Crema is the term for the layer of foam found on the surface of a shot of espresso coffee. Generally golden to dark-tan in color, this foam is a result of several factors in the brewing of the espresso, from the type of bean to the pressure employed by the espresso machine. While the crema does not contribute to the taste of the coffee, it is often considered an indicator of the espresso's quality.
Crema was not a common attribute of espresso until 1948 when Achille Gaggia began to market an espresso machine employing a lever-driven piston which provided more pressure to force water through a more tightly packed puck of coffee than could be used in contemporary models. Gaggia billed the resulting foam as Crema Caffe Naturale or natural coffee cream, possibly using it as a selling point to indicate extra-richness in the coffee. From that point, a layer of crema became a requirement for a quality espresso.