Appliance Science: The high-pressure physics of espresso coffee

What happens when engineers make coffee? Espresso. Appliance Science looks at the physics of this beloved (and reviled) coffee drink.

Richard Baguley
Richard Baguley has been writing about technology for over 20 years. He has written for publications such as Wired, Macworld, USA Today, Reviewed.com. Amiga Format and many others.
Colin McDonald
Essentially born with a camera in hand, Colin West McDonald has been passionately creating video all his life. A native of Columbus, Ohio, Colin founded his own production company, Stoker Motion Pictures, and recently wrote and directed his first feature film. Colin handled photography and video production for CNET's Appliance Reviews team.
Richard Baguley
Colin McDonald
4 min read

Espresso by Flickr User Scott Schiller Scott Schiller

There are two schools of thought on espresso coffee. One holds that this type of coffee is an elixir, a powerful, life-giving shot that tastes like paradise. The other is that it is a bitter, overly strong mess that only hipsters and caffeine addicts love. I am a member of the former school, but I have to concede that the latter school may have a point when it comes to badly made espresso. That's because the process of making espresso is complicated and finicky: get something wrong and you've ruined the delicate balance. Espresso is, to coin a phrase, what happens when engineers make coffee.

Most modern coffee is made by the drip method, although other methods exist (such as percolators and the boiled or cowboy-style coffee). In the drip method, the roasted beans are ground to a loose consistency, placed in a filter and hot water is poured over them. As the water passes through the ground beans, it absorbs their flavors and other chemicals, then passes through the filter and drips out into a container. As any coffee nerd will tell you, that's a gross simplification of the process, but the idea is that the end result is the liquid that contains the flavors of the beans, absorbed by the hot water as it passes through the ground beans, but not the grounds themselves.

Espresso coffee is similar, but with most of these conditions amped up. The coffee beans are more heavily roasted, giving them a darker color and creating more flavor. They are ground much finer, producing a texture more like a powder than the sandy grain of drip coffee. And the brewing process uses much hotter water at a much higher pressure, which creates a very different brew that extracts more of the flavor of the beans.

The espresso machine

First developed in the late 19th and early 20th century, the espresso machine looks quite different from the drip coffee maker. Although the details differ, the core components of most machines are the same: a steam generator that boils water to create steam, the group head that collects and pressurizes the steam, and the portafilter that contains the basket that the ground beans sits in.

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How to brew espresso

To brew, you grind the coffee beans to a fine consistency, then put them into the basket. The beans are then compressed using a tamper to form a more solid lump, usually called the coffee puck. The basket goes into the portafilter, and the portafilter fits into the base of the group head, forming an airtight seal.

Colin McDonald/CNET

Next, the water is boiled to form steam. This combination of hot water and steam is pumped into the pressure chamber, above the screen. This is then compressed to somewhere from 8 to 10 times that of normal atmospheric pressure, at a temperature just below boiling point.

This hot water is forced through a metal screen covered in fine holes to stop any debris getting through, and to make the flow more even. Below the screen, the water enters the coffee puck, where it picks up the coffee flavor. On the bottom of the basket is another set of fine holes that the water passes through, then a funnel on the bottom of the portafilter directs it into the cup.

And then you have your final espresso: a small cup of a dark, thick brew. Most fans refer to this as a shot, and the process is often called pulling a shot, after the old-school manual espresso machines where the pressure is produced by pulling a lever, rather like an old-fashioned slot machine. This lever compresses the steam, but most modern espresso machines have replaced this with a series of pumps and valves that create the pressure automatically.

Colin McDonald/CNET

One interesting feature of the espresso brewing process is that it produces a foam that sits on top of the shot, called the crema. This is produced by the oils that are forced out of the ground beans by the pressure, forming a colloid where small drops of oil are suspended in water. This is one of the unique features of espresso, and it is one thing that connoisseurs of espresso use to judge the quality of a shot. A good crema should be mostly dark, but with some white stripes that give it an almost tiger-stripe look, with small bubbles. The crema should last as long as the shot itself, persisting as you drink it.

Espresso is a great example of what happens when people experiment with food, looking for new ways to tinker with the process and create new and interesting foods. That's what appliance science is all about, and how science can make things taste better. Now, if you'll excuse me, it's about time for another cup...

Correction: Monday, 2:30pm EST: Altered the paragraphs starting "First developed in the late 19th and early 20th century" and "Next, the water is boiled" for clarity on the process.