Crema and the laws of physics  August 9th, 2008  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…..  Comment Policy   18
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