Using a vegetable peeler, peel the rind from one orange and one grapefruit, avoiding the white pith. If there is any pith on the underside of the peel, remove it with a sharp knife to leave only the coloured rind. Cut the rind into very fine shreds and place in a pan with enough cold water to cover well. Bring to the boil, reduce the heat and simmer for 1 minute. Drain.