Appliance Science: The fluid dynamics of the AeroPress coffeemaker

I'm a big fan of coffee. This wonderful tasting brew is one of the things that gets my tired, old body moving. I'm also a big fan of espresso, the more-concentrated shot of coffee goodness. But honestly, making espresso is a pain. I used to own a fancy fully automatic espresso maker that made it easier, but it required maintenance and constant tweaking, and it was expensive. Eventually, I gave up, switching over to a new device that produces coffee with a similar taste using just hand pressure and some ingenious design: the AeroPress. This neat device shows how some smart thinking and creative design can make an appliance that is simple, but works just as well as the more complex devices.

I have described the process of making espresso in some detail in a previous column, but the short version is very hot water forced through finely ground coffee at high pressure. The upside of this process is that it extracts more of the coffee essence from the grounds. The downside is that it is complex, fiddly and a pain to clean out afterwards. A more conventional drip coffee maker slowly drips hot water into a filter filled with coffee grounds, and the water absorbs the coffee taste. The AeroPress is somewhere in between, taking aspects of both processes and simplifying them, producing a drink that is somewhere between espresso and more conventional coffee.

It was invented by maverick engineer Alan Adler, professor of mechanical engineering at Stanford University. The AeroPress isn't his first invention: he also created the Aerobie, a frisbee-type product that holds the world distance record for the "longest throw of an object without any velocity-aiding feature" (1,333 feet). Over dinner one day, he was apparently discussing how conventional coffeemakers were great for making large amounts, but not one cup. After several years of experimentation, he came up with the AeroPress, which is designed for making one cup at a time.

The way it works is ingenious, using just hand pressure to force the water through the ground beans. While drip coffee makers use gravity to pull the water through the coffee grounds, the AeroPress uses air pressure, forcing the water through by pushing the plunger down.

The AeroPress is a simple device, with a chamber made from tough, heat-resistant plastic. A plunger fits into the top of this, forming an airtight seal. On the base of the cylinder, a filter cap screws securely into place, holding the filter and the coffee grounds in place.

To use it, you put the coffee grounds and water into the cylinder, stir, then place the plunger in the top. After letting it brew for a few seconds, you press the plunger down slowly, forcing the water through the coffee grounds and the filter, into the cup below.

This has two advantages over drip coffee: speed and efficiency.

In a drip coffeemaker, the grounds are soaked in hot water for several minutes. When hot water is in contact with coffee grounds for too long, it causes some of the chemicals that you don't want in the coffee to dissolve into the water, such as bitter chemicals like quinic acid and furfuryl alcohol that ruin the taste of the coffee. In an espresso maker or AeroPress, these chemicals usually stay in the grounds, because the hot water is only in contact with the grounds for a short time.

The taste of the coffee comes from the chemicals that are extracted from the ground beans by the water. A higher pressure on this water means that more of these chemicals are extracted, so you get more taste from the coffee, and use less, making it more efficient. It also means that that most of the water has been forced out of the coffee grounds, making it easier to clean (I usually just rinse mine off under the tap after use).

The advantages over espresso are obvious: it is simpler, cleaner and a lot less hassle. You don't need to have a complex mechanism to pressurize the water, or use difficult-to-clean filters: it uses a simple paper filter. I just throw the used paper filters out with the coffee grounds, as at $4.99 for 250, they aren't expensive.

The AeroPress proved to be a big hit, especially among coffee connoisseurs who like to be able to control every aspect of the process. The procedure is simple, but it is also consistent, and these blenders of the sacred bean love to be able to control everything about their precious brew. There is also a yearly championship for the best AeroPress brew, with the winners offering a variety of approaches to using this simple machine.

So, the AeroPress is a great example of what happens when a scientist takes a new look at an old problem. By throwing out the conventional wisdom on the "best" way to make a cup, Alan Adler came up with a new approach that is simpler and tastier. And that is how appliance science should be.