The Process at the Wet Mill

May 5, 2015 13973
Coffee cherry pulp is filtered through a water sluice in the wet mill. Photos by Victor Biro

The wet mills are sometimes known as coffee washing stations, and when you know a bit about the process at the wet mill, it’s easy to see why.

Once the coffee cherries have been harvested and delivered they are fed into the pulping machine that separates the cherry skin and fruit from the seed (which is the coffee bean). The pulp that is produced is washed away and the coffee bean flows into a fermentation tank.

In the fermentation tank naturally occurring enzymes in the coffee will start to break down the sticky mucilage layer (the parenchyma) that coats the beans.

The amount of time the coffee spends in the fermentation tank depends on environmental factors such as the climate, as well as the machine that was used to pulp the cherries – the newer, more economical machines remove more of the mucilage layer so the coffee requires less time in the fermentation tank, a matter of hours rather than days.

Awasa, ETH., 28 Nov 2014 -

After fermentation the coffee is washed. Here laborers brush the green coffee through sluices of water to remove the remains of the mucilage. Other unwanted bits and pieces – such as over ripe beans – will also float away.

The beans are then taken off to be dried and sorted on long rows of drying tables. The beans are left to dry naturally out in the sun.

Ideally the water content in the coffee beans will be reduced to less than 12%. The moisture level can be measured by machine, but the wet mill workers often intuitively know when the coffee has been sufficiently dried. Workers will cover the drying racks with tarpaulin if it looks like it’s going to rain.

Workers turn the beans regularly by hand to aid the drying process. They also remove bad beans – ones that are blackened or insect-damaged. Despite the washing and fermenting, the beans on the drying tables still have a membranous coating on them, known as the endocarp. In this state, the beans are known as parchment.

The wet mills have a storehouse where the dried coffee can be kept, packed in burlap sacks. Then finally the beans will be taken into town to be milled – a process that will remove the parchment from the beans. The fresh green coffee is then ready for sale and export.

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