Do smaller thiol increases have a significant sensorial effect?

by | Jan 1, 2021 | Winetech Technical, Oenology research

PHOTO: Pixnio.

Researchers from the South African Grape and Wine Research Institute and Department of Viticulture and Oenology at Stellenbosch University performed a Winetech-funded study to investigate the minimum concentrations of 3MH and 3MHA needed to notably alter the wine’s sensorial properties.

Introduction

The presence of sufficient concentrations of volatile thiols will result in the wine having prominent tropical fruit characteristics with sensory attributes described as passion fruit, guava, grapefruit, tomato leaf and blackcurrant. These compounds are particularly potent in wines due to their very low sensory perception thresholds (Table 1). However, perception thresholds reported for individual aroma compounds provide a general indication regarding the minimum concentration needed for any aroma to be sensorially perceived. The problem with perception thresholds is that they are often determined in a model wine solution and not in real wines. Determining the perception threshold in 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 (volatile and non-volatile) 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 concentrations, usually well 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 scientific publications, 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.2

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.

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 intensity was again reported when the 3MH concentration was increased from level 4 to level 5.

  • Tomato leaf

Even though increasing the 3MH concentration from level 1 to level 3 resulted in a gradual increase in the intensity of tomato leaf, the increase in intensity was not substantial. Only from level 3 to level 4 was the tomato leaf intensity reported as being notably higher.

  • Passion fruit

The intensity of passion fruit also increased gradually from level 1 to level 3, however, at level 4 a decrease in intensity was observed. This confirms that the intensity of attributes originating from specific aroma compounds does not always increase in agreement with the increase in concentration.

  • Green apple

The intensity of green apple was reported as being relatively low for all samples and a slight decrease in intensity was observed from level 1 to level 5.

  • Cooked veg/onion

At most of the 3MH concentrations, the intensity of cooked veg/onion was reported as being very low. However, the judges reported a notable increase in cooked veg/onion when the concentration increased from level 4 to level 5. Therefore, at these extreme concentrations, 3MH could also contribute to the reductive aroma of wine.

3MHA

 

  • Guava

As with 3MH, the guava attribute was scored at the highest intensity when compared to other attributes. Here, a notable increase in intensity was observed when the 3MHA concentration increased from level 1 to level 3. Another increase was reported when the concentration increased from level 2 to level 5.

  • Passion fruit

The intensity of the passion fruit attribute increased as the concentration of 3MHA increased, however, this increase was not always noteworthy.

  • Tomato leaf

As with 3MH, a substantial increase in tomato leaf intensity was only reported when 3MHA was at very high concentrations.

  • Blackcurrant

The panellists reported an important increase in blackcurrant aroma as soon as the concentration exceeded level 4.

Conclusion

When it comes to sensorially relevant increases of thiol-related attributes, it seems that concentration increases need to be relatively large for aromatic changes to take place. Even though the very low perception thresholds reported (in model wine) for these thiol compounds demonstrate the potency of these aroma compounds, in a real wine medium smaller increases might not have the desired effect.

This should be considered before changing winemaking processes according to scientific data alone. Preferably, the effect of viticultural and oenological treatments reported in research studies should not only include chemical results, but it should also report the sensory significance.

The complex interactive effects that occur between aroma compounds in wines, which are not dearomatised, should also be considered. Sensorial differences observed as compound concentrations changes could rely on the presence of supporting compounds or groups of compounds. These are extremely complex interactions and will be unique for each unique wine.

Abstract

Thiols are powerful aroma compounds sensorially described as guava, passion fruit and tomato leaf. These compounds have very low sensory perception thresholds in a model wine solution, however, in a real wine medium, the sensory impact is complex and influenced by interactive effects with other wine constituents. Research studies often show the effects of viticultural and oenological treatments on the concentration of thiols in wine. Whether these concentration changes have a significant impact on the sensory perception of the wine is not always known. A recent Winetech-funded study by researchers, Matija Lesković, Dr. Jeanne Brand 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, investigated the effect of increased thiol concentrations on the sensory significance of a wine. Results showed that the concentrations of 3MH and 3MHA need to increase considerably for sensory changes to be sensorially perceived.

References

  1. Coetzee, C. & Du Toit, W.J., 2012. A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols. Food Research International 45(1).
  2. Du Toit, W., 2020. Factors affecting the perception of thiols in white wines. Winetech Final Projects Report, No. WW WdT 17/02.

– For more information, contact Carien Coetzee at carien@basicwine.co.za.

 

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