Aspergillus Niger, also called “black mold,” is a fungus common in breweries and malteries. The name Aspergillus niger is derived from the dark, nearly black spores of the fungus. A. niger grows in soil around the world and is a frequent food spoiler, while the metabolites of A. niger can also damage many materials—even glass and optical lenses can be corrupted by them. A. niger is useful in the food industry for the production of citric acid. The fungi metabolize acid under low pH conditions and when there is an absence of iron in the substrate. The optimum growing conditions are at a temperature between 95°F and 98.6°F (35°C–37°C), although A. niger is able to grow at temperatures between 42.8°F and 116.6°F (6°C–47°C) and a pH value between 1.5 and 9.8. This robust nature makes the mold difficult to eradicate in the brewery environment, and frequent cleaning may be necessary to keep it at bay.

In breweries A. niger is most commonly found in the fermentation and lagering areas, especially where condensation occurs on refrigerated equipment. It can also be found in bottling areas, where it can cause severe problems because of the high motility of the spores. Although the mold spores normally do not damage beer, spores detectable in packaged beer can be an indicator of poor brewery hygiene. Therefore, in some countries an elevated count of A. niger spores in beer leads to prohibition of sale.

See also beer spoilers.