What is the second crack in roasting?

Second crack is a drier, metallic snap that occurs around 224-230 °C bean temp, typically 2 to 4 minutes after first crack. It signals that the cell-wall cellulose is yielding to advanced pyrolysis, releasing oils to the surface and partially charring residual sugars. It is the threshold that separates dark roasts (French, Italian) from medium profiles.

Where first crack comes from steam and CO2 fracturing a still-elastic structure, second crack is caused by thermal degradation of the rigid polysaccharides (cellulose, hemicellulose) in the cell wall. The sound is drier, sharper, faster — like splitting charcoal. At this stage the bean temperature sits between 224 and 230 °C, mass loss exceeds 18 %, and the oils previously held inside the matrix migrate to the surface: the bean goes shiny, then glossy, then oily if you push further. 2C is basically mandatory to reach an Italian or Neapolitan roast; it is optional for a classic Full City+.

For a roaster, second crack is a crossroads. Many specialty roasts drop before it begins (220-223 °C drop), to preserve acidity and complexity. Others intentionally enter it for a few seconds to build espresso body and dark-chocolate character. Historical French roasts ride 5-10 seconds into rolling 2C; Italian roasts can go 20-40 seconds past it. Beyond a full minute of 2C, carbonisation takes over, acidity collapses and the cup veers toward ash and burn. Fire risk in the drum also rises sharply: surface oils combined with strong airflow regularly produce flue fires, which is why industrial roasters (Probat, Giesen, Loring, Diedrich) build in mandatory fire-suppression devices.

A lesser-known technical point: 2C starts at a lower temperature at altitude (2 to 4 °C lower at 1 500 m versus sea level), because reduced atmospheric pressure affects volatilisation and cell-wall mechanics. For roasteries in Brussels (100 m) or Liège this is marginal, but crews trained in Ethiopia or Costa Rica at altitude see noticeable shifts on the same machine. A Belgian detail: many craft Belgian roasters avoid 2C entirely in their specialty line-up but keep a house dark blend — often labelled 'espresso blend' or 'classic blend' — for customers attached to a more traditional taste, typically paired with speculoos.

Benchmarks around second crack