WINETECH Technical Yearbook 2020

Thiols in red wine (PART 2): Individual thiols and levels

There is no right and wrong in either of the descriptions. Science has shown us that, often times, the tipping point between different aromas for a single compound is determined by its concentration. Even more, the matrix (or type of solution) might also change our sensory perception While many studies have investigated the sensorial nature of thiols in white wine, the effect of these compounds in red wine is highly unexplored. A study recently conducted at Stellenbosch University confirmed the presence of thiols in certain red South African single-cultivar wines (Mafata et al ., 2018) and these chemical results paved the way to explore the topic from an organoleptic point of view. APPROACH As a first step, we generated descriptors for three selected individual thiols. We considered 3MH, 3MHA and 4MMP to be representative of the chemical class. Since the concentration influences the way the aroma is described, three levels of each thiol (zero, medium and high) were included in the experimental design. To take into consideration possible changes caused by the matrix, four cultivars were used (figure 1). The varieties chosen were Pinotage (x1), Cabernet Sauvignon (x2), Shiraz (x1) and Merlot (x1). As base, of this specific compound. PROBLEM STATEMENT

we decided to have a wine that could be recognised as such, but without any specific or typical aroma. To create this base, the chosen commercial wines were de-aromatised under vacuum to avoid oxidation. Each thiol was evaluated on a different tasting day. Addition of individual thiols was done the day before each evaluation to ensure equilibrium. Practically, the wines were evaluated blind by 15 experienced judges from the sensory panel of the Department of Viticulture and Oenology. Each day 15 wines, plus two blind repeats, were evaluated in duplicate. The method we chose to capture the results is called Projective Mapping (PM). It allows to assess a large number of samples by giving an idea of how similar or different they are in relation to each other. As an additional information, descriptors of each wine are generated (for more details, see Part 1). RESULTS The questions we asked were: • Do increasing levels of thiols in red wine generate different descriptors? • What role does the matrix (in this case different cultivars) play for each thiol level? The results point at two types of outcomes. 3MH and 3MHA behaved similarly and have shown that the separation between

JULY 2020

VALERIA PANZERI, GONZALO GARRIDO-BANUELOS & ASTRID BUICA: Department of Viticulture and Oenology, Stellenbosch University, Stellenbosch KEYWORDS: Red wine cultivars, thiols, Projective Mapping

and mimicking Mother Nature’s most appealing aromas. They develop flavours to be used in the food industry with the aim of keeping the consumers happy. They are familiar with the elusive character of those chemicals. Take benzaldehyde as a classic example. It is the base flavouring component of many candies and cough syrups, but while it is mostly described as ‘cherry’, many people agree that it reminds them of almonds and marzipan instead. In the same way, increasing levels of thiols can vastly change the profile of a white wine. For example, 3MHA in low concentrations is perceived as having ‘guava’ aroma, but once the level increases, the description changes to a not so pleasant ‘sweaty’ character.

The split personality of thiols – ‘tropical fruits’, ‘meaty’ or ‘berry jam’? The work presented in this article forms part of series of experiments on the interaction between thiols and red wine matrices as described in Part 1. BACKGROUND A curious fact about chemical compounds is that describing their aroma can be quite elusive, especially because the perception is not linear. In other words, naming the descriptors can be difficult even for experienced tasters, even more so because with increase in concentration, attributes can change (hence the “multiple personalities”). Consider flavour scientists for a moment. They work constantly at reproducing

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