The black mold on your onions also makes most of the world's citric acid
The sooty fungus that spoils grapes, apricots and peanuts is also a factory workhorse. Fed sugar in acidic tanks, Aspergillus niger turns out more than a million metric tons of citric acid every year, plus enzymes that clarify wine, sweeten corn syrup and tame the gassy effects of beans.
Aspergillus niger belongs to the Nigri section of its genus, a group of 15 black-spored relatives that are easy to confuse with one another. It lives in soil and turns up indoors, where its dark colonies are sometimes mistaken for Stachybotrys, another fungus nicknamed black mold. Taxonomy keeps shifting: in 2007 the widely used laboratory strain ATCC 16404 was renamed Aspergillus brasiliensis, forcing updates to both the US and European pharmacopoeias.
It is hardy. The fungus needs oxygen but survives from 6 to 47 degrees Celsius, grows best at 35 to 37, tolerates pH from 1.5 to 9.8, and copes with both very dry air and near-saturated humidity. Its genome of roughly 34 megabases sits on eight chromosomes and holds 10,785 genes. Two strains have been sequenced, one used for industrial enzymes and one descended from citric acid production lines.
Industry grows it in two ways: submerged in liquid, or on moist solid waste such as wheat bran and rice husks, which is cheaper and gives better yields. For citric acid, labelled E330, strains are fed high sugar and mineral salts at pH 2.5 to 3.5. The fungus also supplies gluconic acid, glucoamylase for high-fructose corn syrup, pectinases for cider and wine, alpha-galactosidase sold in Beano, and fructosyltransferase for making prebiotic sugars. US regulators classify its fermentation as generally recognized as safe. It can even grow in gold-mining solutions and soak up copper and manganese from acid mine drainage.
The downsides are real. Some strains make ochratoxin A, a toxin that contaminates grape products and can damage kidneys. Post-harvest rot causes significant economic losses. Its spores are inhaled everywhere, and aspergillosis usually strikes people with weakened immune systems or conditions like asthma and cystic fibrosis.
Source: Aspergillus niger
