In 1850, British inventor John Walker took out a patent for the drum pulper. Ten years later, he took out a patent for a disc pulper (see below). Drum pulpers usually consist of a rolling pin that is traditionally covered in copper that is bulbed — a bit like the ridges in non-slip copperplate flooring. Cherries are drawn into the pulper from a hopper. Then the bulbs in the rotating drum pull the cherries around to where they press against an adjustable pulping bar. The drum rotates at approximately 120 rpm. (J.N. Wintgens, 2004, pg. 719) Parchment is guided into channels by a carefully calibrated pulping channel whilst most of the pulp is expelled out the side of the pulper on the other side of the machine to the pulping channel. This video from Penagos shows an engineer calibrating the pulping bar, and this one shows how the pulping channel is calibrated to ensure the drum doesn’t rub against the channel. Larger cherries require a bigger gap between the pulping bar and the drum.
The video below shows a drum pulper in action on Finca San Luis. In this footage, workers are pulping coffee that has been processed anaerobically (see Chapter 4) in sealed plastic bags. That is why the cherry appear to be a darker shade of purple-red than they would normally be straight off the tree. Wintgens advises growers to attempt dry pulping only when there is almost 100% ripe cherry. Hard, unripe cherry can block up pulpers of this type; water is not available to help wash the material through the pulper (J. N. Wintgens, 2004, pg. 629). Because the growers at Finca San Luis practice selective harvesting, their cherry are uniformly soft. For this reason, it is unlikely that their pulper will become blocked.
Video of a drum pulper in action at Finca San Luis.