The Maillard reaction is a form of nonenzymatic browning  similar to caramelization

Caramelization

Caramelization is the browning of sugar, a process used extensively in cooking for the resulting nutty flavor and brown color. As the process occurs, volatile chemicals are released, producing the characteristic caramel flavor....

. It results from a chemical reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 between an amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 and a reducing sugar

Reducing sugar

A reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test.-Chemistry:...

, usually requiring heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

.

Vitally important in the preparation or presentation of many types of food, it is named after chemist Louis-Camille Maillard

Louis Camille Maillard

Louis Camille Maillard was a French physician and chemist.-Early days:He was admitted to the Faculty of Science in the University of Nancy at the age of 16...

, who first described it in the 1910s while attempting to reproduce biological protein synthesis.

The reactive carbonyl group of the sugar reacts with the nucleophilic amino group

Amine

Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

 of the amino acid, and forms a complex mixture of poorly-characterized molecules responsible for a range of odors and flavors.