In large sauce pot, simmer apples, water, and sugar until apples are tender -- about 20 minutes. In food processor or blender, puree mixture until smooth; stir in lemon zest. 
Transfer mixture to 8- or 9 inch metal pan and freeze until almost solid. Return to food processor of blender and puree until chunks break up and mixture is fluffy. Refreeze until firm. 

226.4 Calories, 0.5 g Total Fat, 0.1 g Saturated Fat, 0 mg Cholesterol, 2.2 mg Sodium, 58.5 g Total Carbohydrate, 3.7 g Dietary Fiber, 0.3 g Protein, 11.5 mg Calcium.