Boiling milk.

I have just had to clean my effing stove top after a pot of milk boiled over (it wasn’t even all milk. 2cups water, 1cup milk). Stupid milk!

Why doesn’t milk boil like most other liquids, why the froth and foam thing? Is it the fat content?

Why when the pot was only a third milk, did it still react as strongly as if it were all milk?

Ahhh. The science that is boiling. No kidding. I remember a lecturer of thermodynamics (very trendy young German guy) going all glassy-eyed and excited as he described the change of phase of water. He tried to tell us that it was very exciting. (It was about that time that I deciced I didn’t really want to be an engineer. They live in such little worlds. But that is another story.)

Let’s back-track a little. You know about surface tension. Sure you do. That’s what enables some insects to walk on water. It is also why little droplets form on the windscreen of your car instead of a continuous sheet of water. And critically, it is what gives water droplets their shape. The forces holding the molecules together at the surface tend to pull tight to form the smallest possible area. Ok. Now imagine a bubble. The “skin” of the water surface curves the other way. That’s ok, but in this case the forces pull tight to reduce the size of the bubble so that they are as small as possible. When the radius of the bubble is very small (microscopic) these forses are great enough to completely collapse the bubble so that the steam inside goes back to liquid.

So, for any boiling liquid there is a critical radius for bubbles. Any bubbles smaller are instable and will shrink and disappear. Any bubbles that are larger are able to grow.

Which then leads to the question, how do you get bubbles in the first place? If the small ones shrink, they never get a chance to grow into big ones. The answer is that the majority of bubbles form at the surface of something. They start as a little lens shaped thing on a flat surface. That means that their radius is not so small, they don’t consume themselves by their own surface tension and they are free to grow. Typically the flat surface that bubbles form on is the sides of the pot or element or something. Just watch the first bubbles form on the immersion element, grow, rise to the surface and another form at the same place. (This is what got my lecturer all excited. Me, I’d prefer to watch a movie.)

Now. Take the case of milk. It is a mixture with very fine solid particles suspended in it. This means lots of wonderful nucleation sites where a bubble can form. When the milk gets hot enough and bubbles start to form, lots and lots of very small bubbles can form all at once throughout the mixture. There are so many nucleation sites and so many bubbles that they do not individually get very big. Also, very small bubbles do not rise to the surface as quickly as big ones. So, you get a very fine foam that expands in the pot instead of a few smaller bubbles that expand, rise and break through the surface.

Which is why you should never turn you back on boiling milk.

If you think it makes a mess on the stove, try it in a microwave.

I think it has less to do with the nucleation sites than it does with the protein in the milk. Sprinkle salt into some boiling water and you’ll see a bunch more bubbles (from the extra nucleation sites), but they will still all pop when they reach the surface and won’t create a foam. Add a little egg white to boiling water and I’ll bet you’ll see a bunch of foam. The proteins stabilize bubbles into foams.

Mmm, maybe. But remember that salt [list=a]
[li]is a lot coarser than milk solids[/li][li]dissolves[/li][li]raises the boiling temperature of the water.[/list][/li]I don’t think we are comparing apples with apples here.
Egg protein denatures at elevated temperatures. (Ever eat a boiled egg?) Milk proten does not; at least not to the same extent at similar temperatures. Again, probably not a valid comparison.

I am not going to deny the emulsifying effect of milk proteins, but I don’t think it is the primary cause here. You can get a pretty good boiling over effect with a water cornstarch mixture too.

As an aside, ever wonder why your can of coke behaves in such an interesting manner if you shake it before opening?
In a closed can (under pressure) the gas is disolved. Once the pressure is off the gas comes out of solution and escapes. Like a boiling liquid, the rate of escape is dependent on the availability of nucleation sites. By shaking the can you introduce a whole lot of stable bubbles – bigger than the critical radius. Thus, instead of a dependence on nucleation sites, the dissolved gas can simply escape into the already present bubbles and expand them. The rate of phase change is much increased and so it froths violently.