Why does coffee need to degas after roasting?

During roasting, a bean accumulates 2 to 4 ml of CO2 per gram, trapped inside its cellular structure. That gas leaves the bean gradually afterwards — quickly in the first 24-72 hours, then slowly over 2 to 6 weeks. Used too early, coffee disrupts extraction: bubbles, channelling, harsh acidity and underextraction. We call this 'degassing' or 'resting' — typical window 5 to 14 days for filter, 7 to 21 days for espresso.

CO2 is a direct by-product of Maillard, caramelisation and pyrolysis reactions. A roasted bean releases 40-50 % of its CO2 in the first 24 hours after the cooling tray, another 20-30 % over the following week, and the rest across several weeks. The release curve depends on roast degree: a dark roast produces more CO2 and releases it faster than a light roast because cell walls are more porous. An Ethiopian light roast can hold residual CO2 for 6-8 weeks post-roast.

In extraction, undischarged CO2 physically blocks water from reaching soluble compounds: it creates bubbles, inflates the coffee bed and redirects water channels. On a V60, this shows up as a powerful bloom (the 'hot-air-balloon' rise); on espresso, it yields a thick, very dark crema but often an unbalanced, sharp, underextracted shot. Specialty roasters typically recommend: filter 5-14 days, espresso 7-21 days, keeping in mind that under 3 days the coffee is 'too wild' and beyond 45 days aromatic freshness begins to decline seriously.

Specialty bags have been fitted with a 'one-way valve' since the 1970s, an invention credited to Italian engineer Luigi Goglio. The valve lets CO2 out but keeps oxygen out, which allows packaging right after roasting with no bursting risk. Without it, early airtight bags would swell until they split. Technical note: if you seal freshly roasted beans into an airtight container without a valve, internal pressure can exceed 1.5 bar within 72 h. In Belgium, micro-roasters in Brussels, Ghent and Liège almost all print an explicit roast date on the bag, and often a recommended consumption window. That transparency is absent on industrial supermarket coffee, which is usually roasted months before it reaches the shelf.

Recommended degassing windows