For a better cup of coffee, look to physics

The cost of raw arabica beans, the core component of most coffee, has spiked in recent years due to four consecutive seasons of adverse weather.

Climate change has added further strain, threatening the delicate temperature balance required by the Coffea arabica plant. This growing pressure has inspired physicists at the University of Pennsylvania to ask: Can we make great coffee with fewer beans? “There’s a lot of research on fluid mechanics, and there’s a lot of research on particles separately,” says Arnold Mathijssen, assistant professor in the School of Arts & Sciences. “Maybe this is one of the first studies where we start bringing these things together.”

Their findings, published in the journal Physics of Fluids, provide a scientific approach to improving extraction efficiency so fewer coffee grounds can go further without diminishing overall quality. “We tried finding ways where we could use less [or] as little coffee as possible and just take advantage of the fluid dynamics of the pour from a gooseneck kettle to increase the extraction that you get from the coffee grounds—while using fewer grounds,” says coauthor Ernest Park, a graduate researcher in the Mathijssen Lab.

The experiment required making the invisible visible, explains coauthor Margot Young, a graduate researcher in the Mathijssen Lab. “Coffee’s opacity makes it tricky to observe pour-over dynamics directly, so we swapped in transparent silica gel particles in a glass cone,” Park explains.

A laser sheet and high-speed camera allowed them to watch water streams create “miniature avalanches” of particles—revealing the flow’s inner workings. Water poured from a height produces the avalanche effect that stirs the bed of particles and enhances extraction.

A key factor in this process is laminar, or smooth and nonturbulent flow—made possible by a gooseneck kettle, even with a gentle pour-over flow. “If you were just to use a regular water kettle, it’s a little bit hard to control where the flow goes,” says Park. “And if the flow isn’t laminar enough, it doesn’t dig up the coffee bed as well.” The team discovered that when water is poured from a height, it creates a stronger mixing effect. “When you’re brewing a cup, what gets all of that coffee taste and all of the good stuff from the grounds is contact between the grounds and the water,” explains Young. “So, the idea is to try to increase the contact between the water and the grounds overall in the pour-over.”

They found that if poured from too great a height, the water stream breaks apart into droplets, carrying air with it into the coffee cone, which can actually decrease extraction efficiency.

The researchers conducted additional experiments with real coffee grounds to measure the extraction yield of total dissolved solids. Their results confirmed that the extraction of coffee can be tuned by prolonging the mixing time with slower but more effective pours that utilize avalanche dynamics.

For thicker water flow, they found that higher pours resulted in stronger coffee, confirming their observations about increased agitation with higher pour heights. When using a thinner water jet, the extraction remained consistently high across different pour heights, possibly due to the longer pouring time required to reach the target volume.

Broad implications that extend beyond the kitchen

The study is a love letter to coffee—and it’s also a window into the team’s broader research. “We weren’t just doing this for fun,” Mathijssen says. “We had the tools from other projects and realized coffee could be a neat model system to explore deeper physical principles.”