1. Overview & Physics
Thermodynamic Phase Transitions
Roasting is a combination of endothermic (absorbing heat) and exothermic (releasing heat) reactions. As bean temperature reaches 196째C, moisture turns to steam, fracturing the bean wood structure with an audible pop. This is the first crack. If roasting continues to 224째C, CO2 pressure fractures the carbon structure again?봳he second crack, introducing oily compounds and dark roast flavors.
The Roast Window
Stop roasting shortly after the first crack ends for bright acidity, or wait for the start of the second crack for rich, low-acid chocolate tones.
2. Practical Calibrations
Managing this requires measuring indicators in real time. Adjust grind size, water dynamics, and flow parameters incrementally. Change only one variable at a time to ensure clean results and isolate target parameters.
Barista Action Checklist
- Weigh dry grounds dry dose exactly to 0.1g.
- Calibrate water temperature using PID sensors.
- Target correct brew ratios (e.g. 1:15 for filter, 1:2 for espresso).
- Swirl or stir to ensure even saturation during extraction.
- Record extraction time and evaluate flavor notes subjectively.
Frequently Asked Questions
What is the first crack in coffee roasting?
The first crack is a physical fracturing of the bean cell walls caused by steam pressure escaping as moisture vaporizes around 196째C.
What is the second crack?
The second crack occurs around 224째C, driven by carbon dioxide pressure and structural breakdown of woody cellulose fibers, producing oily surface beans.
