The final steps in coffee processing involve removing the last layers of dry skin and remaining fruit residue from the now dry coffee, and cleaning and sorting it. These steps are often called dry milling to distinguish them from the steps that take place before drying, which collectively are called wet milling.
Removal of dried fruit residue. The first step in dry milling is removing what is left of the fruit from the bean, whether simply the crumbly parchment skin in the case of wet-processed coffee, the parchment skin and dried mucilage in the case of semi-dry-processed coffee, or the entire dry, leathery fruit covering in the case of dry-processed coffee. The machines that do this range from simple millstones in Yemen to sophisticated machines that gently whack at the coffee.
Sorting by Size and Density. Most fine coffee goes through a battery of machines that sort the coffee by density of bean and by bean size, all the while removing sticks, rocks, nails, and miscellaneous debris that may have become mixed with the coffee during drying. First machines blow the beans into the air; those that fall into bins closest to the air source are heaviest and biggest; the lightest (and likely defective) beans plus chaff are blown in the farthest bin. Other machines shake the beans through a series of sieves, sorting them by size. Finally, an ingenious machine called a gravity separator shakes the sized beans on a tilted table, so that the heaviest, densest and best vibrate to one side of the pulsating table, and the lightest to the other.
Sorting by Color. The final step in the cleaning and sorting procedure is called color sorting, or separating defective beans from sound beans on the basis of color rather than density or size. Color sorting is the trickiest and perhaps most important of all the steps in sorting and cleaning.
Color Sorting by Eye and Hand. With most high-quality coffees color sorting is done in the simplest possible way — by hand. Teams of workers, often the wives of the men who work the fields, deftly pick discolored and other defective beans from the sounds beans. The very best coffees may be hand-cleaned twice (double picked) or even three times (triple picked). Coffee that has been cleaned by hand is usually called European preparation. Most specialty coffees, since they are whole bean and consumers see what they get, are European preparation.
Color Sorting by Machine. Sophisticated machines now can mimic the human eye and hand. Streams of beans fall rapidly, one at a time, past sensors that are set according to parameters that identify defective beans by value (dark to light) or by color. A tiny, decisive puff of compressed air pops each defective bean out of the stream of sound beans the instant the machine detects an anomaly.
These machines are not widely used in the coffee industry for two reasons. First, the capital investment to install these delicate machines and the technical support to maintain them is daunting. Second, and perhaps most importantly, sorting coffee by hand supplies much-needed work for the small rural communities that cluster around coffee mills. The vision of huge rooms filled with women and a scattering of teenage boys patiently picking through piles of green coffee may offend urbanites, but the economic suffering caused by replacing these women with machines and a highly paid technician from the city is not a comfortable alternative either, particularly in small rural communities with strong communal values.
On the other hand, computerized color sorters are essential to coffee industries in regions with relatively high standards of living and high wage demands, places like Brazil and Hawaii, for example. Readers who have seen television depictions of the slums of Rio may doubt that a labor shortage exists in rural Brazil, but it does. The main areas of coffee production in Brazil are quite prosperous, with a per capita income approximately equal to Belgium.
At the other extreme of the coffee/economic spectrum is Yemen, where even the usual battery of machines that sort coffee by density and size are unknown, and hand sorting and cleaning is the only sorting and cleaning this wonderfully idiosyncratic coffee receives.