 IVES 9 IVES Conference Series 9 Measuring elemental sulfur in grape juice in relation to varietal thiol formation in Sauvignon blanc wines.

Measuring elemental sulfur in grape juice in relation to varietal thiol formation in Sauvignon blanc wines.

Abstract

Aim: Sauvignon blanc displays a range of styles that can include prominent tropical and passionfruit aromas. Both sensory evaluation and chemical analysis have confirmed the above-average presence of ‘varietal thiols’ in the Sauvignon blanc wines from Marlborough, New Zealand.1 The varietal thiols are released from cleavage of non-volatile sulfur-containing precursors or an interaction between a sulfur donor and a C6-compound.2 Machine-harvesting is the most common harvesting practice used in New Zealand, by which, there is a higher probability to add some leaves to the must. Leaves and grapes can contain elemental sulfur (S0), which is commonly sprayed in the fields to protect berries against powdery mildew. S0 is known to cause unwanted reductive aromas, including H2S, in certain wines unless remediation steps are undertaken during winemaking. Also, it was shown that extra S0 addition to the crushed grapes could lead to more varietal thiol formation in wines.3 Despite the clear effects of residual S0 present in the must on the final wine quality and aroma4, its measurement is not a regular practice undertaken in wineries due to the lack of easy and applicable methods.

Methods: We have optimized a sulfide sensor for S0 measurement in grape juice samples and investigated the correlation between S0 concentration in grape juice and varietal thiols concentration in final wines. A simple apparatus was designed to reduce S0 to sulfide using dithiothreitol (under acidic conditions, as H2S), followed by an ion-selective electrode (ISE) to measure sulfide concentrations (under alkaline conditions as S2-). GC-MS is being used to analyze thiol concentrations in wine samples to allow comparisons to be made with juice S0 concentrations.

Results: The semi-log calibration curve plotted based on the ISE data showed very good linearity. The results also showed that the reduction process was successful, and the apparatus is working well with both standard and juice samples. The ISE was confirmed to be able to detect the reduced sulfur at concentrations as low as 0.01 ppm.

Conclusion

The methodology allows action between the concentration of S0 residues and the concentration of varietal thiols in the final wines to be investigated. The analysis is applicable in a winery setting to evaluate the potential of grape juices to form varietal thiols and/or reductive compounds in wines

DOI:

Publication date: September 7, 2021

Issue: Macrowine 2021

Type: Article

Authors

Bahareh Sarmadi

School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand,Paul A. Kilmartin, School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand Brandt P. Bastow, School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand

Contact the author

Keywords

Sauvignon blanc, elemental sulfur, varietal thiols

Citation

Which heat test really represents the haze risk of a white Sauvignon wine ?

AIM: Different heat tests are used to predict a white wine haze risk after bottling. The most used tests are 30-60 min. at 80°C. Nevertheless, there is a lack of information about the relationship between the wine haze observed after such tests and the turbidities observed in the bottles after the storage/transport of the wines in more realistic Summer conditions (35-46°C during 3-12 days)

Long-lasting flavour perception of wines treated with oenological additives considering the individual PROP taste-phenotype

The use of oenological additives is becoming a common practice due to the technological and sensory properties they provide to the wines. However, the number of studies focused on the impact that these additives might induce on wine flavor perception during wine tasting is still quite scarce. The aim of this work was to evaluate the effect of three different types of common oenological additives: two oenotannins (ellagitannin and gallotannin) and a commercial preparation of yeast mannoproteins on the long-lasting flavor perception (aroma and astringency).

Exploring the inner secrets of grapevine: a journey through plant-microbe interactions

Throughout centuries of anthropocentric breeding, plants have been selectively bred to enhance their quality traits and yield, often overlooking the importance of neglected attributes, like those involved in the interactions with beneficial microorganisms. This phenomenon led to an alteration in the distribution of photosynthetic products, shifting from defence mechanisms to growth, commonly described as ‘domestication syndrome’. Addressing the losses stemming from this condition is imperative just as unravelling the concealed communication between grapevines and beneficial microorganisms.

The use of elicitors in viticulture: a tool to obtain highly colored wines with a reduce alcohol content?

Climate change is causing a gap between the technological and phenolic maturity of grapes, resulting in wines with high alcohol content and low polyphenol concentration. Another phenomenon associated with high temperatures and whose effect is more pronounced if the harvest is delayed is the decrease in the acidity of the grapes, mainly in malic acid, and an increase in pH caused by the accumulation of potassium derived from the increase in temperature. Therefore, climate change and the effects it causes on the vine leads to unbalanced wines, with high alcohol content and lack of color, with green tannins, astringency and excessively low acidity if not corrected.

Using δ13C and hydroscapes as a tool for discriminating cultivar specific drought response

Measurement of carbon isotope discrimination in berry juice sugars at maturity (δ13C) provides an integrated assessment of water use efficiency (WUE) during the period of berry ripening, and when collected over multiple seasons can be used as an indication of drought stress response. Berry juice δ13C measurements were carried out on 48 different varieties planted in a common garden experiment in Bordeaux, France from 2014 through 2021 and were paired with midday and predawn leaf water potential measurements on the same vines in a subset of six varieties. The aim was to discriminate a large panel of varieties based on their stomatal behaviour and potentially identify hydraulic traits characterizing drought tolerance by comparing δ13C and hydroscapes (the visualisation of plant stomatal behaviour as a response to predawn water potential). Cluster analysis found that δ13C values are likely affected by the differing phenology of each variety, resulting in berry ripening of different varieties taking place under different stress conditions within the same year. We accounted for these phenological differences and found that cluster analysis based on specific δ13C metrics created a classification of varieties that corresponds well to our current empirical understanding of their relative drought tolerances. In addition, we analysed the water potential regulation of the subset of six varieties (using the hydroscape approach) and found that it was well correlated with some δ13C metrics. Surprisingly, a variety’s water potential regulation (specifically its minimum critical leaf water potential under water deficit) was strongly correlated to δ13C values under well-watered conditions, suggesting that base WUE may have a stronger impact on drought tolerance than WUE under water deficit. These results give strong insights on the innate WUE of a very large panel of varieties and suggest that studies of drought tolerance should include traits expressed under non-limiting conditions.