Oxidation (exposure to air) can cause the fruit of an avocado and/or guacamole to turn brown. By brushing the fruit or guacamole with an acidic agent such as lemon, lime, vinegar or even oranges and placing the avocado or dish in an air-tight container in your refrigerator, you can help delay this process.
Placing the avocado seed in guacamole may help maintain the color of the guacamole because the seed reduces the amount of surface area that is exposed to air (minimizing oxidization). We recommend covering your guacamole with clear plastic wrap or placing it in an air tight container if you prepare it in advance and want to store it in the refrigerator for a few hours.