How a coffee is processed after harvest can have a dramatic effect on the resulting cup, so it has become increasingly important part of how it is described and sold.
After harvest the coffee cherries are taken to a wet mill to separate the beans from the flesh and dry the beans so that they are safe for storage. Coffee beans start with a moisture content of around 60 per cent, and should be dried to around 11-12 per cent to ensure they do not rot while waiting to be sold and shipped.
The wet mill processes coffee from the cherry stage to the parchment stage, when the bean is dry but still covered with its layer of parchment or pergamino. Most believe that the coffee is pretty well protected by this outer layer, and that it does not really begin to degrade untill the coffee is hulled to remove the parchment at the last possible moment before the coffee ships.
Coffee cherries can be processed in 3 main ways: natural, washed and hybrid.
The natural process
Also known as the dry process, this is the oldest method of processing coffee. After harvest, the coffee cherries are spread out in a thin layer to dry in the sun – either on brick patios, or special raised drying tables, which allow a better airflow around the cherry, resulting in more even drying. The cherries must be turned regularly to avoid mould, fermentation or rotting taking place. Once the coffee is properly dry, the outer husk of skin and dried fruit are removed mechanically, and the raw coffee is then stored before export.
This is the common process in places of the world, producing coffee, where there is no access to water, like Ethiopia, and also parts of Brazil. This process will often add fruit flavours to the coffee, regardless of variety and terroir. These are usually described as hints of blueberry, strawberry or tropical fruit.
The washed process
The goal of the washed process is to remove all of the sticky flesh from the coffee seed before it is dried. This greatly reduces the chance of something going wrong during drying, so the coffee is likely to be worth more. However, this particular process is much more expensive than the others.
After picking, the coffee cherry has its outer skin and most of the fruit flesh stripped off using a machine called a depulper. The coffee is then moved to a clean tank or through of water where the remainder of the flesh is removed by fermentation. After fermentation, the coffee is washed to remove the leftover debris, then it is ready to be dried.
Compared to other processes, wet-processed coffees tend to present a higher level of acidity, increased complexity and what is described as a cleaner cup. “Cleanliness” is an important term used in coffee to indicate the absence of any negative flavours, such as off tastes or unusual harshness and astringency.
Hybrid processes vary and can be mainly described as: pulped natural (mainly used in Brazil), honey (Costa Rica and Salvador), semi-washed/wet-hulled (Indonesia).