The industrial processes for the preparation of coffee beans must start immediately after the harvesting to prevent the fermentation of the pulp and spoilage of the product. The preparation can be done using the dry or wet method.
When processing the coffee using the dry method you will get the "natural" coffee. The freshly picked cherries are spread out on 50m² surfaces per hectare plantation area and dried in the sun for about 4 weeks. In order to dry the cherries uniformly, they are raked and turned throughout the day. The dry seeds are mechanically hulled by feeding them through two rollers, arranged at a calibrated distance to each other, so that the beans fall out. Beans and hulls are separated by sieving.
When processing the coffee using the wet method you will get the "washed" coffee. The cherries are rough precleaned and soaked in water for 36 hours. Here, the under or over ripe beans float to the top, while the heavier ripe beans sink to the bottom (immersion). To separate the pulp from the bean the ripe coffee cherries are either pressed through a mesh or milled using rollers.
The slick layer of mucilage still attached to the parchment will be removed through fermentation. When fermentation is complete residues will be rinsed away with lots of fresh water.
Due to the high water consumption, the wet method with fermentation can't be used for coffees with sustainability requirements, although it produces a higher quality green coffee. Today, the fermentation process to break down the mucilage layer can be replaced by rather water-efficient machines.
Polishing is a process where any silver skin that remains on the beans is removed. The skin not only makes the bean look bad, it can also affect the flavour during the roast. Not all green coffee producers polish their coffee beans.
When the processing of the green coffee is finished it needs to be classified. Since this is done using several methods, such as sieving or visual inspection, the coffee is automatically scanned for foreign objects. Small branches, stones, hull remains or other foreign objects are being sorted out. The fact that almost every grinder built-in in a bean-to-cup coffee maker has some kind of stone detection shows that some unwanted objects may pass the final inspection of the natural product coffee.