Pasta pot boils over...

the froth is starch.

It doesn’t matter why. Leave the lid off and move the pot slightly off the burner after the rolling boil resumes. The starchy, hard to clean up, froth will come, the stove will suffer, even with the lid off.

Add some olive oil and it will stop doing that.

I appreciate all the advice, but I know how to keep the starchy froth down. The question is more of a scientific inquiry into why it comes up so fast only when the lid is on.

Pressure. When you put the lid on, it adds pressure which increases the heat and the pot boils faster, and voila: starchy overflow!

Interesting all the different solutions. I just turn down the heat.

adding pressure would make the pot boil at the same “speed”, but a higher temperature. I think we’re onto something here though, hotter water = more froth in the starch?

I think it’s hotter water = boiling harder; this will happen without any pasta in there too.

Water doesn’t get hotter just because it is boiling faster.

If you put the lid on, the pressure inside the pot goes up somewhat. Water has a higher boiling point at higher pressure, so the boiling water could get hotter (this is the concept behind pressure cookers. Still don’t know what (if) this has to do with frothiness

A pot with the lid off allows heat to escape more quickly. The lid keeps the heat in and encourages what might be just a simmer into a full-on boil.

I’ve had the crazy frothy boil with the lid off, too, but I had to have the fire much higher to do so.

As a side question, does anyone else blow on the water when it looks like it’s going to be a froth explosion? Or is that just me?

No, I do too. It works! I think it breaks the surface tension of the froth, as well as helping some of the heat escape. I just heard of a guy who installed a fan on top of his beer-brewing pot to avoid having the grains boil over, too.

i think that you’re barking up the wrong physical science tree. i think there’s something in there to do with the volume of steam, and perhaps condensation, that allows the starch to “climb” up the sides. still working on this one.

So think of this. Water turns to steam at 212. It boils and is not going to be water. So of course the steam=pressure. i.e the steam engine gets a-going because of this phenomenon. So a lid will build pressure but won’t cause the water to get hotter. You’ll get a build up of steam, ie the pressure cooker. So yes your lid off allows the escape of the steam. Lid on and the steam underpressure and the water in the rolling boil will cause whatever is in there to want to lift the lid and overflow. How’s that. So the people who say put a little oil in…etc. This will slow down everything because oil has a higher boiling point. Here is a chart I copied from another web site:Safflower – 510 F (266 C)
Soybean – 495 F (257 C)
Corn – 475 F (246 C)
Peanut – 440 F (227 C)
Sesame – 420 F (216 C)
Olive – 375 F (191 C)
The oil will smoke but not boil and the water around it will boil. We could go further..i.e. do the oil molecules bind to the pasta molecules and keep the starch at bay? Obviously, water alone is allowing the starches to sluff off the pasta and be available for tossing and bouncing upward as the water turns to steam. Your turn.

In a open pot when a liquid boils, it forms hot gas bubbles and floats to the top. When the bubbles hits the surface it is exposed to the cooler temperature and contracts to tiny bubbles.
In a covered pot, boiling temperature in the pot is retained therefor the bubbles don’t contract when it hits the surface. Which will rapidly fill the pot and boil over.