When we think of food and beverages, we usually assume that the fresher something is, the better it is. The same assumption applies to specialty coffee. One of the main reasons why you should buy your coffee directly from a specialty roaster rather than from a supermarket is this: in general, this is fresher than a coffee that may have been sitting on a supermarket shelf for months. But as with all specialty coffees, it's unfortunately not that simple. The reason? Ideally, the coffee needs to rest after roasting. To better understand why and for how long, let's look at the key principles. When you roast coffee, it's effectively cooked. This process triggers a series of chemical reactions that change the composition of the coffee through processes such as the Maillard reaction. These processes produce CO2, and a lot of it. Most of it is carried away, but some is absorbed by the coffee. Over time, this CO2 volatilizes and slowly escapes. This absorbed CO2 causes problems during brewing, as the humidification of the coffee releases this CO2. This is how the bubbles form in a pour-over or the crema in an espresso. This gas emission can cause problems during brewing, as it prevents the water from properly reaching the coffee and effectively extracting it. This is especially true for more intense brewing methods like espresso. Therefore, the coffee must rest to allow this CO2 to dissipate. There are a few factors to consider that determine how long a coffee needs to rest. Density The higher the density of a coffee, the less CO2 it absorbs during roasting - there is simply less space. The lighter the roast of the coffee and the higher the altitude at which the coffee was grown, the higher the density. Therefore, light roasted coffees from higher elevations don't need to rest much. Darker roasted coffees have a much lower density and therefore absorb much more CO2, so they need to rest longer. Degree of roasting The stronger a coffee is roasted, the more CO2 is produced. This means that more CO2 is trapped in the finished coffee. This is another reason why dark roasted coffees need to rest longer. Packaging Most specialty coffees are packaged in bags with one-way valves that allow CO2 to escape. This allows them to be effectively rested and stored in the bags. However, not all bags have these valves. Some also prefer to store their coffee in vacuum-sealed bags or containers. If the CO2 output is inhibited, the resting time must be longer. If you want to keep your coffee in vacuum bags for a long time, you should let it rest before sealing it. Temperature The warmer the ambient temperature, the faster the coffee will degas. If you live in a cooler place, you should let it rest a little longer; if you live in the desert, it will degas much faster. Brewing method The way you brew your coffee also affects how long it needs to rest. Filter coffee doesn't need to rest as long and can be enjoyed the very next day, but tastes best after 3-5 days. Espresso needs a resting time of about 8-10 days to be at its best. Still, resting time is not a magic trick that will make your coffee taste a million times better. It's a matter of optimization - if you know how your coffee rests and develops, you'll know when it tastes best. But don't stress too much about it, and be aware that coffee that's too fresh and too old can still taste good.