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So I have observed a couple of things when milk boils.

  1. First things first is that the volume of milk expands significantly when brought to a boil.
  2. Bubbles form which causes significant foaming.

So here’s my questions

  1. Why does the volume of milk expand when boiled?
  2. Why does milk form bubbles when boiled?( I don’t know the temperature at which milk boils therefore I’m assuming that the bubbles may not be water vapor)
  3. This bubbling causes significant foaming. Does this mean that the surface tension of milk is higher than water?( since it would take more energy to break the bonds holding them together)
  4. I was wondering what if milk was put in a very large vessel and then boiled. Due to the large vessel I know it will not overflow. This will also cause the water to completely evaporate. What will be left behind in the vessel?( I know this maybe a chemistry question but still it would be interesting to find out what would happen if it boils in a larger vessel)

Please correct me if I’m wrong on any of these.

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  • $\begingroup$ Please split up these questions. As it stands the question is too broad. $\endgroup$
    – auden
    Commented Mar 22, 2018 at 0:35
  • $\begingroup$ @heather but the question is only about milk and its characteristics $\endgroup$ Commented Mar 22, 2018 at 0:35
  • $\begingroup$ Yes, but there are multiple questions (your different numbers). Each of those numbers should be a different question. $\endgroup$
    – auden
    Commented Mar 22, 2018 at 0:50
  • $\begingroup$ bubbling probably means that surface tension is lower, but as @heather says there is too much here... but see scientificamerican.com/article/bring-science-home-best-bubbles $\endgroup$
    – tom
    Commented Mar 22, 2018 at 12:09
  • $\begingroup$ @tom thanks. That why I wrote to ‘correct me if am wrong’. $\endgroup$ Commented Mar 22, 2018 at 12:14

1 Answer 1

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Milk is mostly water, so the bubbles are just steam.

The mineral content raise the boiling point slightly, but it is likely not a major effect.

However, milk also contains a number of dissolved substances that changes its behaviour during boiling. Most importantly, there are proteins like casein that denature, unfold and form long strands that tangle together (fats and calcium also contribute, according to McGee's On Food and Cooking 2nd ed. p. 25-26). This increases the viscosity a fair bit. On the surface they form a skin that increases surface tension but more importantly hinders bubbles from bursting. The result is that bubbles get trapped underneath and the whole liquid expands. There are also internal bubbles that keep on expanding (with their own internal skins and steam entering them from the sides) adding to the expansion as they lift liquid above.

As for question 4, isn't that how you make condensed milk? Actually, not exactly: while heating is used, too much can change taste and it is costly energy-wise, so one also uses reduced pressure to evaporate off water.

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