Identification
Aspergillus niger forms fast-growing colonies whose white to yellowish mycelium is soon covered by a dense layer of black to dark brown spore heads, like a sprinkling of soot; the reverse of the colony stays pale. Under the microscope the long conidiophores end in a large, round vesicle covered with two layers of cells (metulae and phialides), which produce chains of rough, dark brown conidia.
A. niger belongs to Aspergillus section Nigri, the black aspergilli. Several species in this section, such as A. tubingensis, A. welwitschiae and A. carbonarius, look very similar and are separated with DNA sequencing. Many older reports of A. niger may refer to these relatives.
Habitat and ecology
Black aspergilli are cosmopolitan and especially common in warm climates. A. niger lives in soil and on decaying plant material and is a frequent contaminant of stored foods. It causes black mould of onions and garlic, black rot of grapes and spoilage of dried fruit, nuts and spices. It tolerates relatively dry and acidic conditions and high temperatures. Indoors it can be found on damp materials and in house dust.
Uses and importance
A. niger is one of the oldest and most important industrial fungi. In submerged fermentations on sugar, selected strains secrete large amounts of citric acid, which is used as an acidulant and preservative in drinks and foods and in cleaning products and pharmaceuticals; most of the world’s citric acid is made this way. The fungus also produces gluconic acid and many enzymes, including glucoamylase for starch processing, pectinases for fruit juice, proteases and phytase for animal feed.
Its strong secretion capacity makes it a host for recombinant proteins, and its genome has been sequenced for several strains. A. niger is also studied for converting agricultural side streams and for producing other organic acids.
Cultivation
Industrial citric acid is produced in large, aerated fermenters with sucrose or molasses as the carbon source. High citric acid yields depend on specific conditions, including high sugar levels, low pH, limited manganese and other trace metals, and strong aeration. After fermentation, the citric acid is recovered from the broth and purified. Enzyme production uses similar submerged or solid-state processes. This is industrial work with selected, non-toxigenic production strains, not home cultivation.
Safety and edibility
A. niger is not a food: mouldy onions, fruit, nuts or other foods with black mould should be discarded rather than cleaned and eaten, especially soft items. Some strains of A. niger and other black aspergilli produce ochratoxin A and fumonisin B2, mycotoxins of concern in grapes, wine, coffee and dried fruit. Industrial production strains are selected and checked to avoid toxin production.
A. niger is a less frequent cause of human infection than Aspergillus fumigatus, but it is a common cause of otomycosis (fungal ear infection) and can cause lung infections or allergic disease in people with weakened immune systems or lung disease. Its spores are also indoor allergens. Seek medical advice for symptoms such as persistent ear pain or discharge, and do not self-treat. Laboratory work requires standard microbiological safety practice.
If you suspect mushroom poisoning, call your local emergency number or poison centre straight away, even before any symptoms appear, and keep the remains of the mushrooms.
Look-alikes
Other black aspergilli, such as Aspergillus carbonarius (a major ochratoxin A producer on grapes) and A. tubingensis, are almost identical and require DNA identification. Aspergillus oryzae and A. flavus are yellow-green rather than black, and Aspergillus fumigatus is blue-green to grey-green. Black spots on food can also be caused by Rhizopus sporangia or by dematiaceous moulds such as Alternaria and Cladosporium.
Around the world
Worldwide, A. niger is among the most familiar moulds: as black mould on onions in kitchens and markets, as a contaminant in warm-climate crops, and as an industrial producer whose citric acid ends up in foods and drinks everywhere.
