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Aims Volatile thiols are very potent aroma molecules that contribute to the aroma of many beverages. The characteristic thiols of certain wine varieties such as Sauvignon blanc are partly released during the yeast-based fermentation from plant-synthesized glutathione- or cysteine-conjugated and dipeptic precursors present in the must. In this work, we aimed at the construction and characterization of yeast strains with the ability to synthesize volatile thiols from respective precursors. Methods and Results Besides genome integration of the Escherichia coli gene tnaA , which encodes an enzyme with high β -lyase activity, a glutathione synthetase and glutathione- S -transferases were overexpressed. Up to 8.9 μg L−1 3-mercaptohexan-1-ol could be formed with the strain from externally added trans -2-hexen-1-ol. Well-characterized thiols such as 2-methyl-2-butanethiol, 3-mercapto-3-methylbutan-1-ol, and 8-mercapto- p -menthan-3-one, as well as several so far undescribed thiol compounds could be synthesized. Conclusion Volatile thiols could be produced by feeding alcohol, alkenol, aldehyde, or ketone precursors like trans -2-hexenal, trans -2-hexen-1-ol, cis -2-hexen-1-ol, 3-methyl-2-buten-1-ol, 3-buten-2-one, and pulegone to the optimized yeast cells. [ABSTRACT FROM AUTHOR]

The biotransformation of hop aroma, particularly by the cysteine S-conjugate beta-lyase enzyme (CSL), has been a recent topic of tremendous interest among brewing scientists and within the brewing community. During a process often referred to as biotransformation, yeast-encoded enzymes convert flavorless precursor molecules found in barley and hops into volatile thiols that impart a variety of desirable flavors and aromas in beer. Two volatile thiols of particular interest are 3-mercaptohexan-1-ol (3MH) and its acetate ester, 3-mercaptohexyl acetate (3MHA), which impart guava and passionfruit flavors, respectively. In this study, a parental Saccharomyces cerevisiae brewing strain that displayed low thiol biotransformation activity was genetically manipulated (GM) to substantially increase its thiol biotransformation potential. Construction of this GM strain involved integration of a gene encoding a highly active CSL enzyme that converts thiol precursors into the volatile thiol, 3MH. Three additional strains were subsequently developed, each of which paired CSL expression with expression of an alcohol acyltransferase (AAT) gene. It was hypothesized that expression of an AAT in conjunction with CSL would increase production of 3MHA. Fermentation performance, sensory characteristics, and 3MH/3MHA production were evaluated for these four GM strains and their non-GM parent in 1.5hL fermentations using 100% barley malt wort hopped at low levels with Cascade hops. No significant deviations in fermentation performance (time to attenuation, final gravity, alcohol content, wort fermentability) or finished beer chemistry were observed between the GM strains and the parent strain with the exception of the speed of vicinal diketones reduction post-fermentation, which was quicker for the GM strains. The GM strains produced beer that had up to 73-fold and 8-fold higher 3MH and 3MHA concentrations than the parent strain, achieving concentrations that were up to 79-fold greater than their sensory detection thresholds. The beers were described as intensely tropical and fruity, and were associated with guava, passionfruit, mango, pineapple and sweaty aromas. These experiments demonstrate the potential of genetic modification to dramatically enhance yeast biotransformation ability without creating off flavors or affecting fermentation performance. [ABSTRACT FROM AUTHOR]

