Make and ferment the poolish: Combine the water and yeast in a medium bowl. Let stand 1 minute, then stir with a wooden spoon until yeast is dissolved. Add flours and stir until the consistency of a thick batter. Continue stirring for about 100 strokes or until the strands of gluten come off the spoon when you press the back of the spoon against the bowl. There will be lively bubbles on the surface. Cover with a clean damp towel or plastic wrap and put in a moderately warm, draft-free place until it is bubbly and has increased in volume.
Prepare the sesame seeds: Place the sesame seeds in a small dry skillet. Cook over medium heat, stirring constantly, until lightly toasted, 2 to 3 minutes. Transfer immediately to a plate, place in the refrigerator, and cool completely.
Mix and knead the final dough: Measure the remaining ingredients. Bring the bowl with the poolish to your work space. The poolish should be soupy, bubbly and puffy and it should have a wheaty aroma. Scrape the poolish into a 6-quart bowl. Add the water and yeast. Break the poolish up well with a wooden spoon and stir until it loosens and the mixture foams slightly. Add the whole wheat flour and cooled sesame seeds; stir until well combined. Add the salt and enough bran flour to make a thick mass that is difficult to stir. Turn out onto a well-floured surface. Knead, adding more flour as needed, for 10 minutes. Gradually knead in the raisins and continue kneading until the dough is soft and smooth, 15 to 17 minutes total. The dough is ready when a little dough pulled from the mass springs back quickly.
Ferment the dough: Shape the dough into a ball and let it rest on a lightly floured surface while you scrape, clean, and lightly oil the large bowl. Place the dough in the bowl and turn once to coat with oil. Take the dough's temperature: the ideal is 78 degrees. Cover with a clean damp towel or plastic wrap and put in a moderately warm (74 to 80 degrees) draft-free place until doubled in volume. The dough has risen enough when a finger poked 1/2-inch into the dough leaves an indentation.
Divide and shape the dough: Deflate the dough by pushing down in the center and pulling up on the sides. Transfer the dough to a lightly floured work surface and knead briefly. Cut into 2 equal pieces. Flatten each with the heel of your hand using firm, direct strokes. Shape each piece into a 9-inch-long log, sealing firmly and pinching closed.
Proof the loaves: Place the loaves seam side down in lightly buttered 9 by 5 by 3-inch baking pans. Cover with a clean damp towel or plastic wrap and put in a moderately warm draft-free place until dough rises just to the rim of the pan.
Bake the loaves: 45 minutes to 1 hour before baking, preheat the oven and homemade hearth or baking stone on the center rack of the oven to 450 degrees. The oven rack must be in the center of the oven. Gently roll the loaf onto a lightly floured peel, seam side down. Score the loaf with a sharp razor blade or serrated knife making quick shallow cuts. Place the pans on the hearth and bake 15 to 20 minutes. Reduce heat to 400 degrees and bake until loaves are a rich caramel color and the crusts are firm, another 15 to 20 minutes.
To test the loaves for doneness, remove from the pans and hold them upside down. Strike the bottom firmly with your finger. If the sound is hollow, the breads are done. If not, bake 5 minutes longer. Cool completely on a wire rack.