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Danilewicz J.C.,44 Sandwich Road
American Journal of Enology and Viticulture | Year: 2016

When model wines that contain polyphenols are oxidized, hydrogen peroxide and quinones are produced. Sulfur dioxide reacts with the hydrogen peroxide, preventing ethanol oxidation by way of the Fenton reaction, and in the case of (+)-catechin, sulfite reduces the quinone near quantitatively back to the catechol. Consequently, the O2:SO2 molar reaction ratio is close to 1:2 in ideal experimental conditions. Here, eight wines (three red and five white wines) were studied to investigate whether this ratio might be similar in practice, so as to assess how effective SO2 might be as an antioxidant in real wine. The reaction ratio was found to be decreased down to 1:1.7 in most wines. To determine the reason for this decrease, a white wine was treated with a large amount of benzenesulfinic acid. This substance reacts very efficiently with quinones and would therefore prevent their interaction with sulfite. The molar reaction ratio was then reduced to 1:1, as has been previously observed in model wine. This result was taken to indicate that sulfite is fully effective in removing hydrogen peroxide and that the reduction in the molar reaction ratio from the theoretical 1:2 ratio was due to limited interaction with polyphenol oxidation products. Two white wines, which were found to be rapidly oxidized with much reduced O2:SO2 molar reaction ratios, were found to contain ascorbic acid. The effect of adding ascorbic acid to a white wine on the reaction of oxygen was therefore also examined. © 2016 by the American Society for Enology and Viticulture. All rights reserved. Source


Kreitman G.Y.,Pennsylvania State University | Danilewicz J.C.,44 Sandwich Road | Jeffery D.W.,University of Adelaide | Elias R.J.,Pennsylvania State University
Journal of Agricultural and Food Chemistry | Year: 2016

Sulfidic off-odors arising during wine production are frequently removed by Cu(II) fining. In part 1 of this study (10.1021/acs.jafc.6b00641), the reaction of H2S and thiols with Cu(II) was examined; however, the interaction of iron and copper is also known to play an important synergistic role in mediating non-enzymatic wine oxidation. The interaction of these two metals in the oxidation of H2S and thiols (cysteine, 3-sulfanylhexan-1-ol, and 6-sulfanylhexan-1-ol) was therefore examined under wine-like conditions. H2S and thiols (300 μM) were reacted with Fe(III) (100 or 200 μM) alone and in combination with Cu(II) (25 or 50 μM), and concentrations of H2S and thiols, oxygen, and acetaldehyde were monitored over time. H2S and thiols were shown to be slowly oxidized in the presence of Fe(III) alone and were not bound to Fe(III) under model wine conditions. However, Cu(II) added to model wine containing Fe(III) was quickly reduced by H2S and thiols to form Cu(I) complexes, which then rapidly reduced Fe(III) to Fe(II). Oxidation of Fe(II) in the presence of oxygen regenerated Fe(III) and completed the iron redox cycle. In addition, sulfur-derived oxidation products were observed, and the formation of organic polysulfanes was demonstrated. © 2016 American Chemical Society. Source


Kreitman G.Y.,Pennsylvania State University | Danilewicz J.C.,44 Sandwich Road | Jeffery D.W.,University of Adelaide | Elias R.J.,Pennsylvania State University
Journal of Agricultural and Food Chemistry | Year: 2016

Sulfidic off-odors as a result of hydrogen sulfide (H2S) and low-molecular-weight thiols are commonly encountered in wine production. These odors are usually removed by the process of Cu(II) fining, a process that remains poorly understood. The present study aims to elucidate the underlying mechanisms by which Cu(II) interacts with H2S and thiol compounds (RSH) under wine-like conditions. Copper complex formation was monitored along with H2S, thiol, oxygen, and acetaldehyde concentrations after the addition of Cu(II) (50 or 100 μM) to air-saturated model wine solutions containing H2S, cysteine, 6-sulfanylhexan-1-ol, or 3-sulfanylhexan-1-ol (300 μM each). The presence of H2S and thiols in excess to Cu(II) led to the rapid formation of ∼1.4:1 H2S/Cu and ∼2:1 thiol/Cu complexes, resulting in the oxidation of H2S and thiols and reduction of Cu(II) to Cu(I), which reacted with oxygen. H2S was observed to initially oxidize rather than form insoluble copper sulfide. The proposed reaction mechanisms provide insight into the extent to which H2S can be selectively removed in the presence of thiols in wine. © 2016 American Chemical Society. Source


