Half an avocado in the fridge turns brown in about four hours. The brown part is not rotten — it is just oxidation, same thing that turns an apple brown. But it looks disgusting and nobody wants to eat brown guacamole. I used to just throw away the other half if I only needed one half for toast. A dollar in the trash every time.

I tried the pit-in-the-half trick. You know — leave the pit in the unused half and somehow it stays green? It sort of works. The pit blocks air from contacting the surface directly underneath it but the rest of the exposed flesh still turns. It is a half-measure. Literally.

What Actually Works

The enemy is oxygen. Keep oxygen away from the surface and the avocado stays green. Here are the methods ranked from best to worst:

1. Water submersion. Put the cut half face-down in a container of cold water. Completely submerged. Water blocks oxygen completely. This keeps an avocado green for up to 48 hours in the fridge. I did not believe this until I tried it — the avocado comes out looking exactly like it went in. Pat dry before eating. The texture is unchanged.
2. Plastic wrap pressed directly against the surface. Not wrapped around the whole thing — pressed against the cut flesh with your fingers to squeeze out all the air. Good for about 24 hours.
3. Lemon or lime juice brushed on. The citric acid slows oxidation. Works for 4-8 hours — fine if you are making guacamole for a party but not overnight storage.
4. Onion in the container. Supposedly the sulfur compounds from a cut onion prevent browning. I tested this. It does not work very well and your avocado tastes faintly of onion. Pass.

If It Already Turned Brown

Scrape off the top layer. The brown is surface-level oxidation — the avocado underneath is still fine.