Geoffrey R. Scollary

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.

Antioxidant action of glutathione and the ascorbic acid/glutathione pair in a model white wine

Glutathione was assessed individually, and in combination with ascorbic acid, for its ability to act as an antioxidant with respect to color development in an oxidizing model white wine system. Glutathione was utilized at concentrations normally found in wine (30 mg/L), as well as at concentrations 20-fold higher (860 mg/L), the latter to afford ascorbic acid (500 mg/L) to glutathione ratios of 1:1. The model wine systems were stored at 45 °C without sulfur dioxide and at saturated oxygen levels, thereby in conditions highly conducive to oxidation. Under these conditions the results demonstrated the higher concentration of glutathione could initially provide protection against oxidative coloration, but eventually induced color formation. In the period during which glutathione offered a protective effect, the production of xanthylium cation pigment precursors and o-quinone-derived phenolic compounds was limited. When glutathione induced coloration, polymeric pigments were formed, but these were different from those found in model wine solutions without glutathione. In the presence of ascorbic acid, high concentrations of glutathione were able to delay the decay in ascorbic acid and inhibit the reaction of ascorbic acid degradation products with the wine flavanol compound (+)-catechin. However, on depletion, the glutathione again induced the production of a range of different polymeric pigments. These results highlight new mechanisms through which glutathione can offer both protection and spoilage during the oxidative coloration of a model wine.

A robust method for quantification of volatile compounds within and between vintages using headspace-solid-phase micro-extraction coupled with GC-MS - Application on Semillon wines

A headspace-solid-phase micro-extraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) method has been developed to quantify a range of volatile compounds in Hunter Valley Semillon wines. The fibre selected for the method was a 50/30 microm divinylbenzene/carboxen/polydimethylsiloxane fibre, a three-phase fibre that allows extraction and desorption of a wide range of volatile compounds of different chemical functionalities and polarities. Four internal standards, methyl isobutyl ketone, n-dodecane, 4-methyl-2-pentanol and ethyl nonanoate were used to monitor the SPME fibre extraction efficiency and integrity. Fibre integrity was monitored by plotting the ratio of the peak area for each internal standard divided by the sum of the peak area for all internal standards as a function of analysis number. The advantage of using four internal standards for better quality control of the fibre integrity is described. The identity of twenty-one volatile compounds was ascertained by comparison of their chemical characteristics (retention indices, mass spectra) with reference compounds using two columns of different polarities. Quantification was achieved using calibration curves constructed for each compound with linear regression equations having correlation coefficients (R(2)) ranging from 0.9717 to 0.9999. The method was applied to two Semillon wines (recent vintage and aged) representative of the Hunter Valley styles. As is typical of white wines, 3-methyl-1-butanol was quantified as the most concentrated volatile compound (83 and 66 mg L(-1) for the 2006 and the 1996 wines, respectively). The study highlights the use of well-defined procedures to ensure integrity of quantitative data where several fibres may be required during an extended study over one or more vintages.

Study of the influence of micro-oxygenation and oak chip maceration on wine composition using an electronic tongue and chemical analysis

The influence of micro-oxygenation (MOX) and maceration with oak chips treatments on wine was studied on wine samples from three vintages produced in the Yarra Valley, Australia. A full factorial design was employed where two factors (MOX and oak chips treatments) had two levels and one factor (vintage) had three levels. Three replicated treatments were run for each factors setting. Wine samples were analysed using conventional laboratory methods with respect to the phenolic wine compounds and colour attributes since the phenolic fraction of wine is most affected by both MOX and oak maceration treatments. The same wine samples were measured with an electronic tongue based on potentiometric chemical sensors. The significance of treatments and vintage effects on wine phenolic compounds was assessed using ANOVA and ANOVA-Simultaneous Component Analysis (ASCA). Cross-validation was used for the ASCA sub-model optimisations and permutation test for evaluations of the significance of the factors. Main effects of vintage and maceration with oak chips were found to be significant for both physicochemical and the ET data. Main effect of MOX treatment was also found significant for the physicochemical parameters. The largest effect on the phenolic composition of wine was due to its vintage, which accounted for 70% and 33% of total variance in the physicochemical and ET data respectively. The ET was calibrated with respect to the total phenolic content, colour density and hue and chemical ages 1 and 2 and could predict these parameters of wine with good precision.

Size fractionation of metals in wine using ultrafiltration

Ultrafiltration has been used to examine the size fractionation of lead, copper, iron, aluminium, calcium, potassium and sodium in white and red wines. Fractionation patterns demonstrated that the behaviour of lead is significantly diferent to the other metals considered. In red wine, there was a sudden decrease for lead between 100 000 and 30 000 nominal molecular weight cut off (NMWCO) and in white wine, a gradual decrease in the lead concentration was observed from 100 000 NMWCO. Iron was the only other metal which showed, for red wine, a size distribution pattern with a reduction in the iron concentration between 30 and 50% from 100 000 to 1000 NMWCO. Potential binding agents for lead and iron are discussed. The absence of any fractionation pattern for the other metals examined has been interpreted in terms of the metals existing as aquated cations (potassium and sodium), metal tartrate complexes (aluminium, copper and calcium) and either tartrate or phosphate for iron in white wine. The possibility of ultrafiltration disturbing kinetically facile processes, particularly for copper and calcium, is identified.

