What it is
Citric acid is a colorless triprotic organic acid with the formula C6H8O7 that occurs naturally in citrus fruits. In cosmetic formulas it is most often added in small amounts to fine-tune and stabilize pH, and it acts as a chelating (sequestering) agent that binds metal ions which can otherwise degrade a product. As a member of the alpha-hydroxy acid family it can also serve as a mild exfoliant at higher concentrations, though it is generally considered less active for that purpose than glycolic or lactic acid.
Safety & warnings
Citric acid is widely used and generally well tolerated at the low levels typical of pH adjustment. Because it is an alpha-hydroxy acid, its irritation potential rises with concentration and with lower formula pH, so products that use it as an active exfoliant can cause stinging or sensitivity, particularly on compromised or sensitive skin. The European Commission has evaluated alpha-hydroxy acids, including citric acid, in formal cosmetic safety assessments.
Origin & sourcing
Citric acid occurs naturally and abundantly in citrus fruits; lemons and limes can contain it at up to about 8% of their dry weight. Commercially it is produced almost entirely by microbial fermentation: strains of the mold Aspergillus niger are grown on sugar-containing media and convert the sugar into citric acid, a route that has dominated industrial manufacture since the early twentieth century.
History
Citric acid was first isolated in 1784 by the chemist Carl Wilhelm Scheele, who crystallized it from lemon juice. Industrial fermentation production became practical after 1917, when James Currie discovered that certain strains of Aspergillus niger could efficiently generate citric acid from sugar; this microbial process replaced citrus extraction as the main source.
Questions everyone asks
Why is citric acid in my sunscreen?
It is usually present in small amounts to adjust and stabilize the product's pH and to bind trace metal ions that could degrade the formula over time.
Is citric acid natural?
It occurs naturally in citrus fruits, but the citric acid used in cosmetics is almost always made by fermenting sugar with the mold Aspergillus niger.
Will citric acid exfoliate my skin?
It can act as a mild alpha-hydroxy acid exfoliant at higher concentrations, but in most products it is present only at low levels as a pH adjuster, not an active exfoliant.