What is Coffee Bloom and Why Do I Need to Do It?


I’m sure you’ve heard the term “coffee bloom” or “bloom your coffee,” but what does it mean?

You probably have other questions too.

Such as why does coffee need to be bloomed?

And how do you bloom coffee?

This post will provide you with the answers to those questions. You’ll find out that blooming your grounds will make your coffee taste less sour and be more consistent from cup to cup.

Coffee Bloom

Coffee bloom is the release of carbon dioxide (CO2) from ground coffee beans when water is initially poured over the coffee.

Carbon dioxide forms inside the coffee bean during the roasting process. The degassing of CO2 from roasted coffee beans occurs at a slow rate when the coffee bean is whole.

But when you grind coffee beans and add hot water to them not only does the degassing occur at a faster rate, but the degassing is visible as the CO2 escapes through the water.

In the video below, you can see the CO2 escaping and creating a bubbly foam on the water surface.

The ground coffee in the video is in a shallow cupping bowl, and you can see that the CO2 gas has raised the grounds up and above the edge of the glass.

Coffee Bloom in a Pour Over

In a typical pour over coffee, you won’t see the coffee grounds rise up and out of the filter, but you will see a very active bubbly foam rise above the coffee as you pour water into your filter bed.

Remember that the water level is decreasing as water is running through the grounds and out of the filter into the cup or container.

The Grind Matters

The size of the grind and the consistency of the grounds matter when you bloom coffee.

In the previous video, the grind came from an inexpensive electric grinder and the size of the grounds is somewhat inconsistent.

The grounds in the next video were crushed in a hand grinder and are more consistent and smaller in size.

When coffee is ground fine, there is more surface area. The increase in surface area allows water to reach and react at a much faster rate and release more CO2.

Remembering that under-extracted coffee is usually sour. The first correction I make with the next cup of coffee is to reduce the size of the grind.

Besides the fact that this increases the surface area and draws more flavor out of ground coffee, it also releases more CO2 and reduces the probability that the cup will be sour.

Compare the amount of bloom between the video above and the one below. The ground coffee is from the same batch of beans.

Coffee Bloom or Pre-wetting?

Adding water to coffee grinds at a 3:1 ratio (3 times the weight of water compared to the weight of coffee grounds) before adding the remainder of the water serves a couple of reasons.

These reasons are coffee bloom and pre-wetting.

We’ll leave prewetting to another post, but I wanted you to know that adding a small measure of water instead of adding the full volume in one pour is an important step.

Blooming and pre-wetting occur at the same time and are inter-related processes that will improve the taste of your coffee.

To bloom coffee, add three times the weight of water as coffee to the filter and stir the slurry.

Some people disagree with this step but for consistent results mixing the slurry, so all of the coffee grounds are wet will produce a better cup of coffee on average than hoping water reached the grounds.

Why is Blooming Important

The reason we want to bloom coffee or degas the CO2 from the freshly ground beans is to remove some of the acids.

Yes, acids are what makes coffee taste great!  But not all acids.

If the CO2 in the coffee beans is not allowed to degas, and become trapped in the extracting process, the CO2 and water will form carbonic acid.

What does carbonic acid taste like?

Carbonic acid, like all acids, has a sour taste, but if you have ever tasted flat carbonated water, you know it isn’t a good sour taste.

Yes, blooming does release some of the other more tasty acids such as malic and citric acids. But CO2 is released sooner in the blooming process.

This isn’t to say that some carbonic acid in your cup of coffee is entirely bad, but it’s good to remove the excess CO2 from the coffee.

Another reason we want to bloom coffee is if the CO2 rapidly escapes your grounds, it pushes away the water.

This doesn’t allow the water to sufficiently make contact with the grounds. This results in a less even extraction or an under extracted sour coffee.

Trapping CO2 in the Bean is Good

That statement might seem contradictory, but we want CO2 to remain in the whole beans as long as we can. Why?

Because CO2 in the coffee bean will keep oxygen away from the compounds in the bean, if oxygen reaches the compounds, oxidation will start, and the coffee will become stale.

That is why freshly ground coffee smells better than pre-ground coffee, and coffee made from freshly ground coffee tastes better to most people.

We want CO2 in our whole beans, but we want it removed when we brew.

What Can the Coffee Bloom Tell Us?

The coffee bloom can tell us (if there isn’t a date on the bag) if the coffee is fresh.

Carbon dioxide is released from roasted whole coffee beans too, but at a much-reduced rate from ground coffee. The longer you store roasted coffee; the more CO2 is released.

As mentioned, CO2 preserves freshness, so once removed, oxygen reacts with the compounds in the beans and they start becoming stale.

If a freshly ground coffee doesn’t have a vigorous bloom, there is a good chance that the coffee is far removed from roasting or stored improperly.

The amount of bloom is only a relative gauge of freshness, as many factors contribute to CO2 levels in roasted coffee beans. Such as:

  • Coffee bean storage temperatures – beans stored warm lose more CO2 (but don’t refrigerate beans).

  • Humidity – dry conditions release more CO2.

  • The hardness of the bean – harder, less fractured beans lose less CO2.

  • The origin-age-storage-transportation of raw beans – as you can guess all of these factors influence the finished product.

  • Roasting level – darker beans usually have more CO2 in them.

With so many factors it is impossible to gauge how long ago a bean was roasted, but seeing a good bloom on freshly ground coffee is an indicator that the coffee will have a lot of flavors.

Coffee Bloom Summary

Adding hot water to ground coffee at a 3:1 ratio then stirring the slurry will remove excess carbon dioxide from the ground coffee and pre-wet all of the grounds for an even extraction and a less sour coffee beverage.

A 3:1 ratio is enough water to coffee is sufficient to wet the coffee grounds and release CO2 but not allow sour coffee to reach the brewing pot/cup.

After 30- to 45-seconds (depending on how fresh the roasted coffee is) add the remaining hot water to the grounds.

Blooming your coffee before brewing will result in a much better tasting coffee!

  • Steve says:

    Great info. Did you post the temperature slurry test on your site?

    • caffeine says:

      Thanks, Steve! I’ll work on that post as soon as I update the website to a new look. There will be more equipment and coffee reviews, as well as, so-called experiments.

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