How to use physics to make a better, cheaper cup of coffee

Coffee beans are seen in grinders at Vigilante Coffee, Wednesday, Sept. 1, 2021, in College Park, Md. (AP Photo/Julio Cortez) (Julio Cortez/AP)

Scientists at a University of Pennsylvania (uPenn) physics and biology lab have put their minds to one of life’s great mysteries: How do you make the perfect cup of coffee, for cheap?

For pour-over coffee at least, one of the secrets lies in how high you pour from, according to a new study published in Physics of Fluids.

After years of adverse weather in the tropics, the supply chain for arabica beans has come under strain, a uPenn press release reads. The university’s Mathijssen Lab was inspired to ask: How can you produce cups of coffee with fewer beans without sacrificing quality?

The goal was to improve what researchers call “extraction efficiency,” or interactions between the coffee and water on a micro-scale.

“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,” said coauthor Margot Young. “So, the idea is to try to increase the contact between the water and the grounds overall in the pour-over.”

First, they ran tests with a silica substance that mimicked coffee, but wasn’t too dark or opaque to see through. Using a high-speed camera, the study authors examined the inner workings of how water flowed through the particles, as well as how it moved them around.

They found that for the best results, the flow needs to be smooth and strong enough to stir up the particles at the bottom of the container, increasing opportunities for particles to contact one another.

Researchers recommend a pour-over setup with a gooseneck kettle, and to pour the hot water from a significant height. That creates what they call the “avalanche effect,” which is shown in later tests with real coffee to increase the extraction efficiency.

Slower, “more effective” pours from a narrow-necked kettle can help as well, the study found.

“Instead of increasing the amount of beans, the sensory profile and the strength of the beverage can be adjusted by varying the flow rate and the pour height,” it reads.

“In this way, the extraction efficiency could be better controlled to help alleviate the demand on coffee beans worldwide.”

It’s not a foolproof system, though. Researchers found that if you pour from so high that the stream breaks into droplets, air enters the mixture, getting in the way of a good brew.

While the study may be helpful for coffee fans hoping to stretch their grounds further, researcher Arnold Mathijssen says it also offers a window into the lab’s broader research in physics.

“We weren’t just doing this for fun,” he said in the uPenn release. “We had the tools from other projects and realized coffee could be a neat model system to explore deeper physical principles.”

Mathijssen and Young note that the same dynamics could apply to erosion for things like waterfalls, dams and even water treatment systems.

“You can start small, like with coffee … and end up uncovering mechanisms that matter at environmental or industrial scales,” Mathijssen said.