The Maillard Reaction

The Maillard reaction is the name given to the set of chemical reactions between amino acids and reducing sugars that causes browning of foods, such as meats, breads, cookies, and beer. The reaction is also used in sunless tanning formulas. Like caramelization, the Maillard reaction produces browning without any enzymes, making it a type of non-enzymatic reaction. While caramelization relies solely on heating carbohydrates, heat is not necessarily needed for the Maillard reaction to occur and proteins or amino acids must be present.

Many foods brown due to a combination of caramelization and the Maillard reaction. For example, when you toast a marshmallow, the sugar carmelizes, but it also reacts with the gelatin through the Maillard reaction. In other foods, enzymatic browning further complicates the chemistry.

Although people have known how to brown food pretty much since the discovery of fire, the process was not given a name until 1912, when French chemist Louis-Camille Maillard described the reaction.

Chemistry of the Maillard Reaction

The specific chemical reactions that cause food to brown depend on the chemical composition of the food and a host of other factors, including temperature, acidity, the presence or absence of oxygen, the amount of water, and the time allowed for the reaction. Many reactions are occurring, making new products that themselves begin reacting. Hundreds of different molecules are produced, changing the color, texture, flavor, and aroma of food. In general, the Maillard reaction follows these steps:

1. The carbonyl group of a sugar reacts with the amino group of an amino acid. This reaction yields N-substituted glycosylamine and water.
2. The unstable glycosylamine forms ketosamines through the Amadori rearrangement. The Amadori rearrangement signals the start of the reactions that cause browning.
3. The ketosamine may react to form reductones and water. Brown nitrogenous polymers and melanoidins may be produced. Other products, such as diacetyl or pyruvaldehyde may form.

Although the Maillard reaction occurs at room temperature, heat at 140 to 165 °C (284 to 329 °F) aids the reaction. The initial reaction between the sugar and the amino acid is favored under alkaline conditions.