Why does toast turn brown?
Quick answer
Bread turns brown in a toaster because of the Maillard reaction, a chemical reaction between sugars and amino acids (the building blocks of proteins) that speeds up at around 140–165 °C (280–330 °F). It creates hundreds of new flavor and aroma compounds and brown pigments called melanoidins.
Named after a French chemist
Louis-Camille Maillard first described the reaction in 1912 while studying how amino acids and sugars combine. Food scientists later realized it explained the color and taste of countless cooked foods.
Why bread needs a dry, hot surface
The Maillard reaction needs high heat, and that can’t happen while food is wet, since water can’t get much hotter than 100 °C. A toaster first dries out the bread’s surface, then the temperature climbs and browning begins. That’s also why boiled foods don’t brown.
Foods that owe their flavor to Maillard
| Food | What you notice |
|---|---|
| Toast and bread crust | Golden color, nutty smell |
| Seared steak | Brown crust |
| Roasted coffee | Rich aroma; see how much caffeine is in coffee |
| Chocolate (roasted cocoa beans) | Deep flavor |
| Fried onions | Sweet, savory taste |
| Beer (roasted malt) | Color and toasty notes |
Maillard vs caramelization
| Maillard reaction | Caramelization | |
|---|---|---|
| Needs | Sugars and amino acids | Sugar alone |
| Starts at | About 140 °C | About 160–180 °C (depends on the sugar) |
| Example | Toast, steak | Caramel sauce, crème brûlée |
Both often happen together, for example in toasted sweet bread.
Don’t burn it
If toast keeps cooking, it turns black as the surface carbonizes. Very dark toast and fries also contain more acrylamide, a chemical formed during high-heat cooking. Food safety agencies such as the UK’s Food Standards Agency suggest aiming for a golden yellow rather than dark brown.
For bread itself, see how to make sourdough bread.
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