1.7 million metric tonnes of citric acid are produced worldwide per year, and most of that is made by Aspergillus niger. The ability of A. niger to make citric acid was originally identified in 1917 by James Currie and industrial level production began two years later by Pfizer. Up to that point when citric acid was needed citrus fruit juices were employed.
The process of citric acid production by A. niger is pretty simple, the Wikipedia entry is as follows:
In this production technique, which is still the major industrial route to citric acid used today, cultures of Aspergillus niger are fed on a sucrose or glucose-containing medium to produce citric acid. The source of sugar is corn steep liquor, molasses, hydrolyzed corn starch or other inexpensive sugary solutions. After the mold is filtered out of the resulting solution, citric acid is isolated by precipitating it with lime (calcium hydroxide) to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid.More modern techniques separate the growing fungus from the sugary liquid using a rotatory biological contactor (RBC). The fungus grows on the RBC and is rotated allowing alternate exposure to the liquid, then the air, then liquid again some 10 times per minute.
This makes the process more efficient as there is no longer a need to filter out the fungus when purifying the product and the sugar solution can be replaced many times before the fungus has to be replaced.