A good espresso coffee is sexy as hell. It flows out of the machine at a languid pace, initially dark and brooding, before shifting into a golden foam that would bat its eyelashes at you if it had them. Once settled in the glass, it breathes out and releases an intoxicating scent that is earthy and sweet, capturing everything from the aromas of freshly sawn timber and rich dark chocolate to delicate floral and fruity cherry scents.
Making the perfect espresso is almost a high art, especially for those who practise at the elite level – and Australia has plenty of those. Ranked one of the biggest coffee markets in the world, the Australian coffee market is worth more than $9bn and Australia coffee fiends sip and savour around 2kg of coffee beans each year. One in four of us reckon we can’t get through the day without a brew.
The hard science of the drink so many of us love and rely on is simultaneously fascinating and illuminating. Understanding it can help elevate even the cheapest bean and most basic coffee-making equipment into something far greater than the sum of its parts.
To begin at the beginning, with the bean. The Coffea plant – arabica or robusta – is famously picky about its habitat. It wants plenty of sunshine but without the heat, so it only grows in the cooler, high-altitude areas of the “coffee belt”; a band that extends between the tropics of Cancer and Capricorn.
These growing conditions allow for long, slow development of the bean, according to the University of Queensland agricultural scientist Prof Robert Henry. The bean – or the coffee cherry, as it is known at this point – is so precious about its growing conditions that even the microclimate within an individual tree makes a big difference to the quality. Henry and colleagues found that the coffee made from cherries growing at the bottom of an individual coffee bush was much better than the coffee made from those at the top of the plant.
The contrast between the conditions in which the bean is grown and what happens to it after it is picked must come as quite a shock to it.
“Coffee is prepared in a very extreme manner,” says Prof David Hoxley, whose day job as a physicist at the La Trobe University focuses on the world of semiconductors, but who was lured into the coffee den as something of a scientific side hustle. “It’s heated up to the point where the bean explodes, maybe twice, and then it’s ground, like brutally.”
First, the cherry is rid of its plump red skin and pulp to reveal the pistachio-coloured bean. This is then fermented, dried and hulled. What’s left is a pale golden bean that gives little hint of its inner chemical glory; possibly thousands of chemical compounds that are still being discovered and described.
‘Roasting is everything’
Now, the roasting. At temperatures of about 200C, interesting things happen to the cellular structure of the coffee bean, according to the chemical scientist Dr Monika Fekete from Breville Australia. “After what we would call first crack in roasting, the cell walls explode with steam and other gases, like popcorn,” she says.
The first flavour that develops in roasting is acidity but then a chemical process called the Maillard reaction happens. This is the interaction between amino acids, sugars and heat that gives us everything from the comforting aroma of toasting bread to the mouth-watering smell of a sirloin on the barbecue. “As you keep roasting further, you get more chocolatey, nutty caramelly flavours,” Fekete says.
Roasting is everything in coffee; it’s why there are competitions just for coffee roasters. Even a fairly ordinary bean can be elevated by the careful application of heat, although that won’t provide the glorious complexity and richness associated with fundamentally good beans. The roasted bean is packaged in airtight and lightproof packing – oxygen, moisture and light are the enemies of good coffee, Fekete says, so keep the coffee beans in their original packaging as long as possible and only take out what you need to grind.
The physics of coffee grinding
Then, it’s time to grind.
But don’t rush. “Often coffee is best around 14 days after roasting,” says Melissa Caia, a judge at the World Barista Championships and a coffee expert at Melbourne’s William Angliss Institute – a specialist training centre for industries including food and hospitality. In general, coffee beans should be used within two to three weeks of being roasted. They should also only be ground just before brewing to preserve their freshness; Caia says even grinding the beans a few hours before use is too long.
Grinding is another process that gets physicists excited, so much so that a study published last month likened the physics of coffee grinding to what takes place in the ash clouds of volcanoes and on a moon of Saturn.
