Shape dough into three 18-inch-long, 1/2-inch-thick ropes. Arrange the ropes on the prepared baking sheet and brush them lightly with the reserved egg white. Bake them for 20 minutes. Cut the rolls into diagonal slices about 1/2 inch wide. Reduce the oven heat to 225 degrees , and bake again until the biscotti are crisp and browned, 20 to 30 minutes.