The Dry Method. In this, the oldest of processing methods, the coffee fruit is simply picked and put out into the sun to dry, fruit and all. It is spread in a thin layer and raked regularly to maintain even temperatures from top to bottom of the layer. Drying takes anywhere from ten days to three weeks, and, on larger farms, occasionally may be accelerated by putting the coffee into mechanical driers. The hard, shriveled fruit husk is later stripped off the beans by machine. In the marketplace, coffee processed by the dry method is called dry processed, unwashed, or natural coffee.
The Wet Method. Here the fruit covering the seeds/beans is removed before they are dried. The wet method further subdivides into the classic ferment-and-wash method, and a newer procedure variously called aquapulping or mechanical demucilaging. Regardless of which of these procedures is used, coffee processed by the wet method is called wet processed or washed coffee.
In the classic ferment-and-wash version of the wet method, the fruit that covers the beans is taken off gingerly, layer by layer. First the outer skin is gently slipped off the beans by machine, a step called pulping. This leaves the beans covered with a sticky fruit residue. The slimy beans then are allowed to sit in tanks while natural enzymes and bacteria loosen the sticky residue by literally beginning to digest it. This step is called fermentation. If water is added to the fermentation tanks it is called wet fermentation; if no water is added and the beans simply sit in their own juice it is called dry fermentation. The fermentation step is one of the main ways coffee mill operators can nuance the taste of the coffees they process. Dry-fermented coffees usually are more complex and sweet than wet-fermented coffees, which tend to be brighter and drier in taste.
After the fermentation step the coffee is gently washed and then dried, either by the sun on open terraces, where the thin layer of beans is periodically raked by workers, or in large mechanical driers, or in a combination of the two. This leaves a last thin skin covering the bean, called the parchment skin or pergamino. If all has gone well, the parchment is thoroughly dry and crumbly and easy removed. Coffee occasionally is sold and shipped in parchment or en pergamino, but most often a machine called a huller is used to crunch off the parchment skin before the beans are shipped. A last, optional step is polishing, which gives the dry beans a clean, glossy look important to some specialty roasters. Other roasters condemn polishing as pointless and detrimental to taste owing to the friction-generated heat it applies to the beans.
Machine-Assisted Wet Processing. The mechanical demucilage or aquapulp variation of the wet method is essentially a short cut approach which removes the sticky fruit residue from the beans by machine scrubbing rather than by fermenting and washing. This mechanized short cut is increasingly popular for two reasons, one admirable and one not-so-admirable.
The admirable reason: Mechanical demucilaging cuts down on water use and pollution. Ferment and wash water stinks, and communities downstream from coffee mills understandably object to having stinky water injected into their fisheries and water supply.
The not-so-admirable reason: Removing mucilage by machine is easier and more predictable than removing it by fermenting and washing. Unfortunately, machine demucilaging has been accused of limiting the taste palate of coffee by prematurely separating fruit and bean. By eliminating the fermentation step, the practice definitely robs mill operators of the most important expressive option they have at their disposal to influence coffee flavor. Furthermore, the ecological criticism of the ferment-and-wash method increasingly has become moot, since a combination of low-water equipment plus settling tanks allows conscientious mill operators to carry out fermentation without polluting.
The Semi-Dry or Pulped Natural Method. This procedure is practiced regularly in only two regions of the world: Brazil and certain parts Sumatra and Sulawesi. The outer skin is removed as it is in the wet process, but the troublesome sticky fruit residue is allowed to dry on the bean and later removed by machine along with the parchment skin.