Pumpernickel, on the other hand, is always to be had, for the Westphalians pray daily: give us this day our daily pumpernickel....For you will barely be able to eat pumpernickel, and Christelchen‡ won't be able to at all; it's almost as if one were eating the grain raw. I have often tried and once had a piece served me which contained about twenty peasant-size bites....At times all the chewing made me laugh, and I gave the remaining 19½ bites to the horses....At times my fancy became more daring: let's wait until the sawdust has been transubstantiated, then it will suit you better — but in any case the horses could count on the 19½ peasant-size bites. I haven't been able to make any more progress so far. The pumpernickel before its transubstantiation into —— is something ghastly, but after transubstantiation, something the like of which no mortal baker has ever baked or will bake. — So much about the pumpernickel and its transubstantiation.I suspect that "transubstantiation into ——" must mean "transubstantiation into excrement."
‡ Dieterich's wife.
Pumpernickel hingegen kann man allemal haben, denn die Westphälinger (Gottlob, daß die Seite herunter ist) beten täglich: unsern täglichen Pumpernickel gib uns heut....Denn Pumpernickel wirst Du kaum, und Christelchen gar nicht essen können, es ist beynah als wenn man das liebe Korn roh äße. Ich habe es offt versucht und ließ mir ein Stück geben, das etwa 20 Bauernbissen enthalten mochte....[Z]uweilen gerieth ich über dem Kauen in ein Lachen, und gab die 19½ übrige Bissen den Pferden.... [Z]uweilen wurde der Einfall muthwilliger: Wir wollen warten, bis sich die Sägspäne verwandelt haben, da sollen sie wohl besser gehen, allemal aber konten doch die Pferde auf die 19½ Bausernbissen Rechnung machen. Weiter habe ich es noch nicht bringen können, der Pumpernickel vor der Verwandlung in —— ist etwas abscheuliges, nach der Verwandlung aber ———— etwas, desgleichen kein Sterblicher Becker je gebacken hat noch bakken wird — So viel vom Pumpernickel und dessen Verwandlung.
On the etymology of pumpernickel see Anatoly Liberman, "Multifarious devils, part 3. 'Pumpernickel,' 'Nickel,' and 'Old Nick'," Oxford Etymologist (June 12, 2013):
Of real importance is the fact that already in the first third of the seventeenth century Pumpernickel sometimes meant "devil." The name as applied to Westphalian bread appeared in documents and books some time (though not considerably) later, and perhaps the short chronological gap reflects reality: first the devil (a rather than the devil), then "devilish" bread. Despite the general uncertainty surrounding the derivation of pumpernickel, the origin of the first element poses no difficulties. In the post on bogey, I listed some b-g and p-g words that denote swelling, a noisy explosion, and so forth. Bomb and pomp were among them. Pump, pamper, and even pimp belong there too. A pimp (like the German Pimpf) was a youngster, a weakling unable to produce a big pumpf, that is, fart. Pamper refers to stuffing one with food (hence spoiling). Pumper-, as has been known for a long time, carries the same connotations. Despite the occurrence of the word from Osnabrücken to Vienna, it must have been coined in the north, for otherwise it would have had pf in place of p, at least after m. Whoever Pumpernickel was, he must have been able to produce a lot of noise, probably by breaking wind, though it is not improbable that he, like Bogey, deafened people in some other way.....The bread soldiers ate in the seventeenth century was indeed heavy and produced more than one "Pumpf," or great flatulence (to use a polite, sufficiently Latinized word). It deserved being called "fart Nickel."