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Soft matter and liquids

Soft matter and liquids

Brewing coffee

10 Nov 2016 Robert P Crease
Taken from the November 2016 issue of Physics World

There’s far more physics to a humble espresso machine than meets the eye, as Robert P Crease finds out

Simple but complex

Peter Stephens pours water into the pressure vessel of my espresso maker, screws on the top, and presses a switch. An experimental physicist at Stony Brook University, he’s dropped by my office to drink coffee and discuss physics. He knows my device well. It was once his; he gave it to me after being appointed associate dean and moving to a larger office with its own coffee maker as a literal perk.

At first glance the Europiccola device – made in Italy by the long-standing Milanese firm La Pavoni – looks like it belongs in a museum of 19th-century physics equipment. About 25 cm big and made of brass, its main feature is a pressure vessel sprouting gauges, tubes, knobs and a manual lever over a bell-shaped attachment. The only hint it’s a 20th-century device is a 110 V electrical cord trailing from the base.

After a minute, the device begins to groan and make soft pops like it’s muttering to itself. When a thin ribbon of steam curls above the safety valve, Stephens announces: “We’re almost ready!” To be sure, he twists the knob on the steamer, which responds by issuing a confident jet. He pumps the lever, pushing a shot of coffee into our cups. An invigorating aroma wafts into the corridor.

“Cheers!” Stephens says as we drink up.

Functioning antique

Coffee is often the fuel of choice for physicists wishing to stay alert to solve a thorny theory or eke out a crucial experimental finding. But most rely on coffee from a pot or vending machine, and it’s a shame not everyone can have a once-popular Pavoni. Mine gets us chatting about the physics of coffee-brewing.

The Pavoni’s main parts are a pressure vessel to heat water, a lever-driven piston and a filter basket. As the water heats, some turns to steam, which drives hot water into the piston chamber mounted on the pressure vessel. Raising the lever pulls up the piston, opening a valve that lets water flow into the chamber. Pressing the lever down forces this water through coffee grounds in the filter basket into one or two cups waiting below.

Yet the Pavoni’s interesting physics, Stephens explains, has nothing to do with Boyle’s law or the bare mechanics of the water flow, but with other aspects of coffee-making. Take the temperature differential between the water in the pressure vessel and the water flowing through the coffee grains. The temperature of the former is about 120 °C, while the optimal temperature of the latter is 91–97 °C. Any cooler and the liquid absorbs the coffee poorly; any hotter and the water burns the grains and makes the coffee taste bitter. The piston cylinder, however, serves as a heat sink, making the water temperature drop.

There’s also interesting physics in how substances get extracted from the coffee grains into the liquid. Coffee beans are made of hundreds of different types of molecule that give the drink its flavour. Brewing an espresso – “quick” in Italian – involves passing water through ground roasted beans. The trick is to extract the tasty flavours and the caffeine, which tend to come out quickest, and leave behind the bitter compounds. It’s a problem that sends Stephens to the blackboard to draw flow diagrams.

Extraction depends on the grains’ size and shape, he explains, as well as the density of their packing, the water temperature and the liquid flow rate. The larger the grains, for instance, the smaller the surface-to-volume ratio, diminishing the percentage of substances absorbed. But if the grains are too fine they turn into a wet paste, slowing the flow.

The Pavoni leaves most such factors to the user: other machines, in contrast, use pods with pre-ground beans, gauges to control the water temperature, and springs or pumps to regulate the water flow. I’ve heard it dismissed as a “functioning antique” doomed to be retired to a user’s storage closet. I kind of get it: a coffee maker shouldn’t act like a finicky experimental facility. But I like that the user is in control of the process rather than it being a matter of inserting pods, adjusting settings and pushing buttons.

The critical point

My Pavoni is neither the simplest nor the most complex home espresso maker. A good account of their history is found in Ian Bersten’s book Coffee Floats, Tea Sinks: Through History and Technology to a Complete Understanding (1993 Helian). Mechanically simplest is the famous stovetop Moka. You pour water in the base, insert a basket with grounds in, screw on an upper chamber and put the whole pot on the stove. Steam drives the hot water up through the grains; it’s flow-up rather than drip-down. I bought my first at a flea market in Rome for about $1 as a grad student and have used it for decades.

Even in this simple device the physics is complicated, as Stephens and I discover (2008 Am. J. Phys. 76 558). It hadn’t occurred to us, for instance, that the pressure in the bottom vessel would be anything other than the saturated vapour pressure of the heated water. But because some air is initially present, the rising temperature creates an “overpressure” through plain old PV = Nrt. “The more mechanically simple, the harder to analyse,” Stephens says.

Stephens has to return to the dean’s office before we get round to discussing how to froth milk or roasting and grinding beans. “These are the kinds of things that interest me about the natural sciences,” Stephens says as he leaves. “Exploring the interface between what is readily quantified – pressure, temperature and volume – and complex intangibles like the flavour of a cup of coffee.” I won’t look at a cup of coffee in the same way again.

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