Combine milk and saffron; let stand 10 minutes.
Dissolve yeast and 1 teaspoon sugar in warm water in a large bowl; let stand 5 minutes or until foamy. Stir in milk mixture. Lightly spoon flour into dry measuring cups; level with a knife. Add 5 cups flour, raisins, 1/4 cup sugar, butter, and salt to milk mixture, stirring to form a soft dough. Turn dough out onto a floured surface. Knead until smooth and elastic (about 8 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel sticky).
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 1/2 hours or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down; cover and let rest 5 minutes. Divide in half. Shape each portion into a 5-inch round loaf. Place loaves, seam sides down, 3 inches apart, on a large baking sheet coated with cooking spray. Make 2 diagonal cuts 1/4-inch-deep across top of each loaf using a sharp knife. Cover and let rise 30 minutes or until doubled in size.
Preheat oven to 375º.
Uncover dough. Bake at 375º for 30 minutes or until loaves are browned on bottom and sound hollow when tapped. Remove from pan; cool on wire racks.