What is caramelization during roasting?

Caramelisation is the thermal breakdown of sugars — mostly sucrose, glucose and fructose — which starts around 160 °C, intensifies past 190 °C and peaks in the development phase after first crack. It drives the sweetness perceived in the cup, the notes of caramel, honey, toffee and dried fruit, and the brown pigments that colour the bean on top of Maillard melanoidins.

Unlike Maillard which involves amino acids, caramelisation is purely sugar-driven: a single family of molecules undergoes dehydration, condensation and fragmentation. Sucrose accounts for 6 to 9 % of the weight of green arabica. From 160 °C sucrose hydrolyses into glucose and fructose, and those monosaccharides then dehydrate into hydroxymethylfurfural (HMF), furfural and cyclic derivatives that carry caramel, biscuit and nutty aromas. Between 190 and 210 °C caramelisation accelerates; past 220 °C a share of the sugars pyrolyses into bitter compounds (heavy 5-HMF, phenolic derivatives).

Caramelisation hits its sensory optimum in the development phase (post first crack, 205-215 °C), exactly where perceived sweetness peaks in the cup. This is why a well-developed specialty filter tastes 'sweet' in the cup even without added sugar — nothing is added; sweetness is the reading of successful caramelisation. Cup of Excellence coffees scored above 87 points almost always show above-average sweetness, often thanks to sugar-rich varieties such as Geisha, Bourbon or SL28. Lesser-known fact: the chemistry is the same as in pastry caramel, but it happens inside a solid matrix (the bean), so heat diffusion is slower and the kinetics stretch out.

In practice, the roaster reads caramelisation visually (uniform deep brown), by smell (biscuit, cooked sugar) and by the time spent above 200 °C. Too-short development (< 1 min after 1C) leaves incomplete caramelisation and residual sugars that taste of raw cereal. Too-long development (> 4 min) flips caramelisation into carbonisation and bitterness explodes. On the Belgian side, the speculoos tradition — a biscuit built on caramelised brown sugar and spices — offers a useful flavour parallel: a well-developed medium-roast coffee literally shares several aromatic molecules with speculoos, notably maltol and 2-methylbutanal. That is one reason why the coffee-and-speculoos pairing is so popular across Belgium and the Netherlands.

Caramelisation inside a coffee bean