The reddish-brown layer of foam that sits on top of a freshly pulled espresso shot. Made of CO₂ trapped inside a colloidal suspension of microscopic coffee oils, melanoidins, and dissolved compounds. Crema only happens under pressure — pour-over and immersion brews don't produce it because they don't extract under nine bars.
Why it's there
Espresso machines force water through finely-ground coffee at ~9 bar. That pressure does two things at once: it dissolves a lot of compounds quickly (which is why an espresso is 8-12% TDS instead of filter's 1.3%), and it forces dissolved CO₂ out of the bean's interior into the brewed liquid. As the espresso exits the basket and pressure drops, the dissolved gas comes out of solution as tiny bubbles. The bubbles get stabilized by surfactant compounds — coffee oils, proteins, melanoidins — and form a persistent foam.
Older beans have less CO₂ left inside them and produce thinner, less persistent crema. Very fresh beans (under 5 days off-roast) can produce too much crema, with a wild gas-rich shot that's hard to dial in.
What crema doesn't tell you
A common myth: thick crema means a good shot. It doesn't. Crema thickness is mostly a function of bean freshness and roast level. A perfectly extracted shot from a 21-day-old light roast can have minimal crema; a poorly extracted shot from a 5-day-old dark commercial blend can have a thick, dramatic crown. Looking at crema tells you about the bean's gas state, not about the cup's quality.
What crema is useful for: as visual feedback during a pull. Even, slow-flowing crema = even extraction. Streaky, uneven crema = channeling. The colour of the streaks (white = under-extracted, dark brown = over) gives you a fast read.
For filter coffee, crema is irrelevant — it doesn't form. The bloom is its conceptual cousin: same gas escaping, just into the brewer rather than into your cup.