Dry process seems simple: Pick the fruit, lay it out in the sun until it turns from red to brown to near-black, and then hull off off the thick, dried outer layer in one step to reveal the green bean. It is a method suited to arid regions, where the sun and heat can dry the seed inside the intact fruit skin.
It's often referred to as "natural coffee" because of its simplicity, and because the fruit remains intact and undisturbed, a bit like drying grapes into raisins. Since it requires minimal investment, the dry process method is a default to create cheap commodity-grade coffee in areas that have the right climate capable of drying the fruit and seed.
But it’s a fail in humid or wet regions. If the drying isn't progressing fast enough, the fruit degrades, rots or molds.
Dry-processing of coffee can also be wildly inconsistent. If you want a cleanly-fruited, sweet, intense cup, dry process (DP) takes more hand labor than the wet process. Even the most careful pickers will take green unripe or semi-ripe coffee off the branch as they pick red, ripe cherry. If these are not removed in the first days of drying, the green turns to brown that is hard to distinguish from the ripe fruit.
Interestingly, there are arid zones where the coffee starts to dry while on the tree. Areas of Brazil in particular have very distinct rainy and dry seasons, and in the dry times when coffee is harvested, the heat is intense enough to parch the coffee fruit while is is on the branch. These are called "pasas" in Latin American areas, raisins, and they have a leathery fruit texture. Cherry forgotten on a tree will also dry, sometimes fully to the stage where the seed is rock hard. These are often called "pods" and are considered a defect when they appear in sorted, hulled, ready-to-export coffee.
For areas that don't have intense heat and distinct seasons, the coffee is picked ripe, just as a wet-process coffee would be. The coffee is laid on raised drying beds that allow air to circulate around the fruit. The beds use metal screens, or metal bands that support a nylon woven cloth (preferable). Sometimes locally woven grass mats are used.
As the cherry dries, the skin and attached pulp (exocarp, mesocarp) attaches to the parchment layer (endocarp). The mucilage layer that is degraded by fermentation in the wet process is sandwiched in-between. The silverskin (chaff when roasted) often stays attached to the bean.
In wet-process (WP) the fruit skin is peeled off by the pulper, and the remaining fruit is fermented and washed off. But in DP the coffee seed dries encased by the thicker layer of fruit skin. The active transference of water and ionic bonding with compounds in the fruit skin, is a likely way that the fruited notes of a DP coffee are infused into the drying seed.
The bulk of DP coffee in the world is commercial grade coffee, with dirty/earthy and musty flavors. Many farmers receive little for their DP coffee so it receives little care: The coffee is dried on tarps or directly on dirt. Coffee that falls to the ground or is re-wet while drying is recovered and sold to traders. It takes incredible intention and care to create a premium DP coffee.
All DP is milled the same: With a relatively crude purpose-designed huller machine, the thick dried fruit skin is removed with friction burrs. The skin can be fanned and cleaned to use as a tea (called Qishr in Yemen, Geshir in Ethiopia).
Because DP skips all the steps in WP to remove defect beans (floating the wet parchment, the “criba” in the pulper, density grading in washing channel, etc.) all defect beans must be removed visually, or in the dry mill. After the critical density table step, coffee is again hand-picked. Even the best DP coffee has a less-uniform look because of the silverskin still attached to many beans.
Natural coffees are perhaps the original method to process coffee. If the first coffee grown as commercial crop, for trading, was in Yemen, the climate is well-suited to dry processing. The western parts of Ethiopia such as Harar are traditional dry-processing areas. Older coffee-growing areas of Brazil as well as newer ones (Cerrado) have distinct seasons ideally suited to dry process method. If a farm can dry coffee from ripe cherry to hard, dried pod in 20 days, it is probably well-suited to DP methods.
Generally speaking, we will cup but usually not buy naturals from areas that cannot dry the coffee in the proscribed amount of time. Naturals from Central America present a good example, although we are always open to new areas and enterprises that can do it right.
In recent years, many areas that are do not traditionally produce dry-processed coffee have began to do so for a variety of reasons. The spread can be attributed to market demand, as many buyers find the fruity tastes of DP coffees easy to distinguish for themselves and their customers. The distinction of exactly what kind of fruity flavors are good and which are bad is an open question.
Certainly there is a matter of personal preference, which is influenced by cultural preferences as well. The distinction between clean fruit notes, winey fruit and vinegar seems universal, but on the cupping table there can be disagreements. One persons "ferment" is another persons "fruited". Go figure. I find many new buyers or those from countries with a newer coffee culture are either quick to over-praise or over-punish these fruit notes.
One experience with naturals that is not so relative to personal taste is the stability of the coffee over time. DP coffees that are not fully rested, or were too-slow to dry will tend to fade. A marginal fruit note tasted in a fresh arrival of such a coffee might be pleasant in the present, but in several months it might fade. What remains might be flat woody or earthy notes that few find attractive, unless you add sugar!
Beyond climate, a quality DP coffee requires tons of hand labor, for multiple iterations of picking. Many areas simply do not have the amount of labor available, or the labor is not affordable. In Brazil they must use only the dry mill machinery to clean the coffee of defects: Even selective hand-picking requires too much labor and would drive up the price of coffee to untenable levels.
But in this resurgence in the dry-process technique, some wonderful new coffees have come to light that have the consistency, uniformity and clean taste that rival wet-process coffees, but have complex layered fruit flavors not found in their counterparts.