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 mold 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: moldy onions, fruit, nuts or other foods with black mold 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 Poison Help at 1-800-222-1222 or 911 in an emergency straight away, even before 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 gray-green. Black spots on food can also be caused by Rhizopus sporangia or by dematiaceous molds such as Alternaria and Cladosporium.
In the United States
In American kitchens Aspergillus niger is best known as the sooty black mold on onions and some fruits. Note that in US usage, "black mold" in homes usually refers to a different fungus, Stachybotrys chartarum, which grows on water-damaged building materials.
In industry, A. niger is one of the most important production organisms for citric acid and food enzymes, so most Americans consume its products every day without knowing it.
