** Barm should be fed, bubbly, expanding up the sides of the bowl, and at room temperature.
SPONGE: In the mixer bowl, place the barm, flour, yeast and warm water. Mix with the paddle attachment until thoroughly hydrated but no not overbeat, about 20 seconds. Scrape down the sides of the bowl, using a spatula rinsed in cold water. Cover the bowl with plastic wrap and let sit in a warm place (72-75 deg.F.) until bubbly and expanding, about 3 to 3-1/2 hours.
DOUGH: In a separate medium bowl, whisk together the bread flour, rye flour, rye bread improver and Deli Flavor if used, brown sugar, salt, yeast, caraway seeds if used, and baking soda; make sure it's all well-mixed with no lumps. Add the flour mixture to the barm, then add the oil and buttermilk. Using the paddle attachment, mix on low speed until the dough begins to come together. Add only 1/4 cup of the water and mix until the dough begins to come away from the sides of the bowl, dribbling in water only if needed. Switch to the dough hook and knead 3 minutes. Let this sit 5 minutes to allow the gluten to begin to develop.
Turn the dough out onto a lightly floured board and knead about 3-4 minutes by hand, adding as little flour as possible to keep it from sticking to the board, until the dough is still a little soft, somewhat elastic, tacky but not sticky (rye flour becomes gummy if kneaded too much). Place the dough into an oiled bowl; cover the bowl with plastic wrap and let rise at room temperature until doubled in size, about 1 to 2 hours.
For loaves, lightly oil two 8-1/2" x 4-1/2" pans. For free-standing boules, sprinklle a shallow baking pan with semolina or cornmeal.
Gently remove the dough from the bowl onto a lightly floured board. Using a bench knife, divide the dough into 2 pieces. Shape into boules or loaves and place in or on the pan(s). Lightly spray the tops with oil, cover loosely with plastic wrap and let sit at room temperature until about 1-1/2 times the size, about 1-1/4 to 1-1/2 hours.
Preheat oven to 350 degrees with the rack in the middle of the oven. If desired, brush tops of loaves with egg wash (I usually just spritz mine lightly with water). Bake 35-40 minutes or until internal temperature registers between 185-195 deg.F. on an instant thermometer. Remove loaves to a rack to cool completely.