Laura Nicolau

The yeast IRC7 gene encodes a β-lyase responsible for production of the varietal thiol 4-mercapto-4-methylpentan-2-one in wine.

Three varietal thiols are key aroma compounds in Sauvignon Blanc wines: 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexanol (3MH) and its acetylated derivative 3-mercaptohexyl acetate (3MHA). Screening of Saccharomyces cerevisiae strains identified a clinical isolate with elevated 4MMP production after fermentation. Bulked Segregant Analysis of a cross between this isolate and the laboratory strain revealed a single major locus for 4MMP production near the telomere of chromosome 6. Deletion of the IRC7 gene from this region in YJM450 reduced 4MMP production below detectable levels, but did not affect yields of 3MH, in Sauvignon Blanc wine. Sequencing revealed that the IRC7 gene in YJM450 had been introgressed from a strain of Saccharomyces paradoxus. Most strains of S. cerevisiae, including the laboratory strain S288C, have a 38-bp deletion that inactivates IRC7. Overexpression of a full-length S. cerevisiae allele of IRC7 in a wine yeast, Zymaflore F15, increased 4MMP production in Sauvignon Blanc wine from undetectable levels (<10 ng L(-1)) to concentrations of 1000 ng L(-1), and also increased 3MH and 3MHA. Biochemical analysis of soluble protein extracts showed that both the cerevisiae and paradoxus IRC7 proteins show β-lyase activity, with a substrate preference for cys-4MMP over cys-3MH.

Influence of juice pressing conditions on polyphenols, antioxidants, and varietal aroma of Sauvignon blanc microferments.

The impact of juice press fractions upon the content of varietal thiols in Sauvignon blanc has been examined for wines fermented at the laboratory scale (750 mL). Wines made from pressed juices (taken at 0.25 and 1.0 bar) contained less than half the concentration of 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA), important contributors to the tropical and passion fruit character of Sauvignon blanc wines, compared to wines made from free run juices. The pressed juices and wines exhibited lower acidity values, more rapid decline in glutathione content, and more advanced polyphenol oxidation. Supplementation of the juices with glutathione (at 67 mg/L) prior to fermentation led to lower varietal thiol concentrations in the finished wines, typically by several percent, whereas treatment with polyvinylpolypyrrolidone (PVPP) made no impact on wine parameters. Pasteurization of pressed juices increased 3MHA content in the finished wines, but also led to a decline in 3MH concentrations.

Identifying the Chemical Composition Related to the Distinct Aroma Characteristics of New Zealand Sauvignon blanc Wines

A comprehensive set of aroma compounds was quantified in 79 Sauvignon blanc wines from different international producers. Emphasis was given to understanding the chemical differences that can explain the unique character of Marlborough Sauvignon blanc. Quantification revealed the potential importance of several volatile compounds in addition to the polyfunctional mercaptans and methoxypyrazines already known to be important to the aroma of Sauvignon blanc wines. Multivariate statistical approaches, including canonical variate analysis, classification tree and partial least square analysis, established correlations between the chemical and the sensory profiles of the wines. A significant role of 3-mercaptohexyl acetate and 3-mercaptohexanol in the unique tropical, fruity character of Marlborough Sauvignon blanc wines was demonstrated, together with important variations in their concentrations, pointing to different styles even within the Marlborough wines.

Influence of Grape-Harvesting Steps on Varietal Thiol Aromas in Sauvignon blanc Wines

The intense tropical fruit aroma of Sauvignon blanc wines has been associated with the varietal thiols 3-mercaptohexanol (3MH), derived from odorless precursors in the grape, and 3-mercaptohexyl acetate (3MHA), arising from 3MH during fermentation. Grapes and juice were sourced from five locations in Marlborough, New Zealand, taking hand-picked grapes and samples at four stages during the mechanical harvesting process and pressing, which were then fermented in replicated 750 mL bottles. With each set of juices, the highest concentrations of Cys-3MH and Glut-3MH were found in the juices pressed to 1 bar, but these juices produced wines with lower 3MH and 3MHA concentrations. With three of the juices, there was an increase in varietal thiol content for wines made from juices that had been machine harvested compared to the hand-picked samples, which matched earlier findings of lower 3MH and 3MHA levels in wines made from hand-picked grapes. Juices that were more oxidized, and which showed a higher absorbance at 420 nm, were found to produce wines with lower 3MH and 3MHA concentrations.

Evaluation of key odorants in sauvignon blanc wines using three different methodologies.

In this study three different approaches were employed to identify key odorants in Sauvignon blanc wines. First, the concentrations of the odorants were compared to their respective aroma detection thresholds. The resulting odor activity values (OAV) were transformed into a normalized and weighted measure that allows the aroma profiles of different wines to be compared and the contribution of a single aroma in a complex mixture to be evaluated. Based on their OAV, 3-mercaptohexanol and 3-mercaptohexyl acetate were the two most important aroma compounds in many Marlborough Sauvignon blanc wines. Due to limitations with the OAV approach, the study was extended to include aroma extract dilution analysis (AEDA), which revealed that β-damascenone, together with the varietal thiols, esters, and higher alcohols, are key odorants in Sauvignon blanc wines. The final approach undertaken was aroma reconstitution and omission tests using a deodorized wine base and the creation of a model Marlborough Sauvignon blanc. Single compounds and groups of compounds were omitted from the model to study their impact on the sensory properties of the model wine. Reconstitution and omission confirmed that varietal thiols, esters, terpenes, and β-damascenone are all important contributors to Sauvignon blanc aroma. The methoxypyrazines showed an important but relatively low impact in all three of the approaches undertaken in this study.

