A major myth around coffee is that freshly roasted makes the best coffee. Now, coffee does go stale after a few months have passed, more quickly if it is not stored well. But the extrapolation back in time that coffee declines in quality after it has been roasted is not true. Coffee greatly benefits by giving it time to rest, with recommendations ranging from at least 7 days to sometimes more than two weeks. One roaster, Kozlow Koffee, recommends at least a 4-week rest for their coffees.
At Fir Beans Coffee, the pattern we generally encounter with our roasts is that a coffee tastes fine the first week after roasting, often tastes flat during the second week after roasting, and can taste awesome once two weeks have passed. We are firm believers in giving coffees at least two weeks to rest before brewing. Now why does coffee need to rest? That is currently unknown.
In the roaster community, resting is often treated the same as degassing. After roasting, coffee gives off CO2, more prominently during the first week after being roasted. This is why sealed coffee bags have a one-way gas valve, so the bags don't explode. We also see this when brewing coffee closer to its roast date; introducing water for brewing creates a foam from CO2 that we see less of as a coffee is further away from being roasted. Espresso particularly benefits from giving coffee time to let off CO2 because brewing coffee under high pressure is partially disrupted by the additional release of CO2 when hot water is introduced.
The off-gassing of CO2 from recently roasted coffee is often interpreted as evidence of CO2 trapped in roasted coffee beans, and they use that interpretation for how they think about resting coffee and how the flavor of coffee changes in the weeks after roasting. However, even when we grind coffee into fine grains for espresso, the CO2 is released when water is introduced for brewing rather than escaping when the coffee is ground into dust.
As an alternative hypothesis, let's think about resting and degassing as chemical processes. There are all kinds of acids and organic compounds found in roasted coffee (Birke Rune, et al. 2023). When a solvent like water is introduced to ground roasted coffee beans, much of that gets extracted as coffee the beverage. In solution, those acids and compounds can more easily react with each other, with CO2 a common output of all kinds of chemical reactions. What about the CO2 released before roasted coffee is brewed? Our current working hypothesis is that the acids and organic compounds in dry, room temperature coffee are still able to react with each other, just much more slowly than they would in a water solution. What this means is CO2 is not trapped in coffee; it is released as an outcome of post-roast acids and organic compounds continuing to react with each other. This view provides a better explanation for why the flavor of coffee changes during the first few weeks after roasting before stabilizing.
That still does not provide a guideline for how long coffee should rest, and this is where we believe it varies by the coffee itself and how it is roasted. Cafe Kreyol, a coffee importer and roaster, conducted a study and found while some coffees scored best in taste profiling 4 days after roasting (though with not much decline afterwards), other coffees benefited in their taste scoring with at least 22 days rest. A two-week rest is a reasonable rule of thumb, but keep in mind that it can vary.
References
https://kozlowkoffee.com/blogs/news/the-basics-of-brewing-light-roasted-coffee
Christina J. Birke Rune, Davide Giacalone, Ida Steen, Lars Duelund, Morten Münchow, Mathias Porsmose Clausen, Acids in brewed coffees: Chemical composition and sensory threshold, Current Research in Food Science, Volume 6, 2023. https://www.sciencedirect.com/science/article/pii/S2665927123000539