Similar to apples and bananas, the flesh of an avocado can turn brown when it is exposed to the air for a long period of time. An easy way to slow this oxidation process is to add some fresh squeezed lime juice to your guacamole. It also helps to reduce the amount of air your guacamole is exposed to. You can do this by putting your guacamole in between two other layers in your 7 layer bean dip.