**5.3 The role of sulfur dioxide**

The addition of sulfur dioxide at 46.9 μM (30 mg/L) prevents significant development of the propagation stage, at pH 2.5 and 4.5. This is due to the reaction of bisulfite with the hydrogen peroxide formed during initiation, preventing it from accumulating to the level required for a significant propagation reaction to develop. This reaction is known to be rapid [26] at pH 2.5 and 4.5; it is essentially complete within seconds. When the addition is made during the propagation stage, the reaction is terminated even when oxygen and Fe(II) are still present. The implication for wine making is that sulfur dioxide is acting on the hydrogen peroxide production at the point of its formation in the initiation step, essentially preventing the downstream chain reactions that would normally occur (**Figure 12**).

### **5.4 The role of copper**

The addition of copper(II) as CuSO4 displays very different responses at pH 2.5 and 4.5. At pH 2.5 the addition at the beginning of the reaction prevents the development of a propagation stage, suggesting that it is reacting with hydrogen peroxide formed in the initiation stage. This can be overcome by a late addition of hydrogen peroxide wherein the reaction quickly goes to completion – data not shown [9]. A similar result occurs when the addition is in mid-propagation, causing subsequent termination due to its reaction with hydrogen peroxide. In contrast, at pH 4.5 the addition at the beginning results in an enhanced initiation reaction, presumably due to the action of Cu(II)-tartaric complex providing additional oxygen activation and diminished free Cu(II) availability. A late addition of hydrogen peroxide allows the

#### **Figure 12.**

*Oxygen consumption with SO2 additions. Autocatalytic reaction with 0 μM ( ) and 30 mg/L SO2 ( ) at pH 2.5, and 0 μM ( ) and 30 mg/L SO2 ( ) at pH 4.5 at initiation (t = 0); reaction of 265 μM Fe(II) in airsaturated 26.7 mM tartaric acid.*

*The Kinetics of Autoxidation in Wine DOI: http://dx.doi.org/10.5772/intechopen.103828*

#### **Figure 13.**

*Oxygen consumption with copper additions. Time traces with 0 μM ( ), 7.87 μM Cu(II)SO4 ( ) at propagation midpoint, and 7.87 μM Cu(II)SO4 ( ) at initiation (t = 0); reaction of 265 μM Fe(II) in air-saturated 26.7 mM tartaric acid at (a) pH 2.5 and (b) pH 4.5.*

reaction to go to essentially the same level of completion as when no addition is made [9]. The oxidation of tartaric acid in the presence of Cu(II) is known to occur [27] (**Figure 13**).

#### **5.5 Autoxidation of wine**

The connection between the classic tartaric acid oxidation studies of Fenton, Warburg, Wieland and Franke, and Smythe and those occurring in wine comes from investigations of the oxidation of dihydroxymaleic and tartaric acids in the presence of iron (II) salts in wines [28]. Rodopulo [29] described oxidation and rearrangements resulting from tartaric oxidation into intermediates of dioxytartaric, dioxosuccinic, mesoxalic acids and glycolaldehyde and likely final products as glyoxalic and oxalic acids. He noted that in the presence of air dihydroxymaleic spontaneously oxidizes to dioxosuccinic acid, which in the absence of oxygen will react with tartaric acid forming two dihydroxymaleic acids. His opinion was that wines in an anaerobic state would contain dioxosuccinic while those with exposure to air would likely contain traces of glyoxylic and oxalic acids. A key contribution of his work was the demonstration that the addition of a ferrous tartrate precipitate to wine caused further oxygen consumption than that of ferrous sulfate, suggesting the importance of the Fe(II)-tartrate complex in wine oxidation. More accessible descriptions of this finding can be found elsewhere [30, 31]. Baraud [32] further investigated the oxidation of tartaric and dihydroxymaleic acids in wine-like conditions and tried to identify all of the products, including one of the unknown intermediates from these reactions. The relationship between these components has recently been summarized by Duca [27] and referred to as the Baraud cycle.

The proposed (**Figure 6**) incorporates the formation of ferryl ions and radicals, such as the hydroxyl and the tartaric radical, that are expected to be able to extract the α-hydrogen from ethanol to form the 1-hydroxyethyl radical in the presence of even small concentrations of ethanol. Hydroxyethyl radical was identified as the most important radical in beer by Andersen and Skibsted [33] and shown to be the active intermediate in the oxidation of linoleic acid to (E)-2-nonenal, a key impact volatile in "staling" character of beer [34]. It is now known to be the central radical responsible

for the selective oxidation of humulones and hop acid components rather than the hydroxyl radical during the oxidation of beer [35].

