It took me a while to notice that one of the prettiest things about espresso was a little bit confusing.
Watch this video and perhaps you will see what I mean:
In theory it doesn’t make sense. The bubbles contain CO2, which is much less dense than coffee so they should be rising quite quickly and not falling. What would make a bubble sink instead of float? You’ll see the same thing happening if you watch a pint of Guinness settle out – and it was their research that gave me the answer to this question.
What is happening is that plenty of bubbles of crema are rising, but they are rising predominantly in the middle of the shot glass and the pressure they create as they rapidly rise effectively sucks down the smaller bubbles close to the walls of the glass. There is nice explanation of Guinness here (with pictures!)
Another mystery solved…..