The biotransformation of hop aroma, particularly by the cysteine S-conjugate beta-lyase enzyme (CSL), has been a recent topic of tremendous interest among brewing scientists and within the brewing community. During a process often referred to as biotransformation, yeast-encoded enzymes convert flavorless precursor molecules found in barley and hops into volatile thiols that impart a variety of desirable flavors and aromas in beer. Two volatile thiols of particular interest are 3-mercaptohexan-1-ol (3MH) and its acetate ester, 3-mercaptohexyl acetate (3MHA), which impart guava and passionfruit flavors, respectively. In this study, a parental Saccharomyces cerevisiae brewing strain that displayed low thiol biotransformation activity was genetically manipulated (GM) to substantially increase its thiol biotransformation potential. Construction of this GM strain involved integration of a gene encoding a highly active CSL enzyme that converts thiol precursors into the volatile thiol, 3MH. Three additional strains were subsequently developed, each of which paired CSL expression with expression of an alcohol acyltransferase (AAT) gene. It was hypothesized that expression of an AAT in conjunction with CSL would increase production of 3MHA. Fermentation performance, sensory characteristics, and 3MH/3MHA production were evaluated for these four GM strains and their non-GM parent in 1.5hL fermentations using 100% barley malt wort hopped at low levels with Cascade hops. No significant deviations in fermentation performance (time to attenuation, final gravity, alcohol content, wort fermentability) or finished beer chemistry were observed between the GM strains and the parent strain with the exception of the speed of vicinal diketones reduction post-fermentation, which was quicker for the GM strains. The GM strains produced beer that had up to 73-fold and 8-fold higher 3MH and 3MHA concentrations than the parent strain, achieving concentrations that were up to 79-fold greater than their sensory detection thresholds. The beers were described as intensely tropical and fruity, and were associated with guava, passionfruit, mango, pineapple and sweaty aromas. These experiments demonstrate the potential of genetic modification to dramatically enhance yeast biotransformation ability without creating off flavors or affecting fermentation performance.

The effect of ripeness on the concentration of polyfunctional thiol precursors was investigated in 'Gewürztraminer' juices in two vintages characterised by very different climate conditions. An incremental trend of glutathionyl-3-mercaptohexan-1-ol and cysteinyl-3-mercaptohexan-1-ol during ripening was observed. The increase in the last phase of maturation was noticeable in both vintages. The concentration of 4-S-glutathionyl-4-methylpentan-2-one and 4-S-cysteinyl- 4-methylpentan-2-one was confirmed to be low in 'Gewürztraminer', although the latter was found at quantifiable levels in a couple of samples. The management of the harvest date appears to be highly important in order to exploit the potential grapefruit-like note related to polyfunctional thiols of little aromatic 'Gewürztraminer' wines, particularly in case of challenging vintage years. [ABSTRACT FROM AUTHOR]

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 [ABSTRACT FROM AUTHOR]

Thiol compounds responsible for tropical fruit associated aroma have been extensively studied over the last 20 years. The occurrence of their non-aromatic precursors in grapes and musts is reported largely mainly for the cultivar Sauvignon Blanc. The presence of these thiols as precursors or free molecules in grape, juice, and wine has been reported in several different varieties, suggesting that they are more or less ubiquitous both for Vitis spp. and interspecific hybrids. The biosynthetic pathways resulting in these compounds are yet to be completely elucidated, but, in the meantime, industry needs to improve technological knowledge to better manage winemaking steps to enhance the variety-dependent aroma of wine. In this work, we studied the implications of the use of grape skin tannins—rich and poor in thiol precursors—on the final content of 3-sulfanylhexan-1-ol (3MH) and its acetate (3MHA) in wine and the effect in terms of sensory appreciability. The evaluation of 36 vinifications carried out in a semi-industrial scale permitted us to prove that only a tannin originally rich in precursors (High), when added to juice at the beginning of fermentation, enhanced both the concentration of precursors in the juice and the final concentration of aromatic thiols in the resultant wine. The 3MH and 3MHA developed as a consequence of the juice supplementation with tannin High and increased pleasantness and typicality of Gewürztraminer wines. A later supplementation with tannin High at the end of the alcoholic fermentation was sensorially not effective. [ABSTRACT FROM AUTHOR]

