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. I’m wondering why this happens.
We put this to Hugh Hunt, Department of Engineering, University of Cambridge:
I've got an experiment here which makes the effect more apparent. I'm actually going to fill up a mug with just some cold water and then I'm going to put in a spoon of Alka-Seltzer. That's going to make it fizz up. If I tap the bottom of the cup you can hear a noise. Now I'm going to put in a spoon of Alka-Seltzer now and stir it up. Now...
I don't 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. It's pretty clear this has got something to do with the bubbles. When you're frothing milk or you're having Alka-Seltzer into a cup you're 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 that's 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. It's 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.