Hi Sam! Great question! As you said, guacamole browns quickly because of oxidation, reactions that occur when the guacamole comes in contact with the air. The avocado is especially vulnerable to oxidation because its cells contains an enzyme called polyphenol oxidase (PPO). When the avocado is cut up, some of the cells break, releasing this enzyme and allowing it to react with the flesh of the avocado in the presence of oxygen from the air. Polyphenol oxidase changes the chemical structure of compounds called phenols, composed of 6-membered rings, in the avocado, which results in a color change.
So, if the avocado pit prevents the guacamole from turning brown, one of two things much be happening. Either the avocado pit somehow changes the chemical composition of the guacamole, preventing the action of polyphenol oxidase (which would be AWESOME!!) or the pit simply prevents oxygen from coming in contact with the guacamole.
Dr. Harold McGee, a professor at Yale, conducted an experiment to find out what was really going on, as detailed in his book The Curious Cook: More Kitchen Science and Lore. He found after significant exposure to the air, only the area of the guacamole directly under the pit had not undergone oxidation. Replacing the avocado pit with a lightbulb had the same effect! Dr. McGee concluded that it was just the physical blocking of the avocado pit that prevented of oxidation. I found this experiment conducted with photos included on a blog, DesiScrub. The link is listed under my sources.
No visible difference here!
Since the pit won’t completely cover the guacamole, it isn’t very effective in preventing oxidation. Better strategies include covering the guacamole with plastic wrap; in Dr. McGee’s experiment, he found that Saran wrap was most effective since it prevented more oxygen from going through than other brands. The plastic wrap should be gently pressed down onto the guacamole to eliminate as much oxygen as possible; some suggest putting something light, like salsa, on top to ensure that the plastic wrap continues to cover the surface of the guacamole and does not move.
Refrigerating the guacamole can slow down the oxygen of the enzyme as long as the temperature isn’t colder than average-avocados are especially at risk for chilling injury. Also, polyphenol oxidase doesn’t react well in acidic conditions, so adding lime or lemon juice will help prevent browning.
Of course, like most foods, guacamole is best when it’s fresh!
DesiGrub. (2010, December 20). Avocado browning. Retrieved from http://desigrub.com/tag/pit-and-guacamole-browning/
USDA Agricultural Research Magazine. (1998, February). Keeping Freshness in Fresh-Cut Produce.
Harold McGee. (1992). The Curious Cook: More Kitchen Science and Lore. Hoboken, NJ: Wiley.