Brown Gucamole? :-(
The lime juice coupled with the tomatillos in this recipe, will prevent the avocado's enzyme polyphenol oxidase from reacting with oxygen, (oxidation), which is the mechanism that causes guacamole to turn brown. To further reduce oxidation, be sure to place a sheet of plastic wrap on top of the bowl and in direct contact with the quacamole.
Adding the avocado pit to the quacamole, contrary to urban legend, does not prevent gucamole from turning brown, it only shields the gucamole that happens to be directly underneath wherever the pit lands from air, thus preventing oxidation. Citrus juice from the limes, tomatillos, and plastic wrap will do all the preservation you'll need to keep the guacamole green; the primary consideration is to keep air away from the guacamole until ready to serve. As far as the pit is concerned, stick three toothpicks in the pit at opposing angles, then fill a glass up with water, and place the skewered pit on top of the glass so that the pointed end of the pit rests in the water. Wait a couple of weeks, monitoring the water level to keep the tip of the pit wet, and you will have a baby avocado tree you can plant and eventually have your own fresh avocados. You certainly DON'T need the pit anywhere near your quacamole!
Interestingly, in addition to oxygen, light also has an effect on the browning of guacamole. If you leave a bowl of guac on the counter under fluorescent lighting, it will brown faster than if left under incandescent or natural lighting, (NOT direct sunlight!). I found this on the blog "Slashfood":
"...flourescent lights brown avocados faster because they give off more light in the higher frequencies. Higher frequency light (especially UV light) will break the double-bonds in omega-3 fatty acids at a higher rate. Try this experiment: put an avocado under a black light, and you'll see how much more quickly it turns brown. Black lights give off UV light in huge quantities, which is very potent at breaking double bonds.
"It is precisely this bond-breaking action that bleaches color out of bright colors, and turns other things yellow or brown. Sunlight fades colors because it has so much UV, and the colors you see in carbon-based dyes depend on the double bonds and other sensitive bonds in the dyes. Chlorine bleach does the same thing to colors and dyes (it snaps the bonds responsible for color absorbtion), but chemically, rather than through high frequency light."
Puree all ingredients together.
To serve, place guacamole in center of sm plate and spoon sauce around it. Tomato should be just enough to give it a suggestion of red color.