White Wine Phenolics: Varietal and Processing Differences as Shown by HPLC

Abstract

Grape skin extracts and white wines with and without pomace contact were made for two vintages from Chenin blanc, French Colombard, Semillon, and Thompson Seedless. HPLC separated at least 124 substances absorbing in the ultraviolet. Seven of these have been identified and quantitated as gallic acid, caftaric acid, coutaric acid, catechin, epicatechin, astilbin, and engeletin. The retention times for these substances on three different reversed phase columns were different, but in the same sequence with coefficients of variation for known standards (5-7 replicates) typically 1-2% for retention time and 1.2-8.9% for peak area. Detection at 280, 292, or 320 nm gave considerably different absolute and relative peak areas depending on the absorbance maxima of the substances, but as long as the absorbance was appreciable and separation complete, fairly constant amounts were calculated. Contamination of the catechin peak with a substance having more absorbance at longer wavelengths illustrated a difficulty with direct quantitation by peak area alone on a complex chromatogram. Composition was similar within a grape variety and differed between varieties. Vintage differed also, but less so, but pomace contact made vary large compositional differences. Evidence is presented that specified components are added by extraction from seeds or skins and others are augmented or depleted by reactions such as hydrolysis and oxidation during processing.