The fruit is left on the plant to fully dry from the sun and air movement. This method might be used in less mechanized areas or at the end of the harvest due to its unpredictable nature and increased chance of defect formation.
The coffee is spread on the patio to a depth of 2-3cm for washed coffees and 5-6cm for natural process. To facilitate even drying it is necessary to constantly turn (rake) the coffee throughout the day (approx. 15-17 times). Initial drying or “skin drying” reduces the moisture from 55-60% down to 20-25%. At this point the coffee can be layered slightly deeper. The second stage of drying reduces the moisture from 20-25% down to 10-12%. Care must be taken as to not dry the coffee too quickly or slowly as each situation will affect the end cup quality. Average time needed to completely dry natural coffee is between 15 and 20 days. To complete the drying process for washed coffee the average is between 8 and 12 days. Environmental conditions, patio material and attention to detail will directly affect the process.
Vertical Dryers and Horizontal/Rotary dryers are the two styles predominately used. They both are quite large and can dry many thousands of pounds of coffee. A Rotary dryer is a long horizontal cylinder that rotates around a central tube through which heated air is blown. The cylinder rotates about 2-3 rpm. A vertical dryer uses a grain elevator to raise the coffee to the top of a chute. Once at the top coffee falls down the chute through streams of heated air blown through horizontal perforations in the chute. Similar to patio drying, the coffee dried in a mechanical dryer should not be dried too quickly (too high of heat) or too slowly (too low of heat), as each situation will lead to poor quality coffee. The heated air used in mechanical dryers should never reach >800C and bean temperatures should not exceed 400C.
Raised Beds and Solar Drier
A wooden frame that suspends a wire mesh above the ground about waist height. One desirable feature of the raised bed is that air movement is both above and below the coffee, drying the coffee evenly. In a Solar Dryer raised beds are placed inside a greenhouse type structure. This method is typically used in areas where patio drying is difficult due to weather conditions. Ventilation is important so as to not over heat the coffee and minimize condensation buildup.