The first step in the process is that the coffee cherries are sorted either by hand on tables or by placing them in a flotation tank where the ripe cherries will sink and the unripe ones will float. The next stage is the so-called depulping where the coffee cherry is split and squeezed to remove both the outer skin and the fruit pulp. This can be done by using a mechanical depulper (sort of a large blender) or simply by jets of high-pressure water.
After depulping the coffee beans are still covered by a resilient sticky layer called mucilage. Its tenacity is due to a combination of sugars and pectin and the best way to remove it from the coffee beans is by means of fermentation.
The beans are placed tanks for 12 to 72 hours to ferment. During this period the pectin in the mucilage is broken down through the activity of enzymes. The duration of the fermentation depends on a number of factors such as altitude, ambient temperature, volume of coffee, type of beans. The fermentation period will also affect the coffee flavour so knowing when to stop is crucial. If the fermentation takes too long unwanted flavours can creep in, but when properly timed washed coffees can develop a distinctive clean acidity. “Cleanliness” is a term used in coffee to indicate the absence of any negative flavour, such as harshness or astringency. In Kenya it is common to do two fermentation periods to achieve certain quality of flavour or appearance. These coffees from Kenyan are bright and fruity.
Following a wash with clean water the beans are place to dry under the sun. As with natural dry processed beans (see http://www.engrano.nl/blog/2017/02/13/natural-or-dry-processed-coffee/) beans can be dried in patios, lying on the ground or placed on raised tables. The tables have the advantage of lifting the beans from the ground so that contamination with stones or other plants can be avoided and improving air circulation around the beans whilst high temperatures should be avoided. During the drying process the beans should be gently rotated. Farmers also take the opportunity to sort through the parchment-covered coffee beans by hand removing the damaged ones. Depending on the weather conditions the drying period could take up to 21 days.
Once dry, the parchment-covered beans look light beige coloured. To remove the parchment the beans are moved to a dry mill after which comes sorting and packing of the green beans.
This process is more expensive than the natural and hybrid process. It involves a precious and sometimes scarce resource: water. On one hand, removing the mucilage greatly reduces the chance of something going wrong during the coffee processing which may lead to a higher value for the coffee. On the other hand, the wet process is not a warranty of quality. Defectives beans are not uncommon and fermentation can be unpredictable. An environmental concern about this process is due to the eventual fate of the waste water, which can be toxic.
Usually the flavour of washed coffees can be described as a clear acidity profile, light-to-medium body, usually with citrus tones. If you have the opportunity to taste coffees from the same origin but that have been processed both as washed and as dried and you will notice the difference!