What Is Espresso? Pressure, Crema and How It Differs From Coffee

Espresso is coffee made by forcing hot water through a tightly packed bed of finely ground coffee at about 9 bar — nine times the pressure of the air around you — for roughly 25 to 30 seconds. The result is a small, thick, concentrated drink topped with a layer of foam called crema, and it is the base of nearly every café drink with milk.

The pressure is the whole point. Everything that makes espresso taste and feel different from other coffee follows from it.

What the 9 bar is for

Fine grounds extract fast. Grinding coffee almost to powder gives it an enormous surface area, so water can dissolve its flavour in under half a minute instead of the four minutes a French press takes.

The catch is that a compacted bed of powder is very hard to push water through. Gravity, which drives a pour-over, would take far too long, and the coffee would over-extract on the way. So an espresso machine supplies the force: a pump, or on older machines a spring-loaded lever, drives the water through the puck at around 9 bar.

More is not better. In a 2002 study at the University of Navarra, researchers brewed arabica espressos at 7, 9 and 11 atmospheres. The 11-atmosphere coffees separated clearly from the other two in their foam and taste, while the 9-atmosphere shots stood out for consistent foam and a high share of the aroma compounds linked to freshness and to fruity, malty and buttery flavours. Nine bar is a convention that happens to have held up.

The two definitions

The Istituto Nazionale Espresso Italiano (INEI), which certifies traditional Italian espresso, sets tight numbers for a single shot:

Modern specialty cafés work differently. They typically use 18 g or more in a double basket, weigh the shot instead of measuring its volume, and often use lighter roasts. Both are espresso; the dialing-in guide covers how to work with either.

Within either tradition, shots are named by how much liquid you take for a given dose. A ristretto stops at about 1 to 1.5 times the dry coffee's weight, a normale at about 2 times, and a lungo at 3 times or more. For an 18 g dose that is roughly 18–27 g, 36 g and 54 g or more in the cup.

How concentrated it really is

Strength in coffee is measured as total dissolved solids (TDS): the percentage of the drink, by weight, that is dissolved coffee rather than water.

• Filter coffee is usually 1.15–1.35%, the target range of the Specialty Coffee Association's long-standing brewing standard.
• Espresso usually lands somewhere between 6% and 12%. A typical modern double — 18 g of coffee in, 36 g of espresso out, extracting 20% of the coffee — works out at 18 × 0.20 ÷ 36 = 10%.

So espresso is roughly five to ten times as concentrated. But concentration is not quantity. Do the sum for that double:

• Double espresso: 36 g in the cup at 10% TDS = 3.6 g of dissolved coffee.
• Mug of filter coffee: 250 g in the cup at 1.35% TDS = 3.4 g of dissolved coffee.

A double espresso and a mug of filter coffee contain about the same amount of dissolved coffee. The espresso just carries it in a sixth of the liquid, which is why it tastes so intense. It is also why an americano works: top that double up to 250 g with hot water and it comes out at 3.6 ÷ 250 = 1.44%, a little stronger than typical filter coffee.

Espresso also feels different, not only tastes different. The pressure pushes oils out of the coffee and suspends them in the drink with tiny fragments of cell wall, which is where its heavy, syrupy body comes from. Paper-filtered coffee holds most of both back.

Where crema comes from

Roasting leaves carbon dioxide trapped in coffee. In measurements reviewed by Ernesto Illy and Luciano Navarini, freshly ground coffee held an average of about 5.7 mg of CO₂ per gram. Under 9 bar, the hot water dissolves far more of that gas than it could at normal pressure. When the espresso leaves the basket and the pressure drops, the drink is supersaturated, and the gas comes out of solution as a mass of fine bubbles. Proteins, polysaccharides and melanoidins — the brown compounds formed in roasting — coat those bubbles and keep them from collapsing.

