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

Sweet Chemistry Under Heat

Caramelisation in coffee roasting is often conflated with the Maillard reaction, but they are chemically distinct processes that happen at different temperatures, involve different molecules, and produce different flavour results. Maillard reactions — the browning interactions between amino acids and reducing sugars — begin as early as 150 °C and are responsible for many of the roasted grain, bread crust, and chocolate notes in a medium to dark roast. Caramelisation proper begins at higher temperatures (around 170-180 °C for sucrose, the main sugar in coffee), and involves the direct pyrolysis of sugar molecules without the involvement of amino acids. The products of caramelisation include compounds with buttery, toffee, caramel, and toasty notes — the family of flavours that many consumers associate with a "classically roasted" coffee and that constitute the sensory backbone of a medium roast.

In the roasting drum, caramelisation contributes directly to the perceived body and sweetness of the cup rather than primarily to its aromatic complexity. The caramelised sugars form high-molecular-weight melanoidins — the same type of browning polymers formed in the Maillard reaction — that contribute to the viscosity and mouth-feel of brewed coffee and provide the visual darkness of the brew. The balance between caramelisation-derived sweetness and Maillard-derived aromatic complexity is one of the central challenges of roast development: push the roast to develop more caramelisation and you gain sweetness and body but lose the delicate aromatic compounds formed at lower temperatures that give light and medium-light roasts their floral and fruity character. The skill of the roaster is in finding the balance point for each specific origin and variety that maximises both dimensions simultaneously.

Practical Recommendations

For home roasters who want to deliberately develop the caramel dimension of their roasts, the most controllable variable is the time-temperature relationship in the mid-development phase of the roast (roughly 170-195 °C bean temperature). Slightly longer exposure at these temperatures — achieved by moderating heat input — allows caramelisation reactions to proceed more fully before the roast tips into the darker Maillard-dominated territory of a full city roast. Reference roasters like those who publish detailed roast curves online (Kafatek, Has Bean, and others) provide examples of roast profiles optimised for caramel development. For consumers, the most reliable proxy for well-developed caramelisation in a roasted coffee is a chocolate-toffee-sweetness impression in the dry aroma before brewing, since caramelisation compounds are among the most volatile and will be immediately apparent when you open a bag of freshly roasted, well-developed coffee.