Curcuminoids powder was kindly supplied by ThaiChina Flavours & Fragrances Industry Co., Ltd. This powder contained 92.04% w/w of curcuminoids (5.25% ± 0.15% w/w of bisdemethoxycurcumin, 18.75% ± 0.42% w/w of demethoxycurcumin and 68.04% ± 1.11% w/w of curcumin). Standard curcuminoids powder (analytical grade, lot no. 69- s3457) was purchased from Sigma-Aldrich Co. Curcumin compound was separated from the standard curcuminoids by preparative thin layer chromatography. Briefly, the standard curcuminoids were dissolved in the solution mixture of methanol (analytical grade; Labscan Asia Co., Ltd., Bangkok, Thailand) and dichloromethane in a proportion of 1 : 1 by volume and then spotted on to a layer of silica plate. The spotted plate was developed with methanol and dichloromethane in the proportion of 1 : 24 by volume. Under ultraviolet (UV) light at wavelength of 366 nm, spot of curcumin, bisdemethoxycurcumin or demethoxycurcumin was detected, scraped and then dissolved in the solution mixture of methanol and dichloromethane. After the silica was separated, the obtained solution was collected and recrystallized to get a crystal of compounds for being used as a standard material in further studies.