WINETECH Technical Yearbook 2021

Various concentrations in-between these extreme values were also tested. The samples were presented blind to a trained sensory panel in randomised order. The judges were given one minute to rate the intensity of predetermined sensory attributes. After the evaluation of each sample, the panellists were instructed to pause for one minute. The timing of evaluation and the resting period was controlled by a leader who instructed the panellists when to start and when to stop the evaluation. RESULTS 3MH As expected, the intensity of the attributes guava, tomato leaf and passion fruit increased as the 3MH concentration increased. However, this increase in intensity was not linear, meaning that the attribute intensity did not increase to the same magnitude as the concentration. • Guava Of all the attributes generated, guava was scored at the highest intensity. The intensity of guava remained relatively similar as the 3MH concentration increased from level 1 to level 2. Only when increasing the concentration from level 2 to level 3 did the judges report a notable increase in the sensory intensity. A substantial increase in

TABLE 1. Main volatile thiols in wine and their perception thresholds (Coetzee & Du Toit, 2012).

Compound

Abbreviation

Olfactory description

Perception threshold (ng/L) in model wine

4-Mercapto-4-methylpentan-2-one

4MMP

box tree, passion fruit, broom, blackcurrant

0.8

3-Mercaptohexan-1-ol

3MH

passion fruit, grapefruit, gooseberry, guava

60

3-Mercaptohexan-1-ol acetate

3MHA

passion fruit, grapefruit, box tree, gooseberry, guava

4.2

a model wine solution helps to standardise this threshold, however, the medium is deprived of many compounds naturally present in wine. A real wine has thousands of compounds present (volat i le and non-volat i le) and several interactions (enhancing/ suppressing) will determine the ultimate perception of individual compounds, as well as the wine aroma as a whole. For individual flavour compounds to be perceived above the general vinous aroma, it needs to be present at sufficient concentrat ions, usual ly wel l above the determined perception threshold. Therefore, the presence of volatile thiols in wine does not guarantee fruity aromatics and the intensity of thiol attributes are dependent on the concentrations present. In sc ient i f i c publ i cat ions , studies often report the effect of viticultural or

oenological treatments on the volatile thiol concentration in the finished wine. However, even though the concentration differences in treatment outcomes might be statistically significant, it does not necessarily relate to sensorially significant results. Therefore, on paper, it might seem to have a large impact on the wine, but when evaluated sensorially, the difference might be marginal or negligible (Du Toit, 2020). MATERIALS AND METHODS For this study, a wine absent of any prominent aroma attributes was prepared. A Sauvignon blanc 2018 wine was stripped of volatile thiols by fining the wine with 5 g/L activated carbon. After filtering, the wine was sparged with nitrogen gas for 10- 15 minutes to remove any dissolved oxygen. The dearomatised wine was chemically analysed to determine the concentration of residual aroma compounds. Results

showed that the fining process completely removed all the volatile thiols, while some aroma compounds, such as esters, were still present at low concentrations. The wine, therefore, had a neutral vinous character. The dearomatised wine was spiked with increasing concentrations of 3MH and 3MHA one hour before sensory evaluation. Results are reported in concentration levels: • 3MH Level 1 = 0 ng/L. Level 5 = 12 000 ng/L, which is a substantial amount of 3MH. Various concentrations in-between these extreme values were also tested. • 3MHA Level 1 = 6 ng/L. Level 6 = 400 ng/L.

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