Latest news with #ArnoldMathijssen
New York Times
23-04-2025
- Science
- New York Times
The Physics of the Perfect Pour Over
More than a billion cups of coffee are consumed daily: French-press, espresso, cold brew, whatever it takes. Arnold Mathijssen, a physicist at the University of Pennsylvania, is partial to pour-over coffee, which involves manually pouring hot water over ground beans and filtering it into a pot or mug below. Surely, he figured, applying the principles of fluid dynamics to the process could make it even better. With two students of similar mind, Dr. Mathijssen began studying how to optimize the pour in a pour over. Their science-backed advice: Pour high, slow and with a steady stream of water. This ensures the greatest extraction from minimal grounds, enhancing the coffee's flavor without added beans or cost. The findings, published this month in the journal Physics of Fluids, highlight how processes that unfold in the kitchen — from making foie gras to whipping up a plate of cacio e pepe — can inspire new scientific directions. In turn, science can enhance the art of cuisine. 'Kitchen science starts off with a relatively low entry barrier,' Dr. Mathijssen said. 'But it's more than just cute. Sometimes fundamental things can come out of it.' Dr. Mathijssen primarily studies the physics of biological flows, such as the way bacteria swim upstream in blood vessels. But when he lost access to his lab during the Covid-19 shutdown, he started playing with his food — literally. He shook up bottles of whiskey, tested the stickiness of pasta and slid coins down slopes made of whipped cream and honey. The interest culminated in a 77-page review, structured like a menu, of the physics involved in making a meal. 'It got totally out of hand,' Dr. Mathijssen said. 'You just realize science is everywhere.' Dr. Mathijssen has since returned to the lab, but the passion for kitchen physics has stuck. The coffee study was partly inspired by a scientist in his group who kept detailed notes about pour-over brews prepared in the lab each day. The notes included information about where the beans had come from, the extraction time and the brew's flavor profile. Ernest Park, a graduate student in the lab, designed a formal experiment. Using silica gel beads in a glass cone, the scientists simulated the action of water being poured over coffee grounds from different heights, recording the dynamics of the system with a high-speed camera. Then they brewed pots of real coffee, pouring from a gooseneck kettle, at varying heights. The resulting liquid was allowed to evaporate in an oven until all that remained were the coffee particles extracted from the grounds. They found that more coffee particles remained when they had poured slowly, which increased the time the water was in contact with the grounds. Holding the kettle higher helped with the mixing, preventing the water from draining along the sides, between the grounds and the filter. This type of flow caused what the researchers described as an avalanche effect. The water eroded the center of the pile of coffee grounds, thus suspending some of the grains, which settled and built up on the sides. Eventually, the sides collapsed inward and the process started again. This increased the flavor extracted from the coffee grounds, but only as long as the water was allowed to flow continuously. 'Your jet of water coming out should look like a smooth column all the way down,' said Margot Young, a graduate student — and former barista — involved in the study. 'If you see it starting to break up, or you can see droplets, then you have to pour from lower down.' The scientists conducted informal taste tests, although these did not make it into the final publication. 'Taste-wise, it's very subjective,' Mr. Park said. 'So we always suggest that you try it yourself.' Mr. Park noted that the study examined only water poured into the center of the coffee grounds, although future experiments could explore other techniques, like making swirls or spirals. Scientific phenomena observed in the kitchen typically have analogues outside its walls. The dynamics between a jet of hot water and a bed of coffee grains, for instance, are similar to the erosion of land that can occur around waterfalls and dams. A stirred pot of soup assumes the same shape as the liquid mirrors of some telescopes. Observations of soap bubbles by Agnes Pockels, a 19th-century German homemaker, gave rise to the field of surface science and laid the groundwork for nanotechnology. In 2022, Dr. Mathijssen helped assemble an array of studies, produced by scientists around the world, into a collection called Kitchen Flows. He is now helping compile a second collection, which so far consists of more than 30 studies, including insights into the behavior of an egg yolk, the sloshing of a bottle of beer and the most efficient way to boil pasta. Dr. Mathijssen also plans to continue exploring the many paths to perfect coffee, such as the physics behind the formation of the milk and espresso layers in a latte. 'I want to do some more work in this direction,' he said. 'And then maybe also something about cold brews.'
