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When hot water first hits freshly ground coffee, the dissolved CO₂ from roasting rushes out, causing the grounds to puff up — the "bloom." Pouring just enough water to saturate the grounds (roughly twice the dose by weight) and waiting 30–45 seconds before continuing the pour gives the gas time to escape so the rest of the brew can extract evenly. The bloom is also the first sensory check: a vigorous bloom means the coffee is fresh.