What is second crack in coffee roasting?

Second crack is the second audible event in a roast - sharper, faster, and more rapid-fire than first crack. It happens when the now-brittle bean structure fractures again under pressure from built-up gases. Where first crack is driven by water vapour, second crack is driven by CO₂ and other gases produced by ongoing roasting reactions.

It marks the transition into medium-dark and dark roast territory. From this point, oils begin migrating to the surface, caramelisation is well advanced, and origin character starts to give way to roast character. Many specialty roasters targeting light to medium profiles will never hear second crack during a normal roast - they've dropped the beans well before it arrives.

If you're developing a darker profile, second crack is your key marker. Dropping just at the onset of second crack produces something in the full city plus to Vienna range - still some sweetness, but increasingly roasty. Push well into second crack and you're into French roast territory: oily, bitter, and roast-dominant, where origin has largely stepped aside.