What is the Maillard reaction in coffee roasting?

The Maillard reaction is the chemistry behind most of what makes roasted coffee smell and taste the way it does. It's a series of reactions between amino acids and reducing sugars that begin around 150°C and produce hundreds of new flavour and aroma compounds - the nutty, chocolatey, caramel, and toasty notes you associate with freshly roasted coffee coming out of the drum.

It's the same reaction that browns bread, steak, and biscuits. In coffee it runs throughout much of the roast profile, overlapping with caramelisation and intensifying as the development phase progresses. The specific compounds it produces - and therefore what the coffee smells and tastes like - depend on the temperature curve, the time spent at each stage, and the composition of the green coffee itself.

Roasters who understand the Maillard reaction not just as a fact but as a process - something they're actively managing through heat application decisions - have a much clearer mental model of what they're doing and why the cup turns out the way it does.