I make drip coffee (pour over for North Americans) at home and I would like to know: what exactly is coffee bloom?

When I pour the initial amount of water the grounds swell up, and sometimes there is a minor eruption if I'm not careful, but is this just air trapped in the grounds and water solution trying to escape or is this CO2?

Does this affect flavour? What affects the volume of the bloom?

5 Answers 5


The gases themselves are largely composed of CO2 and moisture trapped in the grinds. My understanding is that releasing the gases ahead of time prevent the gasses from interfering with an even extraction throughout the brewing process.

As when brewing coffee, we're trying to expose the grounds evenly to the hot water, the release of gas fights against this, causing channeling or uneven extraction. This can affect your flavor profile by overextracting some of the coffee, and not extracting enough from the rest.

From my own experience, fresher, better-quality coffees tend to produce more gasses. I would imagine this being because the gasses escape on their own over time from the coffee when it's just sitting around.

  • I believe this is a more-correct answer in terms of both 1- the physical process of blooming (when you wet the grounds, gases are released and water is absorbed); and 2- the benefit of blooming: by blooming ahead of brewing, you permit better, more even extraction from the grounds with many benefits. +1 for breadth! @EdChum - no offense: great question and answer about things that affect bloom!
    – hoc_age
    Commented Apr 15, 2015 at 15:38

According to this site it's a by product of the roasting phase and occurs naturally hence the reason coffee bags have a degassing hole.

When you grind the coffee the trapped gases are released and when the hot water hits it, this releases the gases quicker.

Factors aside from storage that can affect this phenomenon are:

  • Temperatures that the beans were stored at. Hotter means more gas release.
  • Humidity levels during storage. Dryer levels allow more gas to escape. Of course, high humidity levels may encourage mold and fungus growth so you need to find a happy medium.
  • Bean hardness. Harder beans mean more density for the gas to make its way through.
  • Roast level. Roast level will have a large influence on bloom. Extremely dark, oily, Italian roasts have a much lesser amount of out-gassing compared to exact same coffee roasted Full City.
  • Origin. Some coffee origins are known to have more out-gassing than others.

if your coffee doesn't bloom at all or very little then it's likely to be stale or over-roasted.


I hope this isn't a trade secret or anything, but at Starbucks we recently got new measuring pitchers for pour-overs that have a long narrow snake-like spout. It allows you to more or less just pour straight down the center. The narrow "bore" so-to-speak gives a precisely controlled rate of pour.

Confusingly, all the documentation still refers to a bloom. But with this fancy snake-neck pitcher, there really isn't one. And the whole process is much faster, not having to stop in the middle.

  • 1
    the reason for the snakey spout that is common on pour over kettles is to make it easier to control the pour: the rate of flow and the area over which pouring. Its suprising to hear you're being taught to pour "straight down the center" as the BORE is exactly what people do not want in a pour over. the only mechanism in pour over i'm aware of where that is actually a recommended method because of the type of filter is a laser hole cut KONE coffee filter {youtu.be/Q0pF38ZCJ2c} - is that what you are using?
    – m.c. von s
    Commented Feb 5, 2019 at 17:53
  • Honestly, I'm not sure what's being taught anymore. Perhaps it's the very dark roast or being many weeks after roasting, but it doesn't produce what I call a bloom. That is, a "explosion" of frothing and foaming when the first sploosh of watch hits the grounds. With the gooseneck pitcher I simply cannot strike the grounds with the water with enough force to make that happen. Commented Feb 7, 2019 at 2:33
  • could well be the age of the bean that you're not getting any frothing - it would be cool to see a vid of the process you're doing. As when the beans are fresh (try it at home with some fresh grinds and slow pour of hot water from anything with a spout - see if you get that foaming up)
    – m.c. von s
    Commented Feb 8, 2019 at 17:31
  • Yes, I'll try to add a video. I just try to get even saturation and a nice deep brown "crema" over the surface. I try to avoid pouring too much in any one spot because that can make a pale spot which looks exactly like the pale spots on espresso crema that indicate over extraction. But mostly I go down the middle and then pull over to the darker side if the spot I'm in gets visibly lighter, judging by the "crema". Commented Feb 9, 2019 at 1:47

As per Kyle above, YEs coffees fresher to roasting are expelling more gas than older ones that have degassed more. indeed a definition of staleness may include no more CO2 is released.

hitting beans with water shows that this gas is happening. But why bloom? that is supposed to get the gas out so water can come in.

Does it? if so for how much of what's ground? just cuz you get a mushroom effect does that mean all the grounds in there got wet??

there's a pretty good debate about "blooming" the coffee - and focussing on creating a bloom rather than focussing on really "wetting" the grounds

To this end, coffee afficianados like perger and rao among others are skipping the bloom and just stirring.

as a scientist gotta say i go with the chemistry of that: if the goal is even absorption of water by even release - agitating those grinds is going to to the business better than a bloom that offers little insight into the evenness of the wetting. Stirring ensures far less likelihood of channeling as water passes through too - would recommend reading/viewing rao on this point if interested.

so why not try it? forget bloom and just stir. see what that does to taste?

I've also found that coffees that are supposedly past their prime can be surprisingly flavourful given a good stir for ten secs and soak for another half minute and then the rest of the pour.

In other words another answer to the question "what is coffee bloom" - it may just be "a ritualised futzing with CO2 gas in coffee that has little proven benefit for flavour extraction compared to stirring but provides more "show" for charging 4.50 at a coffee bar"?

best m.c.

ps - a nice way to see if you've got the gas out of the fast 10sec stir of the slurry is that there are NO more bubbles coming out after that ten secs and as the coffee then sits for 30-45 secs total.


Bottom Line: When someone allows their coffee to bloom, they are pouring just enough hot water on the ground coffee to allow the gasses to be released but not so much that a lot of water starts dripping through.

The presence of CO2 is indicative that the ground coffee is fresh. However, we do not actually want CO2 in the coffee we drink. Therefore, it is important to let your freshly ground coffee "bloom" for one minute to allow the majority of the CO2 to be released to avoid sour tasting coffee that results from pouring over too quickly after grinding the coffee beans.

This article was informative. Really garbage singing at the beginning of this video but I found it pretty informative and the visuals helpful.

The bloom is caused by the roasting procedure. Whenever coffee beans are roasted, the heat causes carbon dioxide (CO2) to become trapped in the bean. Once the roasted process is completed, the beans then begin to slowly discharge these gases a little bit at a time in a process known as “degassing.”

If you are using coffee roasted in within a ten day time frame, much of the carbon dioxide will be retained by the beans even though the degassing process has begun. As soon as the beans are ground, the gases begin to escape much more quickly. When hot water touches the ground coffee, they immediately begin to purge themselves of the carbon dioxide creating the bloom effect.

  • 1
    Hi SCott - please allow me to push a little on your "bottom line" Yes - the bloom is caused by gasses being released, but there's a kind of huge gap between gas for bloom and that that bloom means that all the grounds have given up their gas - which is important for even dissolution/extraction. This is WHY one is encouraged to TEST stirring and you will see water does NOT rush through the slurry BUT you can be confident of much higher wetting degree - so stir and let sit - way more effective than bloom which can leave many grounds untouched, channelling, uneven extraction. REALLY TRYIT
    – m.c. von s
    Commented Feb 8, 2019 at 17:26
  • Great! Thanks @m.c.vons. I've updated it to chill out on the "all the gasses" line :-) Commented Feb 8, 2019 at 17:49
  • great Scott - keen to hear how you enjoy the taste.
    – m.c. von s
    Commented Feb 8, 2019 at 20:04

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