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 (Courtesy: iStock/Hispanolistic) 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 scientistsCoffee 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.

From farm to cup A cup of coffee starts with the coffee farm workers planting the seeds and picking coffee berries – but climate change is starting to affect crop yields. (Courtesy: iStock/SupawadeeAdam)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 pressureLet’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.1 Espresso basics(Courtesy: iStock/GerasimovSergey)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 (and a bunch of lingo). Here’s a quick and very basic guide to making an espresso with an espresso machine: 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” (top left) To ensure this coffee is evenly distributed, baristas then lightly tap the basket, and some may manually move it around Next the coffee is pressed down in the basket using a “tamper” to make it tightly and evenly compacted (top right), creating the coffee “puck” Before the portafilter is attached, it’s a good idea to purge the machine with water to ensure no contamination from the previous brew Now it’s time to actually “pull the shot”. The portafilter clicks into place (bottom left) and, if following the historical definition, 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 (bottom right) 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.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 lessBut 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.2 Infiltrating the puck(CC BY 4.0 NC Physics of Fluids 37 013383)When water is initially pushed through a dry espresso puck, it can take up to one-third of the brewing time to permeate the entire bed of ground coffee. But according to mathematician Ann Smith and colleagues, this process remains relatively neglected by mathematical models of coffee extraction.“Understanding the infiltration process gives insights into the extraction rate across the coffee bed,” says Smith, who is based at the University of Huddersfield in the UK. “Over-extracting coffee results in bitter taste while under-extraction leads to both weak brews and wasted resources.