Coffee beans are not beans at all in a botanical sense. They are the twin seeds of a red (sometimes yellow) fruit that grows to about the size of the tip of your little finger. Growers call these coffee fruit coffee cherries. Before the coffee can be shipped and roasted, the bean or seed must be separated from the fruit. Nature has been lavish in its packaging of the coffee seed, and removing the three sets of skin and one layer of pulp from around the seed is a complex process. If done properly, the coffee looks better, tastes better, and demands a higher price.
The worst preparation or processing would be as follows: The coffee berries are stripped — leaves, unripe berries, and all — onto the ground. This mixture is then scooped up, sifted, and dried in the sun (and sometimes in the rain, which is one of the problems with such coffees). Later the dried, shriveled fruit is stripped off the bean. Some beans may be small and deformed, shriveled, or discolored. In very poorly prepared coffee all the beans, good and bad, plus a few twigs, a little dirt, and some stones, are shipped together. The various flavor taints associated with cheap coffee — sourness, mustiness, harshness, composty taste — all derive from careless picking, fruit removal and drying.
The best preparation would run like this: The coffee cherries are selectively picked as they ripen. The same day they are picked, the outer skin is removed, exposing the pulp. The pulp-covered beans are then subject to controlled fermentation in tanks. The ferment-loosened, flabby pulp is then gently washed off the beans and they are dried, after which the last layers of skin, now dry and crumbly, are stripped from the bean by machine.
Between these two extremes — carelessly picked coffees simply put out into the sun to dry and selectively picked, wet-processed coffees — are coffees that have been dried in the old-fashioned way, with the fruit still clinging to the bean, but have been picked selectively and dried with care. These high-quality dry-processed or "natural" coffees can be superb, alive with fruity nuance.