The effects of sulphur dioxide on wine metabolites: New insights from ¹H NMR spectroscopy based in-situ screening, detection, identification and quantification

The wine industry routinely employs sulphur dioxide (SO2) as an additive for antiseptic, antioxidasic and antioxidant activities. Some of the most important wine metabolites such as acetaldehyde, diacetyl, pyruvic acid, a-ketoglutaric acid, hydroxymethylfurfural, furfural and acetoin are known to react with SO2 over time to form adducts. H-1 NMR spectroscopy of wine can be tricky as metabolites exist either in original form or as O-adducts or as S-adducts at different SO2 concentrations. We are proposing a methodology for metabolites identification including all the possible SO2 generated species. H-1 NMR spectroscopy is able to highlight the structural changes that a metabolite undergoes in wine in the presence of SO2. The H-1 NMR spectra of the main wine metabolites known to react with SO2 were recorded in wine-model solutions at SO2 concentrations at 0, 50, 100 and 150 mg/L that are all below the maximum level allowed by the European Union regulations. Acetaldehyde was completely transformed into the corresponding hydroxysulfonate at a SO2 concentration of 100 mg/L. The concentration of this metabolite was assayed in ten commercial Moscato wine samples including the H-1 NMR resonances of all its identified forms. The results corresponded to those determined according to the OIV standard protocols.