Made from ripe avocados, a hint of spice, a bit of cilantro, fresh tomatoes and onions and a pinch of salt, guacamole takes center stage on the chip-and-dip table. It's not difficult to make but it does take some time, which is always at a premium and especially so when you're entertaining. Making it ahead can be a real convenience, though guacamole does tend to darken and discolor over time.

Exposure to air causes an avocado to brown quickly, so within a few hours, your guacamole loses its bright green color and turns a drab olive. Within a few more hours, it becomes unappetizing brown goo. The process, called oxidation, results in enzyme action within the avocado that results in the color change. Preventing exposure to air and slowing down the enzyme action, enables you to make your guacamole up to two days ahead of time, as long as you keep it refrigerated. The United Stated Department of Agriculture recommends that you store cold foods like guacamole, at 40 degrees Fahrenheit or colder, in the refrigerator to prevent the growth of harmful bacteria.

Most guacamole recipes include lime juice, either bottled or freshly squeezed. Not only does lime add flavor, it also slows the enzymes so the guacamole stays bright green. Although lime is the usual recommendation, you can substitute lemon juice in your recipe if you don't have any limes on hand. If your favorite recipe doesn't already include lemon or lime, add ½ tablespoon of juice per avocado to minimize browning and to prolong storage.

The second half of the guacamole equation is air exposure. When packaging the dip, use an airtight storage container. Some air will become trapped beneath the lid, so you must prevent it from browning your guacamole. Cutting a piece of plastic food wrap to size and placing it directly on top the dip solves the problem. Press the plastic down so it's in full contact with the guacamole and so air bubbles aren't trapped beneath. Place the lid on the container only after sealing the guacamole surface with the plastic wrap.

If you store guacamole for the maximum of two days recommended by the University of Nebraska-Lincoln extension, you may see a little discoloration on the top. Fortunately, browning doesn't indicate spoilage, so you can simply skim off the brown area before serving it. A quick stir mixes in any remaining discolored areas so that the dip once again looks bright green and freshly made.