Antithiamin Activity of Polyphenolic Antioxidants

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Abstract
Two flavonoids, quercetin and dihydroquercetin, and two cinnamic acid derivatives, caffeic acid and chlorogenic acid, were selected as being representatives of plant phenolic antioxidants. Tests of antioxidant activity showed the two flavonoids were more effective than the two cinnamic acid derivatives. However, in thiamin degradation tests, cinnamic acid derivatives showed a greater degradation of thiamin than did the flavonoids. The effect of caffeic acid on thiamin degradation was found to be pH dependent. The influence of phenolic compounds at pH 5.5 was extremely slight; the degradation of thiamin was considerably more rapid at pH 7 than at pH 5.5. Spectral and thin‐layer chromatographic analyses demonstrated that thiamin disulfide was the major product of thiamin degradation created by phenolic oxidation compounds.