When I worked in the restaurant business, I noticed many of the hispanic prep cooks would put a avocado pit in the guacamole. Their reasoning was that is helps it from turning brown. I had never heard this and figured it was some old Mexican wives’ tale. I found myself going along with it and putting my own avocado pit in my guacamole that I make at home and I have been doing ever it since. Today I googled it to find out for myself what makes this work.
From straightdope.com: Most fruits and vegetables change color when their flesh is exposed to the air due to oxidation–that is, reaction with oxygen in the air. Some fruits and vegetables, such as the avocado, are more susceptible than others because they contain an enzyme called polyphenol oxidase. This enzyme works on phenolic compounds in the flesh of the avocado, changing their chemical structure and thus their color.
So there are two culprits in this browning process–the enzyme in the avocado, and the oxygen in the air. Logic suggests that if the avocado pits prevents browning, one of two things must be happening: Either the avocado pit chemically changes the guacamole, or it prevents oxygen from getting to the guacamole in the first place. I originally proposed to Ed that I do some hard-core experimentation with multiple guacamole preparations, but I was talked out of it, and referred instead to the book The Curious Cook by Harold McGee. McGee did experiments with guacamole and avocado pits and discovered that the secret was simply that the avocado pit physically blocked air from oxidizing the guacamole. In fact, the best way to prevent oxygen intrusion is to take plastic wrap and seal it over the guacamole, pressing it down into the surface of the food so no air is trapped above the surface.