Soften yeast in warm water to soften and allow yeast to proof for about 5 minutes.
Stir in sugar, salt and 3 Cups of the flour; beat well to develop gluten
Stir in melted shortening and gradually add additional flour to make a soft dough.
Turn dough out on floured surface and knead until smooth.
Place in a greased bowl turning once to grease the top; cover and let rise until doubled in bulk, 1-1 1/2 hours.
Punch dough down in bowl and divide dough in half; shape into balls or loaves and place in well greased pans.
Mix egg white and 2 tablespoons of water and brush tops of loaves when they are almost risen; sprinkle with potato flour and continue rising until loaves are almost doubled in size, about 45-60 minutes.
Bake loaves at 400° F. 35-40 minutes. Remove from oven and cool on wire racks.
My own comment: Don't be fooled by that "knead until smooth" stuff. It gets smooth very quickly and it comes out smooth before you even knead it! So, knead the heck out of it or it won't rise.