Cooking science

Maillard reaction

The Maillard reaction is what makes cooked food smell like cooked food. Roast meat, bread crust, toast, coffee, chocolate, fried onions and beer all owe most of their aroma to the same family of reactions between proteins and sugars.

What is happening

A sugar and an amino acid meet, join, and then rearrange through a long cascade of further reactions. There is no single product: the cascade generates hundreds of different aromatic compounds, plus the brown polymers that give browned food its colour.

Which compounds you get depends on which sugar and which amino acid were present and on the temperature — which is why roast beef, toast and roast coffee smell entirely different despite being the same chemistry. The reaction is a family rather than a single event.

It is not the same as caramelisation, which involves sugar alone with no protein and produces its own narrower set of flavours. Most browned food is doing both at once, which is why the two names are so often confused.

What it needs

Four conditions, and understanding them turns "brown the meat" into something you can actually control.

Heat

It runs slowly at low temperatures and fast above roughly 140°C. Below the boiling point of water it is negligible on a kitchen timescale.

A dry surface

Water caps the surface at 100°C. This is why a wet surface will not brown and why patting things dry is not fussiness.

Protein and sugar

Both must be present. Very lean, sugar-free food browns poorly, which is why a marinade with sugar or dairy in it helps.

Time

It is a cascade, so it deepens as it runs. A longer, gentler browning gives more complex flavour than a brief fierce one.

Alkalinity helps

The reaction runs faster in alkaline conditions, which is why a pinch of bicarbonate speeds up caramelising onions and why pretzels are dipped in an alkaline bath.

Encouraging it

• Dry the surface — pat meat dry, steam potatoes dry, salt in advance and blot.
• Do not crowd the pan. Released steam stops the surface drying, and the whole thing stalls at 100°C.
• Use a heavy pan that holds its temperature when cold food arrives.
• Add a sugar source where the food has little — honey, milk, a little sugar in a rub, or a dairy marinade.
• Leave it alone. Contact time at temperature is what builds the reaction.
• Give it a little alkalinity where it suits — a pinch of bicarbonate in onions or in a stir-fry marinade.

Where you meet it

Almost everywhere that food smells cooked, which is why it is worth recognising as one process rather than many.

• Roast and seared meat — the crust, and most of what makes it smell like roast meat.
• Bread crust and toast, where the sugars and proteins of the dough react at the drying surface.
• Roast coffee and cocoa, both of which owe their aroma to it running inside a bean.
• Fried onions, alongside caramelisation.
• Beer and dark malts, where roasting the grain does the same job.
• Browned butter, where the milk solids are the protein and sugar source.
• Roasted nuts, toasted spices, seared scallops, grilled halloumi — every one of them is this reaction and nothing else.

Common questions