stop the froth, although it does improve the taste. A splash of oil can calm the foam, but it also coats the pasta, and then the sauce slides off it.

So the next time a pot tries to escape from the stove, it is not really boiling harder than usual. It has just been given a blanket of starch bubbles, and the steam underneath wants out.