Wet milling, the step that comes after harvesting, is focused on separating the fruit of the coffee cherry from the under-ripes and over-ripes after they have been picked. This can be done in a variety of ways, using different machines and techniques.
One of the more common approaches involves the use of a pulper. This spiked drum works by tearing the pulp of the coffee from the beans which then fall into a tank. Higher grade coffee is more dense and will sink to the bottom of the canal. Lower grade coffee will float past the denser coffees.
The coffee ferments in these tanks to loosen any remaining fruit so beans can then be thoroughly washed.