Place potato and water in a small saucepan. Bring to a boil, reduce heat, cover, and cook 10 to 15 minutes until tender. Drain, reserving 1 cup of the cooking liquid. Mash the potato without milk or butter and set aside.

Cool the reserved cooking liquid to 110°F to 115°F. In a large bowl, dissolve yeast in 1/4 cup warm water.

Add warm milk, butter, sugar, salt, 4 cups flour, mashed potato, and the reserved cooking liquid. Beat until smooth. Stir in enough remaining flour to form a soft dough.

Turn onto a floured surface and knead 6 to 8 minutes until smooth and elastic. Place in a greased bowl, turning once to grease the top. Cover and let rise in a warm place until doubled, about 1 hour.

Punch down. Turn onto a lightly floured surface, divide in half, and shape into loaves. Place in two greased 9 x 5-inch loaf pans. Cover and let rise until doubled, about 30 minutes.

Bake at 375°F for 30 to 35 minutes, until golden brown. Remove from pans to wire racks to cool.