Maillard reaction
Chemical reaction between amino acids and reducing sugars under heat (>150°C). Responsible for caramel, hazelnut and chocolate aromas developing during roasting. Distinct from pure caramelisation.
Background & Context
The Maillard reaction is a non-enzymatic browning reaction between amino acids and reducing sugars — the most important chemical process in coffee roasting and one of the fundamental flavour-generating reactions across all cooked foods. Named after French chemist Louis-Camille Maillard, who described it in 1912, the reaction begins at approximately 150°C in coffee roasting (after drying and before caramelisation) and continues throughout the roast. In coffee, the Maillard reaction generates hundreds of aromatic molecules including furans (caramel, sweet), pyrazines (nutty, roasted), pyrroles (earthy, coffee-like), and melanoidins — large brown polymer molecules that contribute to coffee's colour, body, sweetness, and antioxidant capacity. Melanoidins, produced by Maillard reactions and caramelisation together, constitute 20–25% of roasted coffee's dry weight and are the primary source of coffee's characteristic dark brown colour.
Practical Use
The Maillard reaction explains several counterintuitive coffee roasting phenomena. Roast colour depends on Maillard product (melanoidin) accumulation, not primarily on bean surface temperature — which is why two roasts with different temperature profiles can produce identical colour at drop if their Maillard development is equivalent. The rate of rise (RoR) during the Maillard phase (approximately 150–196°C) determines how fully the reaction proceeds: a crashing RoR during this phase "bakes" the coffee, leaving Maillard reactions incomplete and producing flat, bready, cereal-like flavours. Extending roast time at low temperature ("slow roasting") maximises Maillard reaction completeness at the cost of some volatile aroma loss. For home brewers, understanding the Maillard reaction contextualises why fresh coffee smells so differently from green coffee — all that aromatic complexity was built during this process.
Related Terms
Related terms: Roasting, First crack, Development phase, Rate of rise, Body (melanoidins).