WTF is the Maillard reaction?

When our ancestors discovered fire, and someone had the bright idea of throwing food on it, our expectations of aroma and flavour were changed forever. As food is browned, new molecules are created and enticing flavours come to life. This process is referred to as the Maillard reaction. In the case of coffee, this reaction is what unleashes the varied and delicious flavour notes we know and love.

Louis-Camille Maillard was the French scientist who in 1912 first described the process whereby amino acids, carbonyl compounds and sugars react to produce new molecules, which in turn react to form newer, often volatile compounds. In the real-world coffee roasting process, there are additional levels of complexity: intermediate molecules are produced, which are the precursors for further reactions. The Maillard reaction is a cascade of consecutive and simultaneous reactions that occur in many parallel pathways.

Lab conditions

When coffee is roasted, Maillard reactions take place on the surface and in the interior of each bean (or to be more precise, inside each cell). In a sense, every bean is a micro-laboratory that’s an enclosed system. Each cell contains hundreds of compounds, including amino acids, proteins and sugars – the precursors of colour, odour and flavou.

The dizzying complexity of the Maillard process in coffee arises partly from the huge variety of chemical compounds in the raw coffee bean – even leaving aside compounds that do not directly participate in the process, although they undergo their own specific transformations and contribute to the flavour profile. Poor-quality raw coffee contains lower amounts of critical flavour precursors and is likely to develop flavours poorly during roastin.

A coffee roaster will also pay attention to the water content of the raw coffee as well as the water activity, a relatively new concept referring to how tightly bound water is to the coffee bean. Water content and activity levels can either inhibit or promote the speed and pattern of Maillard reactions, affecting the formation of aroma and flavour.

Roast with the most

The coffee roaster can’t control green quality or moisture content. Their role is to manipulate the application of heat to control the process of roasting, and their precision makes the difference in the quality and intensity of the aromas and flavours developed as the Maillard reaction is controlled. As heat is applied and roasting progresses, gas pressure inside the coffee bean rises dramatically. This creates conditions for the chemical reactions to take place at a different pace or intensity, which can produce vastly differing results. The speed and intensity of the Maillard reactions have a huge impact on the quality and intensity of odour and flavour in the roasted coffee.

When drinking coffee, what you’re tasting is a sensory impression triggered by various volatile compounds that you inhale and ingest. It’s the roasting process that produces aromatic molecules (particularly some esters, aldehydes and ketones) to give those fruity, nutty, caramel, chocolate and floral flavour impressions. There’s more to flavour than the Maillard reaction, but it plays a key role in creating the flavours in your coffee, and the skill of the roaster is in how those flavours are developed.

Edgaras Juška is a freelance coffee roaster and consultant.