Crema is a dense foam that floats on top of a shot of A small coffee beverage, about 20 ml, prepared on an espresso machine where pressurized hot water extracted through compressed coffee.: In its most stripped-down, basic form, this is a working definition for espresso: A small More. It ranges in color from blond to reddish-brown to black. Blond crema may be evidence of under-extraction or old coffee, while black crema is a sign of over-extraction or an overly hot boiler. Crema like other foams in the result of the physical phenomenon called surface area tension.