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Physics behind milk foaming

  1. Feb 16, 2014 #1


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    So I was making a cappuccino and I started to wonder - how does the foam form?

    I have no problem with the foam itself, milk contains a lot of surfactants that can stabilize the bubbles. But what is inside bubbles?

    My espresso machine makes steam, that is bubbled through the milk, creating foam. What I don't get is why the steam doesn't condense and why the bubbles don't collapse after the foam is cooled down?
  2. jcsd
  3. Feb 16, 2014 #2


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    Is it pure steam, or steam mixed with air?
  4. Feb 16, 2014 #3
    I would guess it is similar to making polystyrene foam, which gets polymerised in the process. In cappuccino foam, the heat acts on the proteins and whatever else in the foam, making it stickier and more rigid.
  5. Feb 16, 2014 #4


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    Frothing is an art form.
    I make myself Cafe Latte every day and I am learning the technique of getting foam in the milk. The essential thing is to get air down under the surface of the water. The steam that comes from the nozzle (a single 1.5mm hole seems best for me) pushes the milk into an indentation in the liquid surface which carries air down with it, causing air bubbles on the surface. The steam condenses, of course, and raises the milk temperature. The next phase is to break these bubbles up and to force them further beneath the surface, the smaller bubbles will be pushed down and stay down for longer (upthrust / viscous force ratio being less). As the larger bubbles break down more and more, the foam becomes 'wetter and wetter' until it becomes a bit like Guinness foam, coloured with the espresso coffee when you pour the wet foam into the cup of espresso. (Beautiful.)
    For Cappuccino foam, the bubbles are kept nearer the surface and include less milk. This makes a white 'structural' foam, which can be used to make sculptures on the surface (mice. flowers etc)

    There are high tech frothers that take all the skill out of the process and they usually have a tube or groove, down which the air is sucked. Starbucks and Costa may not have the time to devote to each cup, of course, so they are excused for taking short cuts.
    Last edited: Feb 16, 2014
  6. Feb 17, 2014 #5

    Andy Resnick

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    More than you ever want to know:


    Apparently, the milk proteins denature during 'foaming', increasing foam stability. Also, since the foam is a fluid-fluid system, as the interior fluid cools, the lamellae (!) can flow, increasing bubble wall thickness.
  7. Feb 20, 2014 #6


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    Thank you guys, definitely several things to think about.

    Good question. I had to clean it so I took apart the nozzle. Apparently it should be capable of mixing steam with air, but at the same time it is sealed with two separate rubber parts, which make it difficult to suck the air.
    Last edited: Feb 20, 2014
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