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

It takes more than 150 people to make a cup of coffee, from farmer to barista. But if that final person makes a mistake, your precious coffee is wasted. Michael Allen investigates whether physics can help us brew smarter so we don’t waste a product that’s threatened by climate change

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.

Espresso basics

If you’ve ever had an espresso-based coffee, you’ll know that making that base shot of concentrated caffeine is more than just putting some beans in a machine and pressing go. Like with any experiment in a lab, there is a strict process with a range of variables:

• The coffee beans are ground down into a powder-like substance
• The ground coffee is then measured out in a “basket” at the end of a “portafilter”
• The coffee is pressed down in the basket using a “tamper” to make it tightly and evenly compacted, creating the coffee “puck”
• High-pressure hot water (9–10 bars, 90–96 °C) is pushed through the puck for 20–30 seconds resulting in 25–35 ml of espresso

The real science behind the brew

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, Maciej Lisicki, a physicist at the University of Warsaw, and his colleagues looked at the complexity of flow in coffee brewing.

Fluid flowing through a porous medium 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. Ultimately, the work confirmed that there is no point pushing the pressure beyond about 8 or 9 bars.

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 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 dissolution is optimal.