When contracts are signed for the sale of a particular lot of coffee being prepared for shipment, this triggers the final stage in processing. Parchment will be taken from a temporary storage facility such as a bodega and moved to the dry mill. The milling process is the most easily automated part of coffee processing, and even basic mills will often have augers and conveyor belts to move parchment and coffee beans through the various stages. Before cherries are ready to leave the mill, five operations will take place:
- Destoning
- Hulling
- Repassing
- Size grading
- Density separation
The best hulling systems for specialty coffee are known as ‘cross-beater hullers’. Unlike many older designs that created a lot of friction between the beans, cross-beater hullers don’t generate excessive heat throughout the process. They also do not polish beans. Polishing refers to the removal of the silverskin from the surface of the green beans. This can be desirable for huge-scale commodity roasting, where freshiness is not a primary concern, but for specialty coffees the silverskin is retained on the beans to add protection prior to the roasting process. With unpolished lots, the silverskin will eventually come free from the beans during roasting and will be sucked out of the roasting drum and captured in the cyclone of a roasting machine.