Coffee Blooming (Pre-Wetting)

Pre-wetting (blooming) is the step just before you add most of the water to your coffee grounds. To make a bloom, a little water is poured into the ground coffee. Then you wait as the grounds become saturated. This allows the rapid release of carbon dioxide from the coffee.

The volume of water for pre-wetting is calculated based on the ratio of 2-3 grams of water to 1 gram of coffee. That is, for a 15 gram serving of coffee, you need 30 grams of water for pre-wetting. Typically pre-wetting lasts 30-40 seconds. When this time expires, you continue brewing by pouring the rest of the water over the grounds.

According to Maxwell Colonna-Dashwood, author of The Coffee Dictionary, presoaking is a measure of the freshness of the roast, not the quality of the brew. And James Hoffmann, author of The World Atlas of Coffee, points out that there is no scientific basis to prove the need for pre-wetting. However, some research does indicate that pre-wetting promotes the release of carbon dioxide and helps the coffee to brew better.

As it gets wet, the ground coffee increases in volume. This facilitates access to soluble substances, and coffee oils come to the surface. Pre-wetting helps stabilize the brewing process and improves extraction quality.

Why Bloom?

As a rule, pre-wetting is done during the preparation of freshly roasted coffee in order to artificially enhance the degassing process, make the brewing process more controllable, and the coffee more tasty.

Within 2–3 weeks after roasting, coffee actively releases carbon dioxide. When we grind coffee beans, the rate of release increases dramatically due to the increase in the surface area of the coffee. Then, when we pour in hot water, the rate of release increases even more.

Pre-wetting is necessary so that most of the gas is released before the start of the main brewing stage:

  • If carbon dioxide remains in the coffee beans during the extraction process, the coffee will be overly acidic. Removing excess carbon dioxide will produce a tastier coffee.
  • Escaping carbon dioxide repels water and interferes with the brewing process. Water can freely extract aromas and oils from coffee only after the carbon dioxide has escaped. However, if gas is released too quickly, it prevents the water from evenly contacting the ground coffee. As a result, the extraction will be uneven. To avoid this, you need to bloom the coffee.

The pre-wetting process is influenced by water temperature, humidity, roast degree, and the hardness and origin of the coffee beans.