Explainer · bloom

Coffee bloom: why fresh coffee bubbles, and how long to bloom

When hot water first hits fresh grounds, the bed swells, cracks and fizzes. That is the bloom: gas formed during roasting, mostly carbon dioxide, escaping as the coffee gets wet. How strong it is tells you something about freshness; how long to wait is mostly convention.

Updated 2026-10-18 · sources listed at the end

Where the gas comes from

Roasting creates gas inside the bean, and much of it stays trapped in the porous cell structure. The 2018 Journal of Agricultural and Food Chemistry paper by Smrke, Wellinger, Suzuki, Balsiger, Opitz and Yeretzian (Zurich University of Applied Sciences, ZHAW) summarizes earlier work: CO₂ makes up about 1–2% of the weight of freshly roasted coffee, and gas released from whole beans has been measured as 87% CO₂, 7.3% carbon monoxide and 5.3% nitrogen. That is why the authors, and this page, treat "degassing" as CO₂ release.

The gas leaves slowly through the cell walls for weeks. The SCA's Coffee Decoded column on freshness notes a side effect: as long as CO₂ surrounds and permeates the beans it keeps oxygen out, and once it escapes, oxygen-rich air takes its place.

How fast coffee degasses

Degassing was long hard to measure. The ZHAW group weighed samples continuously on a precision balance and tracked the tiny weight loss as gas left. Their figures, measured at 35 °C (warmer than room temperature, to speed degassing up) on El Salvador Arabica:

• Whole beans lost gas steadily for the whole run of at least 400 hours (about 17 days). Light roasts were "still degassing 1 month after roasting".
• Ground coffee behaved very differently: for freshly roasted coffee, "up to 75% of the trapped gas was released during and immediately after grinding", within the first minutes of measurement.
• Some CO₂ never leaves dry coffee. Part of it stays adsorbed and "is only released when coffee is extracted with water".

The SCA column, which draws on the Coffee Freshness Handbook by Smrke, Sage, Wellinger and Yeretzian (SCA, 2018), puts the whole-bean curve simply: degassing is rapid for the first 20 days after roasting and slows dramatically afterwards, and the CO₂ lost over weeks in whole beans is lost in minutes once the coffee is ground.

Roast level changes the amount

Darker roasts hold and release much more gas. In the ZHAW measurements, total weight loss from whole beans was roughly three to four times higher for dark than for light roasts, and for medium and dark roasts a faster roast degassed more and faster. For light roasts, roast speed made no clear difference.

What changes how much a coffee blooms

The last row links bloom to brewing water: the bicarbonate that gives water its alkalinity reacts with the coffee's acids.

How fresh is "fresh"? The figures disagree

• Whole beans. The SCA column says whole-bean coffee "can reasonably be considered 'fresh' within about three weeks of roasting". Wikipedia's coffee roasting article gives an optimal shelf life of two weeks, and a separate, unsourced claim of up to one month if kept cool.
• Ground coffee. The SCA column suggests ground coffee might be considered fresh for about an hour after grinding; Wikipedia gives about 15 minutes.
• Gas lost at grinding. ZHAW measured up to 75%; earlier studies they cite found 40–50% (Radtke) and 59–73% (Shimoni and colleagues) in the first few minutes. Methods and grind sizes differ, so treat these as a range, not one number.

The direction is not in dispute: grind just before brewing, and expect the bloom to shrink as the beans age.

What the bloom does — and what is still unproven

The common hypothesis, as NP's Coffee Science describes it, is that trapped CO₂ can block water from wetting the grounds and so weaken extraction; the bloom gives that gas a head start before the main pours. The SCA column links fresh coffee's trapped CO₂ to stronger blooms in filter brewing and more crema in espresso. Beyond that, measured evidence on bloom timing is thin, and most of it comes from coffee educators and hobbyist labs rather than journals:

• Bloom size. Barista Hustle brewed 15 g of coffee with 250 g of water in a V60, varying the bloom water from 1:1 to 8:1. Larger blooms slowed drawdown, which the team read as fines clogging the filter, and raised TDS, but they could not separate the bloom's effect from the longer contact time. Too little water risked dry clumps; too much risked clogging and channeling.
• Stirring the bloom. In a 2021 follow-up, Barista Hustle reported that the "bird's nest" and NSEW stirring methods had "no statistically significant effect on extraction yields" compared with no stirring, although stirring reduced how much water the bloom absorbed.
• Dissolved CO₂. One hypothesis says unreleased gas dissolves into the brew and tastes sharp. A 2020 hobbyist test on NP's Coffee Science, using a lab carbon analyzer on four bloomed and four unbloomed AeroPress brews of coffee three days off roast, found a difference smaller than the variation between two tap-water samples. The author calls the effect on dissolved CO₂ minimal under those conditions; it is one small test, not a settled answer.

Some well-known recipes do not name a bloom at all. HARIO Europe's version of Tetsu Kasuya's 4:6 method starts with a 50 ml pour at 0:00 and the second pour at 0:45 — in effect a wait after the first pour, just not labeled as one.

How to bloom: step by step

1. Grind right before brewing. Up to three quarters of the trapped gas can leave during and right after grinding.
2. Level the bed and zero the scale. Shake the dripper gently so the water meets an even layer.
3. Pour 2–3 times the coffee's weight in water. Barista Hustle recommends "between 2 and 3 times the weight of the coffee"; the Kalita Wave guide gives 40–60 ml (2–3×) for 20 g of coffee. For 15 g, that is 30–45 g.
4. Wet every particle. In the Barista Hustle test, too little bloom water left dry clumps.
5. Wait 30–45 seconds. Hario UK's basic V60 guide waits about 30 s; the Kalita guide 30–45 s. Fresher and darker coffee has more gas to release.
6. Stir or swirl only if you like. The 2021 Barista Hustle test found no significant effect of stirring methods on extraction yield.
7. Continue with the main pours of your pour-over recipe.

Checklist: reading your bloom

• Note the roast date on the bag; expect a vigorous bloom in roughly the first three weeks (how to read bag dates, and how packaging and freezing slow staling: coffee freshness; containers and freezer portions: how to store coffee beans).
• Flat bloom with whole beans ground fresh? The coffee is probably older, or a light roast with little gas to begin with.
• Flat bloom with pre-ground coffee is expected: most of its gas left at grinding.
• Dry clumps after the bloom: use a little more water, up to 3× the coffee weight.
• Slow drawdown after a large bloom: try less bloom water before changing the grind.
• Change one thing at a time and write down bloom water, wait time and total time.

All research figures above are from the studies and guides listed below, under their own test conditions. Degassing measured at 35 °C runs faster than in a kitchen; use the numbers to understand direction and scale, not as a schedule.