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The discoloration of shikonin (1) and beta-carotene (2), occurring during storage of their ethanol solutions in the presence of oxygen in an illuminated room, was remarkably suppressed by hydrolyzable tannins, such as geraniin (4) and tannic acid JP (3) in the solution. The inhibitory effect of tannins was enhanced by the coexistence of metallic ion. The irradiation with ultraviolet lamp (254 and 365 nm) gave, at the first stage of the discoloration, two products, one of which was found to be 5,8-dihydroxy-2-(1-hydroxy-3-oxo-4-methyl-4-pentenyl)-1,4-naphthoquinone (7). The presence of hydrolyzable tannins induced higher accumulation of these two products in the solution, showing that the secondary structural transformations of these two products were strongly inhibited by these coexisting tannins. These results suggest that tannins could be efficient inhibitors of discoloration of natural pigments.

Effect of the Interaction of Tannins with Coexisting Substances. VIII. Inhibitory Effect of Tannins on Discoloration of Natural Pigments