What is the name of the enzyme in avocados that causes the flesh to turn brown when exposed to air?

Polyphenol oxidase is the enzyme in avocados that causes their flesh to turn brown when exposed to air. It is commonly abbreviated PPO and is also associated with browning in many other fruits and vegetables.

Cutting or mashing an avocado damages its cells and brings the enzyme into contact with oxygen and naturally occurring phenolic compounds. PPO helps convert these compounds into o-quinones, which then react and polymerize into dark pigments called melanins. The visible brown color is therefore the result of a chain of oxidation reactions, not simply the flesh “drying out.”

Acidic ingredients such as lemon or lime juice can slow the process by reducing enzyme activity and changing the reaction conditions. Limiting contact with oxygen also helps, which is why tightly covering guacamole can preserve its green surface. Refrigeration slows the reaction further.

PPO is not unique to avocados: it also contributes to browning in apples, potatoes, bananas, and other produce. Browning usually affects appearance more than safety, although prolonged exposure can alter flavor and texture.