Peel potatoes and grate into a mediumsize bowl. Add the corn. In another bowl the onion, flour, and yolks until it forms a smooth paste. Add the milk, salt and pepper and stir to combine. Add the potatoes and corn and mix well.. Fold in the egg whites. Heat the oil in a mediumsize skillet, over a medium heat. Spoon the tortitas de papa cruda, in patties 2 inches in diameter, in the skillet. Cook them until golden brown on both sides. Serve at once.