Synergistic antioxidant interaction between sugars and phenolics from a sweet wine

Musts from sun-dried grapes Pedro Ximénez are rich in polyphenols and sugars, and synergic effects between them have been found. Sugars participate in the stability of polyphenols; so, the autoxidation of (+)-catechin by ferrous iron is inhibited by a mixture of glucose and fructose similar to that found in musts of dried grapes. Phenolic fraction of musts was extracted with SepPack C18 cartridges, and the identification and quantification of phenolic compounds was achieved using HPLC-diode array detection. The evolution of phenolic content along the off-vine process indicated that there was a fourfold increase in most phenolics, catechin being the most abundant phenolic identified. Musts and the phenolic fractions showed a high antioxidant activity when determined by various established in vitro systems, namely β-carotene-linoleate bleaching assay, deoxyribose degradation and pBlueScript SK (-) DNA oxidation. The high antioxidant activity of musts found in the assays involving the formation of hydroxyl radicals, deoxyribose degradation, and DNA oxidation were partially due to the presence of sugars in the musts. The glycation of albumin was also studied by incubating BSA with a concentration of glucose and fructose, similar to that in musts; in these conditions, the phenolic fraction from must inhibited by a 76% the formation of advanced glycation end products (AGEs). A similar inhibition in the AGEs formation was observed when musts were directly incubated with albumin. The results indicate that musts and the phenolic fraction of sun-dried Pedro Ximénez grapes possess a marked antioxidant activity.