Before we get into the technical aspects of adding oxygen to the must, now is a good time to review the other part of the system, the phenomenon of oxidation-reduction and how it effects the chemical matrix of a wine. The term “Redox Potential” effectively refers to a wine’s state of balance between its level of off-smelling sulfur-based compounds (ex: H2S) and the amount of available oxygen. When present, oxygen beneficially counteracts these compounds. However, when a wine contains a higher amount of these negative sulfur-based compounds and not enough available oxygen to mitigate all of them, then by definition you have a reduced pool of oxygen from which the offending sulfur compounds could have been counteracted (but weren’t). This wine is then referred to as being “reduced.” Wines that are in this “reduced” state often have off-sulfur odour defects, so this term is usually used to denote a wine with these flaws. In addition to a wine’s aromas being adversely affected by off-smelling Volatile Sulfur Compounds (VSCs), mouthfeel is also negatively impacted. Negative VSCs aggravate the perception of pH and tannin, and can actually render a wine more aggressive and harsh when they are present.
It is important to note that not all VSC’s are bad. In fact many are quite desirable:
Oxygen, when added to the fermenting must helps to limit the impact of negative VSCs in two ways. The first, as mentioned above, by allowing the yeast to synthesize the fatty acids and sterols needed to keep their cell wall transport mechanisms healthy. This allows them to better cope with the stresses encountered during fermentation and therefore limits the amount of negative VSCs produced in the first place. Second, oxygen counteracts whatever amount of VSCs that may already be present by raising the redox potential of the wine. All fermentations produce sulfur-based compounds and it is not possible (or even desirable) to completely eliminate them from your winemaking. Rather, the goal is to try and limit the impact that the negative ones may have on your wine and oxygen can be a useful tool to help do just that.
Finally, as another positive chemical reaction, in addition to limiting the formation of off-sulfur compounds, oxygen has the added benefit of helping to stabilize color in a red wine. It does this by reacting with the alcohol in the must to form aldehydes, which in turn react with anthocyanin (blue pigment) and tannins to form more stable molecules. Tannins are also chemically changed through oxidative reactions and can evolve to become more complex and rounded.
Check out our complete guide to oxygen and fermentation here.
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