Evolution of polyfunctional mercaptans and their precursors during Merlot alcoholic fermentation

Unravelling copper effect on the production of varietal thiols during Colombard and Gros Manseng grape juices fermentation by Saccharomyces cerevisiae

Nowadays the rapidly increasing organic vineyard management with the utilization of copper as sole fungal control pesticide against downy mildew raises once again the question of copper impact on varietal thiols in wine. For this purpose, Colombard and Gros Manseng grape juices were fermented under different copper levels (from 0.2 to 3.88  mg/l) to mimic the consequences in must of organic practices. The consumption of thiol precursors and the release of varietal thiols (both free and oxidized forms of 3-sulfanylhexanol and 3-sulfanylhexyl acetate) were monitored by LC-MS/MS. It was found that the highest copper level (3.6 and 3.88  mg/l for Colombard and Gros Manseng respectively) significantly increased yeast consumption of precursors (by 9.0 and 7.6% for Colombard and Gros Manseng respectively). For both grape varieties, free thiol content in wine significantly decreased (by 84 and 47% for Colombard and Gros Manseng respectively) with the increase of copper in the starting must as already described in the literature. However, the total thiol content produced throughout fermentation was constant regardless of copper conditions for the Colombard must, meaning that the effect of copper was only oxidative for this variety. Meanwhile, in Gros Manseng fermentation, the total thiol content increased along with copper content, resulting in an increase up to 90%; this suggests that copper may modify the regulation of the production pathways of varietal thiols, also underlining the key role of oxidation. These results complement our knowledge on copper effect during thiol-oriented fermentation and the importance of considering the total thiol production (reduced+oxidized) to better understand the effect of studied parameters and differenciate chemical from biological effects.

New Light on the Varietal Thiols Pathway during Alcoholic Fermentation: Role of 3-S-(N-Acetyl-cysteinyl)-hexan-1-ol (NAC3SH)

For many years, knowledge on thiol precursors has been limited to S-conjugates of glutathione (G3SH), cysteine (Cys3SH), and later on the dipeptides γ-GluCys and CysGly. In this work, we took the parallel between precursor degradation and the glutathione-mediated detoxification pathway a step further by considering a new type of derivative, 3-S-(N-acetyl-l-cysteinyl)hexanol (NAC3SH). This compound was synthesized and then added to the existing liquid chromatography with tandem mass spectrometry (LC-MS/MS) method of thiol precursors. This intermediate was only identified during alcoholic fermentation in synthetic must spiked with G3SH (1 mg/L or 2.45 μmol/L) in the presence of copper with concentration above 1.25 mg/L, which demonstrates for the first time the existence of this new derivative (until 126 μg/L or 0.48 μmol/L) and the capacity of the yeast to produce such a compound. Its status as a precursor was also studied during fermentation, in which a release of 3-sulfanylhexanol was noted corresponding to a conversion yield close to 0.6%. This work completed the thiol precursor's degradation pathway in Saccharomyces cerevisiae in synthetic conditions with a new intermediate, confirming its connection with the xenobiotic detoxification pathway and giving new insights on the precursor's fate.

Unraveling the Mystery of 3-Sulfanylhexan-1-ol: The Evolution of Methodology for the Analysis of Precursors to 3-Sulfanylhexan-1-ol in Wine

Volatile polyfunctional thiol compounds, particularly 3-sulfanylhexan-1-ol (3SH) and 3-sulfanylhexyl acetate (3SHA), are key odorants contributing to the aroma profile of many wine styles, generally imparting tropical grapefruit and passionfruit aromas. 3SH and 3SHA are present in negligible concentrations in the grape berry, juice, and must, suggesting that they are released from non-volatile precursors present in the grape. The exploration of the nature and biogenesis of these precursors to 3SH and 3SHA has proven important for the elucidation of polyfunctional thiol biogenesis during alcoholic fermentation. The development and validation of appropriate analytical techniques for the analysis of 3SH precursors in enological matrices have been extensive, and this review explores the analysis and discovery of these precursor compounds. The development of analytical methods to analyze 3SH precursors, from the selection of the analytical instrument, sample preparation, and methods for standardization, will first be discussed, before highlighting how these techniques have been used in the elucidation of the biogenesis of 3SH and 3SHA in grape wines. Lastly, the future of thiol precursor analysis will be considered, with the development of new methods that greatly reduce the sample preparation time and enable multiple precursors, and the thiols themselves, to be quantitated using a single method.

