Abstract
Varietal thiol precursors in grapes are subject to metabolic changes during post-harvest treatments. Metabolic activity should therefore be limited after sampling to understand their biosynthesis in the berry and genetic regulation. In this study, berries were frozen in liquid nitrogen immediately after harvesting, transported in dry ice, stored briefly at −80 °C, cryo-milled and extracted without being thawed in cold methanol in a ratio of 1:4 (w/v). A UHPLC-MS/MS method for quantitative determination of the thiol precursors 3-S-glutathionylhexan-1-ol (G3MH), 3-S-cysteinylhexan-1-ol (Cys3MH), 4-S-glutathionyl-4-methylpentan-2-one (G4MMP) and 4-S-cysteinyl-4-methylpentan-2-one (Cys4MMP), glutathione, oxidized glutathione and L-methionine in grapes was developed. Reference material was provided through synthesis of precursors and their deuterium labelled analogues. The average thiol precursor content in grapes in 2013–15 was in the range 8–16 μg kg−1 for G3MH, 1–6 μg kg−1 for Cys3MH, 1–4 μg kg−1 for Cys4MMP and 0.3 μg kg−1 for G4MMP. In 2013 and 2014, the highest precursor content in mature Sauvignon Blanc grapes from vineyards located in Italy regarded G3MH, followed by Cys3MH, Cys4MMP and G4MMP. In 2015, G3MH was again the most abundant precursor, but followed by Cys4MMP, Cys3MH and G4MMP.
Highlights
Sulfur-containing compounds which contribute to desirable fruity aromas in Sauvignon Blanc are referred to as volatile or varietal thiols[1,2]
The identification and quantification of thiol precursors in grapes is important in order to understand their biosynthesis in the berry and genetic regulation
The aim of this study was to design an experimental protocol for reliable determination of thiol precursors, reduced glutathione (GSH), oxidized glutathione (GSSG) and L-methionine (Met) in grape berries
Summary
Sulfur-containing compounds which contribute to desirable fruity (grapefruit/citrus, passion fruit/tropical) aromas in Sauvignon Blanc are referred to as volatile or varietal thiols[1,2]. While Sauvignon Blanc grapes are less aromatic, Udine, Department of Agricultural, Food, Environmental and Animal Sciences, via delle Scienze 206, 33100, Udine, Italy Their corresponding wines can exhibit strong flavour, due to varietal thiol formation. Recent studies regarding Sauvignon Blanc thiol precursors have been oriented towards determining possible new precursors[15,16,17], their localisation in the skin and juice of the grape berry[18,19], their conversion to volatile thiols through the use of different yeast strains[3,20,21,22], their evolution during grape ripening[23,24,25,26], their fluctuation due to the time of harvesting (diurnal changes)[27] and modifications taking place during harvest and post-harvest treatments[25,26,28,29]. The content and distribution of the thiol precursors, Met, GSH, and GSSG were determined in Sauvignon Blanc grapes sampled in 5 different vineyards at harvest time in Friuli Venezia Giulia (Italy) in 2013–2015
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