Did you know your barista is actually a secret scientist? For baristas to get the most out of the coffee they’re making, they need to know how to optimise the many variables affecting a particular particle of interest – the coffee grind.
Scientifically speaking, brewing coffee is the process of extracting soluble flavour compounds from roasted and ground coffee beans. For a barista, perfecting the flavour means making sure the extraction hits the sweet spot – literally.
So, what causes coffee to be over- or under-extracted? The size of coffee grinds has a big impact. Larger, coarse particles are more permeable, which means the hot water flows more quickly through them. And if the water flows too quickly, you’re likely to get an under-extracted coffee.
By contrast, smaller particles hold the water in amongst themselves and slow its flow. But when these fine grinds are steeped in water for too long, more of the bitter, caffeine flavour seeps out – resulting in an over-extracted coffee.
Pressure for espresso: how tamping impacts particle distribution
Particle distribution adds another variable to the mix. Examining coffee at nanoscale reveals that grinds are, in fact, a combination of particle sizes. Your average grind has particles of millimetre size packed in among particles that are micron size – a 50th of the size of a human hair.
By packing different particle sizes together, you create a strong combination that’s resistant to pressure. This resistance to pressure comes in handy for brewing one of the most popular styles of coffee – espresso.
Espresso is ‘pulled’ from an espresso machine by forcing pressurised hot water through finely ground coffee beans, to produce a rich, oily coffee ‘shot’. The oils in espresso are what make it particularly tasty.
If you have good resistance to the pressure, then you can extract more oils with the water. And the oily parts of coffee are where you get a lot of the deep flavour. To create this resistance, baristas tamp their coffee before they start the extraction. Tamping coffee helps compress the aggregate of different particle sizes, which in turn helps increase the resistance to the hot water that’s being pushed into it.
Tamping also makes sure the density of coffee grinds is uniform. If you’ve got any pockets in your coffee – weak spots with less coffee particles per cubic centimetre – the water will move more quickly through them. And as you now know, that leads to under-extracted coffee.
And while there’s a lot of superstition among baristas about how hard you should tamp coffee, from the perspective of physics, it’s straightforward: “You just need to press hard enough. If you press too hard it doesn’t make any difference.”
