What happens during roasting?

Before reaching our cup, coffee undergoes a spectacular transformation. The green, dense, vegetal, and almost odorless bean becomes brown, crumbly, and intensely aromatic. Under the influence of heat, its structure changes, and hundreds of compounds are formed. Here are the main stages of roasting.

Grain drying

Green coffee typically contains between 8 and 12% water. When the beans enter the roaster, the heat initially causes this moisture to evaporate.

Their color gradually changes from green to pale yellow. A vegetal aroma, sometimes reminiscent of cut grass, hay, or grains, is released. This phase is crucial: if the drying process is too rapid or uneven, the outside of the bean risks cooking before its core. Conversely, a phase that is too long can produce a dull coffee with weak aromas.

Browning and the Maillard reaction

As the temperature rises, the grains turn golden, then brown. The reducing sugars react with the amino acids: this is the Maillard reaction, also observed when bread forms a crust or when certain foods are grilled.

This reaction creates a multitude of aromatic molecules. These contribute to the development of notes of toast, hazelnut, chocolate, caramel, fruit, or spices. It is during this phase that a large part of the coffee's aromatic identity begins to take shape.

The Maillard reaction also produces melanoidins, brown pigments that give coffee its characteristic color and contribute to its mouthfeel.

The caramelization of sugars

When heated, the sugars present in the grain break down and transform. This phenomenon, called caramelization, enhances the sweet and indulgent aromas.

But sweetness shouldn't be confused with sugar content. Roasting doesn't simply make coffee sweeter; it consumes and transforms some of its sugars. A well-controlled roast creates an impression of sweetness and balance. If it's taken too far, the caramelized notes gradually give way to more bitter, smoky, or burnt flavors.

The first "crack"

Under the effect of heat, the remaining water turns into steam and gases accumulate inside the grain. The pressure increases until it causes a series of small crackling sounds similar to the noise of popcorn: this is the first crack.

The bean expands, becomes more porous, and loses some of its density. This stage marks the transition to a truly roasted coffee. The roaster then enters a crucial phase known as the development phase.

The development of aromas

Between the first crack and the end of roasting, the roaster seeks to balance acidity, sweetness, bitterness, body, and aromatic intensity. A few seconds can be enough to significantly alter the result.

A relatively light roast best preserves the characteristics inherent to the coffee's origin: floral, fruity, or acidic notes. A darker roast generally enhances body and aromas of chocolate, caramel, or nuts. The darker the roast, the more the roasting flavors overpower those related to the terroir and variety.

Therefore, there is no single ideal roast level for all coffees. The right profile depends on the bean, its origin, its density, and its processing.

What happens during a very deep roast?

If cooking continues, a second crack can occur. The grain's structure weakens further, and some oils rise to the surface. Thermal degradation reactions, particularly pyrolysis, become increasingly pronounced.

The coffee then develops smoky, spicy, roasted, even charred notes. Its acidity decreases, while its bitterness increases. At a very advanced stage, the differences between origins disappear: the roasting aromas almost entirely dominate the taste of the bean.

Cooling

Once the desired profile is achieved, the beans must be cooled rapidly. Without this step, the heat they have accumulated would continue to cook them, even after they are removed from the roaster.

A powerful current of air then lowers their temperature to a level close to room temperature. This cooling process fixes the roasting result and prevents the development of undesirable flavors.

Degassing after roasting

The transformation doesn't stop immediately. Freshly roasted coffee continues to release carbon dioxide for several weeks. This phenomenon is called degassing.

Part of this gas temporarily protects the aromas from oxidation. However, in excessive amounts, it can disrupt the extraction process, especially for espresso. This is why coffee is generally left to rest before brewing. The ideal resting time varies depending on the type of coffee and the degree of roasting.

An alchemy under control

Roasting is both a science and a skill. The roaster manipulates the temperature, duration, air circulation, and energy input to guide the reactions that occur within the bean.

Its goal is not just to brown the coffee, but to reveal its potential. A successful roast translates the qualities of the bean into perceptible aromas in the cup, while seeking the right balance between acidity, sweetness, bitterness and texture.

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Now all you have to do is move from theory to tasting. Fruity, indulgent, or more intense coffees: explore our different roasts and choose the one that will accompany your next coffee moments.

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