Why does cut avocado flesh turn brown?

Answer: An enzyme helps oxygen react with compounds in the flesh

The fruit’s oils dry into a brown coating on the surface — The brown surface is not simply a dried film of avocado oil. Enzyme-driven oxidation of phenolic compounds produces the pigments, so a moist cut surface can brown too.

An enzyme helps oxygen react with compounds in the flesh ✓ — Cutting damages cells, allowing an enzyme to help oxygen react with phenolic compounds. The products then form brown pigments. The enzyme speeds the reaction; it is not itself the material being turned into brown pigment.

Dust in the air settles on the exposed flesh — Airborne dust is not needed for this browning. Oxygen in clean air is enough to support the reaction, which is why limiting air contact can slow it.