Intraregional profiles of varietal thiols and precursors in Sauvignon Blanc juices and wines from the Adelaide Hills

Aims: To investigate the intraregional variation of varietal thiol precursors and free thiols in Sauvignon blanc grape juices and experimental wines arising from the Adelaide Hills Geographical Indication (GI) in South Australia.

Methods and Results: Vitis vinifera L. cv Sauvignon blanc grape parcels (n = 21, approx. 8 kg each, encompassing 5 clones) were hand harvested from different blocks within seven commercial vineyards in the Adelaide Hills GI during the 2018 vintage. Parcels were divided into subsets for winemaking and freezing experiments. Amino acid (AA) and thiol precursor concentrations in juice were determined using high performance liquid chromatography (HPLC) with fluorescence detection and stable isotope dilution assay (SIDA) using HPLC with tandem mass spectrometry (MS/MS), respectively, and free thiols in wine were quantified by SIDA with HPLC-MS/MS, after derivatisation with 4,4’-dithiodipyridine. Intraregional variations in grape ripeness were evident according to total soluble solids content, pH, and titratable acidity, even within single locations or for the same clones. Significant differences in the glutathionylated precursor to 3-sulfanylhexan-1-ol (3-SH) were found among several locations whereas for the cysteinylated variant of 3-SH, one location was distinct from the rest. Variation in precursor concentrations was also noted from different blocks within a single vineyard location but was not dependent on grape ripeness. Fermentations progressed without any obvious relationship to location, and wines that were high in 3-SH were also usually high in 3-sulfanylhexyl acetate (3-SHA). One location had significantly higher levels of thiols in wine despite the juice not being the highest for grape-derived precursors, and also gave a substantial concentration of 4-methyl-4-sulfanylpentan-2-one in comparison to other locations within the GI. The AA profile of juices was found to vary according to location, and certain AAs were strongly correlated to thiol precursor concentrations, but relationships of AAs with free thiols in wine were generally weak. Additionally, enhancements in the concentrations of precursors in juice (up to 19-fold) and free thiols in wine (up to 10-fold) were revealed from freezing whole grape bunches in contrast to using fresh juice.

Conclusions: 

Intraregional variation was noted for thiols in wine, and precursors and amino acids in juice, for 21 Sauvignon blanc samples collected from within the Adelaide Hills region. The effects of terroir were implicated in explaining the differences in grape composition, and the potential interactions among grape amino acids and thiol precursors in berries and thiols in wine were revealed.

Significance and Impact of the Study: Sauvignon blanc is a significant variety produced in the Adelaide Hills GI but no information was available on the effects of location within the GI on grape and wine composition with respect to varietal thiols. This was the first study of intraregional variations of thiol precursors, amino acids, and free thiols in Sauvignon blanc juices and wines that were produced in a consistent manner. A remarkable enhancing effect of freezing was noted for thiol precursors in juice, and importantly, free thiols in wine.