Impact of Glutathione on the Formation of Methylmethine- and Carboxymethine-Bridged (+)-Catechin Dimers in a Model Wine System

This study was performed to assess the impact of glutathione on the reaction between (+)-catechin and carbonyl compounds in wine-related conditions. (+)-Catechin (0.50 mM) and either glyoxylic acid (0.25 mM) or acetaldehyde (0.25 mM) were added to a model wine system with 0.0, 0.25, and 2.5 mM of glutathione added. UPLC-DAD and LC-MS analysis showed that the formation of carbonyl-bridged (+)-catechin dimers was inhibited in the samples with a glutathione to carbonyl ratio of 10:1 compared to the samples without glutathione. At a ratio of 1:1, glutathione inhibited the acetaldehyde-bridged dimers but only had a minor impact on the glyoxylic acid-bridged dimers. Further investigations showed that this trend of inhibition by glutathione on the glyoxylic acid-derived dimer was independent of temperatures, 20 °C vs 45 °C, or the presence of metal ions, 0.2 mg/L copper(II) and 5 mg/L iron(II). (1)H NMR analysis and LC-MS analysis provided evidence that glutathione inhibited dimer formation via different mechanisms depending on the carbonyl compound. For acetaldehyde-derived dimers, the main mode of inhibition was the ability of glutathione to form a (methyl-glutathionyl-methine)-(+)-catechin complex. Alternatively, the formation of a glutathione-glyoxylic acid addition product impeded the reaction between glyoxylic acid with (+)-catechin. These results demonstrate that glutathione, at sufficient concentration, can have a substantial impact on carbonyl-derived polymerization reactions in wine-like conditions.

Chemistry of ascorbic acid and sulfur dioxide as an antioxidant system relevant to white wine

The impact of the combined ascorbic acid and sulfur dioxide antioxidants on white wine oxidation processes was investigated using a range of analytical techniques, including flow injection analysis for free and total sulfur dioxide and two chromatographic methods for ascorbic acid, its oxidative degradation products and phenolic compounds. The combination of different analytical techniques provided a fast and simultaneous means for the monitoring of oxidation processes in a model wine system. In addition, the initial mole ratio of sulfur dioxide to ascorbic acid was varied and the model wine complexity was increased by the inclusion of metal ions (copper(II) and iron(II)). Sulfur dioxide was found not to be a significant binder of ascorbic acid oxidative degradation products and could not prevent the formation of certain phenolic pigment precursors. The results provide a detailed insight into the ascorbic acid/sulfur dioxide antioxidant system in wine conditions.

Determination of the impact of bottle colour and phenolic concentration on pigment development in white wine stored under external conditions.

The exposure to sunlight of a Sauvignon blanc wine stored in bottles of different colours has been examined. Wine in darker bottles (Antique Green and French Green) showed considerably more colour development than wine in clear (Flint) or lighter (French Green) bottles, provided a high concentration of catechin-type phenolic compounds was present. Xanthylium pigments were identified by LC-MS as one of the main contributing phenolic pigments to the increased colour in the dark bottles. This is the first observation of yellow xanthylium pigments in a white wine. A Principal Component Analysis of the variation in absorbance measurements at Day 59 of the exposure further confirmed the importance of bottle colour on the result. One component, dominated by the darker Antique Green and Classic Green, reflected the development of colour at 440 and 520 nm. The second component, to which the Flint and French Green bottles contributed the most, was based on a decrease in the 280 nm absorbance. The implication of these results for the safe storage of wine is discussed.

Iron(III) tartrate as a potential precursor of light-induced oxidative degradation of white wine: studies in a model wine system.

The potential for iron(III) tartrate to act as a photoactivator in light-induced oxidative degradation of white wine is described. Using a tartaric-acid-based model wine system containing 5 mg/L iron, exposure to light from a xenon arc lamp led to the oxidative degradation of tartaric acid and the production of glyoxylic acid. The critical wavelength of light for the degradation process was found to be below 520 nm. No glyoxylic acid was formed in the absence of iron and/or light. Flint glass offered little protection from the light-induced photodegradation of tartaric acid. Antique Green glass offered more protection but did not stop the photodegradation process.

Determination of titratable acidity of wines and total acidity of vinegars by discontinuous flow analysis using photometric end-point detection

The titratable acidity of white and red wines and the total acidity of vinegars were determined using the flow-based titration technique, discontinuous flow analysis with photometric end-point detection. A flow cell was used which had a low-volume mixing chamber fitted with a high-speed stirrer. Detection of the end-point was achieved using a light emitting diode and photodiode incorporated into the flow cell. A sample throughput of one every 4 s was possible with a precision of better than 1% relative standard deviation (n= 6) being observed for all standards and most samples. The results compared well with those from a standard manual titration.