Analysis of turmeric oleoresin, gardenia yellow, and annatto extract in foods using reversed-phase thin-layer chromatography/scanning densitometry

We established an analysis method of food colors, turmeric oleoresin, gardenia yellow, and annatto extract (including annato, water-soluble), in foods using reversed-phase thin-layer chromatography/scanning densitometry. The method involves the following three steps: 1) clean-up of the colors with a C18 cartridge, 2) separation of the colors by reversed-phase C18-TLC using acetonitrile-tetrahydrofuran-0.1 mol/L oxalic acid (7 : 8 : 7) as a solvent system and 3) measurement of visible absorption spectra of the colors using scanning densitometry without isolation of the colors. In order to investigate the capability of the present method, 89 commercial foods were analyzed, and their chromatographic behavior and spectra were observed. The separation and the spectra obtained were not affected by coexisting substances in the foods. The spots always gave the same Rf values and spectra as the standards with good reproducibility. The present method is considered to be useful for the rapid analysis of turmeric oleoresin, gardenia yellow, and annatto extract (including annatto, water-soluble) in foods.