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
Static Water Tanks
Static water tanks are rarely seen at large washing stations because they require so much labour. However, there is almost no setup cost to this system. In the next video, Nikolai investigates how static water tanks work. In this footage, you can see that the static water tanks are the first step in the quality control. Winnowing and sifting are not needed here because the pickers have carefully hand-selected ripe cherry only. As in almost all of Colombia, the terrain is too steep for mechanical harvesters.
In this video BH Coach and Medellin-based green buyer and Barista Hustle Coach, Nikolai Fürst travels to Finca San Luis to find out how static flotation tanks work.
A cross sectional diagram of a siphon tank.
Siphon tanks, widely used in many coffee-producing countries, are a water-intensive washing method. They usually take the form of a concrete or ceramic V-shaped tank that is filled with water. Smaller versions with capacities of just 1 or 2 cubic metres (264 to 747 US gallons) of water have been introduced in recent times in order to make the method available to smaller producers. Stones simply sink to the bottom of the V as the cherry are added to the tank. Overripe and dried cherry, as well as leaves and twigs, float to the top and are decanted out the side of the tank. The sinkers, which consist of ripe and unripe cherry. Flotation can’t separate unripe cherry from ripe cherry because their densities are similar. Even though ripe cherries contain more sugar, ripe and unripe cherries sink because they both have a high water content. (Wintgens, 2004, pg. 616).
The ripe and unripe cherry are sucked out from the area near the bottom of the tank, above the level where the stones collect. Stones should be too heavy to be drawn into the syphon, but the inlet is positioned a little way above the bottom of the tank to make sure of that. After the sinkers emerge from the siphon, they are diverted directly into the pulper.
A similar technology that doesn’t require the same level of excavation or the construction as syphon tanks are mechanical washer-separators. These machines separate floaters and sinkers as well as washing the skins of the cherries in water. This excellent video from Cuatro M in El Salvador shows a Brazilian made Pinhalense washer-separator in action. It is positioned at the beginning of the processing line directly after the reception tank and is used to clean the cherries and to remove floaters, stones, and light material.