Light-induced changes in bottled white wine and underlying photochemical mechanisms.

Bottled white wine may be exposed to UV-visible light for considerable periods of time before it is consumed. Light exposure may induce an off-flavor known as "sunlight" flavor, bleach the color of the wine, and/or increase browning and deplete sulfur dioxide. The changes that occur in bottled white wine exposed to light depend on the wine composition, the irradiation conditions, and the light exposure time. The light-induced changes in the aroma, volatile composition, color, and concentrations of oxygen and sulfur dioxide in bottled white wine are reviewed. In addition, the photochemical reactions thought to have a role in these changes are described. These include the riboflavin-sensitized oxidation of methionine, resulting in the formation of methanethiol and dimethyl disulfide, and the photodegradation of iron(III) tartrate, which gives rise to glyoxylic acid, an aldehyde known to react with flavan-3-ols to form yellow xanthylium cation pigments.

Comparison of Techniques for the Quantitation of Reductive Aroma Compounds in White Wine: Links to Sensory Analysis and Cu Fractions.

Multiple analytical methodologies allow quantitation of H2S and methanethiol (MeSH) in wine, but confirmation that the determined concentrations are related to perceived off-aromas, or "reductive" faults, is yet to be provided. Fifty white wines underwent sensory evaluation and measurement of free and salt-treated H2S and MeSH concentrations by gas chromatography with sulfur chemiluminescence detection and/or gas detection tubes. The determined concentrations were compared across techniques and different analysis laboratories. Sulfhydryl off-odors in the wines were best described by boiled and rotten egg and natural gas/sewerage/durian aroma attributes. The wines with the highest ratings for both aromas had high concentrations of free H2S, free MeSH, and/or salt-treated MeSH but were unrelated to salt-treated H2S. The free sulfhydryl concentrations and their associated aromas appeared to be suppressed by specific Cu fractions in the wines. This study provides evidence of the relevant measures of reductive aroma compounds and their relation to off-odors and Cu fractions.

Impact of light on protective fractions of Cu in white wine: Influence of oxygen and bottle colour.

Cu(II)-organic acid (fraction I) and Cu(I)-thiol (fraction II) complexes can suppress sulfhydryl off-aromas in wine. This study investigated the impact of light exposure on the protective fractions of Cu of bottled white wine. Fluorescent light-exposed Chardonnay with two initial concentrations of dissolved oxygen (0.5 and 10 mg/L) was stored in different coloured bottles and concentrations of Cu fractions and riboflavin, a photo-initiator at 370-440 nm, were measured during 110 days storage. Light-exposed wines with lower oxygen concentrations resulted in a 100-fold decrease in the Cu fraction I half-life, and a 60-fold decrease for Cu fractions I and II combined. The half-life for Cu fraction I decay during light exposure was extended 30-fold with the use of brown compared to flint glass. Light exposure can rapidly exhaust the protective Cu fractions in wine, and bottles with less light transmission below 440 nm can slow this loss.

Ascorbic acid addition to rosé: Impact on the oxidative and reductive development of bottled wine.

The impact of ascorbic acid on the oxidative and reductive development of rosé wine during bottle ageing has not been previously established. A rosé wine that contained 0.25 mg/L of Cu was bottled with levels of 0, 50 or 500 mg/L of ascorbic acid and different total packaged oxygen concentrations (3 and 17 mg/L), and was stored for 15 months at 14 °C in darkness. Ascorbic acid increased the first-order oxygen consumption rate from 0.030 ± 0.001 to 0.040 ± 0.001 days-1, and decreased the mole ratio of total SO2 consumed to oxygen consumed from 1.0:1 to 0.7:1. Although ascorbic acid did accelerate the loss of a Cu form that can suppress reductive aromas, it did not induce reductive aromas. These results indicate that ascorbic acid can accelerate the removal of oxygen from bottled rosé wine while maintaining higher concentrations of sulfur dioxide, however, it did not promote reductive development.

Suppression of reductive characters in white wine by Cu fractions: Efficiency and duration of protection during bottle aging.

