Rugbrød
The dough may be made exclusively with rye flour or contain up to one third whole rye grains. A small amount of wheat flour, sugar or molasses is often added to adjust the taste or because wheat flour is cheaper than rye (contrary to in former times). Rugbrød was the major staple of most of the population until potatoes became widespread during the late 19th century, and even up to the 1950s, Danes ate much larger amounts of rugbrød than today. It has been discussed why this bread type prevailed better in Denmark than other Northern European countries. A  possible explanation is the need to bring packed lunches, because Danish schools and most workplaces don't traditionally have canteens with hot meals. Rugbrød is implied in the colloquial Danish term for serving prison time, "på vand og brød" ("on water and bread"). Until 1933, prisoners could in some circumstances be punished with an allowance of only water, a fixed amount of rugbrød, salt and possibly lard. Rugbrød is regarded somewhat difficult for home baking. Apart from the sourdough preparation, the loaves must not be leavened for too long, or else the taste can become excessively sour, with the relatively  pungent acetic acid taking the lead over milder-tasting lactic acid, and enzymes can cause the gluten (protein structure) and starches to degrade and collapse, creating cavities or dense lumps inside the bread or even causing it to shrink during or after baking. Rarely, recipes replace some of the water with ale or beer, but this isn't necessary to create the characteristic sourdough taste. Sourdough is almost always used for the base dough, as commercial yeasts are unsuitable. The naturally fermented dough will develop a 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 and yeast in a mixture of flour and water. The yeast produces carbon dioxide which leavens  the dough, and the bacteria produces lactic acid which contributes flavor. The bacteria metabolizes sugars that the yeast cannot, and the yeast metabolizes byproducts of bacterial fermentation. Commercially produced yeast will not accomplish these processes in rye flour. Rugbrød contains little or no added oils and is low in fat. Additional flavourings, other than salt, can include barley malt syrup or sugar. The bread is rich in protein and dietary fiber and not very sweet, unlike Swedish and German rye bread. Due to confusion of the term wholegrain (in Danish: "fuldkorn", "full grain"), many people assume rugbrød with soft -chewy, whole kernels to be healthier than the old-style version which is made of only unsifted rye flour and has a more even, smooth structure. However, both versions are made of wholegrain rye, including germ and bran, and therefore have the same nutritional value and fiber content. Buttered is essentially the base for Danish open sandwich .