Why do avacado pits turn red when cut into?

The seed of avocado contains beside a lot of other substances trimeric A-type proanthocyanidins. By contact with air ( oxygen ) it turns to a similar substance like that, which gives the color to red wine or cranberries.

Its antioxidant property is the reason for people to put an avocado pit into the guacamole or other fresh avocado dishes not to turn brown. ( "Interestingly, the use of avocado seed as antioxidants in some traditional foods and dishes of the Mexican people was proved by the high antioxidant activity of some of the isolated compounds": http://www.cuvillier.de/flycms/de/html/30/-UickI3z...

I can't say for certain, but I think it is the same reason the pulp of apples turns brown after being exposed to air. The Oxygen in the air is "burning" it at a slow rate. Also called oxidizing it.

Rub the cut back side with lemon or lime juice, then tightly wrap the entire element in plastic and placed it interior the refrigerator. Use it day after in the present day, because this might sluggish yet not halt the browning technique.