Which reaction browns both toast and steak?

The Maillard reaction, first described by the French chemist Louis-Camille Maillard in 1912, is what happens when amino acids and reducing sugars meet at high enough temperature. It is not one reaction but a cascade of hundreds, generating a vast array of new molecules - which is why browned food tastes of so much more than its ingredients.

It needs heat and a dry surface, which is the practical lesson. Below about 140 degrees it barely proceeds, and while there is surface water the temperature cannot climb past 100, so a wet steak steams instead of browning. Patting meat dry, getting the pan properly hot and not crowding it are all the same instruction: get the surface above the boiling point of water.

It is distinct from caramelisation, which is sugar breaking down on its own with no protein involved. Both are running in a loaf of bread, and between them they account for the crust, the smell of roasting coffee, the colour of beer and of soy sauce, and the difference between boiled and roasted anything.