The drying stage is one of the most important aspects of processing. The moisture must be removed gently and consistently, not too quickly or too slowly. There is a tendency to have unevenness when the drying happens rapidly. Slow, meticulous drying tends to result in homogeneity of moisture content. Too much heat during the drying process (from mechanical dryers or direct sunlight) can damage the cell structure of the bean. Too little airflow, too many beans piled together, or humid conditions can overly extend the drying process, allowing fermentation, rot or mold to develop and negatively affect the flavor profile. Ideally, the final moisture content of properly dried green beans should settle between 10-12%. There are four typical methods of drying coffee: concrete or clay patios; raised drying beds; covered, raised beds (‘parabolic’ drying beds); and mechanical dryers.
Concrete or clay patios are most common in Central America. They allow producers to dry large amounts of coffee at a time on an easily raked surface. To implement this system, there must be large areas of flat ground and an easy way to transport the clay or concrete to construct the patio (i.e. roads are awesome and the terrain can’t be too crazy).
Sometimes the patios act as heat sinks – soaking up the heat of the day and releasing it slowly over the night. This can be an advantage because the green coffee doesn’t experience a lot of fluctuation in temperature, but it can also create too much heat. The biggest disadvantage of patios is that the green is only getting airflow on one surface. Patio-dried coffee is sometimes finished in a mechanical dryer.
Raised drying beds are made often from wood and mesh screens, at waist height. They are gaining popularity around the world with specialty coffee producers, but they were originally most common in Africa, and are sometimes called African drying beds.
The most important advantage raised beds is the airflow from above and below, increasing the airflow and even drying. For this reason, they are ideally suited to honey process or natural process, in which even drying is of the utmost importance. They have also been found to be the most perfect method to dry washed coffees.
Raised beds are also advantageous because the coffee can easily be hand sorted and turned. In addition, the wood and mesh screen needed to build the beds is easy to transport by hand or horse – ideal for mountainous conditions not suited to patios.
Parabolic Beds (Parabolic Solar Dryer)
Parabolic drying beds are not very different from raised drying beds however they are specifically designed for use in regions that do not have consistently dry conditions during harvest (i.e. Colombia). The beds look like ‘hoop house’ style of greenhouses used in the US, sometimes a little bit larger than typical raised beds, and covered with plastic. It is very important that the drying beds are open at both ends to ensure airflow. If the drying bed is enclosed, the moisture releasing from the beans condenses on the roof and drips back on the parchment.
Mechanical dryers allow producers to dry coffee more quickly, which is important on large farms. The most important factor to be controlled in mechanical drying is the temperature. It must not be set above 40˚C, because the heat damages the coffee. Mechanical dryers are usually used in conjunction with other methods of drying, such as patio drying. The coffee will be often dried on the patio for a few days, and then moved to the mechanical dryers for final drying.
Multistage Drying (Skin Drying, Shaded Pre-Drying)
As one can imagine, the sunlight at midday in Africa is hot and oppressive. In Burundi, it can cause parchment to crack and open on the drying beds. The cracks leave the green coffee beans exposed to the intense sunlight and make them more susceptible to damage. An initial shaded pre-drying stage slows the drying process. After about 48 hours of shaded drying, coffee is piled thickly to ensure slower, gentler drying. Rwanda also utilizes this process.
In Kenya, wet coffee beans are transferred from the soaking tanks to pre-drying beds (Skin Drying) where they are laid out in a thin layer to quickly remove most of the moisture. The beans complete the drying process piled on raised drying beds.
In Colombia, we recently experimented with techniques to enhance drying to protect the coffee during one of the most vulnerable moments, creating more body and higher levels of sweetness. In the initial skin drying stage, the parchment pre-dries under a canopy for 24 hours to allow air to wick moisture, ensuring more uniform drying. The parchment finishes drying over the course of 7-12 days under a parabolic solar drier, uniquely modified with an opaque roof. This modification prevents the parchment coffee from excessive exposure to direct overhead sunlight.
Another technique used in Colombia dries the coffee in stages on tiered parabolic drying beds. This innovative system utilizes airflow and natural temperature fluctuations to control a three phased drying process. In the first phase, the coffee covers the bottom tier which eases the beans into drying due to sufficient airflow, a cooler temperature and shade. After a few days, they raise the lot to the second tier to dry at a higher temperature. They complete the drying process on the top tier while exposed to indirect sunlight and the highest temperature. This is the ideal drying profile for coffee in parchment.