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 commercially 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 using fermentation methods that exploit citrate cycle reactions in the fungal mold Aspergillus niger. Chemically, the distinctive feature of citric acid is its three carboxylic acid groups, which are ionized at physiologic pH. This makes it a good chelating agent (that is, a compound that binds to metals), as well as a flavoring agent.
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