Preheat oven to 190 degrees Celsius (375 F). Sift the flour into a large bowl and add the sugar. In another bowl whisk together the egg whites, lemon rind, lemon juice, oil, water and apple. Add the wet ingredients to the dry and mix well, but don't over beat as this will make the muffins tough. Spoon mixture into greased muffin cups. Bake until golden brown, 25 - 30 minutes. Cool on a wire rack.