Effect of O2 on winecomposition
O2 oxidises phenolic compounds in wine and must. This entails the loss of an electron and proton from the phenolic molecules, as indicated in Fig. 1.
The resulting quinones can then form brown polymers in must or wine. One of the by-products of this reaction is H2O2, which is an even stronger oxidation reagent than O2. When SO2 is added to wine it reacts with the H2O2, keeping the latter from further oxidising other phenolic molecules. In must oxidation enzymes catalyze this reaction. Two types exist in grapes, namely ortho-diphenolpoly-oxidases and laccase. The former occur naturally in grapes and the latter are produced by Botrytis cinerea in rotten grapes. The oxidation enzyme thus oxidises the phenolic molecule (mainly caftaric acid in juice) to the quinone. This quinone can however, be reduced back to the so-called Grape Reaction Product (GRP) by the addition of the sulphur containing compound glutathione. This can prevent further oxidation if no more O2 is added. Laccase can, however, further oxidise the GRP to the corresponding quinone. This can then in turn react with glutathione to form GRP2, GRP3, etc. When the molecules become too large it forms a brown precipitate at the bottom of a tank. This reaction will stop when all the glutathione of the oxygen are exhausted. The glutathione to caftaric acid ration can thus give an indication of the oxidation sensitivity of certain cultivars. Cape Riesling seems to have a low glutathione content, which may contribute to its oxidation sensitivity. In juice this reaction is auto-catalytic and can take place quite fast if large amounts of O2 are supplied. During the subsequent fermentation process however, the oxidised juice is reduced back to a more reductive state. The brown precipitate also associates with yeast cells and is left in the lees after the wine has been racked from it.
Figure 1. The oxidation of a phenolic molecule to the corresponding quinone.
This fact has thus been investigated to produce less oxidation sensitive wines where the substrate for oxidation, the phenol, has been removed prior to fermentation by a process called hyper-oxidation. Hyper-oxidation entails adding large amounts of O2 to the juice, letting it settle and then racking the juice from the brown precipitate just prior to yeast inoculation. These wines seem to be less bitter than those produced in the conventional manner, but the literates are not clear about this, as some states that it does not affect the taste of the wine. This technique can thus also be used to produce white wine with a low SO2 content. At the Department of Enology and Viticulture we had produced successfully good quality white wines like this, but these wines do not seem to last very long during bottle ageing.
O2 can obviously also influence the composition of wine. This oxidation process is, however, a chemical oxidation process and not catalyzed by enzymes as in must. This oxidation process is also slower than that occurring in must. It is normally the vicinal dihydro phenols that are easily oxidised in wine. These include catechin, epicatechin as well as anthocyanin and other phenolic molecules originating from the grapes. Oak tannins and its hydrolyses products like gallic and ellagic acid can also be oxidised easily, actually buffering the grape phenolic molecules in red wine from oxidation to a certain extent.
In white wine chemical oxidation is generally unwanted, but in red wine, however, small amounts of O2 coming into contact with the wine actually enhances the quality of the wine. This is especially true during ageing of the wine in oak barrels. During this process, O2 oxidises small amounts of phenolic molecules. One of the by- products of this reaction is H2O2 as mentioned before, which in turn can oxidise small amounts of ethanol into acetaldehyde. The latter then forms a "bridge" between an anthocyanin and a catechin molecule to bind to each other. Oak tannins actually enhance this process by being oxidised easily due to the many OH groups occurring in these molecules. This process is relatively quick compared to other more anaerobic polymerization reactions, and helps to enhance the colour intensity and stability of red wine during ageing. This then explains the increase in the colour density (higher red and especially brown colour of red wine), percentage of red pigments and the increase of polymerized colouring and decrease of free anthocyanins during ageing. O2 can also be supplied at higher concentrations just after fermentation, especially to press wine, but care should be taken not to add too much O2 to older red wine, as this may lead to high concentrations of acetaldehyde being formed with a too high polymerization and precipitation of colour. The addition of O2 also leads to the wine becoming softer, as the acetaldehyde induces the polymerization of tannin molecules.
O2 can also influence the aroma of wine. In white wine, oxidation initially leads to a loss of fruitiness and later honey, bee wax and acetaldehyde characters occur in the wine. Oxidation of white wine also leads to changes in fatty acid composition with an increase in furfural and eugenol (the latter has a clove-like character). O2 can, however, oxidise unwanted sulphur compounds, like H2S which has a rotten egg smell. It can also reduce green, reductive aromas in especially red wine, which probably originates from the so-called leaf alcohols. The general effect of O2 on the aroma of especially red wine is, however, not well known.
O2 can also influence the microbial status of wine. Acetic acid bacteria (AAB) are aerobic micro-organisms and thus can grow relatively quickly in wine if enough O2 is available. It has been proven that during barrel ageing AAB numbers increase after each racking procedure, which enhances O2 pick-up and then decrease again when the O2 starts to disappear from the wine. AAB produce high concentrations of acetic acid from ethanol, while producing higher concentrations of acetaldehyde under more anaerobic conditions. Our studies so far had, however, showed that AAB could survive under relatively anaerobic conditions in wine. The spoilage yeast Brettanomyces can also survive in wine, with O2 enhancing its growth in both must and wine. Brettanomyces can produce high concentrations of volatile phenols that have a medicinal, horse-like aroma. Normal wine yeast, Saccharomyces cerevisiae, also needs O2 to complete a successful fermentation. S. cerevisiae uses O2 to produce certain fatty acids, which is incorporated into its cell wall which contributes to the yeasts ethanol tolerance.
It is thus clear that O2 can either be a friend or foe of the winemaker. The next article will deal with the addition or prevention of O2 during the winemaking process.
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