Combine all ingredients and stir until sugar melts. Pour mixture into 9 by 13-inch metal pan and place on level shelf in freezer for half an hour. (Mixture should only come about 1/4-inch up the side of the pan.) Remove and use a dinner fork to scrape any ice crystals that have formed on the side or bottom of the pan. Return to freezer and repeat scraping every 20 to 30 minutes for 3 to 4 hours. Once mixture is thoroughly frozen, fluff with a fork and allow flakes to "dry" in freezer another half hour before serving. When served, the granita should look like a fluffy pile of dry brown crystals. Scoop into goblets and top with barely sweetened whipped cream, add additional citrus zest if desired.