First crack and second crack physics
This topic explains the physics of first crack and second crack in coffee roasting, how they differ, and why they are critical indicators for roast development and flavor outcomes.
- Coffee Basics Nerds
- 2 min read
Article 3 of 12 in Roasting Fundamentals/
First Crack
- Temperature range: ~196–205°C (385–401°F).
- Physics:
- Internal bean pressure builds as water vapor and CO₂ accumulate.
- Cell walls rupture, releasing steam and gases.
- Audible popping sounds similar to popcorn.
- Bean changes:
- Rapid expansion and increased porosity.
- Color shifts from light brown to medium.
- Major release of water vapor.
- Cup impact:
- Marks transition into development stage.
- Brighter acidity and fruitiness preserved with shorter post-crack development.
Second Crack
- Temperature range: ~224–230°C (435–446°F).
- Physics:
- Structural breakdown of cellulose and hemicellulose in bean cell walls.
- Oils migrate to bean surface as micro-fractures form.
- Audible snapping sounds sharper and quieter than first crack.
- Bean changes:
- Darker surface color, often oily sheen.
- Greater carbonization of sugars and acids.
- Cup impact:
- Reduced acidity and sweetness.
- Stronger roast-derived flavors (smoky, bitter, charred).
Key Differences
- First crack: Driven by water vapor pressure.
- Second crack: Driven by structural breakdown and oil migration.
- Sensory: First crack → vibrant, balanced coffees. Second crack → bold, dark, roasty profiles.
Practical Roasting Implications
- First crack is the main landmark for development control.
- Second crack often avoided in specialty roasting, except for dark roast styles.
- Timing between first and second crack is critical for balancing sweetness, acidity, and roast character.
Lasting Importance
First and second crack are the audible and physical markers of roasting physics. Understanding their causes and effects allows roasters to navigate roast curves precisely, ensuring coffee achieves its desired flavor profile and market style.