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

From Bean to Cup: The Origins and Detection of Mould in Coffee

Mould in coffee is both a flavor defect and a food safety concern — a rare intersection in the specialty world that elevates its importance beyond simple quality grading. The off-flavor associated with mould is distinctly musty, earthy in the unpleasant sense, and sometimes reminiscent of damp basement or wet newspaper. This is caused primarily by mycotoxin-producing fungi — Aspergillus and Penicillium species in particular — which colonize green coffee beans when moisture content exceeds 12.5% during storage or transport. The fungi themselves may be killed by roasting temperatures, but the mycotoxins they produce — particularly ochratoxin A and aflatoxin B1 — are heat-stable and survive into the cup, which is why mouldy coffee is not merely an aesthetic problem.

Ochratoxin A, the most commonly detected mycotoxin in coffee, has been classified by the International Agency for Research on Cancer as a possible human carcinogen (Group 2B), which prompted the European Union to set maximum limits of 10 micrograms per kilogram for roasted coffee and 15 µg/kg for soluble coffee in 2005. Since then, EU-bound importers have been required to test representative samples from every container, creating a regulatory pressure that has measurably reduced mould incidence in European supply chains. The specialty market has additional motivation to monitor mould: the sensory defect is reliably detected by trained cuppers at concentrations well below the legal threshold, meaning that mouldy lots typically fail quality checks before they fail safety checks.

Practical Recommendations

As a consumer, your best defense against mouldy coffee is provenance transparency: buy from specialty roasters who source from importers with documented quality control and rejection protocols. Single-origin coffees with specific harvest dates and documented moisture readings at import are measurably lower risk than commodity blends assembled from undocumented sources. At home, store beans in an airtight container in a cool, dry location — never in a humid pantry or near the stove — and consume within three to four weeks of the roast date. If a cup ever registers as musty, earthy, or basement-like in its fragrance phase, trust that signal: well-sourced, well-stored specialty coffee should never produce these notes.