First, and most importantly, espresso is not a roasting method. It is neither a bean nor a blend. It is a method of preparation. More specifically, it is a preparation method in which highly-pressurized hot water is forced over coffee grounds to produce a very concentrated coffee drink with a deep, robust flavor. While there is no standardized process for pulling a shot of espresso, Italian coffeemaker Illy’s definition of the authentic espresso seems as good a measure as any:
A jet of hot water at 88°-93° C (190°-200°F) passes under a pressure of nine or more atmospheres through a seven-gram (.25 oz) cake-like layer of ground and tamped coffee. Done right, the result is a concentrate of not more than 30 ml (one oz) of pure sensorial pleasure.
For those of you who, like me, are more than a few years out of science class, nine atmospheres of pressure is the equivalent to nine times the amount of pressure normally exerted by the earth’s atmosphere. As you might be able to tell from the precision of Illy’s description, good espresso is good chemistry. It’s all about precision and consistency and finding the perfect balance between grind, temperature, and pressure. Espresso happens at the molecular level. This is why technology has been such an important part of the historical development of espresso and a key to the ongoing search for the perfect shot. While espresso was never designed per se, the machines –or Macchina– that make our cappuccinos and lattes have a history that stretches back more than a century.