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When I heat milk, at some point it begins to boil over. When I reheat the same milk after it has reached boiling point once before, it doesn't boil over again, it just boils "normally". Why is this?

I haven't done extensive testing, and I can't seem to find information about this anywhere; be it in English or in my native language. But here is what has happened to me:

  1. I make a homemade hot beverage with low fat milk, cocoa, sugar and coffee (a pseudo-Mocha). Enough for 2 cups.
  2. I boil the pseudo-Mocha, and hope to pay enough attention to prevent it boiling over, but there is still the "rising bubble" phenomenon occurring.
  3. I pour out one cup, the rest of the liquid stays in the pan.
  4. A few hours later, I boil the pseudo-Mocha again.
  5. This time, no bubbly phenomenon occurs, just "classical" boiling.
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  • Just to clarify by 'rising bubble' phenomenon, do you mean that the liquid level rises in the pan, as though it might boil over? And when you boil it a second time, the liquid level doesn't rise in the same way? – mfox Jul 7 '20 at 11:56
  • related : cooking.stackexchange.com/a/5228/67 – Joe Jul 9 '20 at 15:34
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I believe there are two processes involved here, one physical and the other one chemical.

When you heat milk (or any other liquid) dissolved gasses are released. Milk can contain up to 10% dissolved gasses, mostly CO₂ and O₂. When heated close to the boiling temperature, these gasses will form bubbles and cause the milk to boil over.

There will also be chemical changes, and it's entirely possible that the proteins will denature to such an extent that they will no longer form a foam as easily.

You could easily make an experiment to determine which effect is the greatest. Boil up some milk, in order to cause both degassing and denaturing. Let it cool down to room temperature, and sit until the next day. Whisk it, in order to reabsorb gasses, and try to boil it again. If it will boil over now, then the dissolved gasses was the dominant factor. If it doesn't, then you will have shown that the denaturing of proteins is dominant. (Then discard the milk - you probably shouldn't drink milk that's been sitting out overnight, even after boiling it.)

Let us know what you find!

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  • I did it for the science, but quite doubful to be honest. I whisked already boiled milk for 1 minutes or so, vigorously, and heated it again... And it boiled over, again ! I'll try to reiterate the experiment though, it could be biased (lower than usual boil over the first time for instance). – Tahn Jul 10 '20 at 13:24
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If you heat food high enough, you can cause chemical changes. Letting it cool back off doesn't necessarily reverse these changes.

Typical changes include:

  • deactivation of enzymes
  • denaturing (breaking down) proteins
  • converting sugars into more complex forms
  • evaporation of part of the liquid

In general, we call this 'cooking', but there's specifically a term in English for that you're doing to the milk: scalding

I suspect that you've denatured the proteins in the milk, and proteins can create foams (such as what happens when you whip egg whites or the liquid from canned beans), but there might be other processes that have happened as well.

I also know that scalding isn't called for typically in modern cookbooks. Part of this is because today's milk is homogenized and pasteurized, and so some of the chemical changes that we get from scalding have already happened. But there are still quite a few people who insist that you need to scald milk for certain types of bread making, or you won't get the proper rise and final texture from the loaves.

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