Truth in wine yeast

Evolutionary history and early association with anthropogenic environments have made Saccharomyces cerevisiae the quintessential wine yeast. This species typically dominates any spontaneous wine fermentation and, until recently, virtually all commercially available wine starters belonged to this species. The Crabtree effect, and the ability to grow under fully anaerobic conditions, contribute decisively to their dominance in this environment. But not all strains of Saccharomyces cerevisiae are equally suitable as starter cultures. In this article, we review the physiological and genetic characteristics of S. cerevisiae wine strains, as well as the biotic and abiotic factors that have shaped them through evolution. Limited genetic diversity of this group of yeasts could be a constraint to solving the new challenges of oenology. However, research in this field has for many years been providing tools to increase this diversity, from genetic engineering and classical genetic tools to the inclusion of other yeast species in the catalogues of wine yeasts. On occasion, these less conventional species may contribute to the generation of interspecific hybrids with S. cerevisiae. Thus, our knowledge about wine strains of S. cerevisiae and other wine yeasts is constantly expanding. Over the last decades, wine yeast research has been a pillar for the modernisation of oenology, and we can be confident that yeast biotechnology will keep contributing to solving any challenges, such as climate change, that we may face in the future.

Evolution and Correlation of cis-2-Methyl-4-propyl-1,3-oxathiane, Varietal Thiols, and Acetaldehyde during Fermentation of Sauvignon blanc Juice

cis-2-Methyl-4-propyl-1,3-oxathiane (cis-2-MPO) was recently identified in wine and proposed to arise from the reaction of 3-sulfanylhexan-1-ol (3-SH) and acetaldehyde. However, the evolution profile of cis-2-MPO during alcoholic fermentation (AF) and storage and its relationship with varietal thiols and acetaldehyde production were unknown. These aspects were investigated by fermenting Sauvignon blanc juice with J7 and/or VIN13 yeast strains and assessing the stability of cis-2-MPO during wine storage. Moderate to strong Pearson correlations verified similar evolution trends between acetaldehyde, 3-sulfanylhexyl acetate, and cis-2-MPO, with initial increases and a peak during the early to middle stages of AF before consecutive decreases until the end. Contrarily, 3-SH correlated moderately only at the end of AF. A consistent decrease observed for cis-2-MPO when spiked into Sauvignon blanc wine and assessed during 1-year storage revealed its general instability, but acetaldehyde addition (100 mg/L), pH 3.0, and storage at 4 °C all appeared to retain cis-2-MPO. These results have implications for wine aroma and the potential for cis-2-MPO to act as a sink (or source) for 3-SH in wine over time.

Spotlight on release mechanisms of volatile thiols in beverages

Volatile thiols are very strong-smelling molecules that can impact the aroma of numerous beverages. Several thiols and thiol precursors have been reported previously in different plants used as raw material for beverages, some of which are fermented. We focused on thiols in beverages and their release mechanisms from precursors during processing. Volatile thiols in beverages can be classified aslow molecular weight volatile thiols (e.g. H₂S, methanethiol) which impact the smell negatively, and volatile thiols with higher boiling points that contribute positively to the aroma profile. The first part of this review is devoted to volatile thiols, without considering small malodorous molecules. The second part deals with thiol precursors and the different release mechanisms induced by processing (e.g. extraction, roasting or fermentation) and by the growing methods (e.g. viticulture), which can impact on amounts of thiols and their precursors.

Thiol precursors in Vitis mould-tolerant hybrid varieties

BACKGROUND

Over recent years, a new wave of disease-tolerant to mildew varieties has flooded the viticulture sector, for reasons of human safety and economic expediency. These hybrid grape cultivars are selected mainly on the basis of their intrinsic capability to counter the attack of the main fungal diseases that affect grape production, such as downy mildew and powdery mildew. However, their organoleptic and oenological characteristics have not yet been studied in depth for purposes of both juice and wine production, due to the high number of newly proposed germplasms and the lack of information about their adaptability to different environments. This work examines the thiol aroma precursors concentration in 64 red and white disease-tolerant hybrid varieties in the vine germplasm collections of Consiglio per la ricerca in agricoltura e l'analisi dell'economia agraria-Viticulture and Enology Research Center and Fondazione Edmund Mach, both from the north-east of Italy.

RESULTS

All cultivars showed the presence of 3-mercaptohexan-1-ol (3MH) precursors, ranging from 4.4 to 1141 μg kg^-1 for 3-S-glutathionyl hexan-1-ol (GSH-3MH), and from 0.3 to 136 μg kg^-1 for 3-S-cysteinyl 3-hexan-1-ol. The concentration of GSH-3MH exceeded 600 μg kg^-1 in eight varieties, with values comparable to those of the richest Vitis vinifera reported so far. On average, red grapes showed higher concentrations of 3MH precursors than white ones did. Only two hybrids had 4-mercapto-4-methylpentan-2-one (4MMP) precursors over the limit of quantification, albeit with a much higher concentration than those normally reported in Sauvignon Blanc.

