Coffee roasting transforms green beans into aromatic dark beans through heat-driven chemical reactions—primarily Maillard reaction and caramelization—controlled by temperature, time, and airflow. Understanding roasting mechanics reveals that coffee flavor is engineered through precise roasting profiles.
Green Coffee Beans
Green (unroasted) coffee beans are seeds from coffee cherries: (1) Composition: ~50% carbohydrates, ~12% protein, ~9% fats, ~7% chlorogenic acids, water/minerals. (2) Aroma: Minimal—green, grassy smell. (3) Taste: Bitter, astringent, unpleasant. (4) Color: Green-gray. Key insight: Green beans contain chemical precursors for flavor—roasting activates these compounds.
Green beans are raw material—flavor development occurs entirely during roasting.
Roasting Stages
Stage 1 – Drying (0-5 minutes, 160-180°C): Water evaporates (removing ~12% moisture). Stage 2 – Browning (5-10 minutes, 180-220°C): Maillard reaction begins, color changes yellow-brown. Stage 3 – Cracking (10-12 minutes, 200-225°C): First crack (internal steam ruptures bean), aroma releases. Stage 4 – Development (12-18 minutes, 220-240°C): Color darkens brown-black, oils emerge. Stage 5 – Second crack (optional, >240°C): Secondary rupture, darker roast.
Roasting is progression of chemical reactions—timing determines final flavor.
Maillard Reaction
Maillard reaction is chemical process: (1) Reactants: Amino acids + reducing sugars. (2) Conditions: Heat (>140°C), moisture. (3) Products: Hundreds of flavor compounds (aldehydes, ketones, pyrroles) + brown color. (4) Timeline: Begins ~160°C, accelerates at 180-200°C. Key point: Flavor complexity emerges from Maillard products—different temperatures create different compound ratios.
Maillard is primary driver of coffee flavor development—roasting essentially controls this reaction.
Temperature Timeline
Light roast: 190-200°C, 9-11 minutes. First crack, origin flavors preserved. Medium roast: 205-220°C, 11-13 minutes. Full first crack, balanced flavor. Dark roast: 225-240°C, 13-15 minutes. Post-first crack, bold flavor, bitter notes. Very dark: >240°C, >15 minutes. Second crack, charred flavor. Key detail: Small temperature differences create significant flavor differences—roasting is precision process.
Temperature control is critical—1-2°C difference alters final flavor profile.
Roast Levels & Flavor
Light roast: Bright acidity, origin characteristics (origin flavors preserved). Medium roast: Balanced acidity/body, sweetness, origin + roast flavors. Dark roast: Low acidity, full body, bold/roasty flavors, less origin character. Trade-off: Darker roasts mask origin differences—all dark roasts taste somewhat similar. Light roasts emphasize origin differences.
Roast level trades off origin characteristics for roast-derived bold flavors—intentional choice.
Heat Transfer Methods
Drum roasters: Rotating drum heated from below/sides. Heat conducted through drum to beans. Air roasters: Hot air flow roasts beans (convection). Faster, more precise. Fluid bed roasters: Hot air suspends beans (convection + radiation). Most precise control. Heat distribution: Uniform temperature crucial—hot spots create uneven roast.
Equipment design affects roasting precision—industrial roasters have precise temperature control.
Roasting Control & Equipment
Control parameters: (1) Temperature: Thermocouple measures bean temperature (not air temperature). (2) Time: Precise timing from dry end to desired final temperature. (3) Airflow: Air circulation affects heat distribution, evaporation rate. (4) Cooling: Rapid cooling stops roasting (quench). Consistency: Good roasters produce ±2°C consistency—resulting in consistent flavor.
Modern roasters are computerized—profiles are programmed, repeated precisely.