Dry process is a method to transform coffee from the fruit of the coffee tree to the green coffee bean, ready for export. Dry processing is the original method, and the wet process was devised later (as well as the very recent pulp natural process). It is a simple method, using less machinery and more hand labor, and has been a tradition in some growing origins for centuries. It risks tainting the coffee with defect flavors due to poor handling, drying, or ineffective hand-sorting. In dry processing the fruit is picked from the tree and dried directly in the sun or on raised screens, without peeling the skin, or any water-based sorting or fermenting. The dried coffee turns to a hard, dark brown pod, and the green seed is torn out from the skin and parchment layers in one step, or pounded out by hand. Because there is no chance to skim off floating defects, or removed under-ripes as with the wet process, most defects must be removed visually, by hand. Dry process coffees generally have more body and lower acidity than their wet process counterparts, with more rustic flavors due to the long contact between the drying fruit and the seed. They also can have more defects, taints, and lack of uniformity both in the roast and in cupping. A dry process coffee is sometimes referred to as natural coffee, full natural, or traditional dry process, or abbreviated DP.
Pulp natural is a hybrid method of processing coffee to transform it from the tree fruit to a green bean, ready for export. Specifically, it involves the removal of the skin from the coffee, like the first step of the wet process, but instead of fermenting and removing the fruity mucilage, the coffee is dried with the fruit clinging to the parchment layer. Pulp natural can be performed with a traditional pulper, or with newer forced demucilage equipment, which allows for greater control of exactly how much mucilage is left to dry on the coffee. Pulp natural coffees tend to have more body and less acidity than their wet process equivalents, and can have a cleaner, more uniform quality than full natural dry-process coffees.
Wet process coffee (or washed coffee) is a method to transform the fruit from the tree into a green coffee bean for roasting. This process uses water at the wet mill to transport the seed through the process, allowing or the removal of defects that float to the surface. In traditional wet processing, the wet mill is where the coffee is pulped (the outer skin removed), floated in water (to remove defective beans), fermented (to break down the fruit mucilage layer), washed ( to remove the fruit) and dried on a patio, a screen (raised bed), or a mechanical dryer. At this point green coffee seed is inside an outer parchment shell, rested for a period of time (reposo) then milled at the dry mill into the green bean. Wet processing often produces a brighter, cleaner flavor profile, with lighter body than dry process coffees or the hybrid natural process. Wet process coffees are referred to also as washed coffees, or fully washed. Note that the coffee seed is not fermented in this process, just the other fruit layer between the skin and the parchment shell. This is a natural action of peptic enzymes in the coffee. In different countries they might use a submerged wet fermentation, or a water-less dry fermentation, which is a faster method.
Wet-hulled process is a hybrid coffee method used in parts of Indonesia, especially Sumatra. It results in a dark, opal-green coffee with little silverskin clinging to it, and a particular low-acid, earthy, heavy body flavor profile. In this method, the farmer picks ripe coffee cherry, pulps off the skin and either dries it immediately for one day, or lets it sit overnight in a bucket (with our without water), then washes it the next day and dries it. In either case, the coffee is partially dried with some or all of the mucilage clinging to the parchment-covered seed. It is then sold at a local market to a coffee processor. They receive coffee at 40-50% mositure content, then dry it to 25-35%, and run it though the wet-hull machine. Friction strips off the parchment, and the bean emerges swollen and whitish-green. Then it is dried on the patio down to 11-14% moisture, ready for sorting, grading, bagging and export.