One thing that can be a huge impediment to producing high quality coffees is actually quite small. In fact, its small size is exactly the problem. Some coffee fruit never develops beyond a very unripe state, going directly from unripe to dry and never achieving full maturation. Like bananas, the more unripe a coffee fruit, the more its resulting beverage will have a burnt rubber-like astringency. This coffee must be removed in order to not taint the final product.
These small unripes, if they have not dried on the tree, will normally sink in water and thus are found in the ripe and developed unripe sinking lot after hydraulic separation. In most cases, farms seeking higher quality coffees will send this coffee through the pulping process. While the pulping process can be used to separate out the larger unripes, the problem with these small unripes is that they get stuck in the pulping screens as seen here:
Once stuck, they eventually get forced through the pulping screens and pulped along with the ripe fruit, thus forming part of the parchment coffee. If this happens, they will impart their strong burnt-rubber astringency to the final beverage.
A lot of farmers dedicated to producing a high quality cup of coffee are using cylindrical screens as seen below. The small unripe coffee falls through the holes in the screen while the larger unripes and ripes make their way through the screen. By placing this screen in-line before the pulping process, the problem can largely be avoided. In general, the longer the cylinder, the better it sorts since the coffee fruits must then traverse a longer distance, increasing the time spent in the cylinder and the chances that the small unripes will fall through the holes.
In some cases, especially later in the harvest, these small unripe coffees will go from unripe to dry on the tree and thus form part of the floaters during hydraulic separation. To obtain a quality lot of naturals (or “raisin coffee” in this case since it floated) this coffee must be removed and the process is very similar to the one described above. A large cylindrical screen should be inserted before the coffee goes to the patio to remove these small dried unripes (call sequinhos or coquinhos in Brazil). The photo below shows the results of this process. On the left is the unripe coffee that dried without maturing. It will impart astringency to the final cup. On the right is the desired “raisin coffee” lot with the dried unripes separated out.