Dissolve yeast in warm water and set aside. Mix dry ingredients in order given, cutting in shortening until mixture resembles coarse crumbs. Stir in milk and yeast mixture. Blend thoroughly. Knead dough lightly on floured surface. Roll out and cut with a large floured biscuit cutter. Let rise slightly (about 1/2 hour) and bake at 400 for ten minutes, or until slightly browned. Makes 15 large biscuits.