Science Questions

Musical Milk

Tue, 10th Feb 2009

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Question

Mouse asked:

I work as a barista and I steam milk. When the milk gets to 140 degrees Fahrenheit the sound it makes abruptly drops pitch significantly. Iím wondering why this happens.

Answer

We put this to Hugh Hunt, Department of Engineering, University of Cambridge:

Iíve got an experiment here which makes the effect more apparent.  Iím actually going to fill up a mug with just some cold water and then Iím going to put in a spoon of Alka-Seltzer.  Thatís going to make it fizz up.  If I tap the bottom of the cup you can hear a noise.

<Sound of cup being tapped by metal spoon>

Now Iím going to put in a spoon of Alka-Seltzer now and stir it up. Now...

<Tapping sound drops in pitch>

I donít know whether you can hear that but the pitch of the sound has gone right down.  Now we wait a bit.  The pitch is coming back up again.  If I give it a bit of a stir, down it goes in pitch and then up it comes again. 

CappuccinoItís pretty clear this has got something to do with the bubbles.  When youíre frothing milk or youíre having Alka-Seltzer into a cup youíre turning water with no bubbles into water with bubbles.  The bubbles add elasticity.  Water is really highly incompressible.  The presence of a few bubbles in the water make it a lot softer.  Anything thatís softer like something bouncing up and down on a soft rubber band will have a lower frequency than something bouncing on a stiff rubber band which will have a higher frequency.  This is commonly called the hot chocolate effect and some people call it the cheap instant coffee effect.  Itís pretty easy to do but the explanation is not straightforward.

When it comes to milk, milk is a bit complicated really because when it gets hot the proteins denature and this is going to affect the way it forms the froth or the foam.  The exact temperature when the effect becomes most pronounced will really depend on how much protein there is in the milk.  But the basic effect is to do with the bubbles.

That can be most easily demonstrated with water.

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What's the boiling point of milk, anyone know? chris, Wed, 11th Feb 2009

I bought an espresso machine a few years back after a trip to Italy.  (Italian espresso was a divine experience).  I wondered the same thing.  After some looking around on the web, I found that the common explanation is that at ~140F the proteins in the milk (whey and casein) begin to denature, and these denatured proteins form the stable foam you're aiming for in a cappuccino.  The pitch change is probably because the froth is more stable and forms larger bubbles when you force hot steam into it (larger cavities generally equals lower pitch sounds), and because the froth has different physical properties than liquid milk, so sound travels differently in it.

Here's the source where I first read about this:
http://coffeegeek.com/guides/frothingguide/milk jpetruccelli, Wed, 11th Feb 2009

So you're making a miniature echo-chamber in the milk jug!? chris, Thu, 12th Feb 2009

The sound you make when frothing milk is due to the steam being forced into the milk inside the pitcher.  The change in pitch is almost certainly due to the change in the physical properties of the milk.  I can see that the froth becomes firmer and more bubbly as it reaches this critical temperature.  I was just guessing that bubble size was involved since I see bigger bubbles forming at this temperature and I know that larger cavities tend to support higher wavelengths/lower frequencies.  There's probably a whole lot of more complicated physics going on because of the change in milk properties. jpetruccelli, Thu, 12th Feb 2009

Better latte than never? lyner, Wed, 18th Feb 2009

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