Control of the air temperature is critical towards the end of the drying process because the coffee starts to gain temperature rapidly
In most types of dryers the coffee will not heat beyond 30°C while the moisture is high even though the air temperature may reach 90°C. It is widely accepted that the color and quality of Robusta coffees benefit from a high air temperature at the initial stages of drying. Air temperature must be lowered as drying progresses. Control of the air temperature is critical towards the end of the drying process because the coffee starts to gain temperature rapidly.
The damage caused by overheating depends on coffee moisture content, the temperature and the time coffee is exposed to overheating. The damage to beans may take several forms: parchment cracking, bending, bleaching, undue widening of the center cut, toasted beans, etc., all of which lead to quality losses. Parchment cracking exposes the beans to attacks by microorganisms during storage. Bent beans indicate severe overheating that some claim causes sour flavors and a flat cup. Overheating at the early stages of drying without a sufficient air flow to remove moisture may cause the stewing of coffee that can give it a cooked flavor. Beans that are overheated may have their surface crystallized, so that moisture can no longer leave the interior of the bean. This impermeable surface will crack in storage and the bean will bleach as moisture gradually moves out. The speed of drying cannot be increased above the speed at which moisture migrates from the center to the periphery of the bean. The speed at which water migrates in response to the temperature gradient artificially created is limited. This is a physical characteristic of the coffee beans and it cannot be changed. Attempts to accelerate drying beyond accepted limits will always leave moisture in the center of the bean. This moisture will move out later, which will have a negative effect on the aspect and the quality of the coffee. Slow drying of coffee is a good solution to homogenize coffees received with uneven moisture contents. [ Cara Publis Jurnal , 2011]
As a rule, coffee that is dried more slowly will have a more homogeneous moisture content and a more uniform color because longer drying grants the beans more time to exchange moisture and to equalize their moisture content. In recent years some experts have associated metallic tastes in brewed coffee with unduly short drying times. Evenly dried coffees fetch better prices because they have a more uniform color, a better aspect and roast better. It is highly desirable to have coffees with uniform moisture content at the end of the drying process. The fact that the moisture meter indicates 12% has nothing to do with homogeneous drying because meters measure average moisture. There may be beans with moisture levels ranging from 9 to 15% although the average moisture may still read the desirable 12%. [ Cara Publikasi Jurnal Internasional ]
As a general, Coffee moisture remains at 12% when air moisture at storage is between 60 and 65 %. In humid areas it is usual to dry coffee to 10 or 11% to increase storage time before coffee picks up moisture from the air. In other areas drying stops at 13 or 14% to account for moisture losses in hulling. Moisture levels below 10 or 11 % increase the breakage of beans at hulling, especially with Robustas that are more brittle. Moisture contents above 12 % increase the risks of quality and weight loss in storage.
Grading. Few countries export ungraded coffee today, and even those that do still remove oversized and very small beans in an operation that requires graders. Size grading generates improved operational efficiency and additional revenues. In some countries color defects are still sorted by hand, with workers sitting on the floor, at individual tables or on either side of long tables equipped with mobile belts. When sorting is done by hand, workers may also sort defects other than off-color beans. There is no technical reason to color sort by hand instead of using machines. Hand sorting is still performed in a few countries in order to maintain employment or because labor is still inexpensive.
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