Coffee Bean Moisture Measurement
Before shipment, coffee is dried and a coffee moisture meter is used to measure coffee bean moisture. Coffee must be dried from approximately 60% moisture content to 11-12% moisture content. Coffee is typically dried on large patios made of asphalt or cement and then transferred to mechanical dryers. The coffee on the drying patios is shifted every 30-40 minutes and is shaped into long rows of no more than 5 cm in height. Next to each row is open ground, which is warmed and dried by the sun. The coffee is then shifted onto the dry portion of the patio, and the section where it was previously is now allowed to dry in the sun. This helps accelerate the coffee drying process and prevents fermentation and moldy beans from developing. This method is widely used in Brazil, but less widely used in Guatemala or Costa Rica where the coffee is more often piled perpendicularly to the old piles. Drying coffee solely by patio takes 6-7 days for washed coffees, 8-9 days for pulped naturals (semi-washed), and 12-14 days for natural (dry-processed) coffees. This is why coffee beans are typically dried on a patio until they reach a moisture content of 15% and are then transferred to mechanical dryers. Once the coffee reaches a 25% moisture content or less, it can be piled at night and covered with cotton cloths to allow the coffee to breath. If it rains, these piles can also be covered with plastic. Coffee should not be covered with burlap sacks since this will impart a distinct burlap flavor and aroma to the coffee.
Coffee Drying Stages
In a study done in Kenya, Kamau reports that there are six stages to drying coffee.
1) Skin drying. Moisture 55-45%.
2) White Stage drying. Moisture 44-33%.
3) Soft Black stage. Moisture 32-22%.
4) Medium Black Stage. Moisture 21-16%
5) Hard Black Stage. Moisture 15-12%
6) Fully dry coffee and conditioning. 11-10%.
In this study he found that sun drying coffee for stage three is mandatory for coffee quality. He also reports that as long as the temperature was between 40-50°C–which means a bean temp of 35°C–then the coffee quality will not be seriously compromised during the other coffee drying stages.
The final two coffee drying stages (15-11% moisture) take just six hours at 40°C in a mechanical dryer.
Coffee Drying Equipment
There are several coffee dryer systems available. Many older dryers are converted grain dryers that are not as efficient as the new horizontal barrel dryers. The new coffee dryers are designed to mix the coffee evenly to ensure uniform drying. Drying coffee by using mechanical dryers accelerates the slowest part of the coffee drying process (15-11%) and helps prevent fermentation. In some environments that have a high humidity the entire drying process must take place in mechanical dryers. Mechanical dryers should never be set higher than 40-45°C and this question should be asked before buying any coffees that have been mechanically dried. At higher temperatures the germ is killed and the flavor potential of the coffee is ruined. At extreme temperatures the bean crystallizes, and when smashed with a hammer, will break like glass.
Other Machinery for Drying Coffee
The best, but least utilized method of drying coffee is by using drying tables. In this method the pulped and fermented coffee is spread thinly on raised beds, which allows the air to pass on all sides of the coffee. The coffee is mixed by hand and the drying that takes place is more uniform and fermentation is less likely. Most coffee from Africa is dried in this manner and select coffee farms around the world are following their lead.
A relatively new method of drying coffee is solar drying. Solar coffeee dryers are more economically sound than mechanical coffee dryers because they don’t rely on fossil fuels or electricity. They are also more efficient than the patio drying technique because their hotter drying temperatures dry coffee beans at a faster rate. While solar coffee dryers have great potential for saving energy, they are not widely used today. For more information about solar coffee drying systems, visit EcoVentures International or the MesoAmerican Development Institute.