Wet, dry, 72 hour or none at all. What does it all mean?
Fermentation is the part of coffee processing that enables the removal of the mucilage (sticky flesh layer) from the coffee seed. The majority of the coffee’s sugars are found in the sticky mucilage, the process of fermentation is where sugars are transferred from the coffee fruit to the seeds. Some farmers and processors are now experimenting with different methods of fermentation to try and increase the complexity of flavour of their coffees. Some coffee growing regions don’t use any fermentation at all, but rather a mechanical mucilage remover.
Traditional washed coffee includes a period of wet fermentation. This means the pulped coffee is soaked in a tank of water. The water allows the sticky mucilage to degrade slowly. Wet fermentation is undertaken in 12-72 hours depending on weather and the farmer’s desired outcome. The hotter the weather, the shorter the fermentation period. Usually the coffee can be rubbed together in the farmer’s hands to indicate whether fermentation is complete or not. Fermentation is complete when the coffee has no stickiness or sliminess left. Longer fermentation times can lead to increased acid complexity (eg. Acetic, Citric and Malic). The coffee is then rinsed and put out to dry. This style of processing encourages clean, bright acidity and sweetness.
Dry fermentation is a much more aggressive process. Coffee is pulped, then placed into large soaking tanks. No water is added, so the coffee is more exposed to the effects of oxygen and weather conditions. Common dry fermentation periods are 8-24 hours. This fermentation style creates a coffee with more body, similar to a honey processed coffee.
Indonesian coffees are often fermented in bags during transport from the smallholders to the drying areas. In Sumatra, it is illegal to sell coffee cherries. This law was made in order to reduce the amount of coffee cherry theft. Thieves wouldn’t own coffee processing equipment, so would be unable to sell stolen cherry at the local market. This means that smallholder farmers have pulping equipment on site. The pulped cherry is placed into bags and taken to the local drying station. The pulped coffee sits in these bags overnight and is then laid out to dry on concrete patios.
Some coffee farmers In Central and South America are using a process of mechanical mucilage removal. This is a spinning cylinder that intensely rubs the mucilage off the seeds.