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Roasting Chemistry

Coffee roasting triggers complex chemical reactions that transform green beans into flavorful roasted coffee. These changes primarily involve heat-induced processes like drying, Maillard reaction, caramelisation, and pyrolysis.12

Green Bean Composition

Green coffee beans contain 8-12% water, carbohydrates (cellulose, starch), proteins (amino acids), lipids, and organic acids like chlorogenic acids. These components drive flavour development during roasting.31

Key Reaction Stages

  • Drying Phase (up to ~150°C/300°F): Evaporates free water; beans turn yellow without major flavor changes.43
  • Maillard Reaction (~150-180°C/300-356°F): Amino acids react with reducing sugars to form melanoidins , creating brown colour, nutty/toasty flavours, and increased viscosity. Shorter duration preserves acidity and sweetness; longer yields body but reduces fruity notes.2561
  • Caramelisation (>180°C/356°F): Sugars break down into caramel, toffee, and chocolate notes.71
  • Development/Pyrolysis (post-first crack): Volatile compounds form; CO2 release causes cracks; avoids full charring.89

Acid Breakdown

Chlorogenic acids decompose into quinic and caffeic acids, reducing brightness in darker roasts (up to 100% loss). Light roasts retain more for acidity.10111

Strecker Degradation

A Maillard subset: Amino acids oxidise to aldehydes (fruity/floral aromas) and CO2, driving second crack (~80% of roast CO2). Peaks early; higher in lighter roasts.1213

Roast Level Impacts

Roast Level Key Chemistry Flavor Notes
Light Shorter Maillard; high chlorogenic acids Bright acidity, fruit16
Medium Balanced reactions Chocolate, nuts6
Dark Extended pyrolysis; low acids Bittersweet, caramel18

These reactions explain roast profile control for desired cup profiles.142 151617181920212223242526

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