As the coffee harvest season draws to a close, this article systematically reviews the wet-processing steps after picking in Yunnan. From floating fresh cherries in water for separation and removing the pulp, to the key mucilage removal and fermentation stages, it explains step by step the actual operations and quality control points of washed processing. Fermentation duration fluctuates between 6 and 72 hours, influenced by temperature, mucilage volume and enzyme concentration, and the timing of termination directly relates to the flavor direction of the coffee. The article also explains the negative effects of under-fermentation or over-fermentation on coffee quality, as well as the regulating effect of the soaking step on bitterness, helping coffee enthusiasts understand the craft logic and flavor causes behind the washed processing of Yunnan arabica coffee.
The harvest season is about to wrap up, so I'm taking this opportunity to add an article sorting out the coffee processing flow. In this piece, I'll walk everyone through the various stages of wet processing carried out locally in Yunnan after coffee cherries are picked.
After the fresh cherries are brought back, if quality control is in place they can be sent directly into the pulping equipment; alternatively, they can first be gathered into a siphon tank filled with water. In the tank, the berries undergo washing and grading separation. High-density stones sink to the bottom and are picked out by hand when the tank is drained. Lower-density berries float on the surface and are removed with a skimmer together with fallen leaves and twigs. The berries still suspended in the water are then collected through a siphon tube and sent to the pulper.
The goal of removing the pulp is to separate the pulp from the parchment of the coffee bean. The specific method is to apply squeezing pressure to the berries. Ripe berries have a high water content, so when squeezed, the parchment-covered coffee beans easily pop out.
Next is removing the mucilage, that is, removing the mesocarp residue that still clings tightly to the parchment after pulping (see the coffee berry structure diagram). This mucilage is insoluble in water and firmly attached to the parchment, so it cannot be removed by rinsing with clean water alone; it must be stripped off through fermentation followed by washing, or by strong friction using equipment called a mucilage remover.
In Yunnan, this step is mostly accomplished through natural fermentation. Strictly speaking, the term fermentation is not entirely accurate, because no biochemical reaction occurs inside the coffee bean. A more apt description is that biochemical reactions or hydrolysis are used to remove the mucilage, and this reaction is triggered by enzymes naturally contained in the coffee berry itself.
Fermentation duration varies with conditions, ranging from as short as 6 hours to as long as 72 hours, depending on factors such as temperature, mucilage quantity and digestive enzyme concentration. The coffee is placed in a fermentation tank, and once the mucilage is completely broken down, it can move on to the washing stage.
The key to ensuring coffee quality lies in accurately grasping the timing to end fermentation, because over-fermentation gives rise to so-called stinker beans. Conversely, if fermentation is insufficient or proceeds too slowly, butyric acid or propionic acid is produced, and both acids have a negative impact on coffee quality.
After fermentation ends, the parchment coffee needs to be washed immediately to halt the fermentation process. However, after the mucilage has been removed and before the formal washing, the parchment coffee can also be soaked in clean water for 12 to 24 hours, which helps reduce the bitterness in the coffee beverage.
The above content is compiled by CoffeeHunters, a coffee news website.
