The original Pumpernickel bread is usually made from only coarse rye grains, dark rye flour and sourdough starter. Since these are dense heavy ingredients, it needs a substantial amount of sourdough starter in order to rise. And the true sourdough is taking up to 24 hours to rise the bread. The traditional pumpernickel bread is also baked for 16 to 20 hours in a steam oven. In order to produce a very dark color, and a soft crust.