What is fermentation in coffee processing?

Fermentation is what happens when bacteria and yeasts get to work on the sugars and organic compounds in the coffee fruit. It's one of the most influential variables in processing - and one of the least visible to anyone who hasn't stood at a washing station watching it happen.

In washed processing, fermentation has a practical function: it breaks down the sticky mucilage layer after pulping, making it possible to wash the beans clean. Leave pulped coffee in a tank for 12–72 hours and the mucilage releases. But what happens chemically during those hours isn't neutral - acids develop, volatile compounds form, and the bean absorbs things that shape how it ultimately tastes in the cup.

In natural and honey processing, fermentation happens more slowly as the drying fruit interacts with the bean over days or weeks. In experimental methods - anaerobic, lactic, carbonic maceration - it's actively manipulated. Well-managed fermentation produces complexity, sweetness, and clarity. Poorly managed fermentation - too long, too warm, contaminated - produces sour, vinegary, or phenolic faults that can ruin an otherwise exceptional lot.