Antioxidant activity of different phenolics fractions isolated in must from Pedro Ximenez grapes at different stages of the off-vine drying process

The evolution of total phenolics, flavonoids and flavonols in the white grapes of Pedro Ximénez were determined during the off-vine drying process. Phenolics compounds determined, browning index and total antioxidant activity increased during the off-vine drying process. The analysis by gel filtration chromatography showed that compounds of high molecular weight are formed during the off-vine process and are partially responsible for the antioxidant activity and browning of grapes. Musts were separated by solid phase extraction in four fractions (I–IV) and the phenolics and antioxidant activity determined. Fraction III containing catechins and procyanidins, and fraction IV containing polymeric procyanidins were greatly enhanced during the off-vine drying process. Lastly, ethanolic extracts from grape pomaces at different drying stages were fractionated and analysed. The antioxidant activity showed similar values to those obtained in musts at the end of the drying process; however total phenolics and the browning index showed a 30% and 50% reduction, respectively, in comparison to those observed in the corresponding musts. This indicates that the ethanolic extracts could be used as a source of phenolic compounds without enhancing browning and as a way to increase the antioxidant activity of wines from dried grapes.