That is crema, and it explains a few things that otherwise seem arbitrary:

• Stale coffee makes little crema. The gas leaks out over the weeks after roasting, and much faster once coffee is ground. One way to measure crema is to time how long it holds a sprinkle of sugar before the sugar sinks through. In an experiment Illy and Navarini cite, crema from coffee ground 36 hours earlier held it for half a second; freshly ground coffee held it for up to 14 seconds.
• Very fresh coffee makes a lot of it. In the first days after roasting so much gas escapes that shots foam up, gush and run erratically.
• Dark roasts hold it for less time. Foam persistence peaked at a medium roast in the studies reviewed and fell off with darker roasting.
• Robusta's reputation is half true. Robusta is famous for crema, but the measurements found no significant difference in foam volume between arabica and robusta. They did differ in how the foam looked and clung to the cup.

Thick, lasting crema is a sign that the coffee was fresh and the shot was brewed under pressure. It is not proof that the espresso tastes good. Illy and Navarini themselves point out that the traditional idea of crema as a quality gauge rests on almost a century of trial and error rather than on reproducible measurement.

There are no espresso beans

Espresso is a way of brewing, not a kind of coffee. The same arabica or robusta can go into a pour-over or an espresso machine.

When a bag says "espresso", the roaster is telling you what they had in mind. Traditionally that means a blend, roasted medium to dark, because darker coffee extracts easily in a short brew and stands up to milk. Many specialty roasters now sell light single-origin coffees for espresso too. They are harder to extract and usually need a longer ratio, but they are no less espresso. The roast levels guide explains what roasting changes.

How much caffeine is in an espresso

The figure you will see quoted most is about 63 mg for a single 30 ml shot, which comes from the USDA's food database. A double is roughly twice that.

Real espressos vary far more. When researchers at the University of Glasgow analysed single espressos bought from 20 coffee shops in the city, published at the end of 2011, caffeine ran from 51 mg to 322 mg — a six-fold range. Four of the twenty were over 200 mg. The biggest factor was most likely simply how much coffee each shop put in the basket. If you are tracking caffeine, an espresso is a much less predictable unit than the tables suggest — the caffeine guide has figures by drink.

Can you make espresso without a machine?

Not quite, and the arithmetic shows why.

A moka pot brews at roughly 1 to 2 bar. It makes strong, concentrated coffee, but not espresso.

An AeroPress is pressed by hand. Its plunger is about 6 cm across, an area of roughly 28 cm². Push down with a hard 20 kg of force and you produce around 0.7 bar. To reach 9 bar you would need about 260 kg of force — more than a quarter of a tonne on a plastic plunger. These are rough estimates, but the gap is not close.

What does work without electricity is a manual lever press. These use a long lever to multiply the force of your arm; the Flair, for example, is built to brew at 9 bar with nothing but a kettle of boiled water and no plug. They are fiddly, but what comes out is genuine espresso.

Where the pressure came from

The earliest Italian espresso machines, built from the early 1900s by makers such as Bezzera and Pavoni, used steam pressure from the boiler to push water through the coffee. Steam could not produce much pressure, and what crema the coffee had was thin.

The change came after the Second World War. Achille Gaggia patented a lever-and-piston mechanism: the barista pulled a lever that loaded a spring, and the spring drove a piston that forced water through the coffee — usually at 8 to 10 bar. Shots took 25–30 seconds and came out with a creamy layer on top, which Gaggia sold under the slogan crema caffè naturale. It is also why we still talk about pulling a shot.

In 1961 the Faema E61 replaced the spring with an electric pump and added a heat exchanger and automatic pre-infusion. Pump machines built on that pattern became the standard in cafés, and most home machines work the same way. The espresso machine guide explains what to look for in one.

Espresso is a base, not a destination

Most espresso is never drunk as espresso. It becomes a cappuccino, a latte or a flat white — the milk drinks guide explains how they differ — or an americano with hot water. Its concentration is exactly what makes it work there: a shot of filter-strength coffee would disappear into 200 ml of milk. An espresso does not.