Mandy Herbst-Johnstone

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.

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.

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.

Ethyl propiolate derivatisation for the analysis of varietal thiols in wine

Varietal thiols [3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA) and 4-mercapto-4-methylpentan-2-one (4MMP)] have been extensively studied in the recent literature. Nonetheless the hardest obstacle for research focussing on this class of compounds is the lack of quick, user-friendly and sensitive analytical methods. The current paper presents the use of ethyl propriolate (ETP) as a novel derivatising agent to quantify varietal thiols and the first time quantification of the thiol-ETP adducts via gas chromatography-mass spectrometry. Method optimisation including choice of the best SPE cartridge, derivatisation pH and adducts stability is presented. Validation of the method via stable isotope dilution was carried out. Detection limits in both model wine (4MMP 7.2ng/L, 3MHA 40.0ng/L and 3MH 91.2ng/L) and white wine (4MMP 24.5ng/L, 3MHA 120.9ng/L and 3MH 194.6ng/L) for the novel ETP-based method were lower than those obtained with the p-HMB method. Finally, 14 New Zealand Sauvignon blanc were analysed with both the new method and the organo-mercury based procure: good correlations were obtained for 3MH and 3MHA. Detection limits obtained with the new methods, its rapidity and reproducibility make this protocol perfectly suitable for oenological purposes.

Evolution of Volatile Sulfur Compounds during Wine Fermentation

Volatile sulfur compounds (VSCs) play a significant role in the aroma of foods and beverages. With very low sensory thresholds and strong unpleasant aromas, most VSCs are considered to have a negative impact on wine quality. In this study, headspace solid phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME/GC-MS) was used to analyze the time course of the biosynthesis of 12 VSCs formed during wine fermentation. Two different strains of Saccharomyces cerevisiae, the laboratory strain BY4743 and a commercial strain, F15, were assessed using two media: synthetic grape media and Sauvignon Blanc juice. Seven VSCs were detected above background, with three rising above their sensory thresholds. The data revealed remarkable differences in the timing and evolution of production during fermentation, with a transient spike in methanethiol production early during anaerobic growth. Heavier VSCs such as benzothiazole and S-ethyl thioacetate were produced at a steady rate throughout grape juice fermentation, whereas others, such as diethyl sulfide, appear toward the very end of the winemaking process. The results also demonstrate significant differences between yeast strains and fermentation media.

Hydrogen sulfide production during yeast fermentation causes the accumulation of ethanethiol, S-ethyl thioacetate and diethyl disulfide.

Hydrogen sulfide (H2S) is produced by yeast during winemaking and possesses off-flavors reminiscent of rotten eggs. The production of H2S during fermentation has also been associated in the finished wine with the rise of additional volatile sulfur compounds (VSCs) with strong aromas of cooked onions and vegetables. To characterize these more complex VSCs produced from H2S, we performed fermentations in synthetic grape juice. H2S production was manipulated experimentally by feeding increasing concentrations of sulfate to mutant strains that are unable to incorporate H2S efficiently as part of the sulfur assimilation pathway. In finished wines from these mutants, three VSCs - ethanethiol, S-ethyl thioacetate and diethyl disulfide - increased proportionally to H2S. (34)S-labeled sulfate fed to the MET17-deleted strain was incorporated into same three VSCs, demonstrating that they are formed directly from H2S.

Aroma Impact of Ascorbic Acid and Glutathione Additions to Sauvignon blanc at Harvest to Supplement Sulfur Dioxide

Higher levels of the varietal thiols 3-mercaptohexanol (3MH) and 3-mercaptohexyl acetate (3MHA) were seen in research-scale Sauvignon blanc wines following applications of 100 mg/kg ascorbic acid or glutathione to machine-harvested juices in addition to a moderate addition of 30 mg/kg sulfur dioxide. Antioxidant effects at the beginning of yeast fermentation were implicated, which can support 3MH formation when oxidative influences are controlled, as monitored through changes in polyphenol content. An increase in 4-mercapto-4-methylpentan-2-one (4MMP) was seen with added glutathione, while similar concentrations of many remaining aroma compounds, including esters, C6 alcohols and higher alcohols, were seen in the wines regardless of the extra antioxidant additions at harvest.

