Citrate Production by Fermentation

The first reaction of the citrate cycle is catalyzed by the enzyme citrate
synthase. The product of this reaction is citrate, which is the conjugate
base of citric acid. This tricarboxylic acid is used to add flavor to beverages,
to preserve processed foods, and for other commercial purposes.
Historically, citric acid was extracted from lemons, but biotechnology now
produces more than 500,000 metric tons of citric acid annually by using
fermentation methods that exploit citrate cycle reactions in the fungal
mold Aspergillus niger. The distinctive chemical feature of citric acid is its
three carboxylic acid groups, which are deprotonated at physiologic pH
to form citrate. The ionization properties of citric acid make it a good
chelating agent (COO^– groups in citrate bind positively charged metals
such as Ca²⁺, Mg²⁺, Fe³⁺), as well as a flavoring agent owing to taste
receptors that detect the sensation of sour when H⁺ ions are released
from citric acid.