What Is the Maillard Reaction?

The Maillard reaction was first discovered in 1912 by French chemist Louis-Camille Maillard, and in 1953, John Hodge formally named it after him. In 1912, Maillard presented his observation to the French Academy of Sciences: when he mixed amino acids and sugars in a water solution and heated it, the solution turned a yellowish-brown color. At the time, few scientists recognized the potential significance of this reaction. Today, however, the Maillard reaction has become integral to modern food science and technology, playing a vital role in meat processing, food preservation, flavor production, and even traditional Chinese medicine research. Current studies also show its close connection to various physiological and pathological processes.

Definition of the Maillard Reaction

The Maillard reaction—also known as Maillard browning, Maillard condensation, or glyco-amino reaction—is a non-enzymatic browning process widespread in the food industry. It occurs when reducing sugars (carbohydrates) react with amino acids or proteins under ambient or heated conditions, producing a series of complex reactions that yield brown-black, high-molecular-weight compounds called melanoidins (also referred to as humanoid pigments). Along with melanoidins, the reaction generates hundreds of intermediate molecules with various aromas—such as reduced ketones, aldehydes, and heterocyclic compounds—that contribute appealing flavors and colors to foods.

The Maillard Reaction in Coffee Roasting

Among the many chemical processes during coffee roasting, the Maillard reaction is one of the most complex and is a key factor in developing coffee’s diverse flavors. In roasting, the Maillard reaction primarily begins once dehydration is complete and the beans transition from yellow to brown, continuing until the roast is finished and the beans have cooled. Around the time of first crack, another important reaction for coffee flavor—caramelization—begins, sharing sugar as a reactant with the Maillard process. This causes the Maillard reaction to slow somewhat at the first crack. Consequently, the main Maillard reaction phase is typically defined as the period from when the beans first turn yellow to the start of first crack. Throughout this stage, melanoidins (large molecular compounds) form continuously, and by the end of roasting, over 600 volatile organic compounds may be present in the beans. It is this process that imparts coffee with complex flavors, body, color, and depth.

Adjusting the Maillard Reaction During Roasting

Roasters can influence coffee flavor by modulating the roasting profile, indirectly controlling the Maillard reaction’s intensity. Since the Maillard reaction’s rate does not significantly increase once the temperature surpasses 80°C (176°F), adjustments are typically made by extending or shortening the reaction time. Longer Maillard reaction durations produce more melanoidins, which result in a heavier mouthfeel and richer, more profound flavor characteristics.