Coffee: First and Second Crack — Exothermic Phase Transitions
First crack in coffee roasting begins at approximately 196°C in an exothermic phase transition as steam and CO₂ pressure ruptures bean cell walls; second crack at ~224°C fractures the cellular matrix.
| Measure | Value | Unit | Notes |
|---|---|---|---|
| First crack onset temperature | ~196 | °C | Bean internal temperature; varies by origin and moisture content |
| Second crack onset temperature | ~224 | °C | |
| First crack phase type | Exothermic | Bean releases heat; RoR spikes briefly | |
| Internal CO₂ pressure at first crack | 25–50 | bar (estimated) | Buildup from CO₂ and steam within cell walls |
| First crack sound | Loud, sharp pop | Similar to popcorn; rapid and distinct | |
| Second crack sound | Rapid, lower crackle | Quieter, rapid succession; like rice crispies |
The audible cracks during coffee roasting are not incidental — they mark precise thermodynamic phase transitions that define the entire structure of the finished roast. Understanding what causes each crack, and what happens to the bean during each phase, is foundational to roast profiling.
Roasting Phases Overview
| Phase | Temperature Range (°C) | Type | Key Sensory Markers |
|---|---|---|---|
| Drying | Charge temp → ~150 | Endothermic | Grassy, hay-like aroma; yellowing |
| Maillard / Browning | ~150 → ~196 | Exothermic onset | Toast, bread, caramel aroma; browning |
| First Crack | ~196 → ~205 | Exothermic | Audible pop; fruity, acidic notes emerge |
| Development | First crack → drop | Varies | Sweetness development; flavor integration |
| Second Crack | ~224+ | Exothermic | Rapid crackle; oils migrate; smoky |
Phase 1: Drying (Endothermic)
Green coffee arrives with 10–12% moisture content. The first stage of roasting is dominated by moisture evaporation. The bean absorbs heat (endothermic) to drive off water, first from the outer layers and then from the interior. The color transitions from green to yellow. The aroma is grassy and hay-like, sometimes described as popcorn or bread dough as early Maillard products form. The turning point — the lowest bean temperature recorded on the probe, typically 75–95°C at 60–90 seconds after charge — marks the moment the bean shifts from absorbing drum energy to retaining heat.
Phase 2: Maillard Reaction and Browning
As moisture decreases and bean temperature climbs above approximately 150°C, the Maillard reaction between amino acids and reducing sugars accelerates dramatically. Hundreds of volatile aromatic compounds form, including furans, pyrazines, and aldehydes. The bean turns from yellow to light tan to medium brown. The aroma becomes increasingly complex — buttery, nutty, caramel-forward. The rate of rise (RoR) typically peaks in this window.
First Crack: The Exothermic Rupture
At approximately 196°C internal bean temperature, first crack begins. Steam and CO₂ that have been building pressure inside the bean’s cellular matrix — estimated at 25–50 bar — exceed the tensile strength of the cell walls. The result is an audible fracture, similar in character to popcorn popping: loud, sharp, distinct individual reports. Crucially, first crack is an exothermic event: the bean releases heat as the cell walls rupture, causing a brief spike in the bean temperature rate of rise. Roasters observe this as a momentary RoR increase on their logging software. The bean expands in volume by 50–100% compared to green weight.
The Development Phase
The period between first crack and the roast’s end point is called the development phase. This is the most consequential window in specialty roasting. Sugar caramelization continues. Volatile compounds either develop fully or dissipate if heat is applied too aggressively. The development time ratio (DTR) — the percentage of total roast time spent after first crack — is the primary metric specialty roasters use to evaluate profile quality, with targets typically between 20–25%.
Second Crack: Structural Collapse
At approximately 224°C, second crack begins. This is a second exothermic fracture event, but of a different character: rather than steam-pressure rupture, second crack is the further fracturing and collapse of the bean’s now-brittle cellular matrix. The sound is faster, lower, and more continuous than first crack — often described as crackling like Rice Krispies. Internal lipids (primarily diterpenes cafestol and kahweol, as well as triglycerides) are forced to the bean surface as channels in the cellular structure open. Surface oils become visible within minutes. Smoky, roasty, ashy aromatic compounds proliferate as more complex organic molecules fragment under heat.
Where Specialty Coffee Stops
The overwhelming majority of specialty-grade coffees are dropped between first crack and second crack — in the development phase window. Stopping before first crack ends produces an underdeveloped, grassy, bready cup. Continuing into or beyond second crack rapidly reduces complexity and origin character in favor of roast-dominant flavors. High-scoring auction lots from Ethiopia, Panama, or Colombia are nearly universally developed in this window.
Instrument-Based Crack Detection
Modern drum roasters equipped with temperature logging software (Cropster, Artisan) allow roasters to detect cracks by watching for the characteristic RoR inflection point at first crack — the brief rise in RoR caused by the exothermic release. This allows more consistent crack detection independent of ambient noise in the roasting environment.