Why Do We Bloom When Brewing Coffee?

If you don't know what blooming coffee is, it's the process in which you first add a little water to your grounds when brewing pour-over and letting it sit for some time, before adding the rest of the hot water to brew the rest of your pour-over.

When making a pour-over, or drip, or filter, or slow coffee (whatever you want to call it), we baristas are always taught that we should 'bloom' our coffee. But why?

The bad thing is that I always accepted the answer; "because it tastes better, because it gives a better extraction", until a friend asked: "Yes, but why?".

Why does blooming the coffee give a better taste? Off to do some research I went and this is what I found...

What blooming actually does is that it gives the coffee the chance to release the CO2 still trapped in the bean. Good starting point right? So lets start there. The CO2 in the coffee bean.

Coffee is roasted (duh). But it's what happens during roasting that makes us want to bloom. The oxidation process during roasting causes the CO2 that is naturally present in the bean, as it is in all other organic matter, to be released. According to Illy 1-2% of the bean mass after roasting consists of CO2. After roasting the CO2, together with other gasses gradually escape. This is called 'degassing'. 

This high level of CO2 needs to decrease a little before brewing, which is why most roasters suggest to wait one to seven days after roasting before brewing.

Especially for espresso a resting period to degas is important, because the CO2 can only escape after brewing, which causes the crema to be huge and very bubbly and breakable. However you should also not wait too long, if almost all the CO2 has already dissipated your crema will be very, very thin, and that's not appealing.

Coffeetroupe's research setup

So for how long a period does CO2 escape? Or, when has all CO2 left the building (or the bean)? Coffeetroupe tried to find out, measuring the amount of CO2 released. Unfortunately the output container was too small for the amount of CO2 being released, resulting in a forced stop after 108 hours, when already 250 ml of CO2 had been released!

Several books and articles suggest it takes up to seven days, but to be honest, I am quite sure this period is longer.

So why not just wait until all CO2 has dissipated before brewing? Then we do not need to worry about it at all, wondering, pondering and questioning why we need to bloom before brewing.

The answer is simple, when you lose CO2 you also lose the volatile aromas that make the coffee fresh, beautiful and interesting. In other words, your coffee turns stale.

And thus we like our  coffees as freshly roasted as possible, making an abundance of CO2 something we need to deal with. And why do we need to deal with it? Now we enter step two in the research.

As said in the beginning, blooming allows the CO2 to be released. The hot water speeds up the process of degassing that is already taking place naturally. Now, as the CO2 rapidly tries to escape your ground beans, it pushes away the water you just poured on to the beans. This means that the water does not have sufficient contact with the grounds. Resulting in a less even extraction. Tadaaaaa...that's all folks.

Step three, what is the right way to bloom your coffee? As blooming is just a method to release the excess

This one bubbled

CO2, it's important that you do not actually start brewing, if that makes sense. To put it differently, you want to add water to cause the CO2 to evaporate, so the water that you want to add after blooming can extract flavour from your grounds evenly.

In order to do that, you do not want liquid coffee to drip out of your grounds, until after the blooming has ended. And thus the magic lies in pouring in enough water that you cover all grounds and have a successful bloom, but not pouring in too much water causing your coffee to drip though prematurely.

A handy dandy tip to achieve this is knowing that ground coffee can hold about twice its own size. Thus for your blooming period you want to add about twice the amount of water to your grounds. Add the water in a small circular motion, making sure that you do not hit the filter paper (which could cause channelling, letting your water bypass your grounds all together). Then you will see your grounds come to life, either bubbling or rising up like a mushroom. And now you wait...

You wait until your bloom dies down again. Depending on the level of CO2 this can take up 30-50 seconds on average. And then your grounds are ready to be brewed.

And that's why we bloom.

P.S According to Blue Bottle, and do try this at home, blooming can have another advantage. It can add depth and vitality to an older roast, if you manage to prolong your blooming period; the older the roast, the longer the bloom. Don't know how old your roast is? Find a quick and simple method here.

N.B Other cool CO2 coffee facts.

One: Coffee is typically roasted in hot air, where the temperature plays an important role. The air needs to be hot enough to cause change of colour, taste and smell, but cool enough as to not burn the bean. To control this roasters sometimes add CO2 to the hot air to control the intensity of the heat versus the moisture content of the bean to prevent the coffee from being singed, which can result in a burnt taste.

Two: CO2 is also often used in the decaffeination process. One way to decaffeinate coffee is to rinse the roasted beans in chemical solvents. Afterwards the coffee beans are steamed or washed in a chamber with CO2. Afterwards  the CO2 is removed from the coffee bean, cleaned, recycled and used again.