New understanding on sulfites reactivity in wine

Abstract

Sulfur dioxide is widely used during winemaking as an antioxidant and antimicrobial agent. Bisulfite (HSO3−), the predominant form of SO2 at wine pH, reacts with several wine components forming sulfonated adducts. Up to now, the compounds that react with SO2 are categorized into strong and weak binders, depending on the stability of the sulfonated adducts that they produce, meaning that compared to the strong binders, weak binders release easily SO2, and under oxidation conditions, they become a pool of free SO2 that will contribute later to wine’s oxidative stability. Carbonyl compounds have been well studied regarding their reaction with SO2, in contrast to peptides and sulfur containing compounds. In this work, 1H NMR spectroscopy has been used in order to monitor directly under wine-like acidic conditions, the kinetics of sulfonation reactions of carbonyls (acetaldehyde and pyruvic acid) and sulfur containing compounds (cysteine and glutathione) with different ratios of SO2 in aerobic and anaerobic conditions. These simulations of wine aging have shown first, that during aging sulfonation reactions to strong carbonyl binders are reversible inducing a decrease on total SO2 level. The dissociation of carbonyls from sulfites is occurred under all conditions. Second, thiol containing compounds appeared to play the key role as metabolic SO2 sink at the late stages of bottle aging. These, until now, unconsidered SO2 binders appeared stables under wine oxidation conditions and represent the truly waste fraction of sulfites during aging. This study puts under reconsideration the way of evaluating the strength of the SO2 binders and could possibly contribute to new strategies for SO2 management in winemaking.