The Importance of the Coffee Bloom

bloom: key step in the brewing process where the coffee grounds are initially wetted so carbon dioxide (CO2) trapped in the beans can be released, allowing the brewing water to make better contact with the coffee resulting in more complete flavor extraction

Originally, that’s what I was going to write – short and sweet (just like me). Then I got into the research and found that coffee bloom is much more interesting and a bit more complex. As you may know, I owned a Kona coffee farm for about a decade and although sometimes it felt like I spent most of my time running the weed whip to keep the jungle at bay, I managed to pick up a little coffee knowledge along the way. I also spent two years as a chemistry major at the College of Wooster and it is surprising how much those two things intersected. If you want your pot of coffee to be all that it can be, then this is the article for you.

As we know, when coffee is properly roasted all the delicious flavors that we love are developed but what is less commonly known is that, in its whole bean form, coffee also outgasses carbon dioxide (CO2). It does so rather aggressively for about a week, and to a lesser extent after that, until it reaches a point of equilibrium with the atmosphere of its container – where the interior pressure of the CO2 trying to escape is equal to the outside air pressure pushing back in. When the beans are ground, more surface area is exposed, so the coffee again aggressively out-gasses CO2. That outgassing CO2 presents a pressure barrier to the brewing water, which means that the water cannot get into immediate full contact with the coffee grounds. But not to worry, this is where we get tricky in order to get the best flavor extraction possible. And this is also where “The Bloom” comes into play.

Carbon Dioxide (CO2) bubbles in my French Press pot
Carbon Dioxide (CO2) bubbles in my French Press pot
Carbon Dioxide (CO2) bubbles in my French Press pot
Even old, grocery store coffee will outgas











If you are a French press or pour over aficionado, you have probably seen this effect: you pour your hot water onto your coffee grinds and it “foams up.” That is the effect of the CO2 being released. If we just pour the entire amount of water necessary for our desired brew, the outward pressure of the carbon dioxide makes complete contact with the coffee difficult and we cannot get a complete flavor extraction. Since flavor extraction is the whole point of brewing our morning (or afternoon or evening) cup, this must be avoided. By initially wetting freshly roasted and freshly ground coffee with hot (not boiling – but that’s a story for another post) water, we can trigger the outgassing on our own terms. After 20 – 30 seconds, a sufficient amount of CO2 has been released so that when we pour the rest of our water, it can make complete contact with the grinds. Complete contact means better extraction. Voila! Good coffee. As an added benefit, you will have little trouble pushing the plunger down; if you are having trouble, you didn’t let the initial bloom happen for a long enough time.

We can use this knowledge to improve all of our brews, whatever the apparatus. For instance, when faced with an automatic drip coffee maker like the kind often found in offices, pouring hot water into the basket before operating the machine will give you a similar effect and improve the resulting brew. Of course, if you are using old, stale, pre-ground coffee out of a plastic pouch, you might not taste much of difference!

Remember, life is too short to drink bad coffee!


I am Tim Bruno, a former Kona coffee farmer promoting the soon to be released Procaffeination: A Coffee Lover’s Dictionary and several other coffee-related products. I will soon be adding a YouTube channel to my efforts. Stay tuned!

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