Processing & Roasting
Coffee Roasting Fundamentals
Roasting transforms green coffee beans into the aromatic, soluble product we brew. This guide covers the essential chemistry — Maillard reactions, caramelization, Strecker degradation — and the physical milestones like first crack, development time ratio, and the critical decisions that shape every cup.
The Transformation of Green Coffee
Raw green coffee beans are dense, grassy-smelling, and nearly flavorless when brewed. Roasting is the thermal process that unlocks coffee's 800+ aromatic compounds, transforms its physical structure, and makes the bean soluble enough to extract. Understanding the fundamentals of this transformation is essential for any serious coffee enthusiast.
The Phases of Roasting
A typical roast cycle lasts 8–14 minutes for specialty coffee (though some traditional approaches extend to 20+ minutes). The process follows three distinct phases:
1. Drying Phase (0:00–4:00, bean temperature 25°C–150°C)
Green beans contain 10–12% moisture. During the drying phase, this water absorbs heat and evaporates. The beans change from green to pale yellow. No significant flavor development occurs yet, but adequate drying is critical — rushing this phase leads to uneven roasts where the exterior develops faster than the interior (known as tipping or scorching).
2. Maillard Phase (4:00–8:00, bean temperature 150°C–200°C)
The Maillard reaction — the same chemistry that browns bread crusts and seared steaks — begins when amino acids react with reducing sugars at temperatures above 150°C. This cascade produces:
- Melanoidins — brown polymers that give coffee its color and contribute body
- Pyrazines — nutty, roasted, cereal-like flavors
- Furans — sweet, caramel-like compounds
- Thiophenes — savory, meaty notes (in small quantities)
Simultaneously, Strecker degradation breaks amino acids into aldehydes responsible for many of coffee's most recognizable aromas — buttery diacetyl, malty 2-methylpropanal, and chocolatey phenylacetaldehyde.
The beans darken from yellow through tan to light brown. Moisture continues to escape, and the beans expand slightly as internal gases build pressure.
3. Development Phase (8:00–12:00+, bean temperature 200°C–230°C)
This is where critical decisions are made. The development phase begins at first crack.
First Crack
At approximately 196–205°C, internal steam pressure causes the bean to fracture audibly — a popping sound similar to popcorn. First crack marks the point at which coffee is technically "roasted" and drinkable. It signals:
- The bean has expanded 50–80% in volume
- Internal moisture has dropped to approximately 3–5%
- Cell walls have ruptured, making the bean porous and grindable
- Maillard products are well-established
Development Time Ratio (DTR)
The development time ratio is the percentage of total roast time that occurs after first crack begins. It is one of the most important metrics in modern specialty roasting:
| DTR | Character |
|---|---|
| 15–20% | Light roast — origin-forward, bright acidity, floral/fruity |
| 20–25% | Medium roast — balanced acidity, sweetness, and body |
| 25–30%+ | Medium-dark to dark — roast-forward, bitter, heavy body |
A roast that is 11 minutes total with first crack at 9:00 and drop at 11:00 has a DTR of 2/11 = 18% — a classic light-medium specialty profile.
Caramelization
Above 170°C, sucrose (which constitutes 6–9% of Arabica green bean weight) begins to caramelize, breaking down into hundreds of compounds:
- Diacetyl — buttery, butterscotch
- Maltol — sweet, cotton candy
- Furanones — caramel, burnt sugar
Caramelization is desirable up to a point. Extended caramelization at higher temperatures produces bitter compounds and eventually carbonization (charring).
The Chemistry at a Glance
| Reaction | Temperature Range | Products |
|---|---|---|
| Maillard | 150–200°C | Melanoidins, pyrazines, furans |
| Strecker | 150–200°C | Aldehydes (butter, malt, chocolate) |
| Caramelization | 170–230°C | Diacetyl, maltol, furanones |
| Pyrolysis | 220°C+ | Bitter phenolics, carbonization |
Rate of Rise (RoR)
The rate of rise measures how quickly bean temperature is increasing, expressed in degrees per minute. Roasters monitor RoR in real time:
- A declining RoR through the roast is considered optimal — it prevents scorching early and ensures thorough development
- A crash (sudden RoR drop to near zero) produces baked, flat flavors
- A flick (sudden RoR increase after first crack) causes roasty, sharp bitterness
Cooling
Once the target roast level is reached, beans must be cooled rapidly — within 3–5 minutes — to halt chemical reactions. Most roasters use a cooling tray with a fan pulling air through the beans. Inadequate cooling allows carryover heat to push the roast darker than intended.