• Acidulant (useful to adjust pH of products to make them more acid / less alkaline)
• chelating effect (neutralizes minerals to enhance activity of antioxidants & preservatives)
• buffering effect (helps to maintain pH between 2.5-6.5 & stabilizes active ingredients)
• at high concentration shows typical AHA properties

It’s also an antioxidant to help preserve oils in a product from degradation.

Some great sources to read up about citric acid:

http://www.ncbi.nlm.nih.gov/pubmed/16469079

http://www.ncbi.nlm.nih.gov/pubmed/23545907

http://www.ncbi.nlm.nih.gov/pubmed/21234435

http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=311&loc=ec_rcs

http://www.ftb.pbf.hr/import/44-2-3.pdf  à  C.R. SOCCOL et al.: Citric Acid Production, Food Technol. Biotechnol. 44 (2) 141–149 (2006)

Berovic, M.; Legisa, M. (2007). Citric acid production. “Biotechnology Annual Review Volume 13″.Biotechnology annual review. Biotechnology Annual Review 13: 303–343. Access:http://www.sciencedirect.com/science/bookseries/13872656/13