It’s about triboelectrification and fractoelectrification, which describes the build-up of static electricity in coffee as the beans are fractured and the fine particles jostle together. It has relevance for space exploration – the build-up of particles that could cling to cameras on a Mars lander, for example – and for volcanologists, who seek to understand the behaviour of rocks and dust during eruptions.
In the terrestrial coffee world, that build-up of static electricity makes ground coffee clump together in the basket, and “when you have these clumps that form, you inherently get variable espresso”, says a computational materials chemist, Prof Christopher Hendon, from the University of Oregon in the US.
His study tried to vary a lot of things – the bean, the roast, the moisture content, the grind size – to see if that changed the static built up. They found the single-biggest factor was moisture: the drier the bean, the more static you get. A simple spray of water on to the beans before grinding would solve the clumping problem.
And here’s where the die-hard coffee tragic might roll their eyes and point out that serious coffee-heads have been doing this for years. It even has a name – the Ross droplet technique.
The size of the coffee grinds is the subject of constant experimentation, from the home brewer to the world champion barista. It’s incredibly complex, which really lights up the dopamine centres in the brain of Dr Jamie Foster, an applied mathematician at the University of Portsmouth in the UK.
Applied mathematics focuses on developing mathematical models of physical processes with a practical goal in mind. Foster admits that coffee is a tricky topic, because those chemical and physical processes intersect with the messy, subjective sensory sciences.
Grind size is key because it governs how the water flows through the coffee as it is being brewed. That’s described by a law that’s more than 100 years old, called Darcy’s Law, that’s more commonly applied to people trying to extract things like oil from sediments – “Essentially anywhere you’ve got a fluid flow in a porous media.”
The general rule is espresso machines need a finer grind than stovetop espresso “moka pots”, which in turn need a finer grind than the French press plunger coffee maker.
Foster’s modelling efforts delved deeper into this and identified a sweet spot of grind size, where the coffee particles aren’t too big (which means overall less surface area in the coffee is exposed to the brewing water) or too small, where the grounds clog together and reduce the overall coffee particle surface area available.
They found an “optimised tasty point” (yes, it is actually called that), where all these factors come together to deliver the greatest amount of extracted coffee from a set weight of grounds. Because each bean, grinder and espresso machine is different, it means experimenting with grind size, the amount of coffee and extraction time to find the perfect combination for your bean, machine and flavour preferences.
Foster says the coffee industry is a bit snooty about his scientific approach. “They kind of felt like, ‘I don’t want your dirty optimum, I want to faff around and find it for myself and create things,’” Foster says.
At last – the brewing
Finally, it’s brewing time. Caia says filtered, unboiled water is a must – it removes sediment and unwanted minerals. For espresso machines, the water temperature must be controlled within a narrow range – no cooler than 88C or hotter than 97C.
Many people, like my coffee-tragic husband, have a complex ritual of tapping and tamping the grounds in the basket used in an espresso machine. Caia says it’s important to ensure even distribution of the ground coffee in the basket, and “the idea with tamping is just to flatten and level so that you’ve got an even ground for the water to penetrate through”. But tamping won’t fix a bad bean or roast, she warns. It will, however, make sure you get the most flavour from the bean.
The proof of all of these countless steps and processes is in the golden-brown layer of foam on the finished espresso shot; the crema. Chemically, the crema is an emulsion of tiny bubbles of carbon dioxide which, when forced through and out of the grounds, in an espresso machine or stovetop maker, get coated in the proteins and oils of the coffee.
As the espresso lands in the lower-pressure environment of a cup, those bubbles of carbon dioxide rise to the top, much like the fizz when a bottle of soft drink is opened. The fresher the beans, the more carbon dioxide they contain, and the better the crema. The colour doesn’t necessarily matter as much – some high-end beans naturally produce a lighter crema – but its persistence and flavour does, Caia says. “We recommend people to just give the espresso a little swirl so that you can blend the flavours through the coffee.”
What happens next is up to you. Fekete starts the day with a double-shot flat white, then just one filter coffee later on. “I try to watch my caffeine consumption,” she says.
