Cut open an avocado and the bright green surface starts changing.
After a while, brown patches appear.
That color change is mainly an oxygen-driven chemical process called enzymatic browning.
Cutting breaks open the cells
Inside an intact avocado, enzymes and other compounds are kept in different parts of the cells.
Slice the fruit and those compartments break open.
Oxygen from the air can now reach chemicals that were previously separated.
Polyphenol oxidase helps start the reaction
One important enzyme is polyphenol oxidase.
With oxygen present, it helps transform naturally occurring compounds in the avocado into new molecules that eventually produce brown pigments.
This is closely related to why cut apples turn brown.
Different fruit, similar chemistry.
Does brown avocado mean it is spoiled?
Not automatically.
A little surface browning is mainly a chemical color change.
The avocado may still taste fine.
Spoilage involves additional changes caused by microbes, age, texture breakdown, and other processes.
Brown color alone does not tell you the whole story.
Why does lemon or lime juice help?
Acidic juice lowers the pH at the surface.
That can slow the enzyme involved in browning.
The juice may also limit some contact between the exposed surface and oxygen.
This is one reason lime juice is especially handy in guacamole.
Does covering the avocado help?
Yes.
Reducing air contact slows the process.
Pressing wrap closely against the surface, using a tight container, and refrigerating the avocado can all help.
Cold temperatures also slow many chemical reactions.
Why does browning happen mostly on the surface?
That is where oxygen has the easiest access.
The deeper interior remains greener longer because air has not reached it as easily.
The same principle helps explain why the inside can still look fine when the top layer has browned.
So if guacamole turns brown, oxygen has been doing most of the work.
The avocado did not suddenly expire the moment the green faded.
And that’s the short explanation.
Related explanations: Why do apples turn brown?, Why do bananas turn brown?, and Why does toast turn brown?