Degassing

Degassing is the process of roasted coffee shedding the carbon dioxide created during roasting. Beans trap a surprising amount of CO2, and they release it gradually over days and weeks once they hit air. This is why a one-way valve appears on most coffee bags: it lets gas out without letting oxygen in.

Why it matters for your cup: very fresh coffee, say one to three days off roast, is so gassy that the CO2 fights the water during brewing. The grounds bubble and resist wetting, leading to uneven, often sour extraction. That is exactly why the bloom step exists, and why a short rest helps.

For most filter coffee, the sweet spot is roughly 4 to 14 days after the roast-date, though darker roasts settle faster and lighter roasts can need longer. Espresso is even fussier: too-fresh beans gush gas and produce wild, unstable shots. Degassing also feeds crema, so the freshest beans foam the most. See why-freshness-matters for the wider picture.