Uncovering the influence of antioxidants on polyphenol oxidation in wines using an electrochemical method: Cyclic voltammetry

doi:10.1016/j.jelechem.2009.05.007

Journal of Electroanalytical Chemistry

Volume 633, Issue 1, 1 August 2009, Pages 165-174

https://doi.org/10.1016/j.jelechem.2009.05.007 Get rights and content

Abstract

The influence of sulfur dioxide, glutathione and ascorbic acid on the cyclic voltammograms of four representative wine polyphenols (catechin, caffeic acid, rutin and quercetin) and five different wines was investigated using a glassy carbon electrode in a model wine solution. Sulfur dioxide increased the anodic current and decreased the cathodic current for all four polyphenols and all wines, pointing to a rapid interaction of SO₂ with the oxidized polyphenol quinones. A similar trend was seen for glutathione, except that in the case of quercetin, addition of glutathione led to the formation of a second set of voltammetric peaks, corresponding to redox activity of a glutathione derivative. However, ascorbic acid produced no additional effect on the cyclic voltammograms of wine polyphenols and wines, beyond that expected for a simple sum of the polyphenol and ascorbic acid responses, with the exception that adsorption of quercetin and rutin on the carbon electrode caused a shift in the ascorbic acid oxidation peak to more positive potentials.

Introduction

Oxidation is one of the major problems encountered in winemaking, especially with white wines. It can lead to colour browning, a loss of varietal aroma and flavour, and the development of bitterness [1]. The first step in the non-enzymatic oxidation of wine is the oxidation of polyphenols [2], which comprise a major reactive component present in wines. In fact, the oxidation of ethanol in wine through to acetaldehyde, in the presence of molecular O₂, only occurs at a significant rate in the presence of polyphenols and catalytic metals; likewise SO₂ does not react directly with oxygen under wine conditions unless in the presence of metal ions [2]. Among the polyphenolic compounds, the most susceptible to oxidation are those containing an ortho-diphenol (catechol) functional group, including caffeic acid and its derivatives, catechin, epicatechin, flavonols such as quercetin, and compounds such as gallic acid with a triphenol group [3]. These polyphenols are catalytically oxidized to quinones, via the formation of a semiquinone radical, while oxygen is initially reduced to hydrogen peroxide, mediated by redox cycling of Fe³⁺/Fe²⁺ [4]. The quinones that are produced are unstable and may undergo further reactions [5], such as combining with other phenolics to form oligomeric and polymeric structures that can be brown in colour [2], [4], and reacting with thiol-containing compounds [6], [7].

Sulfur dioxide is widely used in winemaking to inhibit oxidation. However, SO₂ is toxic [8] to some groups of people and may cause allergic reactions,¹ such as headaches, abdominal pain and dizziness. As a result, in recent years the trend has been to limit the use of SO₂ [9], [10], [11], and to look for suitable replacements [3]. Ascorbic acid has been considered at times as an alternative antioxidant for use in winemaking [1], but issues regarding subsequent pro-oxidant activity have lessened its applicability [12]. In recent studies [9], [13], [14], [15], alternative options have been assessed for the prevention of oxidation during wine storage, including caffeic acid, gallic acid and glutathione (a naturally occurring thiol that has an established role in limiting enzymatic oxidation in grape juice through its reaction with quinones [6], [16]). To find the most suitable compound that could replace or supplement sulfur dioxide, it is very important to understand the chemical roles of SO₂ during wine aging. To date, only limited research [17], [18] has been undertaken in this area.

In present study, we investigate cyclic voltammetry as a method to produce polyphenol quinones in a controlled manner, and thereby study the influence of sulfur dioxide, glutathione and ascorbic acid on the polyphenol oxidation processes relevant to wine oxidation. Previously [19], [20], [21], [22], [23], [24], [25], cyclic voltammetry was successfully applied to characterize the antioxidant properties of wine and wine polyphenols. Here, we examine the effect of sulfur dioxide, glutathione and ascorbic acid on the cyclic voltammograms of four representative polyphenols (catechin, caffeic acid, rutin and quercetin, see Chart 1) and of five different wines.

Section snippets

Chemicals

Catechin, rutin, quercetin, ascorbic acid, reduced glutathione, 4-methylcatechol, l-tartaric acid and NaOH were purchased from Sigma. Other chemicals included caffeic acid (Acros) and Na₂S₂O₅ (Scharlau). The chemical structures of the polyphenols used in this work, and ascorbic acid, are given in Chart 1. Ultrapure water (18.2 MΩ cm) from a Millipore Milli-Q system was used to prepare all solutions.

Cyclic voltammetry

Cyclic voltammograms were recorded using a Bioanalytical Systems Electrochemical analyzer

Cyclic voltammograms and adsorption of polyphenols on the glassy carbon electrode

The glassy carbon electrode was used to monitor the electrochemical behavior of four polyphenols containing a catechol group and representing the main polyphenol types found in wines, namely catechin (a flavan-3-ol), caffeic acid (a hydroxycinnamic acid), rutin and quercetin (flavonols). The cyclic voltammograms of the polyphenols gave a set of anodic (positive) and cathodic (negative current) peaks. The cyclic voltammogram for 0.1 mM catechin is presented in Fig. 1 (solid curve). The reaction

Conclusions

Sulfur dioxide, glutathione and ascorbic acid are all excellent reducing agents that are capable of interacting with polyphenol oxidation products. From the above results it may be concluded that sulfur dioxide and glutathione can readily protect wine polyphenols against oxidation by reacting with quinones and reducing them back to polyphenols or by combining with them to form new sulfite or glutathione derivatives. However, ascorbic acid did not show any sign of this effect over short time

Acknowledgements

The research was supported by funding from the New Zealand Foundation for Research, Science and Technology (contract UOAX0404) with the support of New Zealand Winegrowers.

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Uncovering the influence of antioxidants on polyphenol oxidation in wines using an electrochemical method: Cyclic voltammetry

Abstract

Introduction

Section snippets

Chemicals

Cyclic voltammetry

Cyclic voltammograms and adsorption of polyphenols on the glassy carbon electrode

Conclusions

Acknowledgements

Food Chem.

Food Microbiol.

Food Res. Int.

J. Chromotogr. A

J. Electroanal. Chem.

Electrochem. Acta

Talanta

Anal. Chim. Acta

J. Pharm. Biomed. Anal.

J. Electroanal. Chem.

Curr. Appl. Phys.

Biochem. Pharmacol.

Making good wine: a manual of winemaking practice for Australia and New Zealand

Am. J. Enol. Vitic.

Am. J. Enol. Vitic.

Am. J. Enol. Vitic.

Am. J. Enol. Vitic.

Am. J. Enol. Vitic.

Wine Science: Principles, Practice, Perception

Am. J. Enol. Vitic.