Volatile thiols play an integral role in the passion fruit, grape fruit and guava aromas of different types of white wines. This is especially true for Sauvignon blanc and possibly Chenin blanc wines. The three most important positive volatile thiols in white wines are normally 3-mercaptohexanol (3MH), 3-mercaptohexanol acetate (3MHA) and 4-mercapto-4-methylpentan-2-one (4MMP). These volatile thiols are formed by the yeast during alcoholic fermentation from certain precursors present in the must. The perception threshold values (in other words the lowest concentrations where we start to smell them in wine) are 60, 4.2 and 0.8 ng/ℓ for 3MH, 3MHA and 4MMP respectively. These very low values show the sensitivity humans have towards these compounds and analytical methods should thus be able to detect and quantify these compounds at such low levels. This, together with the fact that these sulphur containing compounds are sensitive towards oxidation and thus not very stable, makes the analysis of these compounds a huge challenge. The Department of Viticulture and Oenology, Stellenbosch University, embarked on developing a new method for volatile thiol analysis in white wines, as well as to perform a screening to determine levels of these compounds in South African Sauvignon blanc wines.

We initially screened the levels of 4MMP, 3MHA and 3MH in 28 different South African Sauvignon blanc wines from the 2011 vintage. We used a GCMS method in a laboratory in Slovenia for this purpose. The levels for 4MMP ranged from 3 – 35 ng/ℓ, with an average of 10 ng/ℓ. The ranges were 12 – 720 ng/ℓ and 453 – 3 452 ng/ℓ for 3MHA and 3MH respectively, with average values of 158 and 970 ng/ℓ. We tried to implement this method at the DVO, but due to various reasons, we were not successful in this regard. We thus decided to attempt to analyse for 3MH and 3MHA with a novel ultra-performance liquid chromatography tandem mass spectrometry method, in collaboration with the University of Milan. In this method, extensive sample preparation is required where the thiols are concentrated and quantified using UPLC-MS/MS after derivatisation. Although this method still requires extensive sample preparation and specialised equipment, it is in general shorter and easier to perform than traditional sampling preparation methods using mercury, which has a potential health hazard. Using this novel method, excellent recovery and precision were achieved for 3MH and 3MHA. Limits of quantification, in other words, the minimum concentration of these compounds that we could accurately determine, were 0.07 ng/ℓ for 3MH and 1.68 ng/ℓ for 3MHA. This is much lower than the sensory perception threshold of these two compounds which make this method ideal for quantification of these compounds in white wines. We then further quantified the 3MH and 3MHA levels in 11 additional South African Sauvignon blanc wines from the 2012 and 2013 vintages. We found average 3MHA levels of 313 ng/ℓ, with a range of 18 – 1 029 ng/ℓ. Average 3MH levels were found to be 1 320 ng/ℓ, with a range of 820 – 2 262 ng/ℓ.

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Differences in 3MH and 3MHA can be due to vintage effect, the usage of SO2 and or glutathione in the must and yeast strains used. The hydrolysis of 3MHA to 3MH also occurs during bottle ageing, even when wine is closed under screw cap. Bottle storage temperature is critical in this regard, with this degradation rate occurring faster at a higher temperature. This new breakthrough UPLC-MS/MS method, funded by Winetech, opens the doors for further research projects on volatile thiols in wine.

Acknowledgements

Winetech for funding and the Central Analytical Facility of Stellenbosch University.

References

Coetzee, C. & Du Toit, W.J., 2012. A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols. Food Research International 45, 287 – 298.

Van Wyngaard, E, 2013. Volatiles playing an important role in South African Sauvignon blanc wines. MSc (Agric) thesis in Oenology, Stellenbosch University.

Piano, F., Fracassetti, D., Buica, A., Stander, M., Du Toit, W.J. & Tirelli, A., 2014. Development of a novel liquid/liquid extraction and UPLC-MS/MS method for the assessment of thiols in South African Sauvignon blanc wines. Australian Journal of Grape and Wine Research 21, 40 – 48.

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