Stability of Varietal Thiols in Commercial Sauvignon blanc Wines

The intense passion fruit-type aroma of New Zealand Sauvignon blanc wines, attributed to high concentrations of the varietal thiols 3-mercaptohexan-1-ol (3-MH) and 3-mercaptohexan-1-ol acetate (3-MHA), is known to diminish rapidly over just one year in the bottle. It is thus important to understand the processes by which the aroma compounds are lost during storage to improve the shelf life of these wines. The loss of varietal thiols has been linked to polyphenol oxidation, catalyzed by metals, a process that can be inhibited by antioxidants such as sulfur dioxide (SO2), ascorbic acid, and glutathione. The evolution of 3-MH and 3-MHA was monitored in commercially bottled New Zealand Sauvignon blanc wines, stored in the dark at 15°C for 7 months, together with changes in polyphenol content and small molecule antioxidants. The acetate ester 3-MHA was the least stable of the varietal thiols and declined steadily in concentration following first-order kinetic decay, whereas 3-MH barely decreased over the first 3 months of bottle storage, and was instead followed by an increase in concentration after a further 4 months. These results pointed to hydrolysis of 3-MHA to 3-MH as the major loss mechanism in the commercial wines during the initial months of storage. Concomitantly, the flavan-3-ols exhibited a greater susceptibility to oxidation and polymerization reactions compared with the more stable hydroxycinnamic acids. Glutathione concentrations also declined steadily over the first year and would thus only provide protection against oxidative thiol losses up to that point. Free SO2 remained relatively stable in the commercially bottled Sauvignon blanc wines after an initial drop in concentration associated with oxygen entry at bottling.

Concentrations of the Volatile Thiol 3-Mercaptohexanol in Sauvignon blanc Wines: No Correlation with Juice Precursors

The volatile thiol 3-mercaptohexanol (3MH) and its acetylated derivative 3-mercaptohexyl acetate (3MHA) are key contributors to the aroma of Sauvignon blanc. The concentrations of both thiols vary at least 20-fold in different wines, with differences among grape juices the major source of variation. Both thiols are produced by yeast from precursors present in the juice. At least four possible precursors have been proposed, although conversion rates for each precursor are low. The concentrations of three putative thiol precursors in 55 different Sauvignon blanc juices were compared to the concentrations of final thiols in wines fermented from each juice under controlled laboratory conditions. There were good correlations between the two thiols in each wine and among all three precursors in each juice. However, there was no correlation between the concentrations of the precursors in the juice and the final thiols in wine. Either these precursors are minor contributors to thiols in the final wine or there are other factors limiting conversion yields in different juices.

Influence of microoxygenation on reductive sulfur off-odors and color development in a Cabernet Sauvignon wine.

The effect of microoxygenation on the composition of a red wine was investigated, where oxygen was applied at both low and high rates (5 and 20 mg/L/month) through a diffuser or via permeation through polyethylene tanks. A control with no oxygen was also included. Each treatment was performed in triplicate, using a commercially made Cabernet Sauvignon wine, in nine 300-L stainless-steel and three 300-L polyethylene tanks at 16°C over 16 weeks. Spontaneous malolactic fermentation occurred simultaneously with the microoxygenation treatments. The results indicated that microoxygenation enhanced the wine color development, in which effects on pigments resistant to SO2 were more marked in the first half of the trial and more obvious influences on color density were seen during the second half of the trial. Microoxygenation, as applied in this study, did not show any effect on the desirable varietal thiol 3-mercaptohexanol, yet led to some decreases in the concentrations of undesirable off-odors, including a lowering in the concentration of methanethiol without an increase in the concentration of dimethyl disulfide. Changes in the concentrations of other reductive sulfur compounds, except the thioesters, were also affected by oxygen and further by the influence of spontaneous malolactic fermentation, indicating the complicated interactions of these compounds in the wine matrix.

Application of an Automated Headspace Solid Phase Micro-Extraction for the GC-MS Detection and Quantification of Reductive Sulfur Compounds in Wines

Aromatic substances are among the key determinants of food and beverage quality, owing to their interactions with the senses of smell and taste. These can then determine consumer acceptance or rejection of a product. The aroma of foods is influenced by different compounds, among which sulfur containing compounds are an important group due to their abundance and aromatic impact. Likewise, wine contains various sulfur containing structures which have a major sensorial impact and play a significant role in wine aroma and flavour. Among these, certain sulfur containing volatiles can contribute to favourable sensory impacts, while others can have detrimental effects on wine quality. The latter refers to the off-odours that have been a major concern for the wine industry.