In a large bowl, combine 3 cups all-purpose flour, rye flour, sugar, salt and yeast. In a saucepan, heat water and butter to 120°-130°. Add to dry ingredients; beat until smooth. Stir in sauerkraut and enough remaining all-purpose flour to form a soft dough.
Turn onto a floured surface; knead until smooth and elastic, about 6-8 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1 hour.
Punch dough down. Turn onto a lightly floured surface; divide in half. Shape into two loaves, about 9 in. long. Grease two baking sheets or 9x5-in. loaf pans and sprinkle with cornmeal. Place loaves in prepared pans. Cover and let rise in a warm place until doubled, about 45 minutes.
Lightly brush tops of loaves with water. With a very sharp knife, make four to five diagonal slashes across the top of each loaf. Bake at 400° for 20 minutes. Reduce heat to 350°; bake 10-15 minutes longer or until lightly browned. Remove from pans to wire racks to cool.
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