Wikipedia
Rugbrød (Danish rye bread) is a very commonly used bread in Denmark. The common rugbrød usually resembles a long brown extruded rectangle, no more than 12 cm high, and 30–35 cm wide, dependent upon the bread pan in which it is baked. The ingredients typically include rye flour, cracked or chopped rye kernels and seeds such as sunflower seeds, linseeds or pumpkin seeds. Sourdough is almost always the base because commercial yeasts are unsuitable. The naturally fermented dough will develop a naturally occurring Lactobacillus culture in symbiotic combination with naturally present yeasts. It is essential in baking rye-based breads because the chemistry of rye flour produces an environment that is acidic. The most commonly present yeast species in the production of naturally leavened dough is Saccharomyces exiguus, which is more acid-tolerant than commercially produced S. cerevisiae, although the latter and other strains may also be present. Research has shown that when creating a naturally fermented starter, any naturally present S. cerevisiae will have died off after a few days.
Sourdough is thus a stable culture of lactic acid bacteria (LAB) and yeast in a mixture of flour and water. The yeast produces gas (carbon dioxide) which leavens the dough, and the LAB produces lactic acid which contributes flavor. The LAB metabolizes sugars that the yeast cannot, and the yeast metabolizes byproducts of LAB fermentation. Commercially produced yeast will not accomplish these processes in rye flour.
The bread may be made exclusively with rye and wheat flour or contain up to one third whole rye grains. Ale or beer can be substituted for some of the water component of a recipe.
The bread is almost always very low in fat, its content comparable to most other varieties of bread. It contains no added oils or fats. Additional flavourings, other than salt, can include barley malt syrup. It is rich in whole grain and dietary fiber and contains little or no sugar, and is thus considered by many Danes as a healthy alternative to whiter types of bread.
Buttered rugbrød is essentially the base on which the famous Danish open sandwich smørrebrød is created.