Why do coffee bags have valves? How does the valve work on a coffee bag?

I like this question because I asked the same when I first saw one. The first specialty coffee bags I bought years ago had punctures and a one-way valve. Back then, I thought that these valves were a smart way to let me smell coffee before buying it, but it isn’t made for that (I know, I can be too self-centered sometimes).Actually, the main reason for having one-way valves in coffee bags is to allow fresh coffee beans to release CO2. To some extent, a coffee bag with a one-way valve is a good sign, because coffee roasters usually pay attention to this detail when they care for coffee freshness. And after some tasting fresh coffee and comparing it with regular coffee, you will notice that fresh coffee is better almost always.Normally, coffee bags with a one-way valve will have a roasting date printed or handwritten on their labels, and they tend to have between 1 to 2 months old after the roasting date. If you live near a coffee roaster, you might get coffee beans roasted less than a week before.However, coffee freshness isn’t that simple. Don’t get me wrong, one of the main reasons I prefer specialty coffee is freshness. So, I was a bit shocked when I learned that coffee beans can be excessively fresh and they need to be degassing for several days after roasting.This happens because coffee beans suffer a dramatic physical and chemical transformation during roasting and it doesn’t stop there. After we roast coffee beans they start to release CO2 and eventually oxidize. These processes occur naturally after roasting and have opposing effects on coffee freshness and quality. To be more specific, oxidation is undesirable, while releasing CO2 is necessary.Oxidation is what ages coffee, making it lose its aromatic properties, alter its taste, and its hardness, changing even its consistency for grinding.On the other hand, if we pack freshly roasted coffee immediately without any degassing or venting system, the CO2 builds up inside the package, affecting the coffee beans taste too.The solution for this dilemma is the one-way valve, which is pretty clever! Before Goglio -an Italian packaging company- created the one-way valve for coffee bags in 1960, roasters had to puncture their bags and let them release CO2 while becoming stale at the same time.The one-way valve allows CO2 to leave while keeping any gas away from entering the bag. This system allows you to keep coffee fresh for a longer time, without suffering the problems related to trapped CO2 with our precious and delicious coffee beans. In short, if you see those tiny punctures in the coffee bag, and you notice that it has a one-way valve, check the roasting date. If it has one, or your roaster can answer when you ask about it, rest assured that you’re getting coffee as fresh as it should be.