Fermentation in washed coffee

Fermentation tanks at Chelbesa Washing Station, Ethiopia

Fermentation is a transformative process used in many of our favourite foods and beverages. In simple terms, it breaks down sugars into various compounds such as acids or ethanol. Coffee fermentation is no different. There are multiple methods for the fermentation stage in coffee processing, and we'll delve into our preferred method: the Washed or Wet-Process. Even within this style, variations exist, including wet, dry, 72-hour, or no fermentation at all. Let's explore the intricacies of these methods.

Fermentation is a crucial step in coffee processing, facilitating the removal of mucilage, the sticky layer attached to the coffee seed. The sugars in the mucilage fuel fermentation reactions, transforming the coffee seed's flavour profile by incorporating the by-products of these reactions into its cells.

Some farmers and processors are 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 forced mechanical mucilage remover. There is no correct way to do things, but if fermentation isn’t conducted with care, the coffee can easily be ruined, gaining fermenty, alcoholic flavours. Temperature and exposure to oxygen are the two major influencers on the speed of the fermentation reactions. The simple rule of thumb is that the colder the conditions, the slower, and more controlled the fermentation can be. Exposure to oxygen and higher temperatures speed up the fermentation reactions.

Wet fermentation

In the traditional washed coffee method, wet fermentation is employed. Here, pulped coffee is soaked in a water tank, ensuring a stable temperature environment and eliminating the impact of oxygen on the reaction. In essence, the water allows the sticky mucilage to degrade slowly. Wet fermentation is usually completed in 12-72 hours depending on the water’s temperature, and the farmer’s desired outcome. The hotter the temperature, 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 in the coffee.

Dry fermentation (aerobic)

Dry fermentation is a much more aggressive process. Coffee is pulped, then placed into tanks. No water is added, so the coffee is more exposed to the effects of oxygen and variable 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. 

Dry fermentation (anaerobic)

Coffee is pulped, then placed into soaking tanks. No water is added, but the coffee is covered in order to avoid the harsh effects of oxygen and weather conditions. Common anaerobic fermentation periods are 8-24 hours. This fermentation style usually leads to coffees with more complex, wild, fruity flavours, more similar to unwashed coffees.

Bag fermentation

Indonesian coffees frequently undergo bag fermentation during transportation from smallholder farmers to drying stations. In Sumatra, selling whole coffee cherries is illegal due to theft concerns, leading to the accidental creation of this fermentation method. The idea is that thieves wouldn’t own coffee processing equipment, so would be unable to sell their stolen coffee cherries at the local market. This means that all smallholder farmers have pulping equipment on site. The pulped coffee cherries are placed into bags and taken to the local drying station. The pulped coffee sits in these bags overnight (at minimum) and is then laid out to dry on concrete patios. The fermentation reactions that occur within these bags is completely uncontrolled or monitored and often lead to unpleasant flavour characteristics like phenol, which tastes strangely medicinal or rubbery, like eating band-aids.

No fermentation

Some coffee farmers In Central and South America are using a process of forced mechanical mucilage removal. This is a spinning cylinder that aggressively rubs the mucilage off the seeds. This can sometimes damage the coffee seeds, or leave small amounts of mucilage on the coffee. If this process is done carefully, the resultant coffee can taste clean and delicate.

While we haven't covered all the nuanced fermentation styles here, hopefully this overview provides a grasp of the basic terminology and complexity involved in the coffee fermentation process. For the majority of farmers, fermentation is a necessary part of coffee production and is viewed as something that should be conducted as efficiently as possible, rather than doing it for the desired flavour profile. For example, farmers that produce washed coffees may have better access to clean water than those that use the Unwashed method. Farmers who want to sell specialty coffee may process their coffee in multiple ways to try and attract higher prices and more discerning customers. Whether the process is well thought through, or done for necessity, it’s an important part of the process to understand, as it has a significant impact on the flavour of the coffee beans we buy, sell and brew. The more you, the consumer, understand about the processes involved in the coffees you purchase, the more likely you are to be able to pick the best coffee for your tastes.