Where roast smells come from
Why Does Food Turn Brown When It Cooks?
Heat pushes sugars and proteins into each other and builds hundreds of new flavours.
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Step 1 of 5
It needs a sugar and a protein together.
Almost all food has both. When they get hot enough their molecules join and then rearrange into hundreds of new compounds.
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Step 2 of 5
The reaction needs a dry, hot surface.
While water is present the surface is stuck at 100 degrees, far too cool. Browning starts only once the surface has dried out.
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Step 3 of 5
The new molecules are what you smell.
One brown crust can hold hundreds of new compounds that were not in the raw food at all. That is the smell of bread, coffee and roast dinner.
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Step 4 of 5
Push it too far and it burns.
Past about 200 degrees the same molecules break down into carbon and bitter compounds. Brown is flavour; black is damage.
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Step 5 of 5
Which is why cooks dry, space and wait.
Patting food dry, leaving room in the pan and not turning it too soon all do the same job: keeping the surface hot enough to react.
This reaction was described by Louis-Camille Maillard in 1912, and chemists are still cataloguing the compounds it makes.
The short version
Browning is sugars and proteins reacting above about 140 °C, creating hundreds of new colour and flavour molecules.
Try it yourself
Toast one slice of bread and steam another over a pan. Only the dry, hot one browns or smells of anything.
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