During fermentation naturally occurring microorganisms interact with sugars in the mucilage in complex ways that can have both subtle and dramatic impacts on flavour. Fermentation in coffee production has long been employed by farmers as a pragmatic means of breaking down pectins to allow the mucilage to be removed before washing or drying. There are numerous chemical changes that take place during fermentation, and many byproducts of the process that can lead to distinct flavour outcomes. There is still much to be studied and learned about the manipulation of fermentation parameters and how the complex flavours in coffee can be both preserved and altered by the application of more deliberate and precise control during this stage of processing. With natural coffees, fermentation takes place while the coffee cherries dry. Since the complex microbiology is transpiring inside the fruit and cannot be easily observed, farmers have less control over fermentation in the dry process than the wet process.
In wet milling, coffee covered with sticky mucilage rests in tanks for periods of 12-36 hours or more as the mucilage breaks down. Growers may choose to ferment in wet or in the open air. The duration of the process depends greatly on ambient environmental conditions, especially temperature. The fermentation process ends when the coffee is washed with water and agitated to remove the remaining mucilage from the seed. Some growers will also subject their coffee to underwater soaking for a period of time after washing, to give the coffee time to rest before drying and to ensure that every last bit of mucilage is removed.
Each region tends to have a single dominant drying process that evolved in response to a blend of influences, including history, culture, climate and geography. In much of Latin America, drying coffee on concrete, stone, or brick patios is normal, although many farmers without access to patio space still dry coffee on plastic tarps or on rooftops, and some have adopted the raised bed approach that is standard in most of East Africa.
Raised bed technology is simple — waist-high wooden table frames with screens on which wet parchment or fresh cherry is spread out to dry — and often yields the best results. These elevated beds give farmers the ability to regulate drying rates more effectively than they can when coffee is resting on concrete patios or other surfaces that retain heat and don’t allow for good air circulation.
There are an endless number of variations on these basic drying technologies that involve permanent and temporary structures for shade, racks for portable drying beds, greenhouses with vents or electric fans to regulate airflow and ambient temperature, even buildings with drying racks in the rafters and roofs on tracked wheels that open and close to regulate the coffee’s exposure to sun.
There are also a range of mechanical drying processes. Some of these, like the Guardiola-style drums fired by wood, gas or by-products of the coffee milling process, are fairly common at larger farms and mills with a high volume of throughput. They are also used most often in places where the environmental conditions are particularly humid and make outdoor drying difficult to execute consistently. Others, fabulously varied, are homespun innovations.
Regardless of how our coffees are processed and dried, they must all be dry milled before they can be exported. At minimum, the dry milling process involves hulling the parchment shell from the green seed within, and some degree of sorting to eliminate defects and separate coffees by grade. Usually these separations happen in three stages: first by size, then density, and finally by colour. These days the final separation by colour is typically done using optical electronic colour sorting machines, although there are still many places where coffees are sorted by mechanically or even by hand, bean by bean. The precision with which coffees are milled varies a lot depending on the desired result. For top quality lots, an especially rigorous standard is applied and often can mean a substantial reduction in volume, which adds to the cost of the coffee. Sorted and hulled coffee is stored in GrainPro bags or vacuum-packed to preserve its quality during its long trip to our Roasting Works.