CO2 and Oxidation
Generally speaking, fresh coffee is better than old coffee – but that's only part of the truth.
After roasting, coffee contains a lot of CO2, which escapes slowly throughout the following weeks. This is also the reason for the valve on the coffee bag (it's not there so you can smell the coffee): CO2 can leave the bag, which won't inflate, and oxygen won't get into the bag, which prevents a quick oxidation.
Back to freshness. If you drink coffee right after roasting, it contains too much CO2 and this will affect your extraction, so you want to avoid super-fresh coffee, especially so when using an espresso machine (as opposed to, say, a v60 pour over brew).
Is Too Fresh Really Bad?
While filter coffees can be drunk as soon as a day after roasting without any problems and taste great, we recommend to wait around two weeks before brewing fresh coffee on an espresso machine (if possible, of course). Your espresso extraction will most likely be influenced strongly by the CO2 and your espresso might be both under- and overextracted at the same time (google «espresso channeling» to find out more about the topic).
There are ways to improve your results with really fresh coffee, like letting your coffee sit for perhaps 15 minutes after grinding before you use it in your espresso machine. This speeds up degassing of the coffee, but it's obviously not ideal and not very practical.
- coffee usually reaches its full potential 2-3 weeks after roasting
- avoid drinking espresso roasts within the first two weeks after roasting if possible – plan ahead when buying
- try your filter roasts sooner after roasting to find out how the aroma and flavor change over time
- shelf life is usually at the very least six months, for most coffees much more
- when it comes to aroma and flavor, better drink up the coffee within 2-3 months
- keep coffee in a sealed bag with pressure valve when consumed quickly or in a vacuum container if kept for more than 2-3 weeks