The major industrial process for the preparation of citric acid involves the treatment of a crude sugar solution (glucose or sucrose) with a mold (Aspergillus niger). The citric acid formed can be separated as calcium citrate by addition of calcium hydroxide and reformed by acidification with sulfuric acid.
Start a culture of Aspergillus niger. This fungus is very common in nature and specific strains have been developed which have a very high production rate of citric acid. These strains are available wherever science and agricultural supplies are sold.
Feed the culture of Aspergillus niger sugar. This may be sucrose (table sugar) or some medium that contains glucose. The source is typically the most inexpensive source of sugar available, such as hydrolyzed corn starch, corn syrup or molasses. Aspergillus niger uses the glucose as food and produces citric acid and carbon dioxide (CO2) as waste products.
Filter the mold out of the culture once the concentration of citric acid in the culture peaks. The remaining solution will be very high in citric acid.
Extract the citric acid. Add calcium hydroxide [Ca(OH)2] to the solution. It will combine with the citric acid to form calcium citrate [Ca 3(C6H5O7)2], a salt that will precipitate out of the solution. The following equation shows this reaction:
3Ca(OH)2 + 2C3H4[COOH]3OH -> Ca3(C6H5O7)2 + 3H2.
Regenerate the citric acid with sulfuric acid. The following equation shows this reaction:
3H2SO4 + Ca3(C6H5O7)2 -> 2C3H4[COOH]3OH + 3CaS04.
NOTE: The additional product of calcium sulfate (CaSO4).