How the Bloom Affects Your Coffee’s Flavor (with actual test results)

When I was new to pour over, I remember watching James Hoffmann’s and Tetsu Kasuya’s V60 recipe videos. Upon trying these for the first time, I was fascinated with how differently the bloom from each of my coffees would behave. 

Now with a lot more knowledge and some friends to help out, we decided to test just how much a difference the bloom makes to the final cup. 

Read to the end to see what we found out!

What is the bloom?

When water interacts with ground coffee, it expands and puffs up like risen dough. This is caused by the rapid release of carbon dioxide trapped within the bean’s cell walls. 

Numerous pour over coffee recipes call for wetting the grounds with a small amount of water with the sole purpose of releasing the trapped gases. In addition, the bloom is also an indication of coffee freshness. 

Newly roasted coffee would have a large amount of trapped gases and a larger bloom compared to coffee that has been aged for a week or two after roasting.

Why is blooming important?

Releasing the carbon dioxide in ground coffee is essential to produce clarity in the coffee’s flavors. 

If the gases do not release at the beginning of the brew process, the water will be infused with carbon dioxide along with the coffee flavour compounds. This will result in a sour tasting brew.

For this reason, numerous pour over recipes require a blooming phase.

Whole bean vs pre-ground coffee

When comparing the blooms of freshly ground coffee versus pre-ground coffee, the bloom of freshly ground coffee will be more noticeable than pre-ground coffee. When coffee is ground, more carbon dioxide and coffee aromatics are released. Hence, having coffee that has been ground for some time will lose a lot of the carbon dioxide.

Besides carbon dioxide, pre-grinding coffee makes it easier for the flavours to be released from the grounds. This is why pre-ground coffee stales quicker.

All baristas and coffee aficionados I know own a grinder for this reason. They buy whole beans from their preferred roaster and then grind their coffee right before brewing.

Other variables that affect the bloom

How the beans were stored

It is important to store coffee in a cool and dry place. Heat and humidity are bad for storing coffee. 

Heat and humidity accelerate the degassing of the beans. These cause the coffee to go stale quicker. In addition, high humidity levels encourage mold and fungi to grow.

Bean density/bean hardness 

Denser beans have milder blooms compared to less dense beans – which usually have puffy blooms. 

Because denser beans are thicker and harder, it is more difficult for gasses and aromatics to escape. This usually means more agitation is needed to get these to escape so they don’t interfere with the brew. 

Roast levels

When they are fresh, the bloom of dark roasts will be much puffier than fresh light roasts. Because dark roasts are roasted much longer, they get coated with more carbon dioxide. And thus, release more gas when bloomed.

The bean itself is also more brittle which makes it that much easier for the gasses to escape. Dark roasts that aren’t as fresh will have a flatter bloom.

Origin

Coffees from different origins share different characteristics. With that being said, some coffees release more gas than others. 

Origin is also related to bean hardness. For example, beans coming from Ethiopia are known to be denser compared to beans from Brazil. 

What I usually do is to check the altitude at which the coffee is grown (most specialty coffee roasters add this information on the coffee’s label). When the coffee is grown in altitudes of up to 1,700 metres above sea level, then I know that the coffee is dense.

Bloom test

Me and a couple of friends wanted to find out how tweaking the amount and time of the bloom would affect the flavour of the brew. When I was learning how to brew pour over for the first time, I watched videos such as James Hoffmann’s The Ultimate V60 Technique and Tetsu Kasuya’s A Coffee Brewing Theory “4:6 method”.

In James Hoffmann’s video, he advised viewers to allow the coffee to bloom for 30 seconds but no longer than 45 seconds. The reason being the “returns diminish” beyond 45 seconds.

In simple terms, going beyond 45 seconds will have no outstanding effect on the coffee and would be a waste of time. For this test, my friends and I decided to try brews of different bloom times and note the differences.

Other than the brew times, I was also interested in how the amount of brew water for the bloom would affect taste. In Tetsu Kasuya’s famous 4:6 method, he said that the first pour in his technique is dedicated to acidity.

If you want to add more acidic flavours to your brew, you add more water and vice versa. For this test, we would try brews of different bloom amounts.

Parameters

For both tests, plastic Hario V60s were used. A medium roast was chosen since I think it would be representative of most coffees bought from most specialty coffee shops.  

The grind size for the brew was medium-fine (similar to the grind size recommended by Hario).  

Test 1: How long should you let coffee bloom?

For the first test, we used different bloom times of 30s, 45s, and 60s. We all agreed that the brew using a 45 second bloom tasted the best. These were what we noticed:

30s bloom	45s bloom	60s bloom
Decent cup overall but not as good as the 45s brew	Good balance of acidity and sweetness	Tasted weaker than the other brews

Test 2: How much water to use for the bloom?

For the second test, we test different bloom amounts starting with the same amount of water to grounds, twice the amount of grounds, thrice the amount of grounds and four times the amount. As a bonus, we discarded the bloom to see how different the brew would taste like without it. 

By the end, we found that while the brew using 2x the dose tasted the best, the brew using 3x the water was a very close second. In fact, even when drinking these two brews side by side, the differences were hard to pick out. 

Here’s what else we found out: 

Discarded bloom	1x of dose	2x of dose	3x of dose	4x of dose
Tasted very dull and flat; seemed under extracted	Tasted weak but not dull and flat like the discarded bloom.	More acidity compared to 1x of dose	More acidity compared to 2x of dose but only a slight difference	Highest acidity overall

Recommendations

From this test, me and my friends were able to confirm what both James Hoffmann and Tetsu Kasuya teach. In this case, that bloom times beyond 45 seconds don’t really do much for the brew. And that more acidity gets extracted if you use more water during the bloom phase. 

Depending on what you’re after, changing your bloom time and bloom amount can significantly alter the coffee’s extraction. Thus, changing your final cup’s taste.

As a starting point, I recommend doing a 45-second bloom – especially if the coffee is quite fresh. The older it gets (3 weeks and older) you can drop that down to 30 seconds. 

As for bloom amounts, either twice or thrice the amount of your dose is a good start. Most recipes will also recommend blooming with twice the amount of your dose. 

If you work with smaller doses (15 grams and below), you might find that it can sometimes be easy to overshoot the bloom and pour more than twice the dose. But this shouldn’t be too big of a problem if you’re within the 2x to 3x range. 

Frequently asked questions

Why doesn’t my coffee bloom?

There are multiple reasons why coffee doesn’t bloom. One reason could be that the coffee is stale and old. If you think this is the case, check the roast date. It is important to note that the bloom is not necessarily an indication of freshness or quality.

Is the bloom only for pour over?

The bloom is found on both pour over and immersion brews. However, the blooming step is more common in pour over brews. Whether or not to bloom in immersion brews is a big debate as some people believe in it and some don’t. 

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