In comparison to aerobic fermentation, anaerobic fermentation produces distinct acids such as lactic acid which can boost the coffee’s cup-profile and create remarkable flavours.
Firstly, ripe cherries are harvested and pulped (where the skin of the cherries are removed). The pulped cherries where the parchment with mucilage are still intact will be placed into an airtight stainless steel tank that has a valve which is used for degassing. In this sealed tank, the temperature is controlled and fermentation will take place for hours or even up to days, depending on the environmental conditions (in this case, 48H).
During fermentation, CO2 is released and pressure starts to build up. The pre-existing pressure and whatever oxygen that was in the tank will be released through the valve attached to the tank. The resultant pressure from the CO2 forces the juices and sugars from the fruits’ flesh into the parchment, altering the flavours of the bean greatly. This is a critical point in the process: the fermentation has to be stopped once all sugars in the mucilage have been consumed.
Thereafter, the beans are gently washed and drying has to be done quickly in order to stop further fermentation as the flavours would be affected. The beans will be dried through brick patios, raised beds or through a mechanical dryer. To ensure even and consistent drying, the beans will be turned regularly. What is left on the coffee beans then, will be a thin layer of skin known as the parchment skin, and this skin will be removed through the last step in the process which is known as hulling.
Although the cup-quality of anaerobic-processed coffees are impressive, this process is difficult and it requires good skill and expertise. Notable cupping notes from this processing method include: fragrant and fruity aroma, sweet and expressive flavour notes like honey, wine, nuts, citrus.