Fermentation

A worker opens the valve to fill the fermentation tanks with water during the harvest at Kayon Mountain's Sawana milling site, Ethiopia
A worker opens the valve to fill the fermentation tanks with water during the harvest at Kayon Mountain's Sawana milling site, Ethiopia

Fermentation in coffee processing traditionally referred to the stage in wet-processing of the coffee. We now understand that fermentation happens more broadly in nearly all processes, including honey (pulp natural) and dry-process (natural) methods.

For traditional wet-processing, a key stage is ferment the coffee mass in bulk, after the outer skin layer is removed by the pulper machine. The purpose is to break down the fruit (mucilage) layer that tenaciously clings to the coffee seed, so it can be washed off. Fermentation should be done soon after picking the cherry from the tree for traditional wet-process results, and lasts 8 – 24 hours depending on temperatures and other factors. When you feel the slimey coffee and the parchment layer feels rough like sandpaper, the coffee is ready to wash.

For wet process coffee, fermentation dynamics, washing and subsequent drying are part of creating the classic “clean cup” flavor profile. There is debate about whether wet-process fermentation contributes flavor to coffee, or to what degree. But historically the main reason behind wet-process fermentation was to help release the fruity mucilage layer from the parchment layer. The fruit coating the outer parchment skin is broken down with the action microbes, enzymes and acidfication.

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