Espresso machines use pressure and hot water to extract concentrated coffee from finely ground beans. During this process, micro-air bubbles form and attach to the oils of the coffee and rise to the top. They form a creamy-looking, aromatic, flavorful foam that rests on the top of an espresso shot. This layer of silky, velvety microbubbles is called “crema” and is the sign of perfectly pulled espresso.