The bloom is the first stage of a pour-over brew. You add a small amount of water to the dry grounds (typically twice the weight of the coffee), wait 30 to 45 seconds, then continue with the rest of the brew.
Why bloom matters
Roasted coffee contains carbon dioxide trapped during roasting. When hot water hits the grounds, that CO2 escapes vigorously. The visible foam and bubbling you see during the bloom is gas leaving the coffee. If you skip the bloom and pour all the water at once, the escaping gas pushes water around the grounds rather than through them, creating uneven extraction.
How to bloom properly
Use a 1:2 ratio of water to coffee for the bloom. With 18g of coffee, pour 36g of water. Pour in a slow circular motion, wetting all the grounds. Avoid pouring water down the sides of the filter where it can bypass the coffee bed.
Then wait. Most brewers benefit from a 30-45 second bloom. Watch the surface: when the bubbling has mostly settled and the bed has stopped expanding, you can start your next pour.
The bloom tells you about freshness
Fresh coffee blooms vigorously, sometimes doubling in volume. Coffee that is one to four weeks past roast date will still bloom but more modestly. Coffee that barely blooms at all is stale; the CO2 has long since escaped, and you will likely brew a flat-tasting cup regardless of technique.
If your coffee is not blooming much, consider that an early warning that the cup will be lifeless. The bloom is both a brewing step and a freshness indicator.