Toast, a seared steak, roast potatoes and the crust of a loaf all turn brown and taste richer through one reaction between sugars and amino acids: the Maillard reaction (say my-YAR).
Bread, meat and potatoes all hold the molecules of life: proteins, free amino acids with their amine group, -NH₂, and a little sugar, such as glucose. At any moment a tiny share of glucose molecules open out of their ring, and an open glucose molecule ends in an aldehyde group, C=O.
In the heat, the C=O group of a sugar meets the -NH₂ group of an amino acid. The two join and give off a water molecule: a condensation, like the ones that make esters. The joined molecule then rearranges, breaks into pieces and joins up again, over and over.
The French chemist Louis-Camille Maillard described the reaction in 1912. Food chemists have since found hundreds of products. The brown ones are big molecules called melanoidins. The small ones carry the smells of baking and roasting, and each amino acid gives its own mix, so toast, roast chicken, coffee and chocolate each smell different.
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Hot and dry
The Maillard reaction runs slowly at room temperature and fast from about 140 °C. Water boils at 100 °C, and while a food's surface is wet, the heat goes into turning that water to steam, so the surface stays at about 100 °C. In a hot pan, an oven or a toaster, the surface dries out, climbs past 140 °C and browns.
| Cooking | Heat, about | Surface |
|---|---|---|
| boiling or steaming | 100 °C | stays pale |
| frying in a pan | 150 to 200 °C | browns |
| roasting or baking | 180 to 220 °C | browns |
So boiled meat stays grey, and cooks pat a steak dry before it goes in the pan. A cut apple turns brown by another reaction, which lesson 17 explains: an enzyme in the apple uses oxygen from the air to join the apple's phenols, small molecules with -OH groups on benzene rings, into brown pigments. That browning happens at room temperature.