1 cup sugar
1 tablespoon lemon juice
Peel six grapefruits, discarding all but the peel from one grapefruit. Remove the zest from the peel and chop into small pieces. Process chopped zest and 1 cup sugar in a food processor or blender until finely chopped, set aside.
Squeeze the juice from each grapefruit until you have 4 cups. Pour juice into a mixing bowl and add the zest mixture and 1 tablespoon lemon juice, mix well. Pour mixture into a 2- or 4-quart ice cream mixer and process according to the manufacturer’s directions. If no ice cream mixer is available, the mixture can be frozen and re-processed in a blender or food processor to break up the ice and add a smooth texture.