WHY ON EARTH IS THERE A BELLY BUTTON ON THE BAG? YOU ALREADY KNOW THE SIMPLE ANSWER: TO LET OUT AIR. OF COURSE, IT IS MORE COMPLICATED THAN THAT. THAT HOLE IS PART OF A BIGGER DISCUSSION OF COFFEE FRESHNESS AND HOW BEST TO STORE ROASTED COFFEE TO MAINTAIN FRESHNESS.
Presumably, since we know the major factors that cause coffee to stale— gas evolution, high temperatures, oxidation, and humidity—we ought to able to control them to extend the shelf life of the coffee. By teasing some of the data available in the myriad of research on the topic, we can make some general statements that will help. However, without direct research to support our hypotheses, and the ones of the coffee industry at large, some of our conclusions will have to be educated guesses.
Let’s address each staling factor individually, starting with gas evolution. Since smaller coffee pieces allow the release of more gas, keeping the coffee as intact as possible will help. Thus, grinding coffee ahead of time is a poor practice. Rather, grinding should occur just prior to brewing. The other potential way to slow down gas evolution (and all chemical reactions) is to decrease the storage temperature; cooler temperatures slow down chemical reactions and chemical mobility. Thus, storing coffee in the refrigerator or freezer will accomplish this. Unfortunately, I can’t find any sensory data that explores specific taste changes when stored at cooler temperatures.
Coffee geeks abhor the idea, but, at best, they have some personal, anecdotal evidence to support it. Freezing coffee could run the risk of creating crystals that could shatter cells, much like grinding. Freezing could also lead to freezer burn, which probably isn’t a flavor anyone wants to introduce to a coffee. Arguably, the biggest reason not to store coffee in the freezer is the risk of condensation forming on the beans as the beans come out of the freezer. This water may then lead to a deterioration of the quality by hastening the natural staling of coffee when the coffee is out of the freezer or by allowing ice crystals to form on the coffee if it is returned to the freezer. Refrigeration doesn’t run the risk of crystal formation, but the condensation is still an issue.
Ultimately, individual drinkers will have to decide this on their own, at least until some new research surfaces. Preventing or minimizing oxidation reactions is as simple as keeping oxygen away from the roasted coffee beans. Of course, with the atmospheric concentration of oxygen at about 21 percent, that isn’t so easy. Simply putting just-roasted coffee in an oxygen-impermeable container and sealing it doesn’t solve the problem since the air trapped in the container is full of oxygen. Besides, even if coffee were sealed up in a container, the container would likely explode as a result of the pressure build-up from all the volatile compounds being released! So, either the air has to be completely sucked out of the container before it is sealed or all the air must be replaced with a gas that is completely inert, like nitrogen.
I have no knowledge that any company packages just-roasted coffee and then evacuates the air before sealing it, though it seems like a worthwhile strategy. Many larger roasters do flush bags with nitrogen before sealing them. Some research supports this as an effective means of extending the acceptability of the coffee farther from the roast date than by using normal air. Lastly, controlling the amount of water coffee is exposed to is fairly simple. If the coffee is packed in an oxygen-impermeable container, then the container is also likely to be water impermeable. After the container in opened, keeping the coffee in an air-tight container that is waterproof should help minimize exposure to any humidity in the air, although, if the air was full of moisture when the coffee was sealed or closed in a container, then the container won’t offer any protection.
So, what’s the story with the bag and its belly button? The bags that have them are made out of oxygen-impermeable materials. Generally, they prevent many gases from passing through. Thus, as mentioned before, if freshly roasted coffee is sealed in a bag, it is liable to explode. The belly button, more formally known as a one-way valve, is a crafty device that allows gas to exit the bag but prevents any gas from entering. It is a release valve; the carbon dioxide and other volatile compounds can escape but oxygen cannot enter. The one-way valve is a fantastic tool but it has its limitations.
For one thing, unless the air trapped in the bag while sealing it is replaced with something inert, preventing oxygen from entering is irrelevant; the bag is already full of it (though the valve still prevents the bag from exploding). Secondly, once the bag is opened by the consumer, any internal protection is lost and the consumer must repackage the coffee as best as possible. Ultimately, we aren’t able to prevent the staling process from occurring. At best, it can be delayed. However, if coffee is drunk within a few weeks of roasting, the need to delay staling is most likely unnecessary. After all, the freshly roasted coffee will still be pretty fresh!
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