Anyone who loves espresso recognizes the thick, pale foam at the top of a freshly pulled shot - crema. Love it or hate it, it's a part of the espresso experience. But what is it and why is it only associated with espresso?
Crema is only produced during the process of making an espresso because it is a byproduct of the pressure needed to extract a small amount of coffee in such a short amount of time. During the roasting process, coffee produces carbon dioxide, most of which is released during the coffee's "resting period" after roasting. However, there will always be some residual carbon dioxide in the beans and depending on how fresh the coffee is when making an espresso, that carbon dioxide is released under the pressure of the process of making the shot of espresso.
Carbon dioxide is a gas. When gas mixes with liquid it produces thousands of tiny bubbles. The bubbles are mixed with coffee compounds and produce a dense, semi-stable foam that rests at the top of a brewed cup of espresso. The amount and thickness of the foam varies based on the type of beans and the freshness. There is no other brewing method that produces crema since high pressure is required and only espresso uses that much pressure. Although, I have made a few Aeropress shots and have produced a very small amount of crema-like foam, but it always dissipated very quickly.
Does crema taste good?
This is where your own taste preferences play a huge part in what you prefer as far as crema goes. If you have ever taken a spoon and skimmed off just the layer of crema on an espresso and tasted it, you probably were not impressed with the flavor. It's bitter and quite harsh. Most baristas suggests stirring the crema into the espresso - thus diluting it, somewhat, and tempering the bitterness with the sweetness of the coffee. Some would argue that bitterness plays a role in espresso, maybe a large role - but again, that is a personal preference. Bitterness can be tolerated when diluted in the cup, but in isolation at the top, where your tongue first meets the cup, it can be jarring.
Occasionally, espresso drinkers might skim most of the crema off before stirring the cup before their first sip. This help makes the espresso more sweet, but also reduces the perceived thickness (mouthfeel) of the drink - which in espresso, is a trademark quality.
Why the name crema?
Crema isn't cream, right? No. But in 1948, the "father of espresso" Giovanni Gaggia introduced a new type of coffee machine that a great deal of pressure to extract coffee from a small amount of grounds. During that process, people noticed there was a pale yellow "scummy foam" at the top of each cup. He decided to use this a a marketing tool and named it "Crema Caffé Naturale" - natural coffee cream. The name stuck and was just shortened to "crema" to describe the foam at the top of an espresso.