Hydroxyethyl radical has been shown to react selectively with the flavonols quercetin and kaempferol, but not with epicatechin [36], and while others found that most flavonoids with a Cn-Cn+1 double bond and caffeic acid were all reactive with the hydroxyethyl radical [37]. The hydroxyethyl radical has been found in a wine held at 55°C, [38] and its selective reaction with some phenol and thiol entities in model wine has been demonstrated [39]. It is known to react with glutathione [40] as well and cinnamates in wine conditions [41]. We propose hydroxyethyl to be the major and more selective oxidizing agent, whose reactivity is likely to be determining the identity and concentrations of the downstream radical products in wine. For this reason we expect there to be little involvement of dihydroxy phenols and tannin in such reactions. As such, hydroxyethyl radical will have a selective influence on specific phenols and glutathione in determining the oxidation outcomes in wine.

The action of sulfur dioxide is to interact with hydrogen peroxide concentrations at the point of initiation and to prevent propagation. The role of ethanol is to divert ferryl ion and/or other radicals into hydroxyethyl radicals and the subsequent radical chain reactions are likely influenced more by reduced glutathione levels and the cinnamates and flavones (but not due to their dihydroxy patterns). As such many of the subsequent reactions associated with oxidation may have little if anything to do with the dissolved oxygen concentration or the quantity of oxygen it has been exposed to.

The autoxidation sequence in wine can be classified into at least 3 periods, those reactions that take place within hours and days, those resulting from that which continue to interact in the days and weeks after, and those that continue to react and rearrange in the subsequent months and years. The first period is the activation of oxygen, the autoxidation described here and the generation of tartaric acid and related radicals. The second period would be the further reactions associated with more stable and long-lived radicals not necessarily in the presence of oxygen and would be selective radical transfer reactions between different wine components, typically not ethanol. The third period would be the long-term aging reactions. These reactions would be disturbed or interspersed with periodic and/or slow diffusional delivery of oxygen, generally in bottles. It is common for wines to be exposed to some oxygen within the winery during transfers, aging and bottling and there can be abrupt increases in concentration of dissolved oxygen or slow diffusional delivery such as though barrels and porous bottle closures.

There appears to be some confusion around the role of certain wine components involved in the initiation reactions compared with those involved in the propagation and termination reactions as well as the time scales involved. It is expected that subsequent radical reactions will continue after the first stage reaction is completed. These will include redox reactions, polymerization reactions and condensation reactions but all would involve relatively stable radicals and are not expected to require additional oxygen. This makes attempts to correlate the extent of product formation in wine with the rate, the extent of oxygen consumption, or the initial wine composition of major components likely to be unsuccessful.

Existing reaction pathways that have been proposed for wine oxidation use mostly free Fe(II) ions as the initial reactive species and all suggest the formation of the hydroxyl radical as the high oxidation state radical [42, 43]. Most of these proposed pathways have a coupled oxidation of a dihydroxy phenol for Fe(II) regeneration but none account for pH or the fact that almost half of the total Fe(II) in wine is in the

Fe(II)-tartrate complex form. None of these proposed pathways can be used to describe the observed autoxidation of tartaric acid in wine conditions. Most propose the formation of acetaldehyde from ethanol as the major oxidation product. The indiscriminate and almost instantaneous reactions with hydroxyl radicals should result in acetaldehyde (and glyceraldehyde) formation directly coupled to oxygen consumption and in a ratio of products proportional to their initial concentrations. Formation of acetaldehyde involving dihydroxy phenols does occur but only at elevated temperatures, 50°C [1]. Such formation has not been shown to be related to either the extent or rate of oxygen consumption at ambient or wine storage temperatures.

The role of other transition metal complexes in the initiation and propagation reactions needs to be investigated further. Here, there are effects due to the presence of malic acid and copper (II) which will vary between wines, especially before and after the malolactic fermentation and as a result of copper additions during winemaking. We expect their effects to be related to the concentrations of their complexes.

Lastly, the recovery of wines to an initial state after exposure to oxygen was reported to take several days [16]. This might be interpreted as being due to a slower return of Fe(II) for further initiation and or propagation reactions due to certain wine constituents that are absent in these model solutions studies. This deserves further research attention.
