This work is part of a comprehensive study conducted at the Department of Viticulture and Oenology and aimed at shedding light on the thiol composition of Chenin blanc and various red cultivars using chemical and sensory tools.
Wine quality is an abstract concept, as well as a controversial one, because it evades objective definition. What we know for sure is that, among other factors, it is directly correlated to the aroma, taste and mouth-feel of the wine. Traditionally the aromas of wine are divided into three groups. According to their origin, primary or varietal aromas derive from the grape itself, secondary are formed during fermentation and tertiary during maturation and ageing.
Recent developments in analytical chemistry offered the possibility to investigate and validate the impact of thiols, a class of compounds thought to play a significant role in wine aroma. The three most relevant representatives of this class are 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA). In white wine they contribute to the perception of ‘passion fruit’, ‘grapefruit’ and ‘guava’ aromas. Thiols are also known to occur in Merlot, Shiraz, Grenache and Malbec. Reports from France indicated the presence of thiols in Bordeaux blends.1 Last year, work done in collaboration between the Chemical Analytical Laboratory at the Department of Viticulture and Oenology (DVO) and the Central Analytical Facility at Stellenbosch University (SU) proved the presence of thiols in South African single varietal Pinotage, Cabernet Sauvignon, Merlot and Shiraz wines.2
There is still a lot we do not understand about the influence of thiols on the perception of red wine aroma and the contribution the non-volatile component (matrix) plays. With the exception of 4MMP, which has been identified as one of the contributors to the ‘blackcurrant’ aroma in Cabernet Sauvignon, there are no studies demonstrating the impact of 3MH and 3MHA in red wine.
As the chemistry already demonstrated that these compounds are present in South African red wines, the purpose of this study was to elucidate the sensory role that 4MMP, 3MH and 3MHA play. Due to the complexity of the investigation, the approach was multi-layered. We were interested in the attributes associated with these thiols, the extent that their concentration influences their perception, and the effect of the cultivar.
Projective Mapping (PM) was chosen as the most suitable method for the task. This method allowed us to evaluate multiple wines in relation to each other by placement of samples according to their similarity/dissimilarity on a 2D surface. At the same time, individual profiles were obtained using the descriptors given by the judges for each wine (Figure 1). This cost-effective method has been used extensively and successfully to evaluate food products and beverages. Furthermore, it gives the possibility to assess a relatively large sample set, not always possible with more traditional and expensive methods, such as Descriptive Analysis. Dr Jeanne Brand gave a detailed description of the PM sensory method in a previous issue of WineLand (April 2016).
FIGURE 1. A photographic display of the process of Projective Mapping. (Photo: Dr Jeanne Brand.)
Figure 2 better illustrates the methodology of the experiment and it can be explained as follows.
The study was broken down into three parts:
- Experiment 1 (single thiol effect) was designed to assess the relevance of individual thiols at different concentration levels in four red wine cultivars.
- Experiment 2 (thiol x thiol interaction) helped us evaluate the interaction between these thiols taking as example Pinotage as base wine.
- Lastly, to determine the interaction between thiols and different red wine matrices, Experiment 3 (thiol x matrix interaction) was performed in the same four red wine cultivars as Experiment 1.
FIGURE 2. Methodology of the thiols in red wine project.
In Experiment 1, to generate specific attributes for 3MH, 3MHA and 4MMP, three levels of each thiol (zero, medium and high) were considered for spiking. De-aromatised wines from various cultivars (Pinotage, Cabernet Sauvignon, Shiraz and Merlot) were used as base. Each compound was spiked individually and the resulting samples evaluated.
To measure the effects these thiols have on the sensory attributes in combination with each other, interaction experiments were ran between 3MH x 4MMP and 3MHA x 4MMP (Experiment 2). For each set, nine combinations of the levels (from zero x zero to high x high) were evaluated, using as base de-aromatised Pinotage.
To determine the role the matrix plays in the perception of the compounds, additional interaction experiments were run between 4MMP x 3MH and 4MMP x 3MHA (Experiment 3). This time, de-aromatised Pinotage, Shiraz, Merlot and Cabernet Sauvignon were used as base wines.
Take home message
As a result, these extensive sensory experiments showed that complex interactions take place between thiols, thiols and other volatile compounds, and thiols and the wine matrix specific to a cultivar. We are only beginning to understand the role this class of compounds plays in red wine. What is clear though, is that, while single thiols can relatively easily be associated with specific aromas, once present in combinations, the synergistic effects and the resulting complexity are still highly difficult to define. Thiol presence in wine seems to be favourable depending on the desired style pursued by the winemaker, but the understanding of the specific combination thiols x cultivar is of crucial importance given the vast difference of aromas associated with each unique combination.
The most recent work done at DVO’s Sensory Unit sheds light on the impact of thiols on red wine aroma. From literature, we know that thiols have a significant impact on wine aroma. There is a lot of information available on white wine, especially Sauvignon blanc and more recently Chenin blanc, but very little on red wine. Due to the complex nature of the red wine matrix, there is a need for systematically elucidating the contribution of thiols on the aroma of cultivars important for the South African wine industry. The results for Chenin blanc were presented in previous issues of WineLand (September and October 2018). Due to the extent and complexity of the experiments, the red wine investigation will be presented in a series of articles.
- Rigou, P., Triay, A. & Razungles, A., 2014. Influence of volatile thiols in the development of blackcurrant aroma in red wine. Food Chemistry 142: 242 – 248. Doi: 10.1016/j.foodchem.2013.07.024.
- Mafata, M., Stander, M., Thomachot, B. & Buica, A., 2018. Measuring thiols in single cultivar South African red wines using 4,4-dithiodipyridine (DTDP) derivatization and ultraperformance convergence chromatography-tandem mass spectrometry. Foods 7(9): 138. Doi: 10.3390/foods7090138.
– For more information, contact Valeria Panzeri at firstname.lastname@example.org or Astrid Buica at email@example.com.