CO2 (in coffee)
also: carbon dioxide, degassing gas
Carbon dioxide trapped during roasting that drives the bloom and crema, then fades as coffee degasses.
CO2 is the gas created inside the bean during roasting, mainly through caramelization and the Maillard reaction. Fresh roasted coffee is packed with it, and over the following days and weeks it slowly escapes in a process called degassing.
That trapped gas drives two of the most visible moments in brewing. When hot water first hits fresh grounds, the CO2 rushes out and the bed puffs up: this is the bloom. Blooming, then pausing, lets that gas vent so water can soak the grounds evenly instead of being pushed away by escaping bubbles. In espresso, the same dissolved CO2 emulsifies with oils under pressure to form crema.
The catch is timing. Too fresh, and aggressive degassing makes brews erratic and shots gush; too old, and there is little gas left, so the bloom is feeble and crema is thin. This is why many brewers rest beans a few days off roast and why staleness shows up as a weak bloom. For the why and how of blooming, see why-do-i-need-to-bloom.