The surface of guacamole turns brown by reacting with oxygen in the air. The guacamole that is directly under the pit won’t turn brown because the pit prevents air from getting to it. Otherwise, the oxidation process turns the exposed surface brown, just as it does on apples and other fruits. You’ll have much better luck protecting the surface from air by pressing aluminum foil or plastic wrap on it.
citrus juice (or any acid) will do the same. As Guacamole always has lime or lemon juice in it, have your citrus halved and ready to squeeze over the avacado. No browning.
Excellent point! Thanks.
Just have your lime juice in the bowl already and drop the avocado halves into it. Never has a chance to get brown. Plastic wrap is still good advice if you leave it in the fridge longer than a few hours though.
airtightness helps, but not in the long run. keep an open avocado in the fridge, even in a tight ziploc bag, and it will still oxidize. Lemon juice is great, like everyone says, but you know what seems to do the trick for me? Rub the avocado with an icecube or icewater as soon as you open it, and will stay green for a very long time, and in my case, it seems to work better than lemon juice.
The best advice I’ve heard for keeping guacamole fresh is to level the surface and pour water on top to cover the entire surface. When ready to serve, pour the water off and stir in the little bit that’s left. Keeps the guacamole fresh longer than any other method I’ve tried.