Place the flour into a mixing bowl. Grate the cold butter into the flour. Using you hands or pastry blender combine the butter and flour until crumbly. Set flour mixture aside. Blend sourdough starter, kefir and water together. I use my KitchenAid Hand Blender to blend the starter, kefir and water smooth. Pour the starter mixture over the flour mixture. Whisk together until you have a moist dough ball. Cover. Allow to ferment for 7 hours at room temperature. When you are ready to bake the bread, preheat your oven to 350 degrees F. While the oven in preheating, add salt, baking soda, and raisins to the fermenting dough. Knead well. If the baking soda is not incorporated properly you will have yellow streaks in the dough after baking. Divide dough in half. Shape into to round loaves. Place loaves on a parchment covered baking sheet. Beat an egg white. Brush over both loaves. Cut an X on the top of each loaf. Bake for 30-35 minutes. Cool. 12 servings.