USA Today
08-04-2025
- Science
- USA Today
Scientists release instructions for how to make a perfect cup of coffee
Scientists release instructions for how to make a perfect cup of coffee Show Caption Hide Caption Coffee tips to make the best brew at home From storing to grinding beans, here's how to get the freshest cup of coffee at home. ProblemSolved, Reviewed A group of fluid mechanics and physics researchers at the University of Pennsylvania have created what they believe is the best pour-over coffee technique. The key is using a goose-neck kettle and a high pour to achieve an "avalanche" in the ground coffee. The technique results in the maximum amount coffee flavor extracted from the minimum amount of coffee beans. Coffee prices have spiked significantly over the past four years as bad weather reduced production. Add in rising tariffs on coffee-producing countries like Brazil and Columbia and that cup of joe is looking more and more like a morning luxury. But there's a newly-published technique designed to make your coffee grounds go further – courtesy of a slightly over-excited group of physicists and fluid mechanics experts at the University of Pennsylvania. About a year ago, staff in Arnold Mathijssen's lab started pondering how to extract the most coffee using the fewest beans as they hung out at their lab's coffee table. They've just published their findings. Titled "Pour-over coffee: Mixing by a water jet impinging on a granular bed with avalanche dynamics," their 10-page paper in the journal Physics of Fluids was published Tuesday. Fluid mechanics and maximizing coffee extraction "It began around our coffee station, which is basically a space on one of our lab benches," said Mathijssen, a professor of physics at the university. One of his Ph.D. students, Ernest Park, is "the main coffee enthusiast in the lab," Mathijssen said. The lab's coffee-making apparatus include an espresso machine, a French press and a pour-over setup. "But pour-over is the drink Ernest really loves," he said. A bunch of fluid mechanics experts, a lab full of equipment and a whole lot of caffeine: What could go wrong? In this instance, it was, "What could go right?" Experimenting at the coffee station Park initially began simply trying out different methods of pouring the boiling water in his pour-over coffee. As any self-respecting science lab does, the group had a log book at the coffee station to record data about what made the best coffee. "Initially, he was just trying out different things, pouring coffee from different heights and such. Then he said, 'Wait. This tastes good but we need to do the actual experiments," Mathijssen said. They moved the effort to another lab bench and began experimenting with pour height and water stream size, but realized they needed better data to determine exactly what was happening in the coffee filter. "We couldn't really prove what the mechanism was," said Mathijssen. Coffee grounds being opaque – and a fluid mechanics lab being a fluid mechanics lab – the effort quickly ramped up. Soon they had assembled a quantity of small silica gel particles about the size of coffee grounds, a see-through pour-over filter, a laser sheet to illuminate the setup from the side and a high-speed camera to allow them to film and analyze the resulting images using some custom-made Python and Matlab computer code. It's all about the coffee ground avalanche After doing dozens of experiments and analyzing the results, Park, Mathijssen and lab member Margot Young came up with a definitive technique to extract the maximum amount of coffee from the fewest number of beans. In science terms, they were studying "the hydrodynamics of pour-over coffees, especially how a liquid jet interacts with a granular bed of submerged coffee grounds." In layman's terms, they have this advice: Pour the boiling water over the ground coffee slowly, so that the water has more time to be in contact with the grounds. The longer the pour time, the greater the extraction. But not too slowly because that can result in the water not mixing completely with the coffee grounds, resulting in some of the coffee being under-extracted and some over-extracted. Increase the pour height to increase the velocity of the water. This helps create an "avalanche" in the coffee grounds that causes them to be maximally exposed to the boiling water. Insure that the water pours as a stream rather than breaking up into droplets. If you pour too slowly or go too high, the stream breaks and the avalanche stops. You want to maintain a smooth, laminar flow of water hitting the coffee. A thin jet of water – most easy produced from a gooseneck kettle – was the easiest way to get a slow, laminar pour. It's possible to get a thin jet from a regular kettle but harder to do. A slow, laminar pour (meaning