The night before making the bread, in a medium sized mixing bowl, dissolve one packet of yeast and the sugar in 2 cups of water. Let stand until creamy, about 10 minutes. Stir in the rye flour until the mixture is smooth. Cover and let stand overnight.
The next day, dissolve the remaining package of yeast in the buttermilk. Add the rye flour mixture,the baking soda, the salt, 4 cups of the bread flour and stir to combine. Add the remaining 4 cups of bread flour, 1/2 cup at a time, stirring well after each addition (you may not need to add all of the flour). When the dough has become a smooth and coherent mass, turn it out onto a lightly floured surface and knead until smooth and supple, about 8 minutes. Sprinkle the caraway seeds on the dough and knead them in until they are evenly distributed throughout the dough.
Lightly oil a large mixing bowl. Place the dough in the bowl and turn to coat with the oil. Cover with a damp cloth and let rise in a warm place for about 1 hour or until the volume has doubled.
Preheat oven to 350 degrees F (175 degrees C).
Turn the dough onto a lightly floured surface and divide into three pieces. Form each piece into a loaf and place in 3 lightly greased 9×5 inch bread pans. Cover and let rise until nearly doubled, about 1 hour.
Bake at 350 degrees F (175 degrees C) for about 35 minutes or until the bottom of the loaves sound hollow when tapped.
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