Make this 5 minutes before you require it. Peel and coarsely grate the apple and sprinkle over the vitamin C powder to prevent discolouration. Combine with the horseradish and set aside for 2 minutes. A little juice will have leached out. Pour this off into a small bowl and evenly sprinkle over the pectin. Using the back of a spoon, work the pectin in until the mixture is a smooth paste, then mix this back into the grated apple mixture. The pectin will hold the juice together with the apple and prevent it from running on the plates.