The field notes
A first pour. A little bloom.
Where do the bubbles begin?
The first splash of hot water can make a bed of coffee grounds swell and bubble. That visible bloom begins with something the water did not create: gas already retained inside the roasted coffee. Follow it backward to the bean, then forward into the brew.
Start inside the roasted bean
Roasting creates carbon dioxide
Roasting produces carbon dioxide, some of which remains within the bean's porous structure. Our cutaway uses labelled dots and enlarged spaces to explain retained gas; their sizes and quantities are schematic.
Follow the gas before brewing
Time and grinding change what remains
Gas gradually leaves roasted coffee before brewing water arrives. Grinding speeds up that release, but some gas can remain in the grounds. The film follows that retained gas rather than showing grinding creating new gas.
Watch water reach the grounds
Retained gas escapes
As hot water wets the grounds, gas escapes and bubbles can rise at the surface. In the illustrated dripper, the filter keeps the grounds above the cup while brewed liquid flows through.
Next time you make a pour-over, watch the first water meet the grounds. Those little bubbles reveal part of a journey that started during roasting.
Read the narration Full video transcript
Why does coffee bubble when you pour?
Watch the first splash of hot water.
The grounds swell, and little bubbles rise.
That's the coffee bloom.
To understand it, look inside a roasted bean.
Roasting creates carbon dioxide, and some stays trapped in the bean's porous structure.
Our cutaway shows that gas sitting in tiny spaces inside the coffee.
Some gas gradually escapes after roasting, before it ever meets brewing water.
Grinding speeds up that release, but some gas can remain in the grounds.
Now return to our dripper.
As the hot water wets the grounds, that trapped gas escapes.
The bubbles make something hidden inside the coffee visible at the surface.
But a bigger bloom isn't a better-coffee score.
Roast, freshness, grind, and storage can all change what you see.
A small bloom doesn't automatically mean bad coffee.
So those bubbles? A little gas from roasting, escaping as your coffee brews.
Follow the bubbles · Sources
- Specialty Coffee Association · Coffee Decoded: What is freshly roasted coffee?Carbon dioxide produced in roasting, gas retained in the porous bean, gradual degassing, grinding and release during filter brewing.
- Moccamaster · What is the coffee bloom?Initial bubbling and expansion, variation with roast, freshness, grind and storage, and why a small bloom alone does not mean bad coffee.
A representative pour-over with schematic cutaways. Gas dots, pores and bubbles explain the process rather than their exact sizes or quantities. No universal bloom time, water amount or taste guarantee is implied. Sources checked October 29, 2026.
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