Coffee with a splash of physics: how to make the most out of your brew

Espresso, flat white, cappuccino, cortado – there are dozens of ways you can get your coffee fix. Every day more than two billion cups of coffee are brewed worldwide, making it one of the most traded products on Earth. In fact, it is the seventh most traded commodity on the planet (after crude oils, natural gas, gold, silver and copper).

Produced mainly in south and central America, south-east Asia and east Africa, coffee sustains the livelihoods of more than 25 million farming households. But its future is increasingly precarious. Coffee plants need the right temperature range, rainfall patterns and altitude to thrive, but climate change is disrupting it all.

This has led to falling yields and rising prices. For example, the price of Arabica beans – the most dominant coffee variety – rose by more than 80% in 2024. In the UK, this led to the price of beans at supermarkets rising 20% and the cost of some instant coffee surging by 40%, while coffee shop prices were up 30% from 2021 to 2024.

And it’s not just that coffee is affected by climate change – the climate is impacted by coffee. It has one of the largest carbon footprints of any plant-based product, mainly due to the clearing of tropical forests, fertilizer and water use, and processing techniques.

So how can those of us making the drinks help with the coffee and climate crisis?

At-home scientists

Coffee is an unusual drink. Unlike products such as whisky, wine and beer, it is brewed at the point of consumption, whether that’s in a café or restaurant, or in a home. “The very last step, which is probably the most complicated, is all done by untrained scientists,” says Christopher Hendon, a computational materials chemist and coffee expert at the University of Oregon.

It is estimated that it takes 155 people to make a cup of coffee – all the way from the farmer who plants the seed in the ground, to the barista who hands you your cup of coffee. “154 people can do their job perfectly,” says Dan Pabst, manager of innovations and product development at coffee provider Melitta North America. “But if that last person doesn’t pay attention, does something wrong and the coffee doesn’t taste right, they’ve just ruined all that hard work.”

This is where physics can help. Beyond longer-term, large-scale solutions such as re-engineering coffee plants or tackling climate change, physics can actually tell us a lot right now about what happens during the seconds and minutes it takes to brew a cup of coffee. It is a surprisingly complex process and by understanding it, we can improve the quality of the drink and even reduce the amount of coffee needed, cutting waste and helping the environment.

Under pressure

Let’s start with the espresso – those concentrated coffee “shots” you get in tiny cups that also form the base for your latte, americano or cappuccino (and many more).

The Specialty Coffee Association (SCA) has historically defined an espresso as a 25–35 ml beverage prepared from 7–9 g of coffee through which water heated to 90–96 °C is forced at 9–10 bars of pressure for 20–30 seconds. “While brewing, the flow of espresso will appear to have the viscosity of warm honey and the resulting beverage will exhibit a thick, dark golden crema,” states the SCA. This can be achieved using an espresso machine (figure 1), or with smaller contraptions at much lower pressures such as a moka pot or AeroPress.

In 2017 the SCA and the Barista Guild of America surveyed baristas around the world to see how they were preparing their cups of espresso and if they were following the historical definition. Turns out that the average barista brews an espresso with 18–20 g of coffee in 25–30 seconds using water heated to 93 °C at 9 bars of pressure. This produces an average shot of 36.5 g.

Go online and you’ll also find all sorts of claims about how to produce a perfect coffee. “From a scientific point of view, this advice is often given with no substantial evidence,” says Maciej Lisicki, a physicist at the University of Warsaw. Keen to understand the real science behind a good espresso, Lisicki and his colleagues looked at the complexity of flow in coffee brewing (arXiv:2512.21528).

One frequent claim by coffee experts is that the optimal brewing pressure for an espresso is around 6–9 bars, and that pushing it higher yields diminishing returns. To find out why, Lisicki’s team rigged a café-grade espresso machine with a pressure sensor at the pump outlet, and a precision scale under the coffee cup so they could calculate flow rate. They prepared the puck using a high-spec coffee grinder and automatic tamper to ensure consistency, and then brewed shots of espresso at pressures ranging from 1 to 12 bars. “We strive for our espresso to be the same every time,” says Lisicki. To visualize their structure, the researchers also took X-ray micro-computed tomography (micro-CT) scans of the coffee pucks before and after brewing.

