Organic production of wine in the past years has known an important augmentation. This type of cultivation practice switches synthetic phytosanitary product for copper-based protection as fungicide. While the effect of copper on soil and vine is understood, few data are published concerning its impact on wine aromas and even less concerning varietal thiol. The aim of this work was to characterize the thiol aromatic potential of Colombard and Gros Manseng grapes and to investigate the effect of copper on thiol precursor biogenesis.
30 parcels were selected (15 conventional and 15 organic) with sampling at harvest for 18 of them and 3 sampling dates during ripening for the other 12 parcels. Chemical analyses of thiol precursors were performed by adapting an UPLC-MS/MS method based upon Stable Isotope Dilution Assay.
With this first characterization, it was demonstrated that both varieties presented concentrations of glutathionylated (G3SH) and cysteinylated (Cys3SH) precursors of 3-sulfanylhexan-1-ol up to 454 µg/kg and 21 µg/kg respectively. No precursors of the 4-sulfanyl-4-methylpentan-2-one were detected. So, Colombard and Gros Manseng were ranked in the top of varietal thiol producers with Sauvignon and Pinot Gris grapes (Pena-Gallego et al. 2012).
By comparing the copper protected parcels to the others, a significant decrease was identified (p-value = 0.01) of G3SH content in organic Gros Manseng grapes which was around 30%. This could represent a loss in 3SH of 1000 ng/kg in finished wines if a conversion rate of 3% between G3SH and 3SH was considered. On the opposite, Colombard grapes remained unaffected by the copper spraying with no effect on the precursors content at harvest.
The accumulation kinetics of thiol precursors under the two protection methods (copper or not) was also analysed. No accumulation of thiol precursors between seven days prior and after the harvest was observed in Colombard grapes which was inconsistent with literature on other grape varieties such as Sauvignon B. or Melon B.. Gros Manseng grapes showed significant accumulation for conventional culture (161 µg/kg to 356 µg/kg) over the two last weeks of ripening. Furthermore, copper treated parcels of Gros Manseng had no accumulation through the two-week study. However, Pearson test did not show a direct correlation between copper content and G3SH suggesting a possible interaction of copper with thiol precursors making them not analyzable under our conditions or a modification of vine metabolism.
For the first time two grapes varieties (Colombard and Gros Manseng) were characterised, varieties that have high potential towards varietal thiols such as Sauvignon. A negative copper effect was identified on Gros Manseng variety for both precursors content at harvest and accumulation kinetics while Colombard remained unaffected.
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First characterization of thiol precursors in Colombard and Gros Manseng: comparison of two cultivation practices
