Preheat oven to 350 degree F. Mix dry ingredients in a bowl, and then make a well in the centre. Combine wet ingredients, except egg whites, in another bowl. Pour this mixture into the dry ingredients and mix until smooth. Beat egg whites until stiff peaks form. Whip a 1/4 of the egg whites into the egg yolk mixture to lighten. Very gently fold in remaining beaten egg whites. Spoon in to an ungreased angel food cake pan and bake 45 minutes. Invert pan on a baking rack and allow cake to cool this way. Work a paring knife along the edges of the pan before unmoulding. Serve cake with lightly whipped cream and, if desired, some sugar sprinkled slices of the various citrus fruits that flavour this cake.