I love homemade guacamole, but I hate the less than appetizing brown shade it turns after it's been made. Why does it go from green and healthy looking to brown sludge? And, more importantly, how can the color-change process be slowed down or stopped?
First things first, guacamole turns brown due to exposure to oxygen. The oxygen sets off a chemical reaction in catechol, a naturally occurring chemical in the fresh fruit, turning the catechol into o-benzoquinon. The o-benzoquinon then helps to protect the fruit from prematurely spoiling. But it is the o-benzoquinon that is the cause of the browning. (It's also the culprit behind the browning of apples, potatoes, etc.) So, why does guacamole brown so much faster than apples or potatoes? Because of a little compound known as PPO (polyphenol oxidase). PPO serves as the catalyst for the entire reaction, and avocados contain a relatively high amount of PPO, causing them to brown very quickly.
So, how to reduce the browning? There are several different methods on how to do so, the most common being adding an acid of some sort (i.e. lemon juice or lime juice). Adding lemon/lime juice does help to slow down the chemical reaction, but if the guacamole is going to be stored for a day or two, it will still turn brown. For longer storage, it's best to minimize the fruit's exposure to oxygen. Do this by pressing a layer of plastic wrap directly onto the surface of the guacamole, then tightly sealing the storage container. The guacamole should stay green for several days this way.
Extra tip: pressing plastic wrap to the surface of custards or puddings will stop these dishes from forming a tough "skin" while in the refrigerator.