What exactly causes Maillard browning?

Maillard browning is a chemical reaction that usually occurs between amino acids (the building blocks of protein) and those carbohydrates known as reducing sugars – although the reaction has been known to occur between reducing sugars and whole proteins. In a Maillard reaction, the reactive carbonyl group of a reducing sugar molecule reacts with the nucleophilic group of an amino acid, causing a change in color (usually darkening of color) and flavor of a food product. Heat (energy) is usually required for a Maillard reaction to proceed. Reactions between reducing sugars and free amino acids occur easily and with very little heat required. Reducing sugars will also easily react with the reactive terminal end amino acids of hydrolyzed proteins and, again, very little heat is required.

Reactions between reducing sugars and amino acids that are part of a whole protein are less common and require more heat (energy) to proceed. In the food industry, the troublesome Maillard reactions that occur over shelf life time are usually those reactions between reducing sugars and free amino acids or small peptides (fragments of proteins) that result from protein hydrolysis that occurred during food processing. The visible result of a Maillard reaction is development of a darker color … called browning. A flavor change usually accompanies the development of the darker color.