I have heard that to stop guacamole from turning black that you should save the avocado pit and put it on top of the guacamole once it is made. How does this work?--Marc
This tip works only if the avocado pit is big enough to completely cover the bowl from edge to edge. What it does is exclude oxygen. Once the avocado fruit has been cut and the contents mashed, an enzyme released from inside the cells of the avocado flesh starts causing the pulp to turn brown in the presence of oxygen. Placing the pit on top of the guacamole keeps oxygen away from the area directly in contact with the pit. The surrounding area not covered by the pit still turns brown.
Any oxygen impermeable barrier will stop browning from happening; an avocado pit, plastic wrap, as Harold McGee found in The Curious Cook: More Kitchen Science and Lore, even a 40 watt light bulb (not recommended). While McGee found that all plastic wraps worked to some degree, Saran Wrap, which allows less oxygen to pass through it than some other brands, was most effective. Whatever material is used, it should be pressed gently down onto the surface of the guacamole to remove as much oxygen as possible.
The citric acid in the lime juice used to make guacamole also helps a bit, but to be completely effective it would need to be used in much larger amounts than most recipes call for.