Varietal thiol precursors in Trebbiano di Lugana grape and must

Abstract

Trebbiano di Lugana (TdL) is a white variety of Vitis vinifera mainly cultivated in an Italian area located south near Garda lake (Verona, north of Italy). This grape cultivar, also known as “Turbiana,” is used for the production of TdL wine with recognized Protected Designation of Origin whose volatile profile was recently determined [1]. The presence of varietal thiols in TdL, namely 3-mercaptohexan-1-ol and its acetate form, conferring the tropical and citrus notes, has been documented. Winemaking strategies were also described with the purpose of protecting and maintain these desired aromas [2]. To the best of our knowledge, the varietal thiol precursors (VTPs) were not previously determined in TdL grape and must. This study aimed to quantify VTPs in both grape during the ripening and must during the pressing. Volatile C6 compounds were also measured in the must fractions.
TdL grapes were sampled during the ripening in two vineyards, differing for the content of readily assimilable nitrogen (RAN), for a total of five samplings each. The musts were produced in an industrial plan collecting the samples during the pressing for a total of nine samplings [3]. VTPs were identified and quantified in grape fractions, grape samples and must fractions by ULPC coupled High Resolution Mass Spectrometry (HRMS) after SPE of samples [4]. Volatile C6 compounds, namely trans-2-hexen-1-ol, trans-3-hexen-ol, 2-hexenal, 1-hexanol, cis-3-hexen-1-ol and cis-2-hexen-1-ol, were determined by SPME-GC/MS [5].
S-3-(hexan-1-ol)-L-glutathione (G-3SH), S-3-(hexan-1-ol)-L-cysteine (Cys-3SH) and S-3-(hexanal)-glutathione (G-3SHal) were detected in both grape and must samples. At harvest, grapes with lower RAN revealed about 3-folds lower levels of G-3SH (79.71±0.97 μg/L vs. 208.66±1.35 μg/L) and G-3SHal (4.7±0.1 mg/L vs. 13.1±0.0 mg/L), and 2-folds lower amounts of Cys-3SH (11.95±0.82 μg/L vs. 21.75±0.47 μg/L). This suggests the level of RAN in grape to affect VPT synthesis. Nonetheless, the musts obtained with the two grapes showed comparable concentrations of G-3SH (50.71±0.37 μg/L as average); Cys-3SH was found at trace levels in both musts, and little amounts of G-3SHal was detected only in the must with higher RAN (29.53±7.37 μg/L). Considering the volatile C6 compounds, trans-3-hexen-1-ol, cis-3-hexen-1-ol and cis-2-hexen-1-ol were similar in the two investigated musts; trans-2-hexen-1-ol was higher in the must with low RAN as well as 2-hexenal and 1-hexanol. No significant correlation was found between the VPTs and volatile C6 compounds in the must fractions analysed.
These data suggest RAN to impact the VPT concentrations in grape. Moreover, pressing was found to play an important role on VPTs content of musts.

References

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