Bloom
also: blooming, pre-wet
The 30 to 45 second rest after first wetting grounds, letting CO2 escape so water extracts evenly.
Bloom is the first stage of a pour-over (and many drip brews) where you pour just enough water to wet the grounds, then pause for roughly 30 to 45 seconds before continuing. The name comes from the way the bed swells and bubbles as trapped carbon dioxide escapes.
Why it matters: fresh coffee holds a lot of CO2 from roasting (see degassing). If you pour all your water at once, that gas pushes back against the water, creating dry pockets and uneven contact. The result is patchy extraction, often tasting both sour and harsh at the same time.
A good bloom uses about two to three times the coffee weight in water (for example, 30 to 45 g for 15 g of coffee). A gentle swirl or stir helps wet every particle evenly, a mild form of agitation. Very fresh coffee blooms vigorously; older coffee barely reacts, which is itself a clue about freshness. Once the bubbling slows, continue your main pours.