The substrate for the fermentation is a carbohydrate, usually molasses or a sugar, such as raw beet, refined beet, or cane sugars, or a syrup. ... Certain inorganic nutrients, such as ammonium nitrate, potassium phosphate, magnesium sulfate, zinc sulfate, and potassium ferrocyanide, are added. The pH is adjusted to between 3 and 7, depending on the carbohydrate source. Sterilization may be batchwise or continuous; the latter uses less energy and is usually faster. After sterilization, the temperature is adjusted as required. The surface of the sterile substrate in the pans is inoculated with A. niger spores, which germinate and cover the surface of the liquid with a mat of mold. After two to three days the surface is completely covered and citric acid production begins, continuing at almost a constant rate until 80-90% of the sugar is consumed.
https://pubchem.ncbi.nlm.nih.gov/compound/citric_acid#section=Methods-of-Manufacturing&fullscreen=true
https://en.wikipedia.org/wiki/Aspergillus_niger
Edit: I'm going to add the wikipedia page for citric acid since someone pointed out that I was confusing the two.
Citric acid occurs in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (about 47 g/L in the juices[11]).[a] The concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes; these values vary within species depending upon the cultivar and the circumstances under which the fruit was grown. Source