How do you keep guacamole from turning brown?

Guacamole browns because cut avocado reacts with oxygen, an enzymatic oxidation, the same process that darkens a sliced apple. So the winning strategy is simply to keep air off the surface. The most effective home method, tested repeatedly, is to smooth the guac flat, then pour a thin half-inch layer of cold water directly on top, seal the container, and refrigerate; the water forms an airtight barrier, and you just pour it off and stir before serving. It keeps guacamole green for a day or two better than anything else.

If water feels odd, pressing plastic wrap directly onto the surface, with no air gap, works by the same principle and is the classic move; a lid alone leaves an air pocket that browns the top. Lime juice helps in two ways, its acid slows the browning enzyme and adds flavor, but acid alone will not save guac left exposed, so treat lime as a supporting player, not the whole defense. And here is the reassuring part: brown guacamole is oxidized, not spoiled. It looks unappetizing but is perfectly safe to eat, and scraping off the discolored top layer reveals green underneath. Toss it only if it smells sour or has been stored too long, not merely for the color.