And yet oxidation, in which juice comes into contact with oxygen, is an unavoidable part of the winemaking process. It suggests that a wine has developed a sherry-like color and matured too soon but is lacking in the good qualities typically associated with wines like sherry.
When oxidation is a fault, the wine—red or white—tends to lose vibrancy in both color and flavor. Whites begin to brown; reds lose their ruddy hue and become russet or orange. If exposed to air too long, a wine can become oxidized to the point that the acetaldehyde converts to acetic acid, turning the wine to vinegar. Oxidative wines usually have some nutty, savory, umami characteristics on the nose and palate. In some wines this might result in notes of raisins or brown apples; others—such as oloroso sherry, for example—may yield hazelnut notes.
Oxidation can begin in the vineyard—the second a grape skin is ruptured and the fruit and juice are exposed to air—and end in the glass.
Additionally, a faulty cork may allow too much air to enter the bottle and affect the wine. And of course, leaving a bottle of wine open for too long can cause it to become oxidized. Waterhouse, a professor of enology in the department of Viticulture and Enology at the University of California at Davis.
After that, winemakers [may] apply oxygen at a number of steps, on purpose, to modify the wine, mostly to get rid of vegetal character and increase color stability. This can all be remedied by the aerating, thus giving our little yeast buddies a bit more oxygen throughout the fermentation.
The quantity of this is minimized by the gassing the wine in its tank from which it is coming from to the bottles with either argon or nitrogen. The wine is bottled, the bottle is gassed and then corked. It arrives in our hands via our wine shops and if we place it in our collection, we store it in hopefully a cool and slightly humid environment on its side.
By action of pouring the wine in the decanter you are imparting oxygen in the wine. The wider space of the decanter allows for the wine to spread out, thus allowing the wine to have more contact with air, than say it might in the bottle.
The other important benefit of decanting your wine is that when you pour it into a decanter, it agitates and mixes with oxygen. This allows your wine to show its true characteristics. Just leaving it in the bottle does little, or really nothing at all. The cork is porous and it will allow for the transfer of oxygen inside the bottle. If the bottle is not on its side, it will do this at a faster rate, thus aging the wine faster than most of us might desire.
If the cork is in contact with the wine while in a prone position, the cork will remain moist, thus expand and allow for less air to enter the bottle. This controls the contact your wine has with oxygen at slower rates. This is an ideal method to allow it gracefully age. In the beginning, this is not a problem as oxygen is indeed present for the yeast both in the saturated form from being dissolved into must during the mechanical processing of the fruit, as well as from surface area exposure at the top of the fermentation vessel.
However, once the fermentation starts in earnest, the yeast will have consumed the dissolved oxygen in the must and the top of the vat will be blanketed with CO2, effectively cutting it off from the oxygen in the surrounding air.
In short, if the yeast is to receive any more oxygen to help it stay healthy and limit the production of undesired sulfur compounds during the remainder of the fermentation, the winemaker must take measures to add it themselves. 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.
When present, oxygen beneficially counteracts these compounds. Negative VSCs aggravate the perception of pH and tannin, and can actually render a wine more aggressive and harsh when they are present.
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.
There are many ways to get oxygen into fermenting musts, some more effective than others. In general, temperature and turbulence determine the rate and the amount that can be dissolved in the following ways:. The venturi effect is when a moving stream of liquid in a hose quickly passes over a small hole that has been voluntarily or involuntarily placed in the circuit, it will create a vacuum and the outside air will get pulled into and then mix with the stream of liquid.
So, with the exception of the first example we can see how the rest of the above techniques do deliver some amount of O2 to the must, but there is quite a variance. Equipment costs need to be considered, but time and logistics should be taken into account as well. For example, you may like the convenience of an in-line venturi. But if, as can happen, you actually require more than 2 to 2. Depending on how many separate fermentation vessels you will be managing at a single time, along with the amount of staff and equipment available, this may or may not be a big problem, but it is worth consideration none the less.
Therefore, in addition to a venturi, perhaps you might also want to have a diffusion stone or a tray with a fan around just in case more oxygen is needed at a given moment. Therefore, young, dense red wines will be able to take more oxygen than delicate ones will.
The following are some good places to start when considering oxygen additions to the must, but these should not be considered a final formula. The wine should be noticeably fresher in aroma and flavour when you finish each treatment. At this stage, the wine does not oxidize as the yeast will take up the oxygen before it can react with the must. The wine should become noticeably softer and rounder after the dosage.
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