A defining feature of wet processing is that the cherry needs to be depulped (see the following lesson). Not all styles of pulpers can handle unripe cherry, and none can handle stones and other inorganic material. A separation process is necessary in order to ensure that only ripe coffee cherry progress to the next stage of processing. Even though stones that are bigger or smaller than the cherry are separated by sifting, stones the same size as the cherry can be separated only by flotation.
Flotation systems vary in price and complexity, ranging from plastic buckets to huge machines called washer-separators. Flotation is not needed if you don’t have any overripe cherry in the mix, but it is a widely used process, used by even the top specialty coffee producers. There are four ways to do it:
- Static water tanks
- Siphon tanks
- Channels with traps
- Mechanical washer–separators