Aspergillus niger is a common fungus cultivated by humans for several purposes and known for the ability to grow on a diverse set of materials. This has enabled them to become widespread around the world. In nature, Aspergillus niger can be found growing on fruit, vegetables, leaves, grain and compost. It is cultivated for the production of citric acid and several enzymes, and also has uses in fermentation.
Classification and Appearance
A. niger is a species classified as an asexual fungal mold. Aspergillus is the genus. According to the University of Adelaide's Mycology Online database, the basal felt is white or yellow, and this is covered by a textured black or brown surface consisting of tiny round heads mounted on stalks.
Environmental scientist Richard Myers says one of the earliest practical uses of Aspergillus niger was discovered in 1917 by James Currie, a chemist employed by the United States Department of Agriculture. Currie's contribution was the discovery of the conditions necessary to produce high levels of citric acid, an important organic chemical. On publication of his findings, Currie was hired by Pfizer, a leading producer of citric acid. Until Currie's findings, their only source of the chemical was citrus fruit, which made it difficult to mass produce. Once Pfizer obtained Currie's expertise, it became possible to produce citric acid in mass quantities. Over time, Aspergillus niger was also found to be important in the synthesis of various enzymes and is a major tool in their production.
Citric Acid Production
There are two methods used today for cultivating A. niger for citric acid. The first involves the use of trays filled with a sugar solution. The surface of the water is inoculated with spores of A. niger, which ferment the solution over the course of several days. The second method, used in more industrialized countries, involves the use of a deep tank filled with the solution. Spores are introduced and the liquid is stirred or aerated. Regardless of the method, the solution containing the mold is then treated with calcium hydroxide and sulfuric acid to separate the citric acid.
Aspergillus niger is cultivated to produce a number of enzymes, including lactase, invertase, pectinases and amylase. It can also be used to perform difficult enzymatic reactions. The enzymes are produced through fermentation and then extracted from the mix of substrate (sugar solution) and mold.
A. niger is also used in the fermentation of various food products such as pu-erh tea. According to a study performed by C. W. Hou and two colleagues, the fungus was found to be important in enhancing the nutritional content of the tea.