How to identify over-fermentation defects in coffee?

An over-fermented coffee displays aceto-alcoholic notes — vinegar, alcohol, solvent — that are not natural coffee aromas but by-products of uncontrolled microbial degradation. In the cup, the main warning signal is a persistent vinaigrette or lacto-alcoholic aftertaste, often accompanied by a sharp, unpleasant acidity that bears no resemblance to the clean fruity acidity of quality coffee.

Over-fermentation is one of the most common defects in mid-grade coffees and can occur at three stages in the production chain. First stage: pulping. In the washed process, depulped cherries must ferment in tanks for 12 to 48 hours to break down the mucilage. If fermentation exceeds this window — due to excessive heat, high pH, or bacterial contamination — yeasts and bacteria continue producing acetic acid, alcohols, and other undesirable compounds that penetrate the bean. Second stage: drying. Natural (cherry-dried) coffees must be turned very regularly to prevent cherries resting on the ground from fermenting anaerobically in an uncontrolled way. Layers that are too thick or a drying interruption due to rain frequently cause localised over-fermentation. Third stage: green bean storage. Grain moisture content above 12 % in a warm warehouse can restart fermentation even after depulping.

In tasting, the sensory markers of over-fermentation are: fermented smell at the dry fragrance stage (vinegar or cheese note), alcoholic aroma when breaking the crust, sharp or 'vinaigrette' attack, persistent fermented aftertaste, and absence of natural sweetness. A fact few consumers know: mild fermentation defects can be confused with intentional 'funky' or 'natural' notes in high-quality natural coffees. The key difference is acidity quality: in an exceptional Ethiopian natural, fermented notes are supported by sweetness and clean fruity acidity; in an over-fermented coffee, they dominate without a pleasant counterpart.

Over-fermentation signals at each tasting stage