PHOTOCHEMICAL AND THERMAL-DEGRADATION OF ANTHOCYANIDINS

Photochemical and thermal degradation of four anthocyanidins (pelargonidin, delphinidin, malvidin and cyanidin) in aqueous acidic media was studied. The kinetics of the degradation were followed by absorption spectroscopy and high performance liquid chromatography. All anthocyanidins studied have shown a similar thermal degradation pattern. Chalcone was detected as an intermediary product, which undergoes cleavage yielding the final decomposition products, identified as (i) 2,4,6-trihydroxybenzaldehyde originated from ring A for all four anthocyanidins and (ii) 3,4,5-trihydroxybenzoic acid for delphinidin, 4-hydroxy-3,5-dimethoxybenzoic acid for malvidin, 3,4-dihydroxybenzoic acid for cyanidin, and 4-hydroxybenzoic acid for pelargonidin (all these originated from ring B). During the photochemical degradation, the formation of the same final products as for thermal reaction was observed, but through a different kinetic pathway involving the excitation of the flavylium cation. The substitution of the hydroxyl in position 3 by a glucoside gives rise to a large increase in the thermal stability, and a decrease in the quantum yield of the photochemical reaction.