Are Your Avocados Turning Brown or Gray?

What makes a cut avocado turn brown? Cut avocados turn brown for the same reason that cut peaches and apples darken: the oxidation of phenolic compounds. Almost all fruits (and avocado is a fruit) contain phenolic compounds, which turn brown when they react with oxygen.
If you want to prevent this – for instance, when you want to keep your bowl of guacamole green overnight – the only real solution is to put the guacamole in an absolutely airtight container. A bowl with a plastic lid isn’t enough; you need to lay a piece of plastic wrap directly on the guacamole.
It’s a myth that the avocado’s pit prevents browning: only the guacamole touching the pit stays green, and that’s because it’s not touching air.
If you want to keep slices of avocado green temporarily, you can slow down the reaction of phenolic compounds with vitamin C. Dip or soak the slices in lemon or orange juice. If those citrus flavors don’t match your recipe, try using a vitamin C tablet dissolved in water. That isn’t as strange as it might sound: the fruit preservative products you buy in the store are nothing but vitamin C, which, of course, is also called ascorbic acid.