Cu in wine can suppress sulfidic-odours, but the active forms and duration of protection are uncertain. Additions of 0, 0.3 or 0.6 mg/L Cu(II) were made to Chardonnay and Pinot Grigio at bottling. Throughout a 12- or 14-month storage period, Cu fractions were determined by colorimetry, and sulfhydryl compounds by gas chromatography with sulfur chemiluminescence detection. After Cu(II) addition, the dominant Cu fractions were associated with Cu(II)-organic acids (fraction I) and Cu(I)-thiol complexes (fraction II), and over 8-months their concentrations gradually fell below 0.015 mg/L. During this time, a fraction of Cu, predominantly attributed to sulfide-bound Cu, increased in concentration. Suppression of free hydrogen sulfide was assured when the combined Cu fractions I and II concentrations were above 0.015 mg/L, while free methanethiol suppression required Cu fraction I concentration above 0.035 mg/L. Decay rates for Cu fractions demonstrated that the duration that Cu can actively suppress sulfidic odours is wine-dependent.

Sulfide-bound copper removal from red and white wine using membrane and depth filters: Impacts of oxygen, H2S-to-Cu ratios, diatomaceous earth and wine volume.

Sulfide-bound Cu in wine is a potential contributor to the reductive development of wine. This study examines the effectiveness of filtration for the adsorptive removal of this Cu fraction. The copper concentration in wine before and after filtration was determined by atomic spectroscopy (total) and by stripping potentiometry and/or adsorptive methodologies (Cu fractions). Membrane filters (4.7 cm2) removed significant amounts of sulfide-bound Cu from 10 mL of wine, including 60-80 % removal using nylon membranes, but they could not efficiently remove Cu from larger volumes. Dissolved oxygen concentration in the wine immediately prior to membrane filtration did not impact Cu removal, while a high sulfide-to-Cu(II) ratio did enhance removal. Depth filters incorporating diatomaceous earth with cellulose (45 mm-diameter, 3.5 mm-thickness) showed the most efficient removal of sulfide-bound Cu from wines even after treatment of 3.0 L. The relevance of these laboratory scale filtrations to winery scale filtration is discussed.

Micro-oxygenation of red wine: techniques, applications, and outcomes.

Wine micro-oxygenation (MOX) is the controlled addition of oxygen to wine in a manner designed to ensure that complete mass transfer of molecular oxygen from gaseous to dissolved state occurs. MOX was initially developed to improve the body, structure, and fruitfulness in red wines with high concentrations of tannins and anthocyanins, by replicating the ingress of oxygen thought to arise from barrel maturation, but without the need for putting all wine to barrel. This review describes the operational parameters essential for the effective performance of the micro-oxidation process as well as the chemical and microbiological outcomes. The methodologies for introducing oxygen into the wine, the rates of oxygen addition, and their relationship to oxygen solubility in the wine matrix are examined. The review focuses on the techniques used for monitoring the MOX process, including sensory assessment, physicochemical properties, and the critical balance of the rate of oxygen addition in relation to maintaining the sulfur dioxide concentration. The chemistry of oxygen reactivity with wine components, the changes in wine composition that occur as a consequence of MOX, and the potential for wine spoilage if proper monitoring is not adopted are examined. Gaps in existing knowledge are addressed focusing on the limitations associated with the transfer of concepts from research trials in small volume tanks to commercial practice, and the dearth of kinetic data for the various chemical and physical processes that are claimed to occur during MOX.

Ascorbic acid: a review of its chemistry and reactivity in relation to a wine environment.

Extensive reviews of research are available on the use of ascorbic acid, and its consequent degradation pathways, in physiological conditions or food matrices. However, very little information can be found for wine-related systems. This review highlights the relevant chemistry and reactivity of ascorbic acid with a focus on its behavior and potential behavior in a wine environment. The review describes the use of ascorbic acid as a complementary antioxidant preservative to sulfur dioxide along with the metal-catalyzed and radical-dependent manner by which it achieves this role. The relevant degradation products of ascorbic acid in aerobic and anaerobic conditions are presented as well as the interaction of these degradation products with sulfur dioxide and other wine-relevant sulfur compounds. Limitations in existing knowledge, especially regarding the crossover between the antioxidant and pro-oxidant roles of ascorbic acid, are identified.

