Explainer · bloom and degassing

Why coffee blooms: CO₂, degassing and bloom time

When the first splash of hot water hits fresh grounds, the bed swells and bubbles. That is gas — mostly carbon dioxide made during roasting — leaving the coffee. How much gas there is depends on roast degree, time since roasting and whether the beans are whole or ground.

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

What the bloom is

Blooming is the pre-wetting step of pour-over: a small amount of hot water goes onto the grounds, which sit for about 30 to 45 seconds before the rest of the water is poured. The pre-wetting makes carbon dioxide come out of the grounds as bubbles or foam. Wikipedia notes that blooming is sometimes wrongly called preinfusion, which is an espresso term. The National Coffee Association (NCA) describes the water bubbling up "quickly as carbon dioxide escapes" and says that, with those gases gone, the coffee bed can be saturated more evenly.

Where the gas comes from

Roasting produces CO₂, which stays trapped in the bean's network of cells and leaks out over time — the process the industry calls degassing. The SCA notes that while CO₂ permeates the beans it keeps oxygen out; once it escapes, off-flavors can develop.

How much gas? Figures differ by source and roast:

The measured weight losses (2.5 mg/g is 0.25%; 11.9 mg/g is 1.19%) line up with the lecture's light-versus-dark figures, so most of the spread comes down to roast degree. Darker roasts hold more gas. Wang and Lim (University of Guelph) also found residual CO₂ rising with roast degree, and faster release from high-temperature, short-time roasts. A 2007 study found that water-quenched beans, which ended up with more moisture, degassed faster than air-cooled ones.

How fast whole beans degas

The most detailed measurements come from the Zurich University of Applied Sciences (ZHAW). Smrke and colleagues weighed roasted beans continuously on precision balances, starting exactly 1 h after roasting, inside an oven at 35 °C — a temperature chosen partly because it gives relatively higher degassing rates. It records total gas lost, not CO₂ alone. Degassing was measured for at least 400 h (about 17 days); one light roast was followed for 800 h (about 33 days). It had lost 3.4 mg/g and was still degassing at 2 µg/g per hour. The authors conclude that light-roasted whole beans are still degassing one month after roasting; their 2022 paper says degassing from whole beans lasts about one month.

The SCA summarizes the same research for consumers: degassing "occurs rapidly for the first 20 days after roasting" and slows dramatically afterwards, and whole beans can be considered fresh within about three weeks of roasting. The SCA's Coffee Freshness Handbook (2018), by the same ZHAW group, collects this work; it is not openly available, so this page relies on the open papers and SCA summaries.

The first 24 hours is a point of disagreement. One UK roaster's guide says 40% of the gases are released within 24 hours of roasting. Computed from the ZHAW models above, the fast dark roast releases about 18% of its modeled total in the first 24 h and the light roast about 6%. The ZHAW readings started 1 h after roasting, so gas lost in that hour is not counted, and other beans may differ — but the published curves do not show a 40% first day.

Why ground coffee blooms right away

Grinding opens the bean's cells. In the ZHAW study, up to 75% of the trapped gas was released during and immediately after grinding (the first reading came at 90 s). The SCA puts it this way: the CO₂ that whole beans lose over weeks, ground coffee loses in minutes, and ground coffee might be considered fresh for only about an hour. Wang and Lim found that release from ground coffee depended mainly on grind size and roasting temperature, and less on roast degree.

That is why the bloom works as a rough freshness gauge. The SCA notes that fresh coffee gives "stronger blooms during filter brewing"; a coffee ground long before brewing, or roasted months ago, bubbles little.

Bloom water and wait time: what guides say

The common ground: two to three times the coffee's weight, 30–45 seconds. Coffee absorbs about twice its weight in water, so less risks dry clumps (Barista Hustle). For dripper-specific schedules see the pour-over guide and the cone-dripper recipes.

Does the bloom change the cup?

The evidence on taste is thin, and sources disagree:

• Even wetting. The NCA's rationale is even saturation of the bed once the gas is gone. Barista Hustle calls blooming "not an essential part" of good coffee, but useful for wetting all grounds.
• Bigger blooms, slower flow. In Barista Hustle's 2021 test (15 g coffee, 250 g water, main pour 30 s after the bloom), raising bloom water from 1:1 to 2:1 to 4:1 lengthened drawdown (an extreme 8:1 bloom shortened it again), and the flow rate kept falling as the bloom grew, which they attribute to fines clogging the filter. Bigger blooms also raised TDS, possibly just from longer contact time.
• Dissolved CO₂. Roaster guides claim that brewing too-fresh coffee extracts CO₂ that tastes sour or astringent, or disrupts extraction. An independent home-lab blog test using an inverted immersion brewer (15 g coffee 3 days off roast, 50 ml bloom for 40 s vs. no bloom) measured dissolved inorganic carbon and found the bloom/no-bloom difference negligible — about the same size as the variation between two tap-water samples. It is a single, unreviewed test.

Resting beans before brewing

Many roasters recommend resting beans for days before brewing. Their numbers differ, and none of the studies cited here sets a rest period:

The 2018 article also says dark roasts degas in 3–7 days and light roasts need 5–10 days. The ZHAW data fit this only in part: dark roasts released gas faster (shorter time constants), but they also held far more of it, and a light roast was still degassing after a month. The ZHAW group's 2022 storage study measured chemistry only and notes that sensory testing is needed to say when consumers would find coffee stale.

Checklist: a consistent bloom

• Grind right before brewing; ground coffee loses most of its gas within minutes (more on grinding in the grinders explainer).
• Weigh the bloom water: start at about 2× the coffee weight, up to 3× if dry clumps remain.
• Wet every particle; swirl or stir the same way each time.
• Wait 30–45 s, or until the bubbling slows, then start the main pours.
• If a brew drains slowly, try a smaller bloom before changing the grind.
• Note the roast date with each brew: fresh coffee blooms more strongly, and long-stored coffee may barely bloom at all.

Bloom guidance is a starting point, not a rule. Change one variable at a time — bloom water, wait, then grind — and keep notes alongside your ratio.