The fruity volatile thiols and the compounds associated with reductive aromas often occupy the same wine due to similar conditions supporting their formation and preservation. These two aroma groups contribute contrasting flavours and understanding the sensory interactive effects between the compounds can help in the management of reduction in wines.
Protecting must and wine from oxygen and the generous use of antioxidants are considered to be reductive winemaking techniques. These techniques help to preserve many oxidation sensitive wine constituents including important aroma compounds. The fruity volatile thiols are potent aroma compounds and contribute to the sensory perception of tropical fruit flavours, such as guava, passion fruit and grapefruit. Protecting the volatile thiols from oxidation (by maintaining a reductive environment during processing) is important to preserve the pleasant fruity aromas. However, reductive handling of wine can also result in the formation of unwanted aromas often simply referred to as “reductive” aroma. Like the volatile thiols, reductive compounds contain a sulphur atom, however, the aromatic attributes used to describe reductive aromas are usually unpleasant with descriptions such as rotten egg, vegetative and burnt rubber.
The preservation of the volatile thiols (which are formed during fermentation) and the formation of reductive aromas thus occur under similar conditions: an oxygen-deprived environment usually in combination with antioxidants, such as sulphur dioxide and ascorbic acid. Consequently, both aroma groups (fruity volatile thiols and reductive compounds), can be present at perceivable concentrations in the same wine at one time. Seeing that these two aroma groups contribute such contrasting flavours, it is important to understand the sensory interactive effects between them and the effect it can have on the wine.
Materials and methods
Two fruity volatile thiols, 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA) were used in this study. Each of the volatile thiols was added to a model wine at two levels based on the thiol content found in commercial South African Sauvignon blanc wines:
Level 1 (considered as “low”) = 500 ng/L.
Level 2 (considered as “high”) = 2 500 ng/L.
Level 1 (considered as “low”) = 100 ng/L.
Level 2 (considered as “high”) = 400 ng/L.
One reductive compound, ethanethiol (EtSH) was added at concentrations just below and above the sensory perception threshold of 1.1 μg/L. EtSH imparts flavours, such as onion, rubber and sulfidic-like aromas, when present at sufficient concentrations.1
During the initial sensory assessment of the wines, the judges reported that the perception of the samples changes, complicating the sensory evaluation procedure. To overcome this, a new sensory method was developed to allow for the evaluation of samples that are similar and change over time. In this method, judges evaluated the wines continuously over a 120-second period generating unlimited attribute intensity data over time.
Figure 1 shows the sensory profile of a single sample over time generated using the data obtained from the new sensory method. The intensity of four attributes, namely guava, passion fruit, cooked veg and H2S, showed specific trends during the study.
- An increase in 3MH increased the perceived intensity of guava, passion fruit and tomato leaf in the samples. However, the intensity of the attributes changed over time. The strongest signal was captured at the beginning of the sensory evaluation and the intensity decreased over the 120-second period.
- When 3MH was added at the higher concentration (level 2), the perception of guava and passion fruit decreased faster over the 120-second period compared to the sample to which 3MH was added at the lower concentration (level 1).
- Interactive effects between 3MH and 3MHA indicated a synergistic effect as the intensity of guava was significantly higher when the samples contained both compounds compared to when the compounds were added individually. This synergism was, however, not seen for passion fruit.
- Interestingly, the synergistic effect of 3MH and 3MHA on the guava attribute intensity was decreased in the presence of EtSH. Thus, EtSH had a suppressing effect on the thiol synergism.
- The intensity of the fruit-associated attributes (guava, passion fruit and grapefruit) decreased at a faster rate over time when compared to the aroma attributes typically associated with reductive compounds, such as H2S and cooked veg. The perception of the reductive aroma thus remained for a longer time compared to the fruity attributes.
- Surprisingly, an increase in EtSH not only increased the intensity of the attributes usually associated with reductive compounds, but it also increased the intensity of tomato leaf, an attribute usually associated with higher volatile thiol concentrations.
FIGURE 1. Sensory profile of a single sample over time (x-axis). Data shows the average intensity scoring (y-axis) of all judges for each attribute.2
In general, the continuous evaluation of a wine sample over time provides valuable insights into how a wine sample is perceived and the changes that can take place during the evaluation. However, the method and the data generated is complex and more work is needed to facilitate the processing of this data into usable information.
The initial higher intensity scoring followed by a decrease in attribute intensity during the 120 seconds was evident for all attributes. The rate at which this decrease took place depended on the compound responsible for the attribute, as well as the concentration.
These effects should be considered, for instance, the persistence of the reductive aroma (compared to that of the fruity volatile thiols) could have significant implications during wine tastings. Convenors of wine competitions should be aware of the effect of time on the perception of the sample. In a non-continuous evaluation, such as the judging at wine competitions, the timing of the pouring of samples should be standardised and optimised and judges should perhaps be instructed to not revisit a sample unless necessary. However, the exact reason for the changing of the aroma profile over time needs further investigation.
For the producer, the complex sensory interactions between the fruity volatile thiols and reductive compounds can become important. Suppressing/enhancing effects could change the overall perception of a wine, which might not be reflected in the chemical analysis. The contribution of compounds to contrasting aromas should also be considered carefully. This effect is often seen when certain aroma compounds are present at higher concentrations, which then intensifies the perception of unwanted flavours and can thus have adverse effects.
A recent Winetech-funded study by researchers, Sebastian Vannevel, Dr Jeanne Brand, Dr Astrid Buica and Prof Wessel du Toit from the South African Grape and Wine Research Institute (SAGWRI) and Department of Viticulture and Oenology (DVO) at Stellenbosch University developed a new sensory method to continuously assess the aromatic profile of a wine over time. This method was used to evaluate samples containing combinations of the fruity volatile thiols and ethanethiol, a reductive compound. Results showed complex interactive effects including synergism and suppression. The contribution of one compound to attributes usually associated with a contrasting compound was also demonstrated. The new method also highlighted the changes that take place during sensory assessment and how the perception of wine changes over a 120-second interval.
- Goniak, O.J. & Noble, A.C., 1987. Sensory study of selected volatile sulfur compounds in white wine. American Journal of Viticulture and Enology 38(3): 223 – 227.
- Du Toit, W., 2020. Factors affecting the perception of thiols in white wines. Winetech Final Project Report WW WdT 17/02.
– For more information, contact Carien Coetzee at firstname.lastname@example.org.