The removal of Cu from wine by copolymer PVI/PVP: Impact on Cu fractions and binding agents.

Specific forms of Cu in wine can influence wine flavour and development. The co-polymer polyvinylimidazole/polyvinylpyrrolidone (PVI/PVP) is known to remove Cu from wine, but its impact on different Cu forms is uncertain. In this study, three Cu fractions in white wine were determined by colorimetry and two Cu fractions in red wine were determined by diatomaceous earth depth filtration and atomic spectroscopy. PVI/PVP, with either silica or chitosan formulations, reduced all three fractions of Cu measured in white wines, and sulfide-bound Cu in red wines. The inefficient removal of organic acid-bound Cu in red wines was linked to the higher pH of red wines. After PVI/PVP treatment, wines showed lower concentrations of hydrogen sulfide, but minimal changes in weaker Cu binding agents. These results demonstrate that PVI/PVP efficiently removes the least desirable form of Cu present in wine, along with its detrimental binding agent (i.e., hydrogen sulfide).

Sulfide-binding to Cu(II) in wine: Impact on oxygen consumption rates.

This study investigates the relationship between the two main forms of Cu in wine and their impact on the rate of oxygen consumption. The Cu forms were differentiated by medium exchange constant current stripping potentiometry, which classified the Cu as either bound to sulfide or not. Oxygen consumption rates were determined in red, white and model wines after saturation with oxygen. The results for white wines showed that the oxygen consumption was sensitive to the non-sulfide-bound Cu concentration when ascorbic acid was present, and the first order rates ranged from 0.02 to 0.11 h-1. However, the same was not true for wines without added ascorbic acid that showed little influence of Cu form on oxygen decay rates. Cu forms were also found to significantly change in some wines during the oxygen decay experiment. Ascorbic acid is critical in enabling the form of Cu to significantly influence the oxygen reaction rate in wine.

Changes in Red Wine Composition during Bottle Aging: Impacts of Grape Variety, Vineyard Location, Maturity, and Oxygen Availability during Aging.

This work investigated the influence of grape variety, vineyard location, and grape harvest maturity, combined with different oxygen availability treatments, on red wine composition during bottle aging. Chemometric analysis of wine compositional data (i.e., wine color parameters, SO2, metals, and volatile compounds) demonstrated that the wine samples could be differentiated according to the different viticultural or bottle-aging factors. Grape variety, vineyard location, and grape maturity showed greater influence on wine composition than bottle-aging conditions. For most measured wine compositional variables, the evolution patterns adopted from the viticultural factors were not altered by oxygen availability treatment. However, contrasting evolution patterns for some variables were observed according to specific viticultural factors, with examples including dimethyl sulfide, phenylacetaldehyde, maltol, and ß-damascenone for vineyard locations, 2-methylbutanal, 1,4-cineole, and linalool for grape variety, and methanethiol, methional, and homofuraneol for grape maturity.

Rapid Quantitation of 12 Volatile Aldehyde Compounds in Wine by LC-QQQ-MS: A Combined Measure of Free and Hydrogen-Sulfite-Bound Forms.

This work outlines a rapid novel methodology for the direct quantitation of 12 volatile aldehyde compounds related to oxidative off-flavors in wine, by measuring the combined free and hydrogen-sulfite-bound forms of each aldehyde compound, consisting of four general aldehydes, four Strecker aldehydes, and four (E)-2-alkenals. The methodology requires minimal preparation of wine samples: the addition of internal standards and 6 g/L sulfur dioxide, and filtration prior to quantitation by liquid chromatography-triple quadrupole-mass spectrometery. Overall, the limit of detection, limit of quantification, accuracy (recovery, 97-114%), and precision (repeatability and reproducibility, RSD ≤ 10%) were satisfactory to enable routine measurement of the 12 aldehyde compounds in wine. The methodology was applied to 20 commercial white and red wines from various varieties and vintages. A general trend of higher concentrations of the aldehyde compounds in white wines compared to red wines was observed.

A GC-MS untargeted metabolomics approach for the classification of chemical differences in grape juices based on fungal pathogen.

