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Author Topic: Milk bubbles  (Read 3527 times)

paul.fr

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Milk bubbles
« on: 05/03/2007 22:39:24 »
this could be in the wrong section, but i will post anyway.

Why is that then when i have a glass of milk, i can blow in to the milk through a straw and get lots of lovely bubbles forming all the way up the glass.
This does not happen when i replace the milk with water, why?


 

Offline neilep

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Milk bubbles
« Reply #1 on: 06/03/2007 12:23:09 »
In the interim between when you originally posted this and before the resident ' bubble scientist ' answers .........Me will again attempt to speculate that it must be because milk is thicker in it's viscosity than water and therefore bubbles in milk are far more cohesive......and so take longer before bursting.

 

Offline eric l

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Milk bubbles
« Reply #2 on: 06/03/2007 13:04:33 »
The answer is pretty much the same as to the question "why do we have foam on beer, and not on champagne (even if we have bubbles) ?"  In both cases it is due to water soluble polymers, mainly proteins.

These can be stretched and more or less uncoiled, which may create a stable foam. 



 

Offline daveshorts

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Milk bubbles
« Reply #3 on: 06/03/2007 13:44:01 »
Milk is a suspension of fat in water, which is not naturally stable, and it should seperate out, however each of the globules of fat are surrounded by surfactants (detergents are surfactants) which stabilise it - in the same way that washing up liquid stabilises the suspension of grease in your wasing up water. The other side effect of surfactants is that they stabilise thin films of liquid - eg bubbles.

Beer has a load of surfactants in it as well, probably because cell walls are made out of them, I am not sure why wine has a lot less, probably it is just better purified.
 

Offline eric l

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Milk bubbles
« Reply #4 on: 06/03/2007 15:35:46 »
Dave, the reason why beer does give foam, and wine does not is mainly that there is a cooking stage in the production of beer, rather than a difference in purification.  The cooking increases the amount of water soluble polymers (by breaking down very long chains into moderately long chains).  These polymers become the surfactants you are talking about, because they have an influence on surface tension, but they work in a way that is very different from detergents. In fact, they counteract each other, and if you have not rinsed the glass with clear water after washing it with detergent, you will not have a stable foam on your beer.

If you distil your beer (or beer like product), you find little or none of these water soluble polymers in the destilate :  they are much less volatile than ethanol or water.  That is why your whisky does not foam.
 

Offline daveshorts

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Milk bubbles
« Reply #5 on: 06/03/2007 20:26:13 »
Cool, that makes sense, I guess the cooking will tend to break down the cell membranes as well making them soluble.

How do they work differently from normal detergents/soaps?
 

Offline eric l

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Milk bubbles
« Reply #6 on: 07/03/2007 12:35:44 »
Detergents and soaps generally work with a polar "head" an apolar "tail", e.g. a fatty acid tail with an ionised carboxyl group as the head (such as R-COONa, with R as the apolar section).
Water soluble proteins (and other water soluble polymers) have repeated ionisable groups, that are less ionic.

The detergents form mi-cellar structures, most often with the apolar tails in the centre and the polar heads facing the surrounding water.  Washing properties do not depend on reduction of surface tension alone, but on the ability to form such a mi-cellar structure around a fatty dirt particle.
Water soluble proteins form networks, where water molecules (or water chains) link the protein molecules together by means of hydrogen bounds.  Even if they have an effect on surface tension, they are not very effective for washing.

Foam is a very interesting subject.  I remember a demonstration with pieces of cardboard (of the kind we use to put under our glass in pubs) and two glasses of beer, one with foam and one without.  If you lay the piece of carboard flatly on top, it will suck up beer instantly on top of the glass without foam, and sink to the bottom in a matter of seconds.  Doing the same thing on top of a glass with foam, the piece of cardboard can remain on top for over an hour.  In this case a foam (a dispersion of a gas in a liquid) behaves pretty much as a solid.
 

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Milk bubbles
« Reply #6 on: 07/03/2007 12:35:44 »

 

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