Crema

Crema is the layer of fine, reddish-brown foam that sits on top of a freshly pulled espresso. It forms when the ~9 bars of pressure in an espresso machine force hot water through finely ground coffee: dissolved carbon dioxide (from recent roasting) and coffee oils emulsify into a foam of tiny bubbles.

Good crema is a sign of fresh coffee and a reasonable extraction, but it is not a measure of quality on its own. Two things inflate it in ways that do not always mean a better cup:

• Freshness. Recently roasted beans hold more CO2 (see degassing), so they produce more crema. After a few weeks, crema thins out even if the coffee still tastes fine.
• Robusta. Robusta beans naturally generate more crema than arabica, which is one reason traditional Italian blends include some.

So a thick crema can come from a great fresh single-origin arabica or from a stale-tasting robusta blend. Judge the coffee by taste; treat crema as a hint, not a verdict. It typically dissipates within a minute or two, and stirring it back into the shot is normal.