Fluid flowing through a porous medium, such as sand, glass beads or packed soil, usually follows Darcy’s law, with flow rate increasing linearly with the pressure of the liquid. But the researchers found that this was only true for coffee up to around 5 bars.  Above that, the flow rate flattened and then fell as pressure increased.

The team observed that as the coffee is brewed, the puck starts compacting under mechanical load, causing its pores to collapse and its permeability to decrease faster than the rising pressure can increase the flow. “The dynamics of espresso brewing is governed by this poroelastic effect,” Lisicki says. “There is an interplay between the porosity and the elasticity of the coffee matrix.”

Ultimately, the work confirmed what the coffee experts had observed. There is no point pushing the pressure beyond about 8 or 9 bars as the flow rate has already peaked.

The coffee in your coffee

Next, to explore how coffee dissolves over time, the team separated an espresso into different vials every five seconds as it brewed. An optical refractometer then measured the total dissolved solids in each vial. This, says Lisicki, tells you “how much coffee is in your actual coffee”.

The work showed that the first few drops of coffee that fall into your cup are very concentrated, but there aren’t many of them because flow rate is initially low. The flow rate then increases, but the amount of dissolved solids falls. This creates a sweet spot at around 15 to 20 seconds where the dissolved solids entering the cup peak, due to the balance between the increasing flow and decreasing solids.

Again, this tallies with expert opinion. “What we see is that most of the substance, the solubles, go into your cup within the first 30 to 35 seconds,” Lisicki says.

After talking to a friend who works as a barista, Lisicki and his colleagues also explored an annoying problem in coffee brewing known as channelling. This happens when the water finds a path of least resistance in the puck and forms a “channel” through the coffee grains. When they brewed coffees with artificially induced channels in the puck, the researchers found that the flow rate was as expected but the total amount of dissolved solids that were extracted was very low. Essentially, the water doesn’t permeate the rest of the puck, so you can’t extract all its coffee.

In fact, the team found that the more careless you are about preparing your coffee puck, the higher the chances of channelling. To mitigate this, you should stir the coffee grounds to make the puck as homogenous as possible and tamp it evenly. “You don’t need to tamp it very strongly, but tamping is important because if you don’t tamp then it’s easier for the water under high pressure to find a preferential flow path,” Lisicki says.

More from less

But even before you tamp your puck, how you prepare your coffee grains affects your drink’s quality. This is where grind size matters (figure 2). Back in 2020 Hendon and an international team were funded by the Coffee Science Foundation – the research arm of the SCA – to study ways to make highly reproducible espresso (Matter 2 631).

The group started by looking at what happens in the coffee grinder. You might think that a more finely ground coffee will maximize the surface area exposed to water, thereby maximizing extraction. Hendon and his colleagues discovered, however, that this was not the case.

As grind size decreases, from coarse to fine, the amount of coffee that gets extracted initially rises but then peaks and falls. If the grind is too fine, the coffee bed clogs so that water can no longer percolate uniformly through it. Much like with channelling, some areas are over-extracted, while others are barely touched

“If you find the tipping point, you’ll realize the amount that you’re extracting on average [with a coarser grain] is actually much higher than if you ground finer,” Hendon explains. This means you also need less coffee to make an espresso of the same concentration.

The bottom line of the team’s experiments and mathematical modelling is that to get the most reproducible shots just use less coffee and grind it more coarsely.

Pabst echoes that advice: “My recommendation for people at home, without knowing anything they are doing, 90% chance that if you use less coffee and grind a little coarser [your coffee] will actually taste better.”

Hendon and his team trialled their “waste reduction protocol” at a small café in Eugene, Oregon in the US. By grinding more coarsely and reducing the dry coffee mass by 25%, from 20 g to 15 g, the business increased its revenue by more than $3000 over a year, without sacrificing drink strength or flavour.

Based on estimated US espresso consumption figures from the time, Hendon and his colleagues suggested that if their findings were implemented across the entire US, it could save the country about a billion dollars per year.

Volcanic coffee

It was not until Hendon teamed up with a volcanologist that he figured out why finer grinds clog the coffee bed and reduce extraction. Joshua Méndez Harper at Portland State University studies electrification in volcanic eruptions, where magma fragments charge up as they grind together in the plume, generating lightning. Coffee grinding, it turns out, creates a similar phenomenon (Matter 7 266, iScience 27 110639).

