What is coffee bloom


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?


Posted 2015-02-04T20:23:12.423

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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.

Kyle Macey

Posted 2015-02-04T20:23:12.423

Reputation: 1 219

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 – 2015-04-15T15:38:55.120


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.


Posted 2015-02-04T20:23:12.423

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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.

luser droog

Posted 2015-02-04T20:23:12.423

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