Pumpernickel is a type of very heavy, slightly sweet rye bread, originally from Germany, traditionally made with coarsely ground rye. It is now often made with a combination of rye flour and whole rye berries. The defining characteristics of Westphalian pumpernickel are coarse rye flour—rye meal—and an exceedingly long baking period. The long slow baking is what gives pumpernickel its characteristic dark color. The bread can emerge from the oven deep brown, even black. Like most all-rye breads, pumpernickel is traditionally made with a sourdough starter; the acid preserves the bread structure by inactivating the highly active rye amylases. The process is sometimes short-circuited in commercial baking by adding citric acid or lactic acid along with commercial yeast.