There are a number of ways to freshen up your garbage disposal.

Grind up some citrus peels with a steady stream of cold water. Sprinkle baking soda over ice cubes and feed them into the disposal. Or mix 1/2 cup baking soda with 1 cup of white vinegar and pour it into the disposal that is turned off. Let it sit for 20 minutes, then flush with hot water. Any of these methods should produce a fresh clean garbage disposal.