a single stream of water and not droplets) resulted in the most flavorful coffee, the scientists determined. But the final part of the experiment was to test the actual amount of coffee solids extracted by analyzing the dissolved compounds. This was done by making cups of coffee using various pouring heights and stream sizes, evaporating the coffee in an oven and then weighing the dissolved solids. Does this add to science's body of coffee knowledge? Admittedly, most of Mathijssen's research is more esoteric than this. Recent papers include "Collective intercellular communication through ultra-fast hydrodynamic trigger waves' and "Enhancement of bacterial rheotaxis in non-Newtonian fluids." The coffee research came about in part because during COVID-19, when their lab was shut down, the team began a food science initiative "because it meant we could do some experiments at home," he said. This resulted in papers such as "Culinary fluid mechanics and other currents in food science," published in 2023. They've since began outreach to local schools to do kitchen science seminars to raise student interest in physics. For the coffee experiments, a literature review showed there had already been research into the fluid mechanics of the liquids involved in espresso machines and also French press coffee makers. "But as far as we know, there's nothing yet on pour-over coffee," Mathijssen said. Until now.
The Guardian
08-04-2025
- Science
- The Guardian
Secret to stronger pour-over coffee with no extra beans unlocked by scientists
Forget expensive beans and pricey filters – if you want a stronger cup of pour-over coffee, just add water slowly, steadily and from a height, researchers say. While there are myriad ways to make coffee – from moka pots to cafetieres and barista-style machines – pour-over coffee is an everyday staple for many. Now scientists say they have discovered how to make a stronger cup using the same quantity of ground coffee. Dr Arnold Mathijssen, a co-author of the study from the University of Pennsylvania, said pouring the hot water slowly from a goose-neck kettle increases the contact time between the water and the coffee grounds, while pouring from a greater height increases mixing, both of which result in more coffee being extracted. But he added: 'If you pour too slowly, or if you go too high, then the jet tends to break up into these smaller droplets, and that's what you want to avoid as well.' Writing in the journal Physics of Fluids, Mathijssen and colleagues report how they carried out a number of experiments involving transparent silica gel particles – representing ground coffee – that were illuminated with a laser sheet, which were recorded with a high-speed camera. While slow pouring increases contact time, the team found that at low heights the velocity of the water was too low to dig into and disturb the bed of particles. Water poured from greater heights resulted in greater agitation of the particles, but the team found that water must flow in an unbroken jet to dig into the bed of particles and displace those at the bottom of the funnel – a process that results in particles building up on the sides and then falling in, creating an avalanche-like effect that increases mixing. 'The increased height compensates for the slow pouring. You only get the avalanche if there is enough energy available,' Mathijssen said. When the team applied their findings to coffee itself, they found pouring from a greater height resulted in stronger brews, provided an unbroken flow of water was used. The team add that a good starting point for those at home is to reduce the amount of ground coffee by 10%, say from 20 to 18 grams per cup, then taste the coffee produced by pouring the hot water at different heights – keeping to a limit of about 30cm for safety. Researchers have previously revealed that to make the perfect espresso every time, it is best to use coffee that is ground slightly coarser than normal. Both that study and the new research found that the proposed adjustments meant less coffee was needed to produce a drink of given strength. Experts say this is important given that climate breakdown is already causing problems for cultivation of the Coffea arabica plant. Prof Jamie Foster, of Portsmouth University, who was not involved with the new research but carried out the study on espresso-making, said it seemed 'totally plausible' that the proposed method would lead to a stronger cup of pour-over coffee and a more optimal use of the coffee grounds, and he saw no reason why similar logic could not be applied to cafetiere-style brewing. 'Of course, there is a cheat open to those who prefer cafetiere coffee,' he said. 'That is, put in a spoon and give the grounds a stir, but perhaps a cleverly chosen pour could save dirtying cutlery.'
