What is the enzyme that causes French fries to darken when exposed to air?

Polyphenol oxidase is the enzyme that causes cut potatoes, including fry pieces, to darken when exposed to air.

Cutting a potato breaks its cells and brings the enzyme into contact with oxygen and naturally occurring phenolic compounds. Polyphenol oxidase converts those compounds into quinones, which undergo further reactions and form dark pigments on the cut surface.

This is called enzymatic browning and happens before frying, while raw potato pieces are exposed to air. Rinsing or soaking cut potatoes in water limits oxygen contact and removes some surface compounds; blanching also helps because heat denatures the enzyme.

A common mix-up is confusing this reaction with the browning that creates a golden crust during frying. High-temperature frying mainly produces non-enzymatic Maillard browning involving sugars and amino acids. Polyphenol oxidase explains the gray, brown or black discoloration of freshly cut potato, not the finished fry’s crisp golden color.