Maillard reaction
US /maɪˈlɑːrd riˈækʃən/
UK /maɪˈlɑːrd riˈækʃən/

1.
a chemical reaction between amino acids and reducing sugars that gives browned food its distinctive flavor
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The delicious crust on roasted meats is a result of the Maillard reaction.
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Browning onions for a long time enhances their flavor through the Maillard reaction.