tags: [] - coffee/roasting - coffee/education aliases: - Science of roasting module - Roasting science course module

Module 3 — The Science of Roasting¶

Tags: #coffee/roasting #coffee/education Aliases: Science of roasting module, Roasting science course module Related: Roasting MOC | Coffee Chemistry | First & Second Crack | Roast Profile | Heat Transfer in Coffee Roasting Status: ✅ Complete

Overview¶

Module 3 covers the physical and chemical science underlying the coffee roasting process — what happens to the bean as temperature rises, the chemical reactions responsible for flavour development, the physical changes observable during roasting, and how roast decisions translate into flavour outcomes. This knowledge forms the foundation for intentional, informed roast profile development.

Module Content¶

3.1 — Physical Changes During Roasting¶

As green coffee is exposed to heat, the bean undergoes a series of observable physical transformations:

Colour change: Green → yellow → tan → light brown → medium brown → dark brown (Maillard and caramelisation development)
Volume expansion: Beans expand approximately 50–80% in volume as moisture and CO₂ are driven out; cell walls rupture
Mass loss: Roasted coffee loses 12–22% of its green mass (moisture and volatile organic compounds evaporate)
First crack: First & Second Crack — audible crack at approximately 196–204°C as CO₂ and steam build and rupture cell walls; marks the boundary of light roast
Second crack: Occurs at approximately 224°C; oils begin migrating to surface; onset of dark roast

3.2 — Chemical Reactions¶

Coffee flavour is created by a cascade of heat-driven chemical reactions:

Maillard reaction: Coffee Chemistry — reaction between amino acids and reducing sugars; produces hundreds of flavour and aroma compounds; occurs from approximately 150°C; responsible for roasted, caramel, and nutty notes
Caramelisation: Thermal degradation of sugars; produces sweet caramel notes and browning; distinct from Maillard reaction
Strecker degradation: Amino acid decomposition; contributes aldehydes and pyrazines to the aroma profile
CO₂ generation: Carbon dioxide builds up in the cell structure during roasting; released over days post-roast (off-gassing); creates bloom during brewing

See Coffee Chemistry Basics for the full chemistry overview.

3.3 — Rate of Rise and Temperature Control¶

The temperature trajectory through a roast — how fast the bean temperature rises and when — determines flavour development:

Rate of Rise (RoR): Roast Profile — the rate of bean temperature increase in °C per minute; a primary roast management metric
Turning point: The moment after loading when bean temperature stops dropping and begins rising
Charge temperature: The drum temperature at which beans are loaded; affects initial heat transfer rate
Development time: Time from first crack to end of roast; critical for sweetness and acidity balance
Development Time Ratio (DTR): Roast Development Ratio — development time as a percentage of total roast time; target typically 20–25%

3.4 — Heat Transfer in Roasting¶

How heat moves from the roasting environment into the coffee bean determines roast character:

Conduction: Heat transfer through direct contact with drum surface
Convection: Heat transfer through hot air moving through the drum
Radiation: Infrared radiation from the drum walls
Drum vs. fluid-bed: Heat Transfer in Coffee Roasting — how different roaster types balance these three modes

3.5 — Roast Level and Flavour¶

The degree of roast (light, medium, dark) determines which chemical compounds are developed or destroyed:

Light roast: Preserves origin acids, floral aromatics; less development; higher perceived acidity
Medium roast: Balances Maillard compounds (caramel, chocolate) with remaining origin character
Dark roast: Maillard and caramelisation products fully developed; origin character suppressed; bitter phenolic compounds increase
See Compare light vs medium vs dark roasts

Key Facts¶

The Maillard reaction (from ~150°C) and caramelisation are the primary flavour-creating reactions in roasting
First crack (~196–204°C) marks the light roast threshold; second crack (~224°C) marks dark roast onset
Coffee loses 12–22% of its green mass during roasting (moisture and volatile compounds)
Rate of Rise (RoR) is the primary profiling control metric; declining RoR is the standard target for quality roasting
Development Time Ratio (DTR) — typically 20–25% — controls post-crack flavour development

References¶

Changelog¶

Date	Change
2026-04-29	Note created

This article is part of All-About-Coffee.com - The comprehensive coffee knowledgebase.