The Science of Roasting: From Green Bean to Brown Bean
Green coffee is nearly tasteless — it smells like fresh grass. Every aroma, sweetness, and complexity you taste in coffee is born inside the roaster during 8-15 minutes of precise chemical reactions.
The Four Phases of Roasting
1. Drying — up to 160 °C
Beans contain 8-12% moisture. This phase drives water out gradually. Color shifts from green to pale yellow. Takes 4-8 minutes.
2. Maillard Reaction — 160 °C to 200 °C
Here the magic begins. Sugars react with amino acids forming:
- Brown color compounds
- 800+ aromatic compounds
- Bread, nutty, caramel flavors
3. First Crack — around 196-205 °C
Steam pressure inside the bean explodes it open — sounds like popcorn. Marks the end of the light roast window. Stop here = Light Roast (fruity, high acidity).
4. Development — after first crack
The time from first crack to drop is called DTR (Development Time Ratio). Usually 15-25% of total roast time.
- Medium: short development
- Dark: long development, even into second crack (~225 °C)
The Rate of Rise (RoR) Curve
RoR is the rate of temperature increase (°C/min). SCA roasters track it minute by minute:
- Start: 25-35 °C/min
- Middle: declining steadily
- End: 5-10 °C/min before drop
Golden rule: RoR must decline smoothly — never spike, never flatten. Flatlining produces baked, lifeless coffee.
The Agtron Scale
A visual measure of final color:
- 95-85: Cinnamon (very light)
- 75-65: Medium
- 55-45: Full City
- 35-25: Dark / French
How Roast Affects Solubility
Darker beans have more fractured cell walls = easier to extract = need coarser grind and lower temp. That's why dark espresso brews at 88 °C. Light espresso at 95 °C.
These aren't just technical details — they're the reason every cup tastes different from the next.