When selling retail coffee, some of the most common questions are about how old the coffee is. With a lot of coffee this is a really easy question to answer, because the roast date is right on the bag (if it’s not… and there is a 12 month used by date… I would put it back on the shelf), but what does it all mean? How old is too old? How fresh is too fresh?
There are a few different things happening to coffee after it has been roasted, that will affect the flavour:
First, is the expelling of carbon dioxide that has been created and trapped inside the beans during roasting. This is why your bag of coffee will have a little one-way valve stuck to it, and why the bag can be puffy.
The second main thing is the oxidization of oils in the coffee, which is another way of saying the coffee is going rancid. 10% of coffee mass is oil and carries a lot of flavour which, when rancid, tastes less than ideal.
Third is the release of volatile organic compounds, which are very important to how we perceive flavours in coffee, like sweetness. These compounds are volatile because they are in a gas state, even when in the freezer.
Keeping these things in mind, how do we know when our coffee is too fresh, too old or just right? Well… we don’t. Not for all coffees anyway. Not really.
A roaster will test their coffee at different ages to see when it starts to taste better, and when it starts to taste worse. Different roasters, using different machinery, with different roasting styles, and different beans will age differently. So keep an eye out for recommendations. If someone has told you how long coffee is good for, take it with again of salt unless they’re talking about their coffee specifically. The craziest I heard was that coffee is only good for the first three days after roast! I question their knowledge.
Keeping oxygen out is the easiest thing to do. Simplest method is to only take out of the coffee what you need to use, then squeeze the air out and seal it back up. You can buy some pretty fancy containers designed to remove the air our that have second lids to get rid of unused head space. For long term storage you can vacuum seal portions of coffee and do away with oxygen altogether.
Hope this helps you understand the ageing process.