CONCLUSION

This is the first detailed survey of 3MH and 4MMP precursors carried out taking into account a considerable number of hybrid grape varieties. The results show that some of these varieties could be interesting for the production of tropical juices or tropical-aromatic wines and soft drinks, through the enzymatic liberation of thiol aromas, as well as for the production of aromatizing tannins to be used in the food industry. © 2020 Society of Chemical Industry.

Revisiting the evaluation strategy of varietal thiol biogenesis

The varietal thiols 3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA), and 4-mercapto-4-methylpentan-2-one (4MMP) are key aroma compounds in wine due to the tropical notes they impart. They are released by yeast during alcoholic fermentation from different precursors. However, a large part of 3MH origin remains unknown. In this study, we focused on dipeptide forms arising from glutathione S-conjugates to 3MH and 4MMP. Using labelled tracers, we showed in spiked must the release of varietal thiols from 4 different compounds. We highlighted the interconversion between different forms of precursors under defined enological conditions. Cysteinyl-glycine S-conjugates are partially degraded into cysteine S-conjugates, contrary to γ-glutamyl-cysteine S-conjugates. Glutathione S-conjugate to 3MH can be partially degraded to γ-glutamyl-cysteine S-conjugate to 3MH. For the first time, all these labeled forms of precursors were found to release 3MH or 4MMP between 0.17 and 1% molar conversion yield. Two different yeasts were compared without any significant difference.

Chiral Polyfunctional Thiols and Their Conjugated Precursors upon Winemaking with Five Vitis vinifera Sauvignon blanc Clones

Five co-located clones of Sauvignon blanc grapes were fermented under controlled conditions at laboratory-scale to investigate the impact of yeast strain, commercial enzyme, or nutrient addition on the concentrations of enantiomers of 3-sulfanylhexan-1-ol (3-SH) and 3-sulfanylhexyl acetate (3-SHA) in resulting wines. The relationship of these enantiomers with the odorless 3-SH precursors present in diastereomeric forms in grape juice was also examined. Possible variations may have existed due to clone type, not only for the diastereomers of 3-SH precursors in juices but also for the enantiomers of 3-SH and 3-SHA in the resulting wines, although there was no obvious stereochemical relationship between precursors and free thiols. From a flavor enhancement perspective, the use of a commercial enzyme in the juice significantly enhanced 3-SH production for some clones. In contrast, less impact on the production of 3-SH and 3-SHA was seen as a result of yeast strain and nutrient regardless of clone type.

Monitoring volatile compounds production throughout fermentation by Saccharomyces and non-Saccharomyces strains using headspace sorptive extraction

Currently, there is a growing interest in the use of non-Saccharomyces yeast to enhance the aromatic quality of wine, with pure or mixed cultures, as well as sequential inoculation. Volatile components of wines were closely related to their sensory quality. Hence, to study the evolution of volatile compounds during fermentation was of great interest. For this, sampling methods that did not alter the volume of fermentation media were the most suitable. This work reports the usefulness of headspace sorptive extraction as non-invasive method to monitor the changes in volatile compounds during fermentation. This method allowed monitoring of 141 compounds throughout the process of fermentation by Saccharomyces cerevisiae and Lachancea thermotolerans strains. Both strains showed a similar ability to ferment a must with high sugar content. The S. cerevisiae strain produced higher amount of volatile compounds especially esters that constitutes fruity aroma than L. thermotorelans.

Spotlight on Varietal Thiols and Precursors in Grapes and Wines

Wine is an amazingly complex natural product that requires dedicated scientists to resolve many of its mysteries. Traditional synthetic organic chemistry and modern analytical techniques are powerful tools at the disposal of wine chemists who tackle the complexities of wine in order to improve scientific understanding and provide practical solutions to industry. Part of this quest for knowledge relates to maintaining or improving wine quality, which underpins consumer acceptance and links to the competitiveness of wineries in a global market. Wine aroma is an important aspect of wine quality and garners much attention from researchers. Grape-derived aroma compounds are one area of particular importance owing to their distinctiveness and ability to impart ‘varietal aromas’ to wines. Varietal thiols imparting tropical and citrus notes that are characteristic of wines such as Sauvignon Blanc have emerged, along with their grape-derived precursors, as an area of interest over the past two decades. These compounds have also caught our attention and we have made some important contributions to this field, including identifying new precursors, developing novel analytical methods, and conducting studies that provide unique insights into the biochemical transformations occurring in grape berries and juice, and during fermentation.