What does a mouldy coffee taste like?

A mouldy coffee is a coffee whose green bean underwent fungal growth during drying or storage. The cup shows notes of wet soil, cellar mushroom, damp cardboard, wet leather or humus. It is one of the seven major SCA defects, almost always disqualifying for the specialty threshold.

Technically, mould appears when green coffee sits above an activity-of-water aw > 0.70, which translates to roughly 12 % residual moisture — while the specialty target is 10-12 %. At that moisture, several moulds develop: Aspergillus, Penicillium, Fusarium, and especially Aspergillus ochraceus, which produces ochratoxin A — a mycotoxin monitored in Europe under EC regulation 1881/2006 at a 5 μg/kg limit for roasted coffee and 10 μg/kg for soluble coffee. A mouldy lot is not only defective in taste; it can be non-compliant on food-safety grounds.

Causes are multiple: rushed drying on wet ground, storage in non-breathable jute bags in tropical zones, unventilated sea-container transport (wall condensation), broken moisture-control chain between the dry mill and the port. Historically higher-risk regions are very humid climates without mechanical drying infrastructure — parts of South-East Asia, West Africa or Central America during the rainy season. Conversely, drying on raised beds (15-25 days at 60 % relative humidity) drops the risk sharply.

On the SCA cupping form, the official descriptor is 'mouldy/musty'. The panel catches it at the fragrance stage (dry grounds), before brewing — which separates it from fermented (tasted) or phenolic (felt in the aftertaste). A lesser-known point: even a dark roast does not erase mouldy aromas — heat does not fully break down geosmin or 2-methylisoborneol, the main compounds responsible. A roasted coffee that smells of dry mushroom or damp cellar when the bag is opened is almost certainly affected. The only answer is to discard and return it to the seller.

In Belgium, prevention at serious specialty roasters rests on three controls: moisture check on delivery (handheld hygrometer, target 10-12 %), systematic cupping before production roasting, and visual inspection of green beans with manual sorting (mouldy beans often show halos or black tips). For a consumer, vigilance is mostly about freshness: a coffee more than 60-90 days past roast, in a poorly sealed bag, can develop cellar notes through oxidation without being truly mouldy — that would be the 'rancid' defect instead.

Coffee moisture and fungal risk