The release of CO₂ trapped inside roasted coffee beans. Roasting produces gas as a by-product of the Maillard reaction; that gas slowly escapes through the bean's cell structure over the days and weeks after roast. Coffee is "degassing" the entire time it sits in the bag.
This matters for brewing because CO₂ in the bed disrupts extraction. When water hits fresh coffee, the gas violently escapes — that's the bloom — pushing water away from the grounds and creating channels. The fresher the coffee, the bigger the bloom.
The freshness curve
Right after roast, beans are gas-saturated and almost unbrewable — too active, too erratic. By day 4–7, degassing has slowed enough that water can interact with the bed normally; this is the start of the peak window. Most coffees taste best between days 7 and 21 post-roast.
After three weeks, degassing has mostly finished. The cup begins to lose aromatic intensity and the upper-end florals first, then the fruit, then the body. By two months off-roast, the coffee tastes flat and stale even if the bag has been sealed.
What changes about brewing
Younger beans need a longer bloom (45–60 s instead of 30) and slightly higher temperature to compensate for gas-driven channeling. Older beans behave more predictably but give you less to work with — fewer notes to coax out, less aromatic ceiling.