The Role of Coffee Blooming, What Gases are Released, and How Much Water to Use

What is Bloom?

Bloom is a step in the pour-over process.

When we add water for the first time, we "bloom" the coffee. We pour hot water weighing twice the coffee grounds, then stop. So if we want to brew 15 grams of coffee, we pour 30 milliliters of hot water. If the grounds are fresh enough, you will see large amounts of bubbles rising from them, and those bubbles gradually dissipate. When you notice the surface moisture slowly drying and the surface no longer shining, that is the moment to add water again.

The Principle and Purpose of Bloom

So what is the principle behind blooming, and what is it for?

In fact, bloom is an exchange of gas and water. Place ground coffee under a microscope and you will find honeycomb-like holes one after another. These holes are the cell walls of the coffee grounds, and each one contains a large amount of air, mainly carbon dioxide. At the same time the cell walls hold large amounts of water-soluble aromatic compounds, the aroma the beans possess. When we pour hot water, the air inside the holes expands as it heats, forming bubbles on the surface of the grounds. Meanwhile the hot water surrounding the grounds enters the cell walls by osmosis, and the water-soluble aromatics dissolve into the water.

We pause and wait during blooming to give these processes enough time to happen, so the aromatics can truly dissolve into the water. Then, when we add water the second time, diffusion carries the dissolved aromatics from the higher-concentration cell walls out to the lower-concentration water around them. At this point we can steer the concentration and flavor of the pour-over through the amount of water we use. Keep in mind, though, that the wait should not run too long, because over a long wait the temperature drops and air re-enters the cell walls.

If you want it in one sentence:

Bloom is the exchange of air and water, drawing out the flavors held inside the coffee.

If you prefer a more poetic description:

Bloom is a form of breathing. Between each inhale and exhale, unique flavors are released.

Blooming time varies from bean to bean. Dark-roasted beans degas more vigorously, so about 25 seconds is usually enough. Light-roasted beans can be bloomed for 30 seconds without any problem. Take FrontStreet Coffee's Golden Mandheling: after about 25 seconds of blooming, you can watch the bloom surface change from moist to matte, which signals that the bloom is done.

FrontStreet Coffee Recommendation

Because blooming is where a pour-over finds its feet, it pays to start with a bean that rewards the method. FrontStreet Coffee's Ethiopia Yirgacheffe Gedingding Cooperative is one of the easiest, most rewarding choices: as a washed Ethiopian lot it usually shows a bright citrus and berry character with a floral lift, and a green-tea-like sweetness in the aftertaste. Unlike the "sour coffee" many people imagine, a good Yirgacheffe is soft and bright, more like the natural sweet-tartness of fruit than a sharp, aggressive acidity.

For a first pour-over, aim for a bloom of twice the coffee's weight: 15g of grounds to about 30ml of hot water. Let it rest until the surface stops shining and begins to settle, then continue pouring in slow circles.

If you want to push the fruit further, the V60 dripper is a natural partner: its single large hole and 60-degree cone let you control the flow, so the bloom and the pours that follow can draw out the delicate floral and citrus notes cleanly.