Flavylium salts, anthocyanidins and anthocyanins II. —Reactions with sulphur dioxide

Reactions between sulphur dioxide and flavylium salts, anthocyanidins and anthocyanins have been studied both qualitatively and quantitatively. The results were consistent with the formation of a complex between sulphur dioxide and the flavylium cation (F + ) of mole ratio 1:1 and have been expressed in terms of apparent formation constants ( K s ). Complex stability (measured at pH 1.05) was reduced by hydroxylation of the 7‐ and 5‐positions of the flavylium structure, the effect of the latter being more marked, but it was increased by hydroxylation in position 3 (anthocyanidins). The anthocyanins formed the weakest complexes of those compounds studied. Although complexes of the anthocyanin 3‐glycosides were significantly weaker than those of the 3,5‐diglycosides at pH 1.05, there was little difference between them at pH 3‐3.5. The variation in their K s values with pH could not be explained on the assumption that the ligand was simply the bisulphite anion HSO 3 . The effect of anthocyanins on the determination of free sulphur dioxide in fruit products is discussed.