Dry heat, and a reaction
Why Does an Oven Brown Food?
Boiling water and a hot oven can reach similar temperatures, but only one of them makes toast. The difference is water.
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Step 1 of 5
Water caps the temperature.
While there is liquid water on the surface, all the incoming heat goes into boiling it and the temperature will not rise. Nothing wet ever browns.
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Step 2 of 5
Dry heat lets the surface climb.
In an oven the surface dries out, and once it does the temperature keeps rising. Past about a hundred and forty degrees a set of reactions begins.
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Step 3 of 5
Sugars and proteins react together.
The browning is a reaction between sugars and the building blocks of protein, producing hundreds of new brown, savoury-smelling compounds all at once.
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Step 4 of 5
Sugar alone does something different.
Pure sugar heated on its own breaks down into its own set of flavours, which is caramel. Most cooking gets a mixture of both reactions at once.
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Step 5 of 5
Which is why a fan oven cooks faster.
A fan oven is not hotter. It stops the food hiding behind its own air. Still air next to food insulates it. Moving the air strips that layer away, dries the surface sooner and delivers heat faster, so a fan oven browns at a lower setting.
Steam ovens do the reverse on purpose: a wet start keeps bread expanding, and the steam is vented at the end so the crust can finally brown.
The short version
An oven browns food because dry heat lets the surface climb past the boiling point, which is where the browning reactions start.
Try it yourself
Toast one slice of bread and steam another. Only the dry one browns.