Espresso is a very complex system for brewing coffee from its powder. The powder (approx. 7 grams per cup) has to be ground very finely to create resistance against the water used to brew it. This water has a very strong brewing power, because it has roughly 92-94°C of thermal power, much of which is spent during brewing, and about 10 bar of input pressure, and this dynamic energy is completely spent during brewing as the Espresso is delivered from the spout at 0 bar.
All this energy allows the water to penetrate the coffee cells and strongly extract the content which we love to find in the cup: the aromas which combine with other gases to form the Espresso froth or “cream”, the sugars and fats that dissolve and emulsify in the liquid part of the Espresso, and also some microscopic woody particles which give the froth its “stripy” effect.