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.
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.
Most fine coffee goes through a beans sorter machine 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.
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