First & second crack
Two sounds split every roast into before and after. Learn to hear them and you can roast on almost any machine, because your ears don't care what scale the thermometer uses. They are the most reliable signposts you have.
First crack — the popcorn moment
As the bean approaches roughly 196 °C (true bean-surface temperature), steam and carbon-dioxide pressure that's been building inside finally overwhelms the rigid cell walls, and the bean ruptures. You hear it: a sharp, irregular cracking, lower and slower than popcorn but unmistakably the same physics. The bean visibly swells, throws off chaff, and the aroma turns from bready to a sweet, toasty roastiness.
First crack isn't a single instant. It's a rolling cluster of pops over maybe 30–90 seconds: a few tentative cracks (FC start), building to a rolling peak, then trailing off. Roasters anchor their timing to FC start — the first clear pops — because everything in the development phase is measured from there. For light, bright filter roasts you might drop as little as ~45 seconds to two minutes after it begins.
Second crack — the oily, smoky one
Push on to about 224–230 °C and you hear a second, very different sound: quieter, faster, higher-pitched — more like Rice Krispies than popcorn. Now the bean's woody cellulose matrix itself is fracturing, and oils are forced to the surface, giving dark roasts their sheen. This is dark-roast country: body climbs, acidity all but vanishes, and roast-and-carbon flavors take over the cup.
Most specialty roasting stops before or just at the edge of second crack. Go deep into it and you risk ashy, burnt flavors — and, genuinely, chaff and oil can become a fire hazard. Treat second crack as a boundary you approach deliberately, not by accident.
That "196 °C" is the physicists' consensus for the bean's surface. Your roaster measures temperature with its own probe, in its own spot, so the number it displays at first crack will differ — often by a lot. On a Kaffelogic Nano, first crack typically shows around 212–226 °C on its scale. Never chase someone else's temperature. Roast a batch, note what your display reads when you hear the pops, and use that.
The milestones at a glance
| Milestone | What you sense | True bean °C | Means |
|---|---|---|---|
| Yellowing | Color turns from green to yellow; bready smell | ~150 | End of drying; browning about to begin |
| First crack (start) | Sharp, popcorn-like pops; bean swells | ~196 | Development begins; earliest drinkable drop |
| Between FC & SC | Pops fade; roasty-sweet aromas | ~200–222 | The window most filter & espresso roasts live in |
| Second crack | Faster, quieter, higher snaps; surface oils | ~224–230 | Dark-roast territory; acidity gone, roast flavors dominate |
Train your ears — it's the most transferable skill you have
- Roast in a quiet room the first few times. The Nano's fan is loud-ish; lean in and listen.
- Mark the time of FC start on every roast. It's the anchor your whole development phase hangs from.
- Use your nose as a backup sensor: grassy → bready → toasty → roasty is a reliable progression.
- When in doubt, trust the sound and smell over the number.
You now know when things happen. Next: the single line on the graph that tells you how fast they're happening — the rate of rise.