When wine and oxygen meet

The possible management of oxygen in the wine industry has basically divided producers into two large areas of wine-making philosophies: those who take advantage of the potential oxidative risks and those who conversely believe that its presence in cellars should be limited. This distinction underlies two completely different technological approaches: on the one hand a total and radical protection of the wine-must from oxygen at all stages of wine-making, from wine containers to bottles (hyper-reduction), and on the other hand, a logical and controlled use in specific cycle moments (hyper-oxygenation and micro-oxygenation).

Factors affecting oxidative phenomena

Oxygen performs different actions on wine, both positive and negative, depending on several factors and basically on dissolved quantities, the moment of dissolution, and type of wine.
The different cellar operations cause a substantial and progressive increase in oxygen content dissolved in wine, until the saturation limit is reached: in particular, while the air-wine surface increases, the turbulence conditions of the liquid significantly facilitate the gas dissolution in the wine.
After dissolution, oxygen combines with oxidizable substances, such as phenolic compounds, sulfur dioxide and other oxide-reducing substances: wine transformations (ferrous precipitations, alteration of aroma precursors, etc.) do not appear, however, immediately after oxygen absorption, but only after its dissolution.
Disappearance rate of dissolved oxygen (as a result of its reacting with other substances) greatly depends on storage temperature and can vary from several months to a few hours, or even a few minutes in case of temperatures above 30 °C.
Equally important is the role played by catalysts (metals, such as copper and iron): in their absence, oxidation of sulfur dioxide, or tannins, appears to be negligible, while it is dramatically accelerated by their presence.
It should also be noted that oxidative reactions are generally faster in must than in wine, since in the must enzymes – known as polyphenol oxidase (PPO) – act as catalysts: these enzymes come both from grapes (such as tyrosinase) and mold, such as laccase, which musts contain as a consequence of Botrytis cinerea (gray mold) attacks.

Oxydation reactions in musts

Oxidation reactions in musts basically occur for the enzymatic transformation of oxidizable substances in the medium. Ripe grapes contain, as mentioned above, an enzyme called tyrosinase, which significantly varies according to the variety genome.
In musts of white grapes, this enzymatic activity preferably oxidizes the tartaric derivatives of hydroxycinnamic acids, the main phenolic compounds in grape pulp: produced quinones are particularly unstable and likely to be involved in different reactions.
Firstly, these quinones can condense with other molecules of phenolic compounds (flavonoids), with the result of creating products which from yellow evolve towards brown, according to condensation degree and the darkening phenomenon described by Singleton.
Quinones are also capable of reacting with a tripeptide, glutathione (GSH), to produce colorless derivatives, the 2-S-trans-glutationil-caffeiltartaric acid, known as Grape Reaction Product (GRP): as shown from Salgues and coll., under conditions of healthy grapes, such compound can no more be oxidized by enzymes and therefore glutathione can stop the chain leading to the darkening phenomenon.

Tyrosinase and laccase

Grapes tyrosinase is active but unstable at must pH and proves to be quite sensitive to sulfur. It is therefore relatively simple to contain the activity: 50 mg/l SO2 are sufficient to denature the enzyme. Lower intakes of sulfur dioxide delay oxidation over time, since bisulfite ions regenerate the potential enzyme substrates, reducing the quinones which have formed. Moreover, tyrosinase can be easily eliminated through fining agents.
Oxidation of phenolic compounds proves to be far more dangerous when grapes suffered Botrytis cinerea attacks, and must therefore contain laccase: unlike tyrosinase, this fungal enzyme is stable at must pH and has better resistance to sulfur dioxide. Laccase is able to oxidize a greater number of phenolic substrates and molecules belonging to other chemical families. In particular it oxidizes to quinine the set between the phenol molecule and glutathione: in this situation, GRP constitutes a substrate on which laccase can act and, for this reason, wine-making of botrytis-affected grapes becomes increasingly problematic due to browning reactions.

Wine oxydation

Unlike what happens for musts, wine oxidation basically occurs in a chemical manner: as already mentioned in the case of musts, the main substrate for oxidation reactions is constituted by phenolic compounds, and the effects of these reactions are browning and loss of color, together with the precipitation of the coloring matter. Such oxidation reactions may also lead to the formation of a series of volatile substances which, in some cases, may also be responsible for important aromatic deviations.
Acetaldehyde is the main volatile compound related to oxygen consumption and is formed as an intermediate step of the oxidation reaction from ethanol to acetic acid, a reaction catalyzed by some heavy metals (iron and copper). The transformation from acetaldehyde into acetic acid is slowed down from the acetic acid itself, when it accumulates in the mean; acetaldehyde concentration in wine may then increase (dismutation). With high quantities of oxygen, oxidation and dismutation convert ethanol into acetic acid. When the mean becomes depleted in oxygen, acetaldehyde accumulates.
As reported by Zironi et al., as for certain oenological practices, such as wine production in wooden containers or micro-oxygenation, acetaldehyde seems to be involved in some of the mechanisms of color stabilization and phenolic substances, but if oxygen dissolution is very intense or prolonged over time, the major quantities of acetaldehyde formed can evolve into other aromatic compounds (acetals) which are responsible for the typical sensory notes of oxidized wines.

 

Sensitivity to the oxidation phenomena significantly depends on the genetic makeup of the grape variety: some so-called aromatic grape varieties, such as Sauvignon blanc, prove to be rather sensitive to contact with air, i.e. a phenomenon related to their composition, which is relatively little provided with natural antioxidants, such as glutathione (GSH), ascorbic acid and others, all substances which increase the resistance rate to oxidative phenomena.

 

Understanding such oxidation mechanisms has determined the development of the pre-fermentative technique defined as hyper-oxygenation: since the destruction of cell structures, the controlled and sufficient addition of oxygen causes the denaturation of tyrosinase during the oxidation reactions which it catalyzes. The must proves then to be stable against oxidation, due to the disappearance of the involved enzyme and the depletion of oxidizable phenolic substrates: obviously, on the basis of what has been previously explained, the technique cannot be applied to botrytis-affected grapes because of the resistance of laccase.

 

This wine-making technique consists in the controlled supply (by means of special installations) of small oxygen amounts to the must-wine, in specific phases of transformation, in order to improve the organoleptic characteristics of the finished product. Among the defects which can be corrected by micro-oxygenation, let us mention reduction conditions and defects of green or vegetable.

 

It is a particular type of redox (oxidation-reduction) reaction, in which one substance is partly oxidized and partly reduced. Disproportionation reactions occur, for example, during photosynthesis, during fixation of CO2 within chloroplasts.

 

by Giovanni Colugnati and Giuliana Cattarossi – Colugnati & Cattarossi Srl – Reana del Rojale (Udine, Italy)

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