Structural characterization of reaction products of caftaric acid and bisulfite present in a commercial wine using high resolution mass spectrometric and nuclear magnetic resonance techniques

Abstract

Reaction products of bisulfite and caftaric acid were found in wines containing sulfites as a preservative. Acidic compounds were separated from wine and analyzed by HPLC combined with DAD and QTOF mass spectrometer. HPLC chromatograms of the expected [M-H]^- ion and UV absorption revealed the presence of five possible reaction products (a-e). These compounds were isolated then characterized by NMR and confirmed to be the reaction products as follows; 5-sulfo-(E)-caftaric acid (a), 2-sulfo-(Z)-caftaric acid (b), 2-sulfo-(E)-caftaric acid (c), (E)-caftaric acid-4-O-sulfate (d) and (E)-caftaric acid-3-O-sulfate (e). UV spectra and high resolution product ion spectra of the five compounds also supported their identity. The reaction products were confirmed to be commonly present in commercial wines across four vintages and two varieties. Their concentration was found to be as much as that of 2-S-glutathionyl caftaric acid, suggesting that bisulfite consistently competes as a nucleophile with glutathione for the o-quinone of caftaric acid.

Keywords: Bisulfite; Caftaric acid; Mass spectrometry; Sulfur dioxide; Wine oxidation.