Maillard Reaction is a type of non-enzymic browning that adds color and flavor to many types of processed food, including beer. The reaction is named after the French chemist Louis-Camille Maillard (1878–1936), who stumbled on it while trying to replicate biological protein synthesis around 1910. In essence, Maillard provided a chemical explanation for these browning processes that occur in everyday cooking and thus had been empirically known since man began cooking food.
Maillard products are the result of a complex series of chemical reactions between the carbonyls of reactive sugars and the amino groups of amino acids. Maillard reactions are favored or occur more readily at higher temperatures, low moisture levels, and under alkaline (basic) conditions with pentose sugars (i.e., arabinose, xylose) reacting more than hexoses (e.g., glucose), which in turn react more than disaccharides (e.g., maltose). Amino acids also have differing propensities for undertaking Maillard reactions, with lysine and glycine being the most reactive.
The most favorable process phase conditions for the formation of Maillard products, proteins or peptides linked to sugars, occur during malt kilning. Kilning, owing to the low moisture content toward the end, is manipulated by maltsters to achieve the various combinations of color and flavor utilized by brewers to produce different styles of beer.
Maillard browning reactions also take place in the kettle during wort boiling and can develop deeper colors in worts. They also occur during mash boiling phases of decoction mashes, and proponents of decoction mashing often claim that superior depth of malt flavor can result.