Step 1Put the water, sugar, zest and star anise in a saucepan, bring to a boil and simmer for 5 minutes. Remove from the heat, allowing the star anise to infuse until the mixture is cool.
Step 2Remove the star anise, or leave them in if you prefer a stronger spice flavor. Mix in the grapefruit juice. Pour the juice mixture into a wide, shallow container and cover with foil or plastic wrap. Put in the freezer and allow it to freeze around the edge, 1-2 hours.
Step 3Take the container out of the freezer and scrape the ice with a fork, mixing it from the edge into the center. Return to the freezer. Repeat this scraping and mixing process every half hour until the entire mixture has turned into small, sequined ice flakes, at least three repetitions. If you have left the star anise in for stronger flavor, remove it before serving.
Step 4Serve the granita at once. If you leave it in the freezer overnight, let it set at room temperature until it softens a little, about 10 minutes, and then scrape it again with a fork to lighten the texture.