Crema is the thin, reddish-brown foam that sits on top of a freshly pulled espresso. It is mostly carbon dioxide gas that formed when the beans were roasted: hot water forced through the coffee at about 9 bar of pressure dissolves that gas, and when the brew leaves the machine at normal pressure the gas comes out as tiny bubbles. The liquid around the bubbles is an emulsion of microscopic coffee-oil droplets in water carrying dissolved sugars, acids, protein-like compounds, caffeine, and fine cell-wall fragments.
A well-made espresso is expected to have crema making up at least 10% of the cup’s volume. Freshly roasted, freshly ground coffee holds more carbon dioxide and so produces more crema, while stale or pre-ground coffee produces less.
Source: Illy & Navarini, “Neglected Food Bubbles: The Espresso Coffee Foam,” Food Biophysics (2011)
Image: CremaWithSwirls by HSwaff, CC BY-SA 4.0, via Wikimedia Commons.
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