The processing of coffee refers to the method used to remove the fruit part of the cherries from the green beans that sit inside, which is done before they are ready to be roasted. Processing includes the striping of three layers; the cherry, pulp (mucilage) and parchment (pergamino); and the method by which the beans are dried.
There are three main ways of processing and drying coffee, with each one producing unique characteristics. They are Natural (Dry), Pulped Natural (Semi-washed) and Washed (Wet).
Once the cherries are picked and the clean ripe fruit separated from unripe/damaged cherries they are laid either on trestles or patios to air dry in the sun. By leaving the bean inside the cherry during this phase there is a closed environment allowing the sugars to ferment into alcohols and soak into the beans. The sugars and alcohols from the cherries and pulp can impart the strong, exotic flavours that naturally processed beans are renowned for.
This process can take up to four weeks for the moisture to reduce to the optimum level of around 10-12%, resulting in some larger plantations investing in machine driers. Once the whole cherries have dried into leathery fruits they are hulled, sorted and graded ready for shipping. Natural processing can produce equally good, if not better beans, however it can also produce inconsistent results and so should be considered on a lot by lot basis.
Pulped Natural (Semi-washed)
During this type of processing the freshly picked ripe cherries are passed through a machine to break the cherry and remove the fruit but leave some of the mucilage on the beans. If a large amount of mucilage remains it may be referred to as 'Honey' processing, which is more common where water is scarce.
Once pulped the beans are dried similarly to the natural process but require frequent movement in order to dry the beans evenly and prevent mould occurring. This method allows the mucilage to dry to the beans infusing flavours into them, although they are not as pronounced as the sugars from the cherries in the natural process.
Pulped Natural or Honey processed coffees tend to have more body than a washed coffee whilst being cleaner and more balanced than a Natural coffee. This is particularly good for beans that are to be used in an espresso.
Instead of leaving the cherry on the bean, wet processing removes the bean from the mucilage and cherry immediately after picking. Once this is done, there beans are often left to soak for up to two days in fermentation tanks where naturally occurring microbes break down the sugars. Some producers choose to use a machine at this stage to scrub the remaining pulp from the beans. Now that the beans are washed and clean, they are dried in similar ways to the other methods (i.e. air dried in the sun), before the crumbly parchment is easily removed during the hulling process.
Coffee that has been through the washed process generally produces a more consistent beans than the natural process. They are known for their bright clean flavours and light bodies. Due to the requirement for large fermentation tanks this process is more common in well developed coffee growing countries.
It's your choice
All three of the processing methods produce unique characteristics in the final cup and the choice is ultimately up to you (well the roaster, but there will always be a choice for the end drinker). As you try new coffees, pay attention to or even ask how the coffees have been processed and which you like best. It can make all the difference.