The effect of ultrasound (20 kHz, 153 μm) on the prefermentation extraction mechanisms in Sauvignon Blanc grapes was studied, focusing on 3-mercaptohexan-1-ol (3MH) and 4-mercapto-4-methyl-pentan-2-one (4MMP) precursors linked to glutathione (GSH) and cysteine (Cys). The treatment determined a positive extraction trend between the duration (untreated, 3 and 5 min) and the conductivity or the concentration of catechins and total phenols, significantly differentiated after 5 min. Nevertheless, the concentration of the thiol precursors in grape juice not only remained undifferentiated, but that of 3-S-glutathionyl mercaptohexan-1-ol showed a negative trend with the treatment time applied (168 ± 43, 156 ± 36, and 149 ± 32 μg/L, respectively, for control, 3 and 5 min). The divergence on the effect between families of compounds suggests an interaction between the sonication treatment and thiol precursor molecules. In order to evaluate the possible degradation properly, ultrasound was applied in a model solution spiked with 3MH and 4MMP precursors, reproducing the conditions of grapes. Except for Cys-3MH, the mean concentration (n = 5) for the rest of the precursors was significantly lower in treated samples, predominantly in those linked to glutathione (~−22% and ~18% for GSH-3MH and GSH-4MMP) rather than to cysteine (~−6%~−8% for Cys-3MH and Cys-4MMP). The degradation of precursors was associated with a significant increase of 3MH and 4MMP. The formation of volatile thiols following sonication is interesting from a technological point of view, as they are key aroma compounds of wine and potentially exploitable in the wine industry through specific vinification protocols.

Both 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA) were largely studied for the last 20 years due to their pleasant olfactory notes conferred to wine. Until now, many analytical methods focused only on the free forms of both 3MH and 3MHA in wine that provided partial information in the wine aroma evolution. Our study proposes new analytical measurements which allow quantification of both free and disulfide forms of 3MH and 3MHA to better understand the redox phenomenon occurring in wines and further, to orientate wine aroma evolution. Free thiols were analyzed by an original method based on maleimide derivatization allowing in-situ disulfide reduction followed by SIDA-LC–MS/MS analyses exhibiting excellent performances. Indeed, the accuracy ranged from 95 to 110% in three different wine matrices and the repeatability and intermediate reproducibility were inferior to 15% (RSD measurements). Our method exhibited very low limits of detection, which are below to 0.5 ng/L and inferior to the perception thresholds of both compounds. Then, this method was applied to three different wines exposed to several oxidative conditions. On the one hand, it was demonstrated that copper sulfate treatment firstly destroyed the total amount of free 3MH to the benefit of thioether and disulfides compounds, with proportions that could be slightly modified by glutathione addition. On the other hand, oxygenation of wines resulted in partial free 3MH destruction to the benefit of thioether compounds. We proposed for the first time an innovative analysis that gives a complete picture of wine aroma, which can be really useful to winemakers to manage wine aroma evolution and to take advantage of the disulfide reservoir. [ABSTRACT FROM AUTHOR]

Background and Aims Wine aroma is influenced by complex interactions between various wine constituents. This study investigated the sensory interactive effects of Sauvignon Blanc impact compounds, 3-mercaptohexan-1-ol and 3-isobutyl-2-methoxypyrazine, with acetaldehyde that typically forms during the oxidation of wine. Methods and Results Spiked model wines were subjected to sensory descriptive analysis using a trained sensory panel. The concentration of each compound varied from below aroma threshold values to high values as reported for wine. Depending on the concentration, acetaldehyde enhanced fruity attributes at a lower concentration, whereas suppression occurred at a higher concentration. Acetaldehyde effectively suppressed the green pepper aroma attribute at certain concentration values, whereas 3-mercaptohexan-1-ol suppressed oxidised green apple associated with acetaldehyde. Changes in attributes used for aroma description also occurred because of change in concentration. Conclusions Complex sensory interactions may occur between Sauvignon Blanc impact compounds and one of the main oxidation-derived compounds, acetaldehyde. Acetaldehyde can enhance or suppress pleasant fruity characters depending on the concentration. Significance of the Study This study showed the potential positive effect of acetaldehyde on white wine aroma when present at a low concentration. Formation of this compound during winemaking and ageing should, however, be controlled because of negative sensory interactions occurring at a higher concentration. This study may also contribute to the sensory characterisation of Sauvignon Blanc wine undergoing oxidation. [ABSTRACT FROM AUTHOR]

