For the majority of coffee growers around the world harvest season is nearing an end. Much of the coffee that will be enjoyed over the next year is entering a critical phase of its journey to your cup – the milling stage.
During the dry mill stage the coffee is sun dried to prevent the beans from over fermenting or spoiling. To dry the beans they are placed on raised beds that allows airflow from both above and below (or a cool patio floor depending on the climate and environment). The beans must also be carefully monitored so they receive the right amount of sunlight and the right amount of shade. It is a laborious process because the beans must be turned during the process and moved in and out of direct sunlight based on temperature, moisture in the air, or cloud coverage (not to mention the careful attention given in regard to wind, rain, insects, etc).
Once the beans are dry they are still covered by a layer of parchment. This is a layer that coats the coffee between the bean and the mucilage. It is like a dry thin shell that either needs to be removed by hand or through a huller. The advantage of a huller is that it enables you to do large volumes quickly. The advantage of hand sorting is that it creates more jobs and is safer for the bean. In our current environment, most hulling equipment is well built and calibrated so damage to the beans is minimal to non-existent.
During the sorting phase we make sure that any defective beans and all other debris are removed from the lot so that only pure high-end coffee remains. If the lot is earmarked for instant coffee then not as much attention is given to the removal of defects. If it is a specialty grade lot then careful attention is given to remove everything but the best of the beans.
Most high end processing plants use a gravity machine and/or color sorter for the final sorting stages. The gravity machine sorts the beans by weight. The color sorter scans each individual beans to make sure that unripe/overripe beans are removed from the specialty lot.
The beans need to be separated by size because roasting different sized beans together can greatly affect the success of the roast. A larger bean will respond to the temperature variables differently than a smaller one so it can lead to an uneven roast if the beans are different sizes.
During the grading process the beans are sifted through sieves which are filled with carefully perforated holes based on internationally accepted sizes. They are numbered 8-20 with each number referring to how many 64ths of an inch the holes are. So a size 8 sieve has holes that are 8/64ths of an inch while a 20 sieve size has holes that are 20/64ths of an inch.
The sieves are stacked on top of each other and the beans are poured into the largest sieve which remains on top. They then fall through the holes until they reach the size they can no longer fit through. Even-numbered sieves are used for Arabica beans while odd-numbered sieves are used for Robustas.