A by-product of the fermentation process of malting rice for use in the manufacture of sake, Japanese rice wine, kojic acid definitely has convincing research, both in vitro and in vivo and in animal studies, showing that it is effective for inhibiting melanin production. (Sources: Cellular Signaling, September 2002, pages 779-785; Biological and Pharmaceutical Bulletin, August 2002, pages 1045-1048; Analytical Biochemistry, June 2002, pages 260-268; American Journal of Clinical Dermatology, September-October 2000, pages 261-268; and Archives of Pharmacal Research, August 2001, pages 307-311.)
However, kojic acid in concentrations high enough to make a difference can be irritating, and far more irritating than the more effective hydroquinone. (Source: Skin Lightening and DepigmentingAgents, emedicine. com, July 2008.)
So why don’t more products claiming to lighten skin contain kojic acid? Kojic acid is an extremely unstable ingredient in cosmetic formulations. Upon exposure to air or sunlight it turns a strange shade of brown and loses its efficacy. Many cosmetics companies use kojic dipalmitate as an alternative because it is far more stable in formulations. However, there is no research showing that kojic dipalmitate is as effective as kojic acid, though it is a good antioxidant.