What is fermentation time in coffee processing?

Fermentation time is how long the coffee spends in the fermentation stage before it's washed or moved to drying. For washed coffees in tanks, typically 12–72 hours - the window in which mucilage breaks down and flavour-active compounds develop.

Get it right and fermentation adds brightness, complexity, and sweetness. Get it wrong in either direction and the results are obvious: under-fermentation leaves mucilage that won't wash off cleanly; over-fermentation produces sour, vinegary, or phenolic off-flavours that are very hard to remove downstream.

Experienced processors read fermentation directly - running a hand through the tank and feeling whether mucilage releases cleanly from the parchment. Temperature is the variable most closely watched: a tank at 30°C completes in half the time of one at 15°C. This is why altitude matters so much - cooler highland conditions allow longer, slower fermentation that tends to produce more nuanced results.