Factors affecting the spectrophotometric quantification of flavonoids in wine

The quantification of flavonoids in wine and grape skin extract by spectrophotometric evaluation at 280 nm wavelength provides essential information to oenologist concerning wine composition and evolution and itis commonly applied in wine labs. The measurement of the absorption peak height at 280 nm reported by DI STEFANO and GUIDONI (1989) prevents from the interferences arising from non flavonoid compounds. However, it showed to be affected by SO2 at low pH or acetone in unpurified grape skin extracts. Moreover, the effect of pH on flavonoids quantification in wine, either containing or not SO2, has been not assessed. The effect of SO2, purification, pH and dilution solvent on spectrophotometric quantification of flavonoids in red wine samples has been evaluated in this work. SO2 can overrate the flavonoids content in red wine when ethanol and Cl- ions are contained in acid dilution solvents. A wine sample dilution with a strong acid solvent is mandatory to attain a reliable quantification of flavonoids due to the low anthocyanins absorption at 280 nm in water solution. A minor effect arises from the ethanol content. Eventually, flavonoids can be quantified in SO2-containing wine diluted with a strong acid solution but a 7% overrating should be expected.