By-product of the fermentation process of malting rice for use in the manufacture of sake (Japanese rice wine). In vitro and in vivo research and animal studies have shown that kojic acid is effective for brightening an uneven skin tone.
Kojic acid’s downside is that it’s an extremely unstable ingredient in cosmetic formulations. On exposure to air or sunlight, it turns brown and loses its efficacy. Many cosmetics companies use kojic dipalmitate as an alternative because it’s more stable in formulations. However, there’s no research showing that kojic dipalmitate is as effective as kojic acid, although it’s a good antioxidant.
References for this information:
International Journal of Molecular Sciences, September 2009, pages 4,066-4,087
Journal of Cosmetic Science, March-April 2004, pages 139-148
Journal of Dermatological Science, May 2003, pages 193-201
The Journal of Pharmacy and Pharmacology, December 1994, pages 982-985