The Maillard reaction is a chain of chemical reactions that creates the characteristic taste, smell and recognizable color of roasted coffee beans (as well as chocolate, toast or many other products). These reactions were discovered by L.C., a French scientist who first described them in the early 20th century. Named after Maillard.
Reaction of sugar and amino acids
Maillard reactions occur between the sugars and amino acids present in the product. To dive into the details, some of the sugars in coffee have a free aldehyde or ketone group in their chemical structure.
These groups contain a double-bonded oxygen atom attached to the carbon chain, which can easily react with amino acids and other compounds. Amino acids (AAs) are the building blocks of proteins. They are very different, but all have an NH2 amino group at one end of their molecule and a carboxyl group at the other, so AA is theoretically open to chemical participation in hundreds of other reactions.
The course of a chemical reaction
During the interaction, the nitrogen from the AA reacts with the sugar, releasing 1 molecule of H2O.
Sugar + Amino Acids = Glycosylamines
The resulting glycosylamine molecule is unstable and changes its structure in a process called Amadori rearrangement. The continued reaction either results in the loss of many water molecules, or with short-chain molecules (such as diacetyl, which gives the butter popcorn flavor) or the molecules reacting again with other amino acids.
As a result, molecules called melanoidin are formed. They actually have a dark brown color that determines the color of roasted coffee. They also have a taste. For example, malt, bread or just bitter/burnt/roasted. During coffee brewing, these substances are responsible for the thickness and density of the foam formed, as well as for the strength of the resulting beverage.
The Maillard reaction begins when coffee is roasted at 140°C. At temperatures above 170 °C, caramelization begins with the use of crushed sugars. Because water molecules are released first, the roasted coffee beans must evaporate their moisture before the coffee begins to brown.
The various reaction pathways combined with the specific amino acids and sugars found in the product mean that a large amount of aromatic compounds are formed. The most famous of these are the toasty, bready or bitter taste of melanoidins and the salty taste of peptides. The reactions also produce a wide variety of small molecules responsible for floral, fruity and caramel flavors, as well as some "unpleasant" notes such as onion or earthy flavors.
The variety of flavors created is critical to roasted coffee and determines its organoleptic quality – what we collectively refer to as 'quality'. The reason why green (immature) grains containing little sugar do not get the desired caramelization and color are the chemical reactions described. As a result, they come out of the roaster pale and tasteless.