What browning reaction gives French fries their golden-brown color?

The Maillard reaction gives French fries much of their golden-brown color by reacting amino acids with reducing sugars during high-heat cooking.

This is a form of non-enzymatic browning. On the fry’s surface, heat drives reactions between compounds derived from amino acids and sugars such as glucose. The process produces melanoidins, brown-colored compounds, along with many aromatic molecules responsible for the roasted, savory flavors associated with crisp fries.

Frying usually happens in two stages. A lower-temperature first fry cooks the potato and gelatinizes its starch, helping create a fluffy interior. A hotter second fry removes more surface moisture and crisps the exterior, allowing browning to accelerate. The Maillard reaction is not the same as caramelization: caramelization is the thermal breakdown of sugars, while Maillard browning requires amino compounds as well. French fries can undergo both kinds of chemistry, but the reaction chiefly associated with their golden-brown crust is the Maillard reaction. Louis Camille Maillard described the underlying chemistry in work published in the early twentieth century.