Twenty rose and ten claret A.O.C. Bordeaux wines made from Cabernet Sauvignon, Cabernet franc and Merlot grapes were separately tasted by the same jury of ten professionals. They were asked to classify the wines according to the intensity of the fruity character. The fruitiest sample was given first rank. The wines’ bdamascenone, b-ionone, phenyl-2-ethanol, isoamyl acetate, phenyl-ethyl acetate (APE), 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (A3MH) contents were also measured. The higher the latter three compounds' values, the higher the wines’ fruity character. Highly significant correlation were found between the sensorial analysis results and these compounds contents. In order to confirm the 3MH, A3MH and APE contribution to the fruity aroma of rose wines, a tasting has been carried out. For this, these three compounds were added to a rose wine up to the concentrations found in the wine judged the fruitiest by the tasters. 100 % of the tasters identified the supplemented glasses, and 90 % prefered the supplemented wine. This second experiment clearly demonstrate that 3MH, A3MH and APE can be considered key components of Bordeaux rose wines’ fruity aroma. The production of APE by yeast has been known for a long time. The 3MH is present in must under cysteinylated precursor form. The transformation of the precursor into aroma is made by the Saccharomyces cerevisiae yeasts during alcoholic fermentation. Thus the choice of yeast strains has a decisive impact on the fruity aroma of Bordeaux rose and claret wines.

Background and Aims Wine aroma is influenced by complex interactions between various wine constituents. This study investigated the sensorial interactive effects of Sauvignon Blanc impact compounds, 3-mercaptohexan-1-ol, 3-isobutyl-2-methoxypyrazine with aldehydes typically originating from oxidation of white wines, methional and phenylacetaldehyde. Methods and Results Spiked model wines were subjected to sensory descriptive analysis using a trained panel. The concentration of each compound varied from below aroma threshold value to high values reported in wine. Combinations of the four compounds were also evaluated sensorially. Depending on the concentration of the compounds, methional had a strong suppressive effect on the intensity of attributes contributed by 3-mercaptohexan-1-ol, such as grapefruit and guava, whereas methional and 3-isobutyl-2-methoxypyrazine enhanced the intensity of certain attributes, which included cooked beans and cooked potato. Conclusions Complex sensory interactions may occur between Sauvignon Blanc impact compounds and compounds related to oxidation depending on concentration of the compounds. Oxidation-derived compounds such as methional can suppress pleasant aroma attributes linked to volatile thiols, while contributing negative attributes especially in the presence of 3-isobutyl-2-methoxypyrazine. Significance of the Study This study highlights the importance of preserving fresh and fruity flavours while preventing the formation of unwanted aldehydes due to interactions that can influence the overall aroma profile of the wine. This study may also contribute to the sensory characterisation of oxidised Sauvignon Blanc wine. [ABSTRACT FROM AUTHOR]

Background and Aims Two compounds that play an important role in the aroma of Sauvignon Blanc wines are 3-mercaptohexan-1-ol (3 MH) and 2-isobutyl-3-methoxypyrazine ( IBMP). Little is known about the sensory interaction of these two compounds in Sauvignon Blanc wine and which aroma attributes they elicit at the concentration range normally found in Sauvignon Blanc wines. Methods and Results 3-Mercaptohexan-1-ol and IBMP were spiked separately or as a mixture to dearomatised Sauvignon Blanc wine at five different concentrations normally occurring in wine, and were assessed by a trained sensory panel. Descriptors were generated by the panel for these compounds when added alone, with overall tropical, guava and passionfruit being generated for 3 MH, while overall green, green pepper and grass were used for IBMP. The contribution of these attributes differed, however, depending on the concentration of 3 MH and IBMP. Conclusions There is sensory interaction between 3 MH and IBMP. The sensory attributes linked to each compound can depend on the concentration of both compounds present in wine. It is possible that wine producers may use this phenomenon to achieve a specific style of Sauvignon Blanc wine. Significance of the Study This study might allow wine producers to predict which aroma profile to expect when 3 MH and IBMP occur at a certain concentration in Sauvignon Blanc wine. [ABSTRACT FROM AUTHOR]

