These volatile thiols have aromas reminiscent of passion fruit, grapefruit, guava, gooseberries and blackcurrant, depending on their relevant concentrations in wines and the presence of other flavour active compounds, which can mask or enhance their perception. The finding is significant, especially in the light of the fact that Chenin blanc is often described in scientific literature and wine related books, as a “neutral” grape variety.
The research was conducted under the supervision of Dr Astrid Buica, an analytical chemist in the Department of Viticulture and Oenology, Stellenbosch University (SU). The research formed part of an M.Sc. study by Christine Wilson. The presence of thiols in Chenin blanc was previously confirmed for the first time in experimental wines investigating the effect of skin contact on Chenin blanc wine sensory properties. Based on these initial findings, the Wilson study analysed 65 commercial wines of various styles and ages for their thiol concentrations. The results were very promising with the concentrations of thiols measured, especially 3MH (3-mercapto-hexan-1-ol), being comparable to what is found in SA Sauvignon blancs! The highest value obtained for a one year old Chenin blanc was 2929 ng/l (3MH odour threshold: 60 ng/l).
This research actually has much deeper roots, dating back to as early as 1981 when two SA researchers, Du Plessis and Augustyn, wrote a paper on the “guava aroma of Chenin blanc and Colombar wines.” These two researchers hypothesized that Chenin blanc most likely contain sulphur containing flavour active compounds, since the addition of copper sulphate (CuSO4) removed the aroma. What is significant about this early research is that it was published long before volatile thiols were officially identified as flavour active compounds in Sauvignon blanc.
The Wilson study also found that in general the 3MHA (3-mercaptohexyl acetate) values were relatively low compared to values obtained for Sauvignon blanc. It should be noted that 3MHA is very sensitive to oxidation and only young wines that were produced in the fresh and fruity style, exhibited significant values. Many of South Africa’s top quality Chenin blanc wines are barrel fermented or aged, a winemaking process that introduces tiny amounts of oxygen to the wine and thus negatively influence 3MHA concentrations. These wines still benefit, however, from the more stable 3MH thiol making a significant contribution to the final wine aroma when present in high concentrations. Due to the low odour threshold for 3MHA (4.2 ng/l), it has a higher impact than 3MH on the aroma of fresh and fruity young Chenins.
In a subsequent study where only young fresh and fruity, less than one year old, Chenin blancs were analysed for their thiol concentrations, higher 3MHA values were measured than in the Wilson study, that contained wood matured Chenins. The fact that volatile thiols are present at appreciable levels in SA Chenin blanc wines is exciting, as it not only opens up new avenues for research, but also provide winemakers with possibilities to achieve the perfect balance of flavour active compounds in the style they desire to produce.
This research was funded by Winetech.
Further reading:
- Wilson, C.L. Chemical evaluation and sensory relevance of thiols in South African Chenin blanc wines
- Thiol levels in young 2016 fresh and fruity Chenin blanc wines