Step 1Peel the grapefruits and mince the whole peel, including the white part. Place the peel in a quart jar with the coriander seed and alcohol. Seal for 1 week.
Step 2Put the sugar in a small pan and heat over medium heat until it turns light brown. Remove from the heat and carefully add water. Stir until the sugar dissolves.
Step 3Strain the peels and coriander from the alcohol and discard. Add the caramelized sugar. Return it to the jar and let it stand until the liquid is clear, about 1 hour, then bottle for use.