Second crack

Roasting sound phenomenon (224-230°C) where bean cells fracture a second time, releasing oils to the surface. Marks entry into dark roast territory. Beyond: very dark roast, carbonisation.

Background & Context

Second crack is the second acoustic roasting event — after first crack — and marks the onset of dark roasting. It occurs at approximately 220–230°C, when the coffee bean's cell structure, already expanded and weakened during first crack, undergoes a second thermal fracture. The sound is distinctly different from first crack: second crack is higher-pitched, more rapid, and quieter — like the crackling of a fine twig rather than the popping of popcorn. At second crack, the bean's cell walls begin to rupture, releasing oils to the surface. These oils migrate over hours and days to form the shiny, greasy surface characteristic of dark roast beans. At and beyond second crack, most of the origin-specific aromatic compounds have been destroyed by heat, and the flavour profile is dominated by roast-derived melanoidins, phenylindanes, and caramelised sugars. Beyond second crack — 'second crack finish' and 'city dark' roast profiles — the coffee begins to develop char notes, and in extreme cases (continued roasting) the bean can catch fire. Professional specialty roasters virtually never roast beyond the onset of second crack, viewing it as a quality ceiling rather than a target.

Practical Use

For home roasters: listen carefully for the distinctly different sound of second crack after first crack ends. If you want a dark roast, drop the beans 30–60 seconds into second crack. If you drop much later, you risk carbonisation. Most modern specialty roasters target dropping before any beans reach second crack — their entire output is 'pre-second crack' by design. Recognising second crack by sound takes practice — it helps to record a roast and compare audio at the expected temperature zones.

Related Terms

Related terms: First crack — the earlier event that precedes second crack. Dark roast — roast profile that begins at second crack. Development phase — the window between first and second crack. Agtron scale — second crack corresponds to Agtron below 30.