Pumpernickel is not a bread that I can eat, because it contains lots of rye. But my boyfriend love this kind of bread very much and I was always fascinated because this bread is more cooked in its own steam than baked for about 16 hours at low temperature. The enzymes in the rye break down the starch to sugar during baking because the dough is heated slowly. Part of this sugar caramelize and part of it reacts in the maillard reaction where amino acids react with reducing sugars. This reactions give the dark brown, nearly black colour and the slightly sweet taste to the bread.
Pumpernickel is baked in closed tins normally, but I used Weck glass jars. But you should only use mold form only because it is really hard to get the bread out of the tulip. I used both and had to struggle to get the bread out. But luckily jar, bread and even me survived this adventure without injuries 😉