Pumpernickel has been long associated with the Westphalia region of Germany, first referred to in print in 1450. Although it is not known whether this and other early references refer to precisely the bread that came to be known as Pumpernickel, Westphalian pumpernickel is distinguished by use of coarse rye flour—rye meal and a very long baking period, which gives the bread its characteristic dark color. Like most traditional all-rye breads, pumpernickel is traditionally made with an acidic sourdough starter, which preserves dough structure by counteracting highly active rye amylases. That method is sometimes augmented or replaced in commercial baking by adding citric acid or lactic acid along with commercial yeast.
Traditional German Pumpernickel contains no coloring agents, instead relying on the Maillard reaction to produce its characteristic deep brown color, sweet, dark chocolate, coffee flavor, and earthy aroma. To achieve this, loaves are baked in long narrow lidded pans 16 to 24 hours in a low temperature (about 250°F or 120°C), steam-filled oven. Like the French pain de mie, Westphalian pumpernickel has little or no crust. It is very similar to rye Vollkornbrot, a dense rye bread with large amounts of whole grains added.
While true Pumpernickel is produced primarily in Germany, versions are popular in the Netherlands, under the name Roggebrood, where it has been a common part of the diet for centuries.