What are aldehydes in coffee?

Aldehydes are a family of volatile organic compounds formed during roasting - products of the Maillard reaction and caramelisation as heat transforms the structure of the green bean. They're one of the main reasons freshly roasted coffee smells so immediately distinctive when you open the bag.

Different aldehydes contribute different notes. Some read as nutty or malty; others as caramel or toffee; some, at lower concentrations, as fruity or even floral. The balance between them shifts depending on the roast curve, the green coffee's composition, and how development time is managed - which is part of why the same green coffee can smell quite different roasted by two different people.

Coffee scientists map aldehydes using gas chromatography, linking specific volatile compounds to specific aromas. As a roaster or cupper you won't be working at that level of detail - but understanding that what you smell has a precise chemical origin makes sensory evaluation feel less like guesswork and more like reading something that was always there.