Chemical Stability of Thiol and Flavanol Sulfonation Products during Wine Aging Conditions

Evolution of sulfonated tannins in red wines with ageing: A targeted metabolomic approach

During wine ageing, tannins could react with sulfur dioxide to form sulfonated flavanols which are anticipated to alter tannin binding to proteins contributing to the reduction of astringency during ageing. Previous studies have identified or quantified monomeric and dimeric sulfonated flavanols in aged wines, but the evolution of sulfonated tannins has been lacking. Here, we quantified sulfonated tannins in three Washington state vineyards over a 20-year period, employing targeted LC-QToF analysis. Analysis of 24 wines revealed a systematic trajectory of sulfonated tannins over 20 years. Sulfonated monomers rose consistently with wine age, becoming the dominant form of sulfonated products in all samples at the 20-year mark. Concurrently, there was a decline in native tannins and sulfonated oligomers, suggesting a process of acid-catalyzed depolymerization of native tannins followed by sulfonation via reaction with sulfur dioxide. Future work is needed to understand the stability of sulfonated tannins and their impact on wine astringency.

1H NMR based sulfonation reaction kinetics of wine relevant thiols in comparison with known carbonyls

Sulfite addition is a common tool for ensuring wines' oxidative stability via the activity of its free and weakly bound molecular fraction. As a nucleophile, bisulfite forms covalent adducts with wine's most relevant electrophiles, such as carbonyls, polyphenols, and thiols. The equilibrium in these reactions is often represented as dissociation rather than formation. Recent studies from our laboratory demonstrate, first, the acetaldehyde sulfonate dissociation, and second, the chemical stability of cysteine and epicatechin sulfonates under wine aging conditions. Thus, the objective of this study was to monitor by 1H NMR the binding specificity of known carbonyl-derived SO2 binders (acetaldehyde and pyruvic acid) in the presence of S-containing compounds (cysteine and glutathione). We report that during simulated wine aging, the sulfur dioxide that is rapidly bound to carbonyl compounds will be released and will bind to cysteine and glutathione, demonstrating the long-term sulfur dioxide binding potential of S-containing compounds. These results are meant to serve as a complement to existing literature reviews focused on molecular markers related to wines' oxidative stability and emphasize once more the importance of S-containing compounds in wine aging chemical mechanisms.