A Beginner's Guide To Resting Coffee by James Hoffmann
The Science of Resting Coffee
The primary reason to rest coffee after roasting is to allow carbon dioxide (CO$_2$) to escape, a process known as degassing.
- CO$_2$ Production:** The roasting process creates CO$_2$ as a byproduct of browning reactions.
- Darker Roasts produce significantly more CO$_2$ but the beans are more porous, allowing the gas to escape quickly.
- Lighter Roasts produce less CO$_2$, but the beans are denser, making it harder for the trapped gas to escape, requiring a longer rest period.
- The Brewing Problem: When water hits the coffee grounds, it rapidly liberates the trapped CO$_2$.
- In filter coffee (like a V60), this causes a visible "bloom".
- In espresso, intense CO$_2$ outgassing is very disruptive to the pressurised brewing process, making it difficult to properly extract the coffee.
- Taste of Unrested Coffee: Coffee that still contains too much CO$_2$ can taste sour or have a carbonic acid tang.
Recommended Resting Times
The ideal resting time depends heavily on the brewing method and the roast level.
| Roast Level | Brewing Method | Recommended Resting Time | Notes |
|---|---|---|---|
| Light Roast | Espresso | ~10 days | Requires the longest rest. Will produce a thinner crema than dark roasts |
| Medium Roast | Espresso | 8–10 days | A good balance of degassing and flavor development. |
| Dark Roast | Espresso | A couple of days | Degasses quickly. The primary concern becomes staling (oxidation) due to exposed oils and porosity, so fresher is better. |
| Light/Medium Roast | Filter | 3–5 days | Resting is less critical. While a 1-day rest is drinkable, 3–5 days allows for better flavor development. Very dense, light roasts may benefit from up to 10 days. |
Factors Affecting Degassing & Storage
- Temperature: Warmer storage environments accelerate degassing and other staling reactions (oxidation). Store coffee at or around 20°C (68°F) if possible. If storing in a colder environment (like a cellar or freezer), you must extend your resting times.
- Packaging Valve: Most coffee bags have a one-way valve that allows CO$_2$ to exit without letting fresh oxygen in. Note that the CO$_2$ escaping does not push out all the air/oxygen already trapped inside the bag.
- Vacuum Canisters: If you put very fresh coffee into a vacuum canister, the vacuum will appear to fail after a few days as the degassed CO$_2$ equalises the pressure inside. This is normal and means the coffee is degassing, not that the canister is faulty.
- Quenching (Roaster Technique): A roaster can "shortcut" the resting period by spraying a fine mist of water on the beans during cooling. This evaporative cooling, called quenching, rapidly accelerates CO$_2$ outgassing but also causes the coffee to go stale faster. Quenching is generally done only when coffee is needed immediately for espresso.