Roasting produces carbon dioxide inside the bean, and roasted coffee holds a lot of it. Wetting the grounds releases it fast, which is the foam and swelling you see.
Why it happens
The Maillard and caramelisation reactions during roasting generate CO₂, which is trapped in the bean’s cell structure.
Grinding releases some immediately. Water releases the rest.
It is a freshness gauge
A dramatic, doubling bloom — the coffee is within a week or two of roast. A modest rise — three to five weeks. Almost nothing — well over a month, or the coffee was stored badly.
This is the most reliable freshness test available without a roast date.
Why it is a problem while brewing
Escaping gas physically pushes water away from the grounds, so the bed does not wet evenly and extraction is lower and patchier.
That is what the bloom step is for: let the gas out first, then brew.
In espresso
Very fresh coffee — under four days — can make espresso behave strangely: gassy, blonding early, with an unstable flow and huge crema that tastes of nothing.
Most espresso is at its best 7 to 21 days from roast, once it has settled.
The valve on the bag
The one-way valve lets CO₂ out without letting oxygen in. A bag that puffs up is doing exactly what it should.
Last reviewed 8 August 2026