The Maillard Reaction

The Maillard Reaction is a non-enzymatic chemical reaction that results in a brown color change in a food product. This is the reaction responsible for the browning of crust, the color of cooked meat, and the brown color of chocolate chip cookies.

A simple explanation of the reaction is:
PROTEIN+REDUCING SUGAR--> BROWN PIGMENT


Of course, it is much more complex than that, but what you really need to know is:
*PROTEIN -a compound which contains an amine group (as in Amino Acid: the building blocks of protein)
*REDUCING SUGAR -a carbohydrate with an aldehyde group or ketone group. It can gain electrons from other molecules and reduce it.
reducing sugars are all monosaccharides (simple sugars-glucose, frutose, galactose) and some disaccharides (ex. lactose-sugar in milk)
This reaction will occur over time, but will occur more rapidly with the help of heat (as in the cookie case, baking)