Extraction of Polyphenols from the Grape

As soon as the grape has been picked from the plant, the seeds and skins of 50 berries are separated from the others parts of the berry, separately immersed in two solutions of 50 mL tartrate buffer solutions containing 1 g/L SO2 and immediately frozen. Before analysis, samples are heated at room temperature and so kept for 4 h in order to allow the extraction to occur. Extracts are then homogenized and centrifuged, the supernatant is transferred in a 100-mL volumetric flask, pellets are re-suspended twice in 15-20 mL of tartrate buffer and, after centrifugation, the liquid phases are collected in the same volumetric flask and the final volume is adjusted to 100 mL by tartrate buffer. 

Extraction of seeds and skins by acetone/water solutions 

After separation from the flashes, skins from 300 berries are rinsed with distilled water, immersed in 100 mL of aqueous solution containing lOOmg/L of S02 and stored at 4°C (adapted from Kennedy et al., 2002). To perform the extraction, the sample - made up of skins and aqueous solution - is transferred to a 500 mL Erlenmayer flask, and 200 mL of acetone is added. The flask is covered and placed on a shaker for 24 h at 20 °C at 100 rpm. After extraction, acetone is removed under vacuum at 35-40 °C, the solution is centrifuged, the supernatant is recovered, diluted at 100 mL with distilled water and stored at —20 °C until analysis. 

Seeds of 20 berries are extracted with 50 mL of a acetone/water 2 1 (v/v) solution at 20 °C for 2 h (Kennedy etal., 2000). Extracts are filtered, acetone is removed under vacuum at 35 °C, and the residual aqueous solution is diluted to 25 mL with distilled water. Ten millilitres of the resulting solution is transferred in a sample tube, frozen, and then freeze-dried. The solid residue is weighed and then dissolved in methanol (10 mg solid in 1 mL methanol). 

Another method of extracting polyphenols from seeds and skins is by immersion of the fine powder, obtained by grinding the grape skins and seeds in liquid nitrogen, in an acetone/water 7 3 (v/v) solution containing 0.1% ascorbic acid added to prevent oxidation (Downey et al., 2003). 

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