What is pyrolysis in coffee roasting?

Pyrolysis is the thermal decomposition of organic compounds when exposed to high temperatures in the absence of oxygen. In coffee roasting, it refers to the broad set of chemical reactions that occur as the bean is heated - breaking down complex molecules like carbohydrates, proteins, and chlorogenic acids into simpler, more volatile compounds that define the flavour, aroma, and colour of roasted coffee.

Pyrolysis begins to play a significant role from around first crack onwards, when roasting temperatures become high enough to drive the more transformative reactions. It's responsible for the development of the hundreds of volatile aromatic compounds detected in roasted coffee - many of which are also found in other roasted or cooked foods. The Maillard reaction and caramelisation both occur within the broader framework of pyrolysis.

The degree of pyrolysis - how far the reactions have progressed - is one way of understanding roast level. A light roast has undergone less pyrolysis and retains more of the green coffee's original compounds; a dark roast has been taken further, with more complex molecules broken down into simpler, more intensely roasted character. Knowing that pyrolysis is cumulative and largely irreversible helps explain why you can't "back off" a roast once it's gone too far - the chemical changes that produce dark or baked character cannot be undone.