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Bloom is the pre-infusion step in pour-over brewing where you pour a small amount of water over the grounds and wait before continuing. The coffee swells, bubbles and releases gas. It looks dramatic and purposeful, and it is — skipping bloom produces a measurably less even extraction in freshly roasted coffee.
During roasting, chemical reactions produce large amounts of CO2 gas that becomes trapped inside the bean. After roasting, this gas slowly off-gases through the bean's pores — a process called degassing that continues for days or weeks. Freshly roasted coffee contains a lot of CO2; stale coffee has much less. When ground coffee contacts hot water, the CO2 is released rapidly. In espresso, this is partly why the crema forms. In pour-over, it creates the bubbling, swelling dome you see during bloom.
CO2 is hydrophobic — it repels water. When brew water contacts coffee grounds that are still releasing CO2 rapidly, the gas creates a barrier that prevents water from fully saturating the grounds. If you pour all your brew water immediately into freshly roasted coffee, some of the water flows through channels created by escaping gas rather than flowing evenly through the coffee bed. The bloom step allows most of that CO2 to escape first, so subsequent pours saturate the grounds evenly and produce a more consistent extraction.
Use approximately twice the weight of water to coffee for the bloom pour — for an 15g dose, pour about 30g of water. Pour gently over all the grounds, ensuring every part of the bed is wetted. Then wait. The coffee will swell, bubble and push upward. The bloom lasts 30-45 seconds for most coffees, though very fresh coffee (roasted within a week) may bloom more vigorously for longer.
The bloom is also a useful freshness indicator. Fresh coffee (7-21 days from roast) blooms actively: the grounds swell visibly, CO2 bubbles vigorously to the surface, and the dome holds its shape for most of the waiting period. Stale coffee barely blooms: the grounds wet immediately without much swelling, and there is little or no bubbling. If your coffee does not bloom at all, it is either very old or was stored badly. This is the same reason espresso from stale coffee has little or no crema.
For coffee that is more than 3-4 weeks old, CO2 has largely off-gassed already and bloom has little practical effect on extraction evenness. You can skip it or shorten it. For very fresh coffee (within a few days of roast), some baristas extend the bloom to 45-60 seconds to allow more gas to escape before the main pours. If you are making coffee in a batch brewer or French press where agitation compensates for uneven saturation, bloom is less critical than in V60 where flow distribution depends entirely on even wetting.
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