In large mixer bowl, soften yeast in warm water. Blend in 2 1/2 cups of the flour, sourdough starter, salt, and sugar. Combine 2 1/2 cups of the flour and the baking soda; stir into flour, yeast mixture. Add enough remaining flour to make a stiff dough. Turn out onto a lightly floured surface and knead until smooth and elastic (5 to 7 minutes). Shape in ball. Place in greased bowl; turn once. Cover and let rise in warm place until double (about 1 1/2 hours). Punch down; divide in half. Cover and let rise 10 minutes. Shape into 2 round loaves. Place on lightly greased baking sheets. With sharp knife, make diagonal slashes across tops of loaves. Let rise in warm place until double (1 1/2 hours). Bake 400ºF. for 35 to 40 minutes. Remove from baking sheets; cool. If desired, brush with butter. Makes 2 loaves. 

Sourdough Bread

1 1/2 cup of sourdough starter
3/4 cup of milk
3 tablespoons of sugar
1 teaspoon of salt
2 tablespoons of oleo or shortening
1/4 cup of warm water
1 package of yeast
5 to 6 cups of unsifted flour

Scald milk, stir in sugar and shortening. Cool to lukewarm. Measure warm water and dry yeast; stir in. Add milk mixture, starter, and 2 1/2 cups flour. Beat until smooth. Stir in enough flour to make a stiff dough. Turn onto floured board and knead until elastic. Place in greased bowl and cover. Let rise until doubled in volume. Punch dough down, divide into 3 equal pieces. Place in greased loaf pans. Let rise. Bake at 400ºF for about 25 minutes or until done. Turn out onto rack to cool.