What browning reaction between amino acids and sugars gives seared steak and toast their flavour?

The browning reaction between amino acids and sugars that gives seared steak and toast their characteristic flavour is the Maillard reaction.

It is a non-enzymatic browning process: reactive carbonyl groups in reducing sugars interact with amino groups in amino acids or proteins. The reaction produces many aroma compounds and brown nitrogen-containing polymers called melanoidins. Together, these compounds create the roasted, nutty and savoury flavours associated with browned foods.

French chemist Louis Camille Maillard described the reaction in 1912 while studying chemical reactions involving amino acids and sugars. Food chemist John E. Hodge later clarified much of its mechanism in 1953. The process usually accelerates at temperatures around 140°C, especially when food surfaces are relatively dry.

A common mix-up is caramelization, which is the heat-driven breakdown of sugars alone. Caramelization can occur alongside the Maillard reaction, but it is not the same process. The Maillard reaction also explains browning in bread crusts, roasted coffee, fried dumplings and toasted marshmallows.