Combine milk, cornmeal, 1/8 teaspoon soda, 1/2 teaspoon sugar, and 1/4 teaspoon salt in a medium-size mixing bowl. Cover tightly with plastic wrap, and place in a lighted warm place (80° to 85°) for 72 hours. (Batter will become puffy and bubbly over the fermentation period.)
Add 2 1/2 cups flour, 1 cup warm water, remaining soda, and 1/2 teaspoon salt to fermented starter; stir well. Cover and let rise in a warm place (85°), free from drafts, 4 hours or until doubled in bulk.
Combine remaining flour, sugar, and salt in a large mixing bowl. Cut in shortening until mixture resembles coarse meal. Add remaining warm water and risen starter mixture; stir until dough leaves sides of bowl. Turn dough out onto a lightly floured surface, and knead 8 minutes or until dough is smooth. Dough will be stiff.
Divide dough into 3 equal portions, shaping each into a loaf. Place in 3 greased 7 1/2- x 3- x 2- inch loaf pans. Brush loaves with melted butter. Cover and let rise in a warm place (85°), free from drafts, 3 hours or until doubled in bulk. Bake at 325° for 10 minutes; increase temperature to 350°, and bake an additional 40 minutes or until loaves sound hollow when tapped. Remove bread from pans immediately; cool on wire racks.