The Science of Degassing Fresh Coffee: Why It Matters and How to Brew Accordingly
What is Coffee Degassing?
Freshly roasted coffee releases carbon dioxide (CO₂) as a byproduct of the roasting process. This degassing phase is crucial because excessive CO₂ in coffee can lead to uneven extraction, causing sourness or overly aggressive crema in espresso. Understanding degassing helps us brew coffee at its peak flavour.
The Science Behind Degassing
When coffee beans are roasted, chemical reactions break down carbohydrates and amino acids, creating CO₂ and other gases. These gases are trapped in the porous bean structure and gradually escape over time. The degassing rate depends on roast level, grind size, and storage conditions.
- Light Roasts Degas Slower – Denser cell structures retain CO₂ longer, requiring more time to stabilise.
- Darker Roasts Degas Faster – More porous beans allow CO₂ to escape quickly but also oxidise faster, leading to quicker staling.
- Ground Coffee Degasses Rapidly – Grinding drastically accelerates degassing due to increased surface area exposure.
Why Espresso Roasts Need Longer Resting Than Filter Roasts
Espresso brewing involves high pressure, meaning excess CO₂ can cause excessive crema, rapid channeling, and uneven extraction. To avoid this, espresso roasts need at least 7-14 days of resting to allow proper CO₂ release and flavour development.
Filter coffee is less affected by CO₂ and can be enjoyed sooner—typically after 3-7 days—while still benefiting from degassing.
Oil Migration & Freshness
During degassing, lipids and oils within the coffee beans migrate from the inner structure to the outer surface. This process is more pronounced in darker roasts, leading to visible oiliness. Once oils reach the surface, they oxidise quickly, causing stale, rancid flavours. This is why dark roasts become stale faster than light roasts.
Best Resting Period for After Hours Coffee
For our roasts at After Hours Coffee, we recommend:
- Espresso Roasts: 7-14 days for best flavour and consistency.
- Filter Roasts: 3-7 days for balanced extraction.
- Light Roasts (for espresso or filter): Add an extra 3-7 days to reach peak clarity.
How to Brew Fresh Coffee if You Can’t Wait
If you absolutely must brew coffee before the optimal resting period, here’s how to adjust:
Espresso Adjustments:
- Lower Your Dose – Use 0.5-1g less coffee to compensate for excessive gas, reducing channeled extractions.
- Grind Slightly Finer (with caution) – Helps control faster flow due to excessive crema, but avoid going too fine as it may cause excessive resistance.
- Lower Brew Temperature (88-90°C) – Reduces the acidity caused by trapped CO₂.
- Longer Pre-Infusion (5-10 sec) – Allows more gas to escape before full pressure extraction.
Filter Coffee Adjustments:
- Use a Coarser Grind – Prevents excessive gas retention, which can cause uneven water flow.
- Increase Bloom Time (45-60 sec) – Helps CO₂ escape, leading to more even extraction.
- Use a Higher Brew Ratio (e.g., 1:16 instead of 1:15) – More water ensures better extraction despite gas interference.
When Does Coffee Become Stale?
- Dark roasts: Start losing vibrancy after 2-3 weeks due to rapid oxidation.
- Medium roasts: Typically stay fresh for 3-4 weeks if stored well.
- Light roasts: Can retain complexity for 4-6 weeks, thanks to denser cell structure.
To extend freshness, store coffee in an airtight, opaque container away from heat and moisture. Freezing small portions can help preserve quality beyond the typical freshness window.
Final Thoughts
Degassing is an essential process that influences flavour, aroma, and extraction quality. While it’s tempting to brew coffee immediately after roasting, allowing it to rest will produce a more balanced and enjoyable cup. If brewing fresh coffee is unavoidable, small adjustments can help counteract excessive CO₂ and ensure a better experience.