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 meal and a very long baking period, which gives the bread its characteristic dark color. Like most traditional all-rye breads, pumpernickel is made with an acidic sourdough starter, which preserves dough structure. The acid destroys the rye protein, amylase, which converts the starch to sugar. That sourdough starter is sometimes augmented, or replaced, in commercial baking by adding citric acid or lactic acid along with commercial yeast.