UV Spectroscopic Characterization and Stabilities of Natural Colorants from Roselle Calyx, Lac Resin and Gardenia Fruit

Abstract

This research intended to study natural water-soluble colorants from three sources; roselle calyx, lac resin and gardenia fruit.Their UV spectroscopic characteristics and acid dissociation constants were determined. Stabilities of the colorants under various conditions (water, acidic, alkaline, low and high temperatures, oxidizing and UV light) were monitored using spectroscopy. Aqueous solutions of roselle, lac and gardenia colorants showed maximum UV absorption wavelengths (λmax) at 520, 490 and 450 nm, corresponding to visualized color, i.e. red, red and yellow, respectively. The observed pKa values of roselle and lac colorants were determined using spectrophotometry to be 3.00 ± 0.08 and 5.96 ± 0.15, respectively. Roselle and lac colorants were very stable under acidic condition while gardenia colorant was stable under alkaline condition. Temperature had no effect on the color of lac colorant. Light and oxidizing agent accelerated color bleaching of roselle, lac and gardenia colorants.