Pumpernickel [puhm-per-nik-uhl] is a typically heavy, slightly sweet rye bread traditionally made with sourdough starter and coarsely ground rye. It is often made today with a combination of rye flour and whole rye grains.
Some mass produced pumpernickel in North America may incorporate natural colorants such as molasses, caramel color, coffee, or cocoa powder among others to imitate the various shades of brown of traditional German pumpernickel, which is derived from long baking times and the maillard reaction (a chemical reaction between amino acids and reducing sugars).
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The Daily Omnivore 