Pre-fermentation fining effects on the aroma chemistry of Marlborough Sauvignon blanc press fractions.

In the wine industry, fining agents are commonly used with many choices now commercially available. Here the influence of pre-fermentation fining on wine aroma chemistry has been explored. Free run and press fraction Sauvignon blanc juices from two vineyards were fined using gelatin, activated carbon, polyvinylpolypyrrolidone (PVPP) and a combination agent which included bentonite, PVPP and isinglass. Over thirty aroma compounds were quantified in the experimental wines. Results showed that activated carbon fining led to a significant (p<0.05) concentration decrease of hexan-1-ol and linalool in the experimental wines when compared to a control, consistent across all vineyard and fraction combinations. Other aroma compounds were also influenced by fining agent, even if vineyards and press fractions played a crucial role. This study confirmed that fining agents used pre-fermentation can influence wine aroma profiles and therefore needs specific tailoring addressing style and origin of grape.

Characterization of the Aroma of Central Otago Pinot noir Wines Using Sensory Reconstitution Studies

The aroma of two representative Pinot noir wines over two consecutive vintages from the Central Otago region of New Zealand underwent gas chromatography olfactory analysis. Forty-two odorants were identified in the aroma extract dilution analysis (AEDA) study with flavor dilution (FD) factors ranging from 3 to 19,683, with over 20 having FD > 81. The highest FDs were for phenylethyl alcohol (rose), ethyl isobutyrate (strawberry), β-damascenone (tea, floral), isovaleric acid (cheese), ethyl isovalerate (fruit, cherry), ethyl cinnamate (honey, cinnamon), and guaiacol (medicinal, smoky). Following AEDA studies, 51 compounds were quantified via GC–MS following liquid-liquid extractions, of which 22 were present at levels above perception threshold. The odorants with the highest odor activity values were β-damascenone, ethyl octanoate, ethyl hexanoate, ethyl isovalerate, isovaleric acid, and 3-mercapto-hexanol, with values ranging from 17 to 95. Using the data from these screening methods, aroma models were prepared by deodorizing two Pinot noir wines with LiChrolut EN resin and reconstituting the wines using purified aroma standards. Five further models for each wine were prepared in the same manner but with the omission of one major aroma family, and the models were presented to a trained panel. When compared to the fully reconstituted model, the omission treatments altered certain sensory descriptors, although overall no profound differences were observed and no genuine impact compounds were revealed.

Influence of Sulfur Dioxide Additions at Harvest on Polyphenols, C6-Compounds, and Varietal Thiols in Sauvignon blanc

The varietal thiols 3-mercaptohexanol (3MH), derived from odorless precursors in the grape juice, and 3-mercaptohexyl acetate (3MHA), arising from 3MH during fermentation, are prominent compounds responsible for tropical fruit aromas in Sauvignon blanc wines. Machine-harvested grapes were sourced from three different locations in Marlborough, New Zealand. Different concentrations of sulfur dioxide (0, 30, 60, 120, and 300 mg/kg) were added to the grapes and juice in the field, followed by transport to the University of Auckland, pressing, and fermentation in triplicate 750 mL bottles using EC1118 yeast at 15°C. Juices transported without SO2 or with a low SO2 addition were more oxidized and contained lower concentrations of most polyphenols. The juices fermented at a similar rate, with a delay in the onset of fermentation of up to a day with the 120 mg/kg SO2 juices, but around 12 days for the 300 mg/kg SO2 additions. In the more oxidized juices, there were higher concentrations of C6-alcohols in the finished wines but lower concentrations of the corresponding acetate esters. With each set the highest concentrations of 3MH and 3MHA were found in wines produced from the juices with a 120 mg/kg addition of SO2 at harvest. Conversely, wines made from juices that were transported with low SO2 additions had much lower concentrations of the varietal thiols.