The varietal volatile thiols give the Sauvignon Blanc a specific flavored property characterized by notes of grapefruit, citric and exotic fruits. They are synthesized from the aroma precursors during the alcoholic fermentation. This study was made in the Vinification Complex of S.C. Promotion Blaj and the biological material used was represented by the Sauvignon Blanc grapes. From a 2010 harvest of Sauvignon Blanc wines there were determined 4-mercapto-4metylpentan-2-one (4MMP) and 3-mercaptohexan-1-ol (3MH). According to the yeasts used to ferment the grape must the quantities are: 1.45-10, 85 ng/L 4MMP and 424,7 and 900,3 ng/L 3MH. [ABSTRACT FROM AUTHOR]

Abstract: The isotopically labelled S-3-(hexan-1-ol)-glutathione d2/d3 (G3MHd2/d3) was synthesized with a strategy based on acid-labile protecting groups. The natural analogue of this compound could be a precursor of 3-mercaptohexanol, a varietal thiol reminiscent of grape fruit, released during alcoholic fermentation. In a first time, deuterated glutathione conjugate was used to perform identification and quantification by stable isotope dilution assay of G3MH in musts from several varieties: Sauvignon, Riesling and Gewurztraminer. In a second time, synthetic and natural musts (Sauvignon Blanc) were spiked by G3MHd2/d3 at different levels from 12 to 150μgL−1 to demonstrate the relationship between 3-mercaptohexan-1-ol (3MH) and G3MH. GC–ITMS/MS analysis of the resulting wines clearly showed the formation of the corresponding deuterated 3MH, that proves the direct connection with G3MH under enological conditions. In Sauvignon Blanc grape juice, a conversion yield of 4.4% was found in duplicate for an initial spiking of G3MH d2/d3 equivalent to 12μgL−1. [Copyright &y& Elsevier]

The effect of ultrasound (20 kHz, 153 μm) on the prefermentation extraction mechanisms in Sauvignon Blanc grapes was studied, focusing on 3-mercaptohexan-1-ol (3MH) and 4-mercapto-4-methyl-pentan-2-one (4MMP) precursors linked to glutathione (GSH) and cysteine (Cys). The treatment determined a positive extraction trend between the duration (untreated, 3 and 5 min) and the conductivity or the concentration of catechins and total phenols, significantly differentiated after 5 min. Nevertheless, the concentration of the thiol precursors in grape juice not only remained undifferentiated, but that of 3-S-glutathionyl mercaptohexan-1-ol showed a negative trend with the treatment time applied (168 ± 43, 156 ± 36, and 149 ± 32 μg/L, respectively, for control, 3 and 5 min). The divergence on the effect between families of compounds suggests an interaction between the sonication treatment and thiol precursor molecules. In order to evaluate the possible degradation properly, ultrasound was applied in a model solution spiked with 3MH and 4MMP precursors, reproducing the conditions of grapes. Except for Cys-3MH, the mean concentration (n = 5) for the rest of the precursors was significantly lower in treated samples, predominantly in those linked to glutathione (~−22% and ~18% for GSH-3MH and GSH-4MMP) rather than to cysteine (~−6%~−8% for Cys-3MH and Cys-4MMP). The degradation of precursors was associated with a significant increase of 3MH and 4MMP. The formation of volatile thiols following sonication is interesting from a technological point of view, as they are key aroma compounds of wine and potentially exploitable in the wine industry through specific vinification protocols. [ABSTRACT FROM AUTHOR]