Pumpernickel is deep, dense stuff. Over 300% as dense as a sturdy white loaf. This is due to certain botanical differences between rye and wheat. The gluten in wheat is composed of two types of proteins: gliaden and glutenin. Rye gluten is made up of gliaden and glutelin. Unlike glutenin molecules which readily form strong end-to-end bonds, glutelin molecules form only very weak bonds. The upshot of that is that rye proteins have a hard time forming the nice bubble-holding networks that we take for granted with wheat . All the active enzymes don’t help either (see below), since they pre-digest the bread’s starch structure as it rises.
The one thing rye does have going for it, however, is a sticky goo called pentosan gum (arabinoxylan), basically microscopic balls of sugars capable of soaking up huge amounts of water (around eight times their weight). The effect of the pentosan is twofold. First, since pentosan makes up about 10% of rye’s total carbohydrates, it renders the bread incredibly moist and resistant to staling, since pentosan doesn’t crystallize the way ordinary starch does as it ages. Second, it helps leaven the bread a bit, since pockets of steam and CO2 get trapped in the all that sticky muck.
Another interesting thing about pentosan is that when it finally dries it becomes a sort of natural appetite suppressant. It’s all that absorptive power, you see. Once it lands in the moist environment of the stomach it expands to eight times its previous weight, creating the sensation of fullness (or satiety, as food chemists like to say). So, chalk up one more important feature that rye offered to the peasantry of the Old World. Of course it offers the very same advantage to today’s dieters. Rye crisps, anyone?