How does the sun-drying process work?

Sun-drying is fundamentally a mass-transfer process where moisture moves from the interior of the coffee seed to the surface and then into the surrounding air. For the producer, this is a high-stakes balancing act: the goal is to reach a target moisture content of 10–12% to prevent germination or microbial spoilage, while avoiding the 'rollercoaster' of environmental variables. Unlike mechanical drying, which allows for precise control over temperature and airflow, sun-drying is at the mercy of local microclimates. A common misconception is that sun-drying is merely 'leaving coffee out'; in reality, it requires constant labor. Producers must manage bed depth, rotate beans frequently to prevent clumping, and cover or move the coffee during rain or high humidity to avoid uneven drying or mold. The mechanism relies on the synergy between solar heat—which increases the vapor pressure within the bean—and airflow, which carries the evaporated moisture away. If the air is stagnant or too humid, the moisture remains trapped, leading to quality degradation. While mechanical dryers are increasingly used to mitigate the risks of unpredictable weather or to finish the drying process, sun-drying remains the cultural and economic backbone for smallholder farmers globally. It is not just a method of preservation; it is a delicate phase where the final sensory profile is locked in. When executed with precision, it maintains the integrity of the bean's cell structure, which is vital for the high-scoring specialty coffees that define the industry's top tier.