Did you know your barista is actually a secret scientist? Dr David Hoxley from La Trobe University’s Department of Chemistry and Physics takes us beyond the coffee machine to reveal how the scientific method impacts your daily brew.
Coffee grinds as particles in a complex system
As a physicist and nanotechnologist, Dr David Hoxley is fascinated by how particle size affects the behaviour of things. 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.
“Baristas have a tough job of deciding how to grind the coffee, how to pack it and how to do the tamping – it’s a complex multivariate system. They’ve got beans coming in that may not always be the same, they’ve got humidity and on their feet they’ve got to work out how to bring it all together,” David says.
“That’s where the science of coffee can really help. A nanotechnologist like myself can help with imaging on grind size, and see how different characteristics and particle distributions affect the extraction.”
From sour to bitter: how particle size impacts coffee flavour
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.
When you’re extracting coffee, you talk about a flavour profile. A coffee that’s under-extracted is really sour. And a coffee that’s over-extracted is really bitter. But the two things combined give you a sweet flavour, somewhere in the middle,” says David.
So, what causes coffee to be over- or under-extracted? According to David, 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.
To demonstrate to coffee appreciators the impact of grind size on flavour, David uses a simple experiment. He extracts coffee from different grinds into separate beakers. Then, using a machine called a reflectometer, he measures how many dissolved solids have been extracted from the coffee.
“It’s a perfect experiment, because it’s just changing one variable – grind size – that everyone can set on their own grinder. But it has a tremendous effect on flavour. As you’d expect, if it’s a finer grind, you extract more caffeine and you get a more bitter flavour. Personally, I liked the coffee that was near the middle, with 1.37 total dissolved solids.”
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.
David likens this mix of coffee particle sizes to the aggregate that’s used to make concrete. 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’. For David, 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,” he says.
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.”
Applying science to give Australian baristas an edge
Of all Australian cities, Melbourne has a reputation for having an over-the-top coffee culture. But Dr Hoxley believes science can help bring the humble brew back down to earth. He’s working with Dr Monica Fekete of the Australian Coffee Science Lab to help local baristas win international competitions by applying physics and chemistry to their brew.
Melbourne is self-aware that we’re a kind of caricature when it comes to coffee, that everyday luxury. But with a bit of science and humility we can get rid of some myths.”
So next time you grab a coffee, take a moment to thank your barista. Behind the apron and the masterful latte art, they’re making sure each particle works the best it can for your brew.
Discover how to apply the scientific method in your everyday life when you study Science at La Trobe University.