Cutting open an avocado only to find it unripe can be highly disappointing, especially if it's the main ingredient of a recipe. While fruits ripen best in room temperature, ripening an avocado that has already been cut open is best done in the refrigerator. While the cold temperature will slow down the process, it will also save you from throwing the avocado away.
Things You'll Need
Squeeze some fresh lemon juice on the exposed flesh of the unripe avocado. The acid from the lemon juice will halt the oxidizing process preventing the flesh from turning brown.
Align and press the two halves back together.
Wrap the avocado tightly with plastic wrap to help seal the two halves back together.
Place the avocado into the refrigerator.
Wait a day or two for the avocado to ripen. It should be soft to the touch.
You can substitute lemon juice with other citric juices like orange or lime juice. You can substitute plastic wrap with an airtight container. Ripening times will vary based on the ripeness of the avocado when it was sliced.