1In a medium saucepan cook potato cubes in water for about 12 minutes, or until tender. Do not drain. Cool mixture to 110 degrees F (43 degrees C). Set aside 1/2 cup of the cooking liquid. Mash potato and remaining liquid; if necessary add warm water to make 2 cups potato mixture.
2In a large mixing bowl, sprinkle yeast on reserved 1/2 cup potato water. Add mashed potato mixture, 2 cups of the flour, sugar, shortening, and salt. Beat with an electric mixer on low speed for 1/2 minute, scraping sides of bowl constantly. Beat 3 minutes at high speed. Stir in as much of the remaining flour as you can mix in by hand.
3Turn dough out onto a lightly floured surface. Knead in enough of the remaining flour to make a moderately stiff dough. Knead for 6 to 8 minutes, or until dough is smooth and elastic. Place in a lightly greased bowl; turn once to grease surface. Cover. Let rise in warm place until doubled in bulk, about 1 hour.
4Punch down, and turn out onto a lightly floured surface. Divide dough in half. Cover, and let rest for 10 minutes. Shape each half into a loaf. Place in greased 8 x 4 x 2 inch loaf pans. Cover. Let rise until nearly double, about 35 minutes. Before baking, brush tops with a little water and dust with additional flour.
5Bake at 375 degree F (190 degrees C) for 40 to 45 minutes. Cover with foil for the last 15 minutes of baking to prevent over-browning. Remove from pan; cool on wire rack.