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. It has been long associated with the Westphalia region of Germany. The first written mention of the black bread of Westphalia was in 1450. While it is not known whether this, and other early references, refer to precisely the bread that came to be known as pumpernickel, there has long been something different about Westphalian rye bread that elicited comment. The defining characteristics of Westphalian pumpernickel are coarse rye flourâ€”rye mealâ€”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. Flour is a powder made of cereal grains. It is the main ingredient of bread, which is a staple food for many civilizations, making the availability of adequate supplies of flour a major economic and political issue at various times throughout history. Wheat flour is one of the most important foods in European and North American culture, and is the defining ingredient in most European styles of breads and pastries. Maize flour has been important in Mesoamerican cuisine since ancient times, and remains a staple in much of Latin American cuisine. Flour contains a high proportion of starches, which are complex carbohydrates also known as polysaccharides. Leavening agents are used with some flours, especially those with significant gluten content, to produce lighter and softer baked products by embedding small gas bubbles. The production of flour has also historically driven technological development, as attempts to make gristmills more productive and less labor-intensive led to the watermill and windmill, terms now applied more broadly to uses of water and wind power for purposes other than milling.