During the roasting process a lot of carbon dioxide and other gases form inside the bean due to chemical reactions. This build-up of gases results in enough pressure to cause cracks in the beans. You might be familiar with first (light), second (medium), and burnt (dark) cracks. This is the process of gases breaking the cell wall as chemical reactions continue to develop inside the bean.
How Much Degassing Occurs?
After the beans are roasted, they will give off roughly 40% of the CO2 in 24 hours. They will continue to seep CO2 at a reduced rate over the following weeks. This process of degassing serves as an indicator of bean freshness and is the reason why it’s important to check the roast date.
Roasters will sell their beans a few days to a week after the roast date. Typically fresher beans offer a better froth, whereas stale beans will create almost none. This froth is referred to as “bloom.”
What is Coffee Bloom or “Froth”?
The bloom or froth is part of the coffee brewing process where the gases from the ground coffee are released as the water hits the grind. It’s causes the grinds to grow and rise as the CO2 is purged and replaced with water.
If you don’t see the bloom it means that degassing has likely already occurred and the flavor compounds have likely diminished. This can happen for two reasons: 1. The coffee isn’t fresh. 2. The packaging didn’t prevent oxidation.
Bloom time has a large impact on the flavor and aromatic properties of coffee and should be sought after. This is also why resting coffee beans is important.
Allowing Your Beans to Rest
To preface, if you were to use freshly roasted beans you could run into a few issues with the brewing process. The resulting grind could lead to uneven extraction of acids and oils leading to a lackluster cup of coffee. The escaping CO2 could also dissolve in the water leaving an acidic taste.
Roasted beans are at their best typically 24 hours to twelve days after roasting. Note that this highly depends on the type of bean and the level of roast you’re using. For instance an espresso bean is usually ready forty-eight hours after roasting.
As a general rule of thumb: Longer roasts (dark) usually degas faster than faster roasts (light). This is because dark roasts are more porous than light roasts. Light roasts are usually best 5-10 days after resting.
Note: Darker roasts will also bloom more because more CO2 is created through the longer roasting process.
The process of holding off on brewing the beans is referred to as resting. The goal when resting the beans is to keep them away from oxygen while allowing the CO2 to leave the beans. Staleness is a result of too much oxygen getting to the beans. This process is known as oxidation.
Oxidation causes apples to brown and metal to rust, leaving behind only a fraction of what they once were. This decay happens in the same way with coffee beans. It deteriorates acids and causes aromatic oils to evaporate.
What Happens Once the Beans are Degassed?
You can think of this similarly to wine, in that, the coffee gets better with time. However, unlike wine, the beans are susceptible to oxygen or oxidation. As I noted above in the resting and bloom sections, oxygenated beans result in stale coffee.
This is why manufacturers created one-way air valve bags for their products (pictured below). They allow CO2 to escape while still keeping the coffee beans fresh by preventing oxygen from coming into contact with the beans. Most enthusiasts also use containers that feature a one way valve that works in a similar manner.
Most home roasters even use mason jars with one way valves. If the valve didn’t exist in either storage option (bag or container) then they risk blowing up due to CO2 build up.
It’s important to remember that you should treat your coffee beans similar to how you would buy bread. Only buy what you would consume over a couple of weeks. Coffee is at its freshest 2-3 weeks after the roast date.
Take the beans out of the package and grind as you need the coffee as opposed to bulk/batch grinding. Grinding the beans causes a lot of CO2 to be released. Finer particles are also more susceptible to oxidation than larger ones.