Coffee: CO₂ Degassing — Why Fresh Coffee Blooms
Freshly roasted coffee beans emit CO₂ for 2–14 days after roasting; espresso crema is approximately 70% CO₂ by volume, with the rest being emulsified oils and water vapor.
| Measure | Value | Unit | Notes |
|---|---|---|---|
| CO₂ absorbed during roasting (per kg of coffee) | 2–12 | liters | Darker roasts absorb more CO₂ and are more porous; degass faster |
| CO₂ degassing period (light roast) | 7–14 | days post-roast | Slower release; denser bean structure retains CO₂ longer |
| CO₂ degassing period (dark roast) | 2–5 | days post-roast | More porous cell structure after second crack; faster degassing |
| Espresso crema CO₂ content (by volume) | ~70 | % | Remaining ~30% is emulsified coffee oils and water vapor; Illy & Viani (2005) |
| Recommended rest time before espresso (light roast) | 7–14 | days post-roast | Excess CO₂ interferes with extraction — causes channeling and uneven flow |
| Recommended rest time before espresso (dark roast) | 3–7 | days post-roast | Faster degassing means it's ready sooner; also oxidizes faster |
| Pour-over bloom time | 30–45 | seconds | Hot water releases CO₂; bloom ensures even saturation before brew |
| CO₂ released during pour-over bloom | visible gas bubbles | Visually observable; absence indicates stale coffee (CO₂ already escaped) |
CO₂ is produced in large quantities during coffee roasting and trapped within the bean’s cellular structure. Its controlled release — degassing — is a critical but often overlooked factor in coffee freshness, extraction quality, and the formation of espresso crema. Understanding degassing explains why specialty roasters print roast dates on their bags and why coffee shops rest espresso beans before putting them on grinder.
How CO₂ Gets Into Coffee Beans
During roasting, several chemical processes generate CO₂:
- Pyrolysis of organic acids: Chlorogenic, citric, and malic acids decompose at high temperatures, releasing CO₂ as a by-product
- Carbohydrate pyrolysis: Polysaccharides and sugars undergoing caramelization and thermal decomposition release CO₂
- Maillard reaction pathways: Various steps in the amino acid–sugar condensation cascade produce CO₂
- Decarboxylation reactions: Organic acids lose a carboxyl group as CO₂
As the bean temperature rises above 196°C (first crack), this CO₂ generation becomes rapid enough to build pressure inside the bean, causing the characteristic crack sound as the internal pressure ruptures the bean cell walls. Second crack (224–235°C) represents a second, more intense fracturing event.
Degassing Timeline by Roast Level
Darker roasts degas faster for two reasons: (1) more CO₂ is generated due to more extensive pyrolysis, and (2) the bean’s cell structure is more porous after second crack, creating easier pathways for gas release.
| Roast Level | CO₂ Absorbed (estimated) | Active Degassing Period | Optimal Rest Window |
|---|---|---|---|
| Light roast | ~2–5 L/kg | 7–14 days | 7–21 days post-roast |
| Medium roast | ~5–8 L/kg | 5–10 days | 5–18 days post-roast |
| Medium-dark roast | ~7–10 L/kg | 3–7 days | 4–14 days post-roast |
| Dark roast | ~8–12 L/kg | 2–5 days | 3–10 days post-roast |
Degassing is not linear — it follows an approximately exponential decay curve, with most CO₂ released in the first 24–72 hours, then tapering off.
CO₂ and Pour-Over Bloom
In pour-over brewing (V60, Kalita, Chemex), the bloom step allows CO₂ to escape before the main brew begins. When hot water first contacts the grounds:
- CO₂ comes out of solution rapidly, creating visible bubbling and foam
- CO₂ escaping from grounds can prevent water from evenly saturating the bed (channels through the grounds)
- A 30–45 second bloom with a small amount of water (~2× the grounds weight) allows CO₂ to escape before the main pour
If grounds barely bloom, the coffee has already lost most of its CO₂ — a reliable indicator of staleness. Specialty brewers use bloom vigor as a qualitative freshness check.
CO₂ and Valve Bags
Specialty roasters package coffee in one-way valve bags immediately after roasting. The one-way valve allows CO₂ to escape outward without allowing oxygen to enter — preventing both bag rupture from CO₂ pressure and oxidative staling from oxygen exposure. Without such valves, packaging freshly roasted coffee in sealed bags would cause the bags to burst from CO₂ pressure within 24–48 hours.
Ground coffee, with vastly greater surface area, degasses in minutes to hours rather than days — which is why pre-grinding dramatically accelerates both CO₂ loss and subsequent oxidative staling.
Related Pages
Sources
- Clarke RJ, Macrae R (1988) — Coffee Vol 1: Chemistry. Elsevier Applied Science
- Anderson BA et al. (2003) — CO2 in Roasted Coffee: Measurement, Release and Effect on Packaging. Packaging Technology and Science
- Specialty Coffee Association — Coffee Freshness Handbook
- Illy A, Viani R (2005) — Espresso Coffee: The Science of Quality. Elsevier Academic Press
Frequently Asked Questions
What causes the bloom in pour-over brewing?
The bloom is caused by CO₂ rapidly escaping from freshly roasted coffee when hot water contacts the grounds. During roasting, CO₂ is generated from pyrolysis of organic acids, carbohydrate decomposition, and Maillard reactions, and becomes trapped within the porous cell structure of the bean. When hot water reaches the grounds, the temperature differential and water contact releases this trapped gas rapidly. A vigorous bloom with bubbling foam indicates fresh coffee. If your grounds barely bloom, the CO₂ has already escaped — a sign that the coffee is stale or was roasted more than 2–3 weeks ago.
Why do you need to rest espresso after roasting?
Excess CO₂ in freshly roasted coffee creates problems in espresso extraction. CO₂ forms a barrier between water and coffee grounds, causing uneven water flow (channeling) through the puck and producing unpredictable, inconsistent extractions. The CO₂ also generates excessive crema volume that masks the true flavor compounds in the cup. Resting for 7–14 days (light roast) or 3–7 days (dark roast) allows CO₂ to degas to a level that permits stable, even extraction. The ideal window for espresso is typically 7–21 days post-roast.
What is espresso crema and what does CO₂ have to do with it?
Espresso crema is the persistent, reddish-brown foam on top of a well-extracted espresso shot. It forms when pressurized hot water (9 bar) forces CO₂ dissolved in the coffee into the high-pressure environment — the CO₂ then comes out of solution as the espresso exits the grouphead at atmospheric pressure, forming fine bubbles stabilized by emulsified coffee oils. Approximately 70% of crema by volume is CO₂. A thick, persistent crema indicates both freshness (CO₂ present) and appropriate extraction. Decaffeinated coffee produces far less crema due to the chemical treatment that removes caffeine also damaging cell structures.