Steps of wet treatment of coffee after picking

As harvest season draws to a close, I would like to add an overview of the coffee processing process. In this blog post, I will introduce you to the wet processing steps after coffee picking in Yunnan.

After harvest, the berries are either sent directly to a pulp collector (if the harvest is done well) or collected in a siphon tank filled with water. Berries are washed and separated in this tank. The stones sink to the bottom of the jar and are manually picked up when the jar is empty. Lighter berries will float on the water, along with leaves and twigs, using a hedge to remove them. The berries suspended in the water are then recovered by a siphon and transported to a pulp collector.

The purpose of pulp removal is to separate the pulp from the seed coat of the coffee bean. The pulp is removed by squeezing the berries. Because ripe berries contain a lot of water, coffee beans with skins pop out easily when the berries are squeezed.

The next step is mucilage removal to remove the mesocarp residue that clings to the seed coat after pulp removal (see illustration of coffee berry parts). Since this slime is insoluble in water and adheres tightly to the seed coat, it cannot be removed by simple washing alone, so it needs to be either washed after fermentation or removed by strong friction in a device called a slime remover.

In Yunnan, this process is mainly accomplished through natural fermentation. The term fermentation is not 100% accurate because no biochemical reactions take place inside the coffee beans. It is more appropriate to refer to this process as mucilage removal by biochemical reaction or hydrolysis. This reaction is caused by enzymes naturally present in coffee berries.

The fermentation time varies from 6 to 72 hours, depending on conditions such as temperature, amount of mucus, and concentration of digestive enzymes. Coffee beans can be cleaned by placing the coffee in a fermenter until the mucus is completely broken down.

The key to coffee quality is to stop fermentation at the right time, because excessive fermentation can lead to so-called stinky coffee beans. If fermentation is insufficient or too slow, butyric acid or propionic acid is produced, both of which adversely affect coffee quality.

Fermented coffee with skin needs to be washed immediately after fermentation to terminate the process. However, after removing mucus and before washing, it may be necessary to soak the coffee with skin in water for 12 to 24 hours to reduce the bitter taste of the coffee beverage.