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Determination of flavonoids by RP-HPLC-ED in different varieties of Portuguese red grapes

Red grapes represent an extremely rich source of flavonoids, which are mainly contained in the skins and seeds of grape berries. One of the major groups of flavonoids found in grapes and wines are anthocyanins. The main naturally occurring anthocyanins in red grapes are 3-O-glucosides of malvidin, delphinidin, peonidin, petunidin and cyanidin, with malvidin-3-O-glucoside being the most abundant in all Vitis Vinifera varieties. Anthocyanins are present exclusively in the cell walls and vacuoles of grape skins, and are directly responsible for the colour of grapes and young red wines. Several flavonoids present in grape skins from different varieties of Portuguese grapes were determined by reverse-phase high-performance liquid chromatography (RP-HPLC) with electrochemical detection (ED). Grapes from Vitis Vinifera var. Alfrocheiro, Jaen, Touriga nacional and Tinta roriz were harvested from the Dã o wine demarcated region in Portugal. The extraction process used in this work was ultrasound. Good peak resolution was obtained following direct injection of a 50 µ L sample of extract in a mobile phase of pH 2.20. LODs obtained for each compound by ED detection were around 50 pg/L, showing that electrochemical detection allowed the determination of a 100 times lower concentration compared with PDA detection, with LODs around 50 ng/L. RP-HPLC-ED is characterized by an excellent sensitivity and selectivity, which is an advantage in comparison with the PDA detector. RP-HPLC-ED should be of particular relevance to analyze these electroactive compounds in red wines and in skins, seeds and pulp extracts of red grapes.

RP-HPLC; electrochemical detection; flavonoids; antioxidants; red grape skins; ultrasound extraction

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