Free and Glycoside Hydroxystilbenes in Grape

Twenty millilitres of skins extract, must or wine sample are extracted 3x10 mL with ethyl acetate. Extracts are combined and organic solvent is evaporated to dryness under vacuum, then the residue is redissolved 

For determination of free and glucoside fnws-resveratrol in grape, 20 berries are separated from seeds and extracted for lh by 30 mL of methanol. After homogenization and centrifugation, the supernatant is separated from the solid parts and methanol is removed under vacuum. 

Two hundred microlitres of the aqueous residue is added to 100 p,L of a lOOmg/L 4, 5, 7-trihydroxyflavanone methanolic solution as internal standard and of 0.5 g NaCl. The volume is adjusted to 5 mL using distilled water, the solution is transferred in a separation funnel and extracted 3x3 mL with ethyl acetate. Extracts are combined and the resulting solution is evaporated to dryness under vacuum. The residue is redissolved with 2mL of methanol/0.5% aqueous formic acid 3 7 (v/v) solution and filtered through a 0.22 xm membrane before analysis (Bavaresco et al., 1997 2002). 

In the chromatogram recorded at 320 nm in the HCTA analysis (paragraph 2.4.2) it is also possible to identify the trans-resveratrol gluco-side peak. The cis isomer is detected at 280 nm, but separation from quercetin-3-glucoside and rutin needs optimization of the chromatographic conditions (column and solvent). 

The total trans-resveratrol content can be determined after enzymatic hydrolysis of the glucoside form (La Torre et al., 2003 Wang et al. 2002). One millilitre of extract is evaporated to dryness under vacuum, then redissolved in 5 mL of a phosphate buffer solution at pIT 6. After addition of 15 mg (3-glucosidase 7.55 U/mg enzyme, the 

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