The Maillard reaction is a chemical reaction between amino acids and sugars starting at around 140 °C. It creates the bean's brown colour and hundreds of aromatic compounds — the same reaction that makes bread crust and seared meat flavourful.

What Happens During the Maillard Reaction in Coffee?

The Maillard reaction is a chemical reaction between amino acids and sugars starting at around 140 °C. It creates the bean’s brown colour and hundreds of aromatic compounds — the same reaction that makes bread crust and seared meat flavourful.

Why that matters

When you sear a steak, bake bread, or toast onions, the same thing happens: amino acids and sugars react with each other and form new compounds — brown colour pigments (melanoidins) and volatile aromatic compounds. French chemist Louis-Camille Maillard described this reaction in 1912. In coffee, it’s responsible for a large part of the flavour.

In the raw coffee bean, the ingredients are ready: free amino acids (from proteins) and sugars (sucrose, glucose, fructose). From around 140 °C bean temperature, they begin to react. This doesn’t happen as a single reaction but as a cascade of hundreds of parallel reactions, each producing different compounds.

The first phase (140–160 °C) produces melanoidins — large, brown molecules responsible for the colour of roasted beans. The longer the Maillard reaction runs, the browner the bean becomes. At the same time, compounds that taste bitter emerge — melanoidins are a major source of bitterness in coffee.

The second phase (160–200 °C) generates most of the volatile aromatic compounds. Over 800 different compounds have been identified in roasted coffee, most of them products of the Maillard reaction: pyrazines (nutty, earthy), furanones (caramel-like, sweet), thiols (roasty, bready), aldehydes (fruity, floral). Which of these compounds dominate depends on temperature, time, and the specific amino acids and sugars in the bean — and therefore on origin, variety, and processing.

Running in parallel with the Maillard reaction is caramelisation: sugar breaks down under heat and forms caramel aromas and additional brown pigments. Both processes are closely intertwined and hard to separate.

For roasters, the Maillard reaction is the key: they must control temperature and time so that enough aromatic compounds form without bitter compounds taking over. Roasted too short — the Maillard reaction is incomplete, and the coffee tastes green and grassy. Roasted too long — the delicate aromatics have evaporated and bitterness dominates.

At Green Wall Coffee

At our café on Sophienstraße 27 in Berlin-Lichtenberg, I like to explain the Maillard reaction using toast as an example: light-brown toast — aromatic and sweet. Black toast — bitter and burnt. Exactly the same thing happens with coffee beans. The roaster controls where on that spectrum the bean lands. Our goal: the golden-brown toast moment, when most aromas are present and bitter compounds are still in the background.

Related Questions

• At what temperature is coffee roasted?
• What is First Crack and Second Crack?
• What is the difference between light, medium, and dark roast?

More depth on this topic in the article How to Make Perfect Espresso. Or stop by at Sophienstraße 27 — Mon–Fri 8am–5pm, Sat 10am–5pm.