Fungal bunch rot of grapes leads to production of detrimental flavour compounds, some of which are well characterised but others remain unidentified. The current study uses an untargeted metabolomics approach to classify volatile profiles of grape juices based on the presence of different fungal pathogens. Individual grape berries were inoculated with Botrytis cinerea, Penicillium expansum, Aspergillus niger or A. carbonarius. Grape bunches were inoculated and blended with healthy fruit, to provide 10% (w/w) infected juice. Juices from the above sample batches were analysed by GC/MS. PLS-DA of the normalised summed mass ions indicated sample classification according to pathogen. Compounds identified from those mass ion matrices that had high discriminative value for classification included 1,5-dimethylnaphthalene and several unidentified sesquiterpenes that were relatively higher in B. cinerea infected samples. A. niger and A. carbonarius samples were relatively higher in 2-(4-hexyl-2,5-dioxo-2,5-dihydrofuran-3-yl)acetic acid, while P. expansum samples were higher in γ-nonalactone and m-cresol.

Analytical strategies for the measurement of different forms of Cu and Fe in wine: Comparison between approaches in relation to wine composition.

The speciation of Cu and Fe in wine was assessed by a number of methodologies and those with superior performance were utilised on 49 wines and compared to compositional data. The adopted analytical strategies were stripping potentiometry, HPLC and an extraction (solid-phase or liquid-liquid) followed by atomic absorption or optical emission spectroscopic measurement. Stripping potentiometry was specific for sulfide-bound Cu in wines, and showed that this was the predominant form of Cu. A solid phase extraction technique provided a hydrophobic fraction of Cu that was specific for sulfide-bound Cu in white wine but red wine matrix effects hindered isolation of sulfide-bound Cu. The other methods assessed either perturbed the sulfide-bound Cu or were not sufficiently sensitive. The solid phase extraction method enabled routine fractionation of Fe in red and white wines whilst the additional techniques surveyed either measured total Fe or suffered from background contamination.

The Chemical Reaction of Glutathione and trans-2-Hexenal in Grape Juice Media To Form Wine Aroma Precursors: The Impact of pH, Temperature, and Sulfur Dioxide.

The aldehyde 3-S-glutathionylhexanal is an intermediate which is produced during the formation of the wine aroma precursor 3-S-glutathionylhexanol, after the reaction of glutathione with trans-2-hexenal. This study was conducted to assess whether the chemical, as opposed to the enzymatic, production of 3-S-glutathionylhexanal could occur at a significant rate in grape juice. LC-MS/MS was used in low- and high-resolution modes, in combination with functional group derivatization, to identify and quantitate products. In comparison to cysteine, glutathione was found to induce less cyclized products on reaction with trans-2-alkanals and the glutathione-derived products were more reactive to hydrogen sulfite. The zero-order rates for 3-S-glutathionylhexanal formation in model grape juice were 1.08 ± 0.08 and 0.45 ± 0.05 mg/(L·day) glutathione equivalents at 25 and 13 °C, respectively, and the reaction rate increased 3-fold by increasing the pH from 3.2 to 3.8. 3-S-Glutathionylhexanal was detected in all five white grape juices examined. The concentration of the aldehyde could be increased by up to 10-fold after being released from hydrogen sulfite, demonstrating a potentially novel source for the production of varietal thiol aroma compounds in wine.

Impact of fluorescent lighting on the browning potential of model wine solutions containing organic acids and iron.

Model wine solutions containing organic acids, individually or combined, and iron(III), were exposed to light from fluorescent lamps or stored in darkness for four hours. (-)-Epicatechin was then added, and the solutions incubated in darkness for 10days. Browning was monitored by UV-visible absorption spectrophotometry and UHPLC-DAD. The pre-irradiated solutions containing tartaric acid exhibited increased yellow/brown coloration compared to the dark controls mainly due to reaction of the tartaric acid photodegradation product glyoxylic acid with (-)-epicatechin to form xanthylium cation pigments. In these solutions, browning decreased as the concentrations of organic acids other than tartaric acid increased. Xanthylium cations were also detected in the pre-irradiated malic acid solution. However, in the malic acid, succinic acid, citric acid and lactic acid solutions, any coloration was mainly due to the production of dehydrodiepicatechin A, which was largely independent of prior light exposure, but strongly affected by the organic acid present.

