Extract derived from citrus fruits and used primarily in small amounts to adjust the pH of products to prevent them from being too alkaline.
Citric acid is an AHA, and as such, in the right formula and concentration, it can exfoliate skin. However, research on this ability looked at much higher concentrations (20%, for example) than are used in skincare products—not to mention there’s proportionately more research on AHAs glycolic and lactic acids. Moreover, the AHAs lactic and glycolic acid have been shown to be more effective and less likely to promote a stinging reaction on skin.
References for this information:
The Journal of Dermatology, January 2006, pages 16-22
Dermatologic Surgery, August 1997, pages 689-694
International Journal of Cosmetic Science, April 1996, pages 75-83