PHOTO: Hendrik Höller (Wosa Library).

When evaluating wines, we have two main tools at our disposal: sensory and chemistry. The chemical profile of a wine is made up of a multitude of compounds, but we cannot reduce a wine to a list of substances, no matter how comprehensive. In contrast, sensory evaluation looks at a wine holistically, but make use of human instruments which are by nature less objective than analytical equipment. Therefore, a combination of the two disciplines should be able to give a comprehensive picture of a wine. This approach has been used to profile the 2017 Absa Top 10 Pinotage wines.

Overall, the top 10 wines were characterised by complex aromas driven by ‘dark berries’, ‘cherries’, ‘spice’ and ‘tobacco’. ‘Complexity’ was also cited as an attribute itself (Figure 1). On the palate, the high scoring wines were considered ‘full bodied’, ‘balanced’, with an after-taste driven by flavour. On the other hand, the lower scoring wines were described as having aromas of ‘red berries’, ‘acetone’, ‘smoke’ and ‘barnyard’, while the palate was described as ‘medium body’, ‘watery’ and ‘unbalanced’ (Figure 1). One aspect that stood out was the agreement between judges on the quality scoring for the top wines, while poor consensus was achieved for the lower scoring wines.

 

FIGURE 1. Example of sensory profiles of a high scoring wine (left) and a low scoring wine (right). The top word clouds refer to the aroma profiles and the bottom word clouds to the taste and mouthfeel.

 

The chemical compounds analysed are linked to aromas that can be traced back to either the vineyard, fermentation, maturation or ageing (Table 1). This was an opportunity to report for the first time the thiol content of South African red wines. Even though present at ng/L levels (parts per trillion), these impact compounds contributed to the overall wine profiles as their odour thresholds are very low. As can be seen from Table 1, the four thiols are 3-mercaptohexyl acetate (3MHA, ODT 4.2 ng/L), 4-mercapto-4-methylpentan-2-one (4MMP, ODT 0.8 ng/L), 3-mercaptohexanol (3MH, ODT 60 ng/L), and furanmethanethiol (FMT, ODT 0.4 ng/L). From the literature, their main aroma descriptors in aqueous or ethanolic solutions, are ‘boxtree’, ‘blackcurrant’, ’grapefruit’ and ‘roasted coffee’, respectively. Another important class of compounds analysed was volatile phenols. This decision was made based on the sensory profiles of the wines, where attributes, such as ‘smoke’ and ‘barnyard’, were used to describe a number of wines. Although the presence of guaiacol, 4-methyl guaiacol, 4-ethyl guaiacol and cresols was confirmed in all the wines, their sensory impact was not negative if the wines were complex and the rest of the aromas could balance the perception of the volatile phenols. In other words, some smokiness in the top wines did not reduce their scoring.

 

TABLE 1. Thiol composition of the wines included in the evaluation.

 

Even though chemical analysis is an objective measure of a wine’s composition, it is not a substitute for human perception. Conversely, sensory evaluation is the tool of choice for evaluating perception, but it is still seen as a more subjective approach. By combining the two fields, there is a lot of information to be gained, to further both fundamental and applied knowledge.

 

– For more information, contact Astrid Buica at abuica@sun.ac.za.

 

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