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.

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 is not necessary to create the characteristic sourdough taste.

It is essential in baking rye-based breads because the chemistry of rye flour produces an environment that is acidic.