7 replies to this topic

[MrShorty]

I hope this isn't too out of line here, but I don't know much of anything about foaming, other than it can occur in distillation towers and cause problems if not properly dealt with. Can we ask non-engineering related question??

I was cooking some noodles for my daughter and, like many times before, the pot boiled over. Like many times, I observed that the pot only boils over if I put the lid on the pot. If I boil the noodles with the lid off, it doesn't foam up as bad. So I asked myself, "Why does putting the lid on a pot of boiling pasta give it a greater tendency to foam up?"

I know it's kind of an unimportant question, but I'm quite curious if we can come up with a good "engineering" type explanation for why this happens. (Who knows, the answer might even extend to explaining foaming in engineering applications).

[djack77494]

MrShorty,
You raise an interesting question that I can only speculate about. Since I find it intriging, I offer the following speculation:

1) The mildly elevated pressure inside a container may somehow encourage the growth of larger bubbles.
2) The large bubbles then transport more liquid water higher in the vessel.
3) This water gets pushed around by the turbulence present, and a fraction is ejected from the container.

Alternately, the slightly elevated pressure may lead to additional dissolution of pasta constituents that affect the surface tension in such a way as to generate larger bubbles, again leading to the above results.

Another explanation could be greater violence when the bubbles burst. This could again be due to the mildly elevated pressure inside the container which increases the vapor density. The increased momentum leads to a greater liklihood of ejection.

Still another explanation is the contained vapors in the closed container impact the surface tension in such a way as to generate larger bubbles, still again leading to the above results.

Interesting isn't it?

Be careful what you ask (an engineer) for,
Doug ;-)

[gvdlans]

Since I like cooking, I had ample opportunity to think this over...

I would say proteins from the pasta act as a surfactant. With the lid on the pot, water vaporization rate is lower than with the lid off. Reason is that steam condenses at the lid and the liquid droplets falling back into the water provide cooling to the water. With the lid off, most of the steam will be removed from the pan so there is no cooling effect. As a result of this, with other conditions (e.g. heat supply, amount of water you started with) the same, pan with lid off will boil dry much faster than a pan with the lid on.

With the higher vaporization rate, the foam bubbles will collapse so there is no stable foam and no boil over.

I do not think the slightly higher pressure inside the pan with lid on will have any effect. This pressure rise will be very small (weight of the lid divided by the area of the lid, I would say about 1 mbar).

[MrShorty]

Thanks for your thoughts and ideas.

As I've thought about it, I've been wondering if it isn't partially due to heat balance DQ=Q(in)-Q(out). Q(in), of course, is from the stove. Q(out) is a result of conduction, radiation, and how much steam escapes. Because water has such a high heat of vaporization, I would expect that most of Q(out) (especially with the lid off) is contributable to the ecaping steam. As noted, with the lid on, less steam escapes, but rather condenses, releasing the heat of vaporization to the lid.

I'm still interested in more comments, if they are still out there.

[gvdlans]

John ng.

Good post! Probably biotechnology is a better background than Chemical Engineering when it comes to boiling pasta...

You wrote "There is hardly any protein in pasta. I had no idea what that posting was on about. " You could be right there, but since pasta is made by mixing flour and eggs (and water and salt), there must be some proteins in there. According to http://food.oregonst...p/compasta.html pasta contains about 5 to 12 weight percent proteins.

Anyway, you don't need a high concentration of surfactants to make a foam.

[chemsep05]

there is a pressure increase with the lid on, also there is a temperature increase since less of the heat energy is given off to the atmosphere, usually it suggests on the pasta box to boil with the lid off for these reasons

[gvdlans]

chemsep05,

As I wrote in a previous post "I do not think the slightly higher pressure inside the pan with lid on will have any effect. This pressure rise will be very small (weight of the lid divided by the area of the lid, I would say about 1 mbar). "

The temperature increase is directly linked to the pressure increase via the vapor pressure curve (we have boiling water remember...), and is therefore not related to "less of the heat energy being given off to the atmosphere".