We get it: fresh-roasted coffee is delicious. But why does it take around seven to ten days for freshly roasted coffee to degas?
The answer is simple: CO2. Carbon dioxide, that is. It's a gas that occurs naturally in the beans as they're growing and roasting, but it takes time for it to fully vent out of the bean after roasting.
The process of degassing is actually quite complex. You see, when you roast coffee beans, you're changing the chemical structure of their cellular material—particularly the oils and lipids that exist inside the bean. As these chemicals begin to break down during the roasting process, they release gas in the form of CO2.
That's why you've probably noticed that your freshly roasted beans have a much stronger aroma than older ones. That's because as soon as they are exposed to air or water (like in your grinder or French press), those oils will begin to release more CO2 into the air around them. This means that over time, as your beans continue to lose volatile compounds like CO2, they also lose some of their flavour and aroma.
As long as you keep your beans sealed in an airtight container or bag (and away from sunlight and strong smelling pantry items), they'll stay fresh and flavoursome for your daily cuppa.