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What Is Coffee Degassing?

Fresh is good, too fresh isn't.

by Maciej Kasperowicz | June 12, 2019

Coffee can actually be too fresh — here's how to make sure it's not!

A focus on freshly roasted coffee has certainly been a hallmark of specialty coffee education, and for good reason. Coffee, when it sits around, slowly loses the aromatics that make it taste so special. It also oxidizes, creating unwanted stale flavors. No matter how good your storage, coffee consumed within a reasonable window after roasting will taste better and more lively than that same coffee after it has been sitting in your cupboard for eight to 11 months. But, is there such a thing as coffee that’s too freshly roasted? Why yes, yes there is!

One byproduct of roasting coffee is the buildup of carbon dioxide inside the roasted coffee beans. This carbon dioxide leaks out of the coffee starting immediately after roasting ends. If you’ve noticed a little plastic circle on your retail bag of specialty coffee with a tiny hole in it, that’s a one way-valve. It’s there specifically to let that carbon dioxide out without letting much oxygen in. Without that valve, you’d either have to have unsealed bags, which would let oxygen in and have the coffee go stale faster, or the release of the carbon dioxide would eventually make the sealed bag explode.

The carbon dioxide in the coffee is also easily observable if you’ve ever brewed coffee in a pour over, French press, or any brew method where you can actually see the water coming into contact with the coffee. Those bubbles that start appearing when the hot water hits the coffee: that’s carbon dioxide. There’s nothing wrong with carbon dioxide being in your coffee, but too much of it does make it harder to make tasty coffee, and it’s easy to understand why with some very basic physics.

In order to extract all the tasty stuff we want from coffee, we need water to penetrate into the coffee grounds, dissolve those flavor compounds, and get them out into our cup. If you have a bunch of gas pushing out from the coffee grounds, it’s simply harder for that water to get in. And if the water can’t get in and can only dissolve particles from the outside of the coffee grounds, it won’t extract evenly, which can potentially lead to cups of coffee that are both too sharply acidic and too bitter at the same time.

Sometimes when we brew coffee, we try to account for this by pouring a little bit of our hot water first, letting some carbon dioxide escape, and then after 30 to 45 seconds continuing with the rest of our water. That process is called a bloom. But a bloom can only do so much if we haven’t let enough carbon dioxide escape between roasting and brewing. And that pre-brewing, resting period we need to let the gas escape is called degassing or off-gassing. Degassing doesn’t involve doing anything special, it’s just letting the coffee rest on your counter, ideally still in its original bag.

This is especially important if you’re brewing espresso, a brew method that has coffee grounds that are packed too tightly and there is not enough time to even allow for a bloom. Espresso's flavors are so concentrated that you’ll be able to taste that sharp, uneven extraction immediately. But even for drip coffee, we recommend four days after the roast date as the ideal time to start brewing.

So when buying fresh coffee (which you definitely should!), getting it any sooner than four days off roast isn’t really a goal worth pursuing if you’re trying to drink it immediately. All it takes is either a little or patience or, more to my liking, just making sure you have enough coffee so that you won’t run out until your new bag is four days off roast.