What is the Maillard reaction?
The Maillard reaction is what happens when amino acids and reducing sugars meet heat, producing hundreds of new aroma and flavour compounds along with brown colour. It is responsible for the taste of seared steak, toast, roast coffee, bread crust, fried onions and beer malt — most of the flavours we consider savoury and appetising.
It is not caramelisation, though the two are often confused. Caramelisation is sugar breaking down on its own, with no protein involved. Maillard needs both protein and sugar, and produces a far wider range of compounds. Roasting a joint of meat involves Maillard; making toffee involves caramelisation.
The reaction accelerates sharply above roughly 140°C, which is the single most useful thing to know about it. Water boils at 100°C, so as long as the surface of food is wet, it cannot brown. This explains most browning failures in a home kitchen: a crowded pan releases steam that cannot escape, the surface stays at boiling point, and the food greys instead of browning.
Hence the standard advice. Pat meat dry before searing. Use a pan large enough that food is not touching. Get the pan properly hot before adding anything. Do not move it around for the first minute.
Other factors help. Alkaline conditions speed it up, which is why a pinch of bicarbonate of soda makes onions caramelise much faster. Sugar on the surface accelerates it too.
The reaction was described by the French chemist Louis-Camille Maillard in 1912, though cooks had been exploiting it for millennia.
Very high temperatures push past flavour into bitterness and burning, so hotter is not indefinitely better.