Danilewicz J.C.,44 Sandwich Road
American Journal of Enology and Viticulture | Year: 2016

Iron plays a key role in wine oxidation. Polyphenols that contain catechol systems are the main reductants, and it has been proposed that the oxidation of these substances is mediated by the redox cycling of the Fe(III)/ Fe(II) couple. At any time, the Fe(II):Fe(III) concentration ratio should depend on the rate of Fe(II) oxidation by oxygen relative to that of Fe(III) reduction by polyphenols. Fe(III) oxidation of polyphenols, although facilitated by sulfite, is somewhat slower than the reaction of Fe(II) with oxygen, which is strongly accelerated by Cu. Alongside this process, Fe(III) inhibits is own formation. Therefore, the Fe(II):Fe(III) concentration ratio is determined by the interplay of a number of competing reactions. However, because of the relative speed of Fe(II) oxidation, oxygen should be a major determinant of this ratio. A simple spectroscopic method involving ferrozine is used to measure Fe(II) concentration in wines collected under nitrogen with minimal disturbance so as to determine Fe(II) levels in the original wine container. However, Fe(III), which becomes a strong oxidant in the presence of ferrozine, oxidizes catechols in wine conditions. Therefore, Fe(II) concentration, which increases as a result of catechol oxidation, was monitored over time and extrapolated back to the moment of ferrozine addition. Total Fe concentration was determined by adding ascorbic acid to reduce the Fe(III). As expected, the Fe(II):Fe(III) ratio was higher in wines bottled with screw caps than in those bottled with natural cork or filled in boxes. Exposure of wines to oxygen lowered the ratio, which reached equilibrium after some days of aerial saturation. However, the ratio attained differed in the different wines, and this difference likely depends on wine constituents that alter the relative rate of Fe(II) oxidation to that of Fe(III) reduction. © 2016 by the American Society for Enology and Viticulture. All rights reserved. Source


Danilewicz J.C.,44 Sandwich Road
American Journal of Enology and Viticulture | Year: 2012

The reduction potential of wines has been thought to indicate their level of oxidation or reduction, but how that relates to wine composition has remained vague. Reduction potentials are generated by redox couples, which are at equilibrium and both adsorbed on the measuring electrode, the magnitude of the potential for any couple being determined by the relative proportion of the oxidized and reduced component. However, redox couples associated with polyphenols, which are most likely to determine reduction potentials, are not at equilibrium in wine due to the instability of quinones. Reduction potentials are highly dependent on oxygen concentration and it is proposed that they are generated by the oxidation of ethanol coupled to the reduction of protons or of oxygen. While the so-called reduction potential is therefore of little value for wine, cyclic voltammetry has proved very useful in determining the reduction potential of wine constituents and estimating the concentration of the most readily reduced polyphenols. The reduction potentials of proposed redox couples involved in the reduction of oxygen and oxidation of polyphenols, ethanol, and sulfite are useful in determining the thermodynamic feasibility of possible interactions. The reaction of polyphenols with oxygen is mediated by iron, which redox cycles between them assisted by copper. Catechol-quinone and oxygen-hydrogen peroxide couples have similar reduction potentials. Consequently, the oxidation of catechols such as (+)-catechin cannot proceed to completion and is accelerated by substances that react with quinones, such as sulfite, which reduces them back to polyphenols. Sulfite, therefore, has multiple antioxidant activities. It accelerates oxygen consumption, prevents loss of polyphenols by regenerating them, and intercepts hydrogen peroxide, thus preventing ethanol oxidation. © 2012 by the American Society for Enology and Viticulture. All rights reserved. Source

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