PHOTOCHEMICAL AND THERMAL-DEGRADATION OF ANTHOCYANIDINS

Photochemical and thermal degradation of four anthocyanidins (pelargon idin, delphinidin, malvidin and cyanidin) in aqueous acidic media was studied. The kinetics of the degradation were followed by absorption s pectroscopy and high performance liquid chromatography. All anthocyani dins studied have shown a similar thermal degradation pattern. Chalcon e was detected as an intermediary product, which undergoes cleavage yi elding the final decomposition products, identified as (i) 2,4,6-trihy droxybenzaldehyde originated from ring A for all four anthocyanidins a nd (ii) 3,4,5-trihydroxybenzoic acid for delphinidin, 4-hydroxy-3,5-di methoxybenzoic acid for malvidin, 3,4-dihydroxybenzoic acid for cyanid in, and 4-hydroxybenzoic acid for pelargonidin (all these originated f rom ring B). During the photochemical degradation, the formation of th e same final products as for thermal reaction was observed, but throug h a different kinetic pathway involving the excitation of the flavyliu m cation. The substitution of the hydroxyl in position 3 by a glucosid e gives rise to a large increase in the thermal stability, and a decre ase in the quantum yield of the photochemical reaction.