Roasting turns hard, odorless green beans into the aromatic beans we know, through a sequence of physical and chemical stages driven by heat and time. Every stage leaves its mark in the cup.
🌡️ Why roasting transforms green coffee
Green coffee beans start out as hard, odorless seeds. Once the temperature rises above roughly 150°C, the first physical changes begin: moisture evaporates, the beans expand, and color starts to shift – setting the stage for the chemical reactions that follow.
🔥 The main stages of roasting
Between the moment beans enter the roaster and the moment they're dropped, they pass through several distinct phases:
Drying phase – remaining moisture evaporates
Caramelization phase – sugars begin to break down
First crack – an exothermic reaction where the beans audibly crack, officially becoming roasted coffee
Development – the period after first crack where the final flavor profile takes shape
Second crack – for darker roasts, a quieter second crack as oils begin to break down inside the bean
🔬 The Maillard reaction during roasting
Between 150°C and 200°C, the Maillard reaction takes place – an interaction between amino acids and sugars. It's one of the key stages of roasting, responsible for deep, roasted notes like caramel, nuts, and chocolate.
🍬 Caramelization and flavor development
As heat increases, sugars inside the beans caramelize, creating sweet, rich flavors and sometimes a slight bitterness, depending on roast level and duration.
💨 Degassing and post-roast freshness
After roasting, beans keep releasing CO₂ for days. This degassing stage is essential for freshness and aromatic balance in the final brew, which is why most roasters recommend a short resting period before brewing.
🌈 How roast level shapes flavor
Light roast: floral, citrusy, bright acidity
Medium roast: balanced sweetness, acidity, and bitterness
Dark roast: full body, chocolatey notes, low acidity
📚 Conclusion
Roasting is a controlled process where every stage – from moisture loss to first crack and development – directly shapes what ends up in the cup. For a closer look at the chemical compounds responsible for aroma and flavor, see our article on coffee's chemical compounds.