The chemistry and physics behind coffee roasting

Roasting is a complex thermodynamic process where hundreds of chemical reactions transform the composition of green beans – from sugars and amino acids to over 800 aromatic compounds.

The main chemical reactions

Roasting typically takes place at temperatures from 180-250°C and lasts 10-20 minutes depending on the desired result. The process can be divided into these key phases:

The Maillard reaction

The central non-enzymatic browning reaction between reducing sugars (e.g., sucrose) and amino acids/proteins. Typically starts at 140-165°C and produces melanoidins (brown pigments), as well as aroma compounds such as furans (caramel notes), pyrroles, and 2-furfurylthiol (strong coffee aroma). This reaction is responsible for much of the complex flavor and color development.

Caramelization

Pyrolysis of sugars (without amino acids) at higher temperatures (>170°C). Reduces sweetness and increases bitterness through the formation of ketones, esters, and aldehydes.

Pyrolysis

Thermal decomposition in an inert environment begins significantly at ~220°C. Releases CO₂, water, and volatile compounds; responsible for the second crack and dark, smoky notes in dark roasts.

Other reactions

Strecker degradation (aldehydes from amino acids), breakdown of chlorogenic acids (contributes to acidity and bitterness), and trigonelline (forms nicotinic acid and pyridines).

Green beans contain almost no aroma – most compounds are formed during roasting.

The phases with temperatures and changes

The roasting shifts from endothermic (heat-absorbing) to exothermic (heat-releasing) around the first crack.

Drying phase
Up to ~150-160°C. Evaporation of 10-12% water. The beans change from green to yellow. Endothermic phase – critical for even roasting.

Maillard and browning phase
150-205°C. Intense Maillard reaction and caramelization. Color changes to light brown.

First crack
~195-205°C (depending on the bean). Exothermic reaction: Steam and CO₂ cause the bean to expand (volume doubles). Marks the transition to development.

Development phase
After first crack. Flavor development – the longer, the more caramelized.

Second crack
~225-230°C. Pyrolysis of cellulose; finer cracking sounds, oils on the surface. Begins dark roasting.

Rate of Rise (RoR) and roasting curves

RoR is the rate of temperature increase per minute (°C/min) in the bean – a key control parameter. A smooth, declining RoR curve avoids "crashes" (too rapid a drop, causing baked notes) or "flicks" (upward spikes, risking scorched flavors).

Typically: High RoR early (10-20°C/min), decreasing towards first crack (5-10°C/min). Development time: 20-25% of total time for balance.

Measurement of roast level: Agtron scale

Professionals use the Agtron spectrophotometer to measure reflected infrared light (higher number = lighter roast).

Light: Agtron 85-105

Medium: 65-85

Dark: <65 (e.g., French roast ~45)

This is more accurate than visual assessment.

When roasting, you can experiment with these parameters. It gives full control over acidity, sweetness, and body. Do you have specific questions about a roasting profile or equipment? ☕ Write to us in the chat or contact us at kontakt@homeroast.dk. Happy roasting! ☕