Photoproduction of glyoxylic acid in model wine: Impact of sulfur dioxide, caffeic acid, pH and temperature.

Glyoxylic acid is a tartaric acid degradation product formed in model wine solutions containing iron and its production is greatly increased by exposure to UV-visible light. In this study, the combined effect of sulfur dioxide, caffeic acid, pH and temperature on the light-induced (⩾300nm) production of glyoxylic acid in model wine containing tartaric acid and iron was investigated using a Box-Behnken experimental design and response surface methodology (RSM). Glyoxylic acid produced in the irradiated model wine was present in free and hydrogen sulfite adduct forms and the measured total, free and percentage free glyoxylic acid values were modeled using RSM. Sulfur dioxide significantly decreased the total amount of glyoxylic acid produced, but could not prevent its production, while caffeic acid showed no significant impact. The interaction between pH and temperature was significant, with low pH values and low temperatures giving rise to higher levels of total glyoxylic acid.

Impact of Fluorescent Lighting on Oxidation of Model Wine Solutions Containing Organic Acids and Iron.

Previous studies have provided evidence that light exposure can increase oxygen consumption in wine and that the photodegradation of iron(III) tartrate could contribute to this process. In the present study, model wine solutions containing iron(III) and various organic acids, either alone or combined, were stored in sealed clear glass wine bottles and exposed to light from fluorescent lamps. Dissolved oxygen was monitored, and afterward the organic acid degradation products were determined and the capacity of the solutions to bind sulfur dioxide, the main wine preservative, was assessed. In the dark controls, little or no dissolved oxygen was consumed and the organic acids were stable. In the irradiated solutions, dissolved oxygen was consumed at a rate that was dependent on the specific organic acid present, and the latter were oxidized to various carbonyl compounds. For the solutions containing tartaric acid, malic acid, and/or citric acid, irradiation increased their sulfur dioxide-binding capacity.

Production and Isomeric Distribution of Xanthylium Cation Pigments and Their Precursors in Wine-like Conditions: Impact of Cu(II), Fe(II), Fe(III), Mn(II), Zn(II), and Al(III).

This study establishes the influence of Cu(II), Fe(II), Fe(III), Zn(II), Al(III), and Mn(II) on the oxidative production of xanthylium cations from (+)-catechin and either tartaric acid or glyoxylic acid in model wine systems. The reaction was studied at 25 °C using UHPLC and LC-HRMS for the analysis of phenolic products and their isomeric distribution. In addition to the expected products, a colorless product, tentatively assigned as a lactone, was detected for the first time. The results show the importance of Fe ions and a synergistic influence of Mn(II) in degrading tartaric acid to glyoxylic acid, whereas the other metal ions had minimal activity in this mechanistic step. Fe(II) and Fe(III) were shown to mediate the (+)-catechin-glyoxylic acid addition reaction, a role previously attributed to only Cu(II). Importantly, the study demonstrates that C-8 addition products of (+)-catechin are promoted by Cu(II), whereas C-6 addition products are promoted by Fe ions.

The impact of aging wine in high and low oxygen conditions on the fractionation of Cu and Fe in Chardonnay wine.

Solid-phase extraction has previously been used to fractionate copper and iron into hydrophobic, cationic and residual forms. This study showed the change in fractionated copper and iron in Chardonnay wines with 1-year of bottle aging under variable oxygen and protein concentrations. Wines containing protein in low oxygen conditions induced a decrease (20-50%) in total copper and increased the proportion of the hydrophobic copper fraction, associated with copper(I) sulfide. In contrast, protein stabilised wines showed a lower proportion of the hydrophobic copper fraction after 1-year of aging. In oxidative storage conditions, the total iron decreased by 60% when at high concentration, and the concentration of the residual fraction of both copper and iron increased. The results show that oxidative storage increases the most oxidative catalytic form of the metal, whilst changes during reductive storage depend on the extent of protein stabilisation of the wine.