Together, Hendon and Harper found that friction between beans and the fracturing of beans during grinding generates static electricity. By passing coffee through a grinder a second time at a coarse setting, which removes fracturing from the process, the researchers discovered that most of the static charge arises from fracturing rather than friction.

According to Hendon, the more times you fragment your coffee, the more static electricity you generate. “You’re making lots of small particles when you grind finer, but they clump together to form an aggregate [because of static], which is effectively impermeable to water,” he adds (figure 3).

The solution, the researchers found, is to squirt a little bit of water on the beans before you grind them. “That totally suppresses the static accumulation,” Hendon says. Wetting whole beans with less than 0.05 ml of water per gram of coffee – or about 0.5 ml for an average espresso shot – resulted in a marked shift in particle size distribution by preventing clump formation.

But again, the coffee experts got here first. In the coffee industry, this is known as the Ross droplet technique and was anecdotally thought to reduce static charge, even if the physics wasn’t well understood. Pabst says that a lot of the recent findings “are not necessarily newer ideas, they are validating what we have taught in the industry for many years”. But he describes it as an exciting time, with science providing deep insight into industry knowledge.

Moisture content of the beans is another key variable, Hendon’s team found, with drier, darker roasts charging most strongly and therefore benefiting most from pre-wetting. The researchers also found that the right amount of water results in near-zero grounds being retained by the coffee grinder, again due to the reduced static charge.

They note that their findings have implications for waste reduction and drink quality. Hendon says that adding water during grinding allows you to reduce coffee mass by about 25%, while maintaining espresso concentration.

Pour-over science

Coffee is not just espresso.

Among the myriad of coffee-making techniques available, pour-over coffee is increasingly popular with enthusiasts due to its reputation for being better able to extract the unique characteristics of different coffee beans. A popular set-up uses a conical filter or “dripper” containing a filter paper, and the process is simple – you put coffee grounds in the filter, pour in hot water, and let the coffee drip out into a cup. There is a wealth of variables – such as grain size, water temperature and water speed – that allows whoever is holding the kettle to experiment and vary the process.

However, Arnold Mathijssen, a physicist at the University of Pennsylvania, has been studying exactly how they should be pouring that kettle (Physics of Fluids 37 043332).

As with an espresso, the challenge is to bring the water into uniform contact with every particle in the coffee bed. If the stream is too slow, for instance, it might run to the edges of the cone, flow through the filter paper and drain away without touching the grains in the centre.

To investigate how pouring technique affects this, Mathijssen and colleagues used a transparent glass cone similar in shape to a popular pour-over filter, and ultrathin filter paper. They then filled it with silica gel particles as a transparent model for coffee grains, and illuminated the set-up with a laser sheet while filming it with a high-speed camera.

The experiment revealed the importance of pour height for a static kettle. Pour from close range and the slow stream fails to effectively disturb the coffee bed. Lift the kettle to around 20 cm above the filter and something different happens. “We found that as you increase the height of the kettle this kind of avalanche dynamic emerges,” describes Mathijssen.

The increased energy in the stream enables it to dig deep into the coffee bed, suspending the particles and creating a hole in the middle. Particles around the side then slide into the centre and are themselves suspended, establishing a recirculating vortex (figure 4).

“Every single particle in that cone is moving up and down through this vortex, so you get very nice and even extraction,” explains Mathijssen. “All of the particles see the water for an equal amount of time.”

But go too high, above about 30 cm, and surface tension breaks the stream into droplets – a process known as the Rayleigh–Plateau instability. The drops fail to dig deep into the bed, so the vortex does not form and coffee extraction falls. There is a sweet spot at a height of around 15–20 cm, Mathijssen says.

When the researchers switched back to coffee, they found that higher pours did indeed create stronger coffees with more total dissolved solids.

One cup at a time

These studies highlight a common theme. Decent coffee extraction is about creating uniform fluid contact with a porous medium that tends to be heterogeneous. And if it goes wrong, it is probably due to some failure of that uniformity.

As climate change squeezes yields and pushes prices higher, the coffee industry faces growing pressure to do more with less. Physics cannot protect vulnerable growing regions from drought and rising temperatures, but its insights can ensure that coffee is not wasted in the final seconds or minutes by a poorly prepared puck, a clumped grind or a lazy kettle lift.

“The best thing we can do,” says Hendon, “to be good custodians of any agricultural product is figure out how to use less of it so that more people can enjoy it.”