What is bloom in coffee brewing?

Bloom - sometimes called pre-infusion - is the initial stage of brewing where a small amount of hot water is poured over ground coffee and left to rest for 20-45 seconds before the main pour begins. During this time, the coffee releases a visible surge of CO₂ gas, causing the grounds to bubble and swell.

The bloom matters because freshly roasted coffee contains significant amounts of CO₂ trapped inside the bean structure during roasting. If you pour all your water at once without blooming, that CO₂ can interfere with extraction - the gas escaping from the grounds creates channels and prevents water from saturating the coffee evenly, leading to an uneven and often underwhelming cup.

For home roasters, the bloom is one of the clearest indicators of freshness. Very fresh coffee produces an energetic, domed bloom that rises dramatically from the grounds. Older coffee blooms less vigorously, or barely at all. If you've just roasted a batch and the bloom is dramatic, the coffee is probably still too fresh - most roasters recommend waiting at least 5-7 days after roasting before brewing, and longer for espresso, to allow CO₂ levels to stabilise.