- First, the Maillard reaction, when sugars and amino acids react non-enzymatically. This commonly causes browning during cooking.
- Second, pattern-recognition reactions in someone's brain, which enables the subject to see the image interpreted as Mary's face in the toast.
Many foods brown when they are cooked, and flavors and aromas develop. This has been known for ages. But the mechanism by which a grilled cheese sandwich browns was first explained by the French biochemist Louis-Camille Maillard (1878-1936) at the beginning of the 20th century.
The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring the addition of heat. Like caramelization, it is a form of non-enzymatic browning. The reactive carbonyl group of the sugar interacts with the nucleophilic amino group of the amino acid, and interesting but poorly characterized odor and flavor molecules result. (Thefreedictionary.com)
Examples of the Maillard reaction include:
- the browning of bread and toast
- the color of beer, chocolate, coffee, and maple syrup
- the flavor of roast meat
Maillard reactions are associated with application of dry heat, since water is one of the reaction products. In high-water-activity systems (e.g. boiling) equilibria do not favor Maillard reactions. The bread of a grilled cheese sandwich, typically fried in butter or other fat, presents the ideal environment for Maillard browning. (Grilled cheese sandwich recipe—this is how we always made it in our house.)
Anyone having a roasted turkey for the U.S. holiday of Thanksgiving this week will notice the skin of the bird turns from white to golden brown during cooking, due to the Maillard reaction.