The combination of high-temperature and natural-air drying procedures has been proposed as a feasible method to dry natural and pulped coffee with moisture contents ranging between 53 and 65% w.b. to approximately 14% w.b. The high-temperature drying stage was carried out in a fixed-bed drier operating at 50°C, and was used to reduce the moisture content at harvest to values in the range from 16 to 26% w.b. Subsequently, natural-air drying in a separate bin was successfully used to complement the drying by further reducing the moisture content of the product to 14% w.b.
The drying curves of the coffee located in the top layer of the bed, the time versus temperature and time versus static pressure histories of the drying air at several locations in a total bed depth of 2•0 m were investigated. The results indicated that under appropriate weather conditions, it is possible to use natural air to dry natural coffee from moisture contents as high as 23-14% w.b. using an airflow rate of about 8 m/min m without providing significant supplemental heat to the drying air. However, it is concluded that the ambient-air drying of natural coffee with moisture content above 26% w.b. can be a hazardous procedure and should not be recommended under poor weather conditions. It was also evident that the low-temperature stage of combination drying is very effective in drying fairly high moisture (37% w.b.) pulped coffee to a moisture content considered safe for long-term storage, i.e. 13% w.b.