What Actually Happens to Coffee Beans While They Rest?

Resting coffee beans is not one process but three: degassing, development, and maturation. Degassing releases carbon dioxide trapped during roasting, development continues the chemical changes begun in the roaster, and maturation lets flavors settle and round out. Treating them as a single vague waiting period is why advice usually amounts to little more than try it and see.

Freshness on a roast date does not equal good flavor. Carbon dioxide escaping too quickly can disrupt extraction, so many beans taste better after several days of rest, and the needed time differs between light and dark roasts. Separating degassing, development, and maturation gives a clearer framework for judging when a batch is ready, whether the coffee is used in a pour-over or in brew in the bag coffee.

When buying coffee beans online, asking about the roast date has almost become a standard move for every enthusiast. But more and more people are realizing that fresh does not equal good-tasting, and resting the beans is the key step that determines how the flavor is presented. However, discussions about resting beans often remain at the vague level of "every shop is different, just try it yourself." This article attempts to break down the three core mechanisms behind resting beans—degassing, development, and maturation—explaining their effects on extraction and flavor respectively, and exploring the differences in maturation needs between light and dark roasts, to help readers build a clearer framework for judging bean resting.

When it comes to the thing people care about most when selecting coffee beans these days, what is the first thing that comes to your mind? I guess most people, before placing an order at an online shop, if they have communicated with the seller, will nine times out of ten throw out this question: "What date were these beans roasted?"

So what exactly does the freshness of the roast date mean for coffee beans?

In the past, most consumers were still at the stage of believing "the fresher the better," and we did not really seem to need to discuss this specifically. But when buyers become more knowledgeable and start asking, "Can I drink this batch of coffee 'right away'?" it shows that everyone has already noticed—coffee is not more delicious the fresher it is.

"How long exactly should beans be rested?" This is a question that has appeared frequently in recent years on Baidu or in WeChat groups. But the responses received often remain at the level of "It depends, every shop is different," "Why don't you try and see how long this shop's beans need to be rested~"—the kind of reply that should be a premise of the question, not the answer.

Therefore, we should first clarify one question: when resting beans, what exactly are we resting?

In the concept of "resting beans," there are three aspects that need to be separated and examined: "degassing," "development," and "maturation."

If we do not discuss them separately, we can only give answers like "It depends, every bag of beans and every shop is different." Today, let us first explain these three parts—"degassing," "development," and "maturation"—clearly, and when we have the chance later, we can analyze the various factors that affect the resting process and the time required.

"Degassing"

After coffee beans finish roasting, a large amount of carbon dioxide remains within the cell walls of the bean body, and because the Maillard reaction continues weakly after roasting is completed, a small amount of carbon dioxide is also generated (this point will be explained later). So when brewing coffee you can see the grounds swell, and the bag storing the beans puffs up after a day or two—these are all manifestations of carbon dioxide release.

In freshly roasted coffee beans, the large amount of carbon dioxide accumulated inside the bean body seriously interferes with extraction. Because the essence of extraction is the process of water entering the interior of the coffee grounds and bringing out the flavor substances. This large amount of carbon dioxide: ① blocks water from entering the interior of the coffee, isolating contact between water and coffee grounds; ② forms many large bubbles, carrying away a large amount of easily volatile aromatic substances from the extraction water; ③ also creates many violent and difficult-to-control disturbances (especially during pour-over).

At this point, if you do not let them sit for three to five days to allow the beans to first release carbon dioxide, you will encounter the trouble of unstable and under-extracted brewing.

So when someone asks, "If I grind the coffee beans and let them sit open for ten minutes before brewing, can that replace resting the beans?" the answer is: "If we are only talking about degassing, it can indeed replace it," but the two stages of "development" and "maturation" cannot be lazily skipped in this way.

"Development"

As mentioned earlier, after roasting is completed the Maillard reaction still proceeds weakly and also produces a small amount of carbon dioxide, which at the same time means aromatic substances are also continuously being generated. When a large amount of carbon dioxide fills the bean bag and completely isolates oxygen, it also "presses" the vast majority of aromatic substances inside the bean body or on the bean surface. At this time the aromatic substances after the Maillard reaction continue to be generated, and the taste of the coffee beans begins to develop more and more "complete." How long this process can last depends on how much material available for continued Maillard reaction was left during roasting. Once the reaction formula reaches equilibrium, that is when we say this bean has entered its "optimal flavor period." (This refers to an ideal state, without considering sensory thresholds and personal preferences for the moment; that is another complicated issue.)

So coffee beans themselves are not a food that is finalized the moment roasting is finished. Strictly speaking, before the Maillard chemical formula reaches equilibrium, it is still in the process of "being processed." This slow yet truly gradual reaction that increases the richness of the coffee beans is what we usually mean by "resting."

"Maturation"

When we place freshly roasted coffee beans in a bean bag filled with carbon dioxide, or even in a sealed bean canister, the space inside the container is in a high-pressure state. This high pressure helps integrate the volatilized aromatic substances into the coffee oils. For now, let us call this state "maturation."

This process makes the coffee as a whole taste more rounded and harmonious, and also allows more aromatic substances to be retained for us to drink than with unmatured beans. Of course, someone will surely ask: then do light-roasted beans have no oils, so they do not go through this process?

This depends on everyone observing carefully. Even when using light-roasted coffee beans to make espresso, or brewing with other metal filters, you can still see a layer of oil on the coffee surface by its reflection. The difference in oil content between light and dark roasts is actually not large. (If you do not believe it, you can take light-roasted beans and try burning them—just be careful not to burn down the kitchen...)

Therefore, light roasts naturally also need the maturation step.

How long does maturation take? Generally speaking, about 10~15 days counted from when the bean bag starts to swell, and for dark roasts about 20 days will be better. But note that once the high-pressure environment is destroyed, maturation will not proceed until the high-pressure environment is restored. So if you really care about this step, please absolutely do not casually press the bean bag, or open the bean canister to smell it whenever you feel like it... (Especially for dark-roasted beans, whether for pour-over or espresso, maturation is a very important step~)

Alright~ with that, we have roughly gone over the three parts of resting beans in crude language. Probably no one wants to see the actual chemical reaction formulas and calculations from Henry's law, so let everyone digest this first, and when we have the chance later we can analyze how to infer from the information provided by the shop just how long the coffee beans you bought should be rested!

The above content is compiled by CoffeeHunters, a coffee news website.