How to Bloom Your Coffee

The first moment hot water and ground coffee come in contact with each other is one of the most important parts of the brewing process. 

If your coffee is fresh, the grounds will immediately begin to expand-- or “bloom” -- as water is absorbed and trapped carbon dioxide is released.

Although the carbon dioxide shows your coffee is fresh, it can interfere with the extraction process and degrade into carbonic acid, which doesn’t taste great. Blooming allows some of this gas to escape before brewing your coffee. 

More importantly, once the coffee grounds are saturated the soluble compounds will begin to dissolve. Evenly wetting your coffee with a sufficient amount of water sets the stage for better extraction later in the brew. 

Prewet Percentage or Ratio?

Roasted, ground coffee can absorb approximately twice its mass in water. This means to thoroughly wet your coffee bed you need at least two grams of water for every gram of coffee. 

It’s unlikely you will wet the coffee bed perfectly, so I recommend giving yourself a little wiggle room. If you use 2.5 to 3 grams of water for each gram of coffee I find it’s much easier to get a proper bloom. But don’t use more than three grams of water per gram of coffee, otherwise too much water will pass through the bed before the coffee is ready to extract, leading to a lower overall potential extraction.

Many commercial coffee brewers and some high-end consumer machines offer the ability to bloom the coffee with a “prewet” setting.

Rather than expressing the amount of water used for wetting as a ratio, these machines use a “prewet percentage.” i.e. the amount of the brew water that is used for the first pulse, followed by a waiting period. If you set your coffee brewer to have a 15% prewet percentage when brewing a liter of coffee, it will use approximately 150 ml of water for wetting. If you use a ratio of 60 grams to a liter, a 15% prewet percentage ends up being comparable to a 1:2.5 coffee-to-water bloom ratio.

For pour-over coffee, however, baristas and home brewers alike tend to describe their bloom as ratio rather than a percentage. 

Just in Time

Most coffee professionals recommend blooming your coffee until the grounds stop expanding. In the case of fresh roasted coffee, this ends up being around 30-40 seconds. But even when brewing coffee that isn't fresh and doesn’t bloom, I recommend allowing for at least 25 seconds to allow the coffee grounds to absorb as much water as possible. Some coffee professionals recommend a full minute or even more, but personally, I find diminishing returns on longer bloom periods. 

Practice Makes Perfect 

To properly bloom your coffee, you need to use the right pouring technique. If you're not careful, you might leave a dry pocket which will lead to uneven extraction: i.e. more sour and bitter flavors in the cup. 

I find the easiest way to wet the grounds is with a simple spiral method. Starting in the middle, I work my way to the edge pouring in concentric circles. I tend to pour rather briskly (9-10 ml a second) with the spout of my kettle relatively close to the coffee bed. I find the extra turbulence helps ensure all of the grounds come in contact with water. Many excellent baristas, however, use the opposite technique: low velocity and minimal turbulence. The most important thing is not the technique but the execution. Does the entire coffee bed receive a similar amount of water and turbulence? 

To Stir or Not to Stir? 

Many baristas opt to stir the bloom with a small wooden spoon or paddle. This technique can help ensure all of the coffee grounds are thoroughly saturated during the wetting stage. Personally, I rarely stir the bloom as flat-bottom brewers like the Etkin Dripper have a shallower bed depth than conical drippers which makes it easier to saturate the grounds. I also don't like how some of the grounds tend to stick to the spoon. 

I do think stirring can be a helpful training tool, and occasionally stir to check on my wetting technique. If I find a dry pocket, I know there’s room for improvement.

A Simple Test 

If you don’t mind wasting a small amount of coffee there is a simple test to measure the effectiveness of your wetting technique.

After blooming the coffee like you normally would, remove the filter and place on a large plate. Carefully open the filter (be careful, it’s hot!). Using a small wooden spoon or chopstick, comb through the grounds looking for dry grounds. If all of the grounds are wet, congratulations, you passed the test! 

If not, you might consider stirring your bloom until you consistently don’t find any dry pockets.  

As always, don't be afraid to experiment with different techniques and find what blooming method works best for you.