How does wine oxidized

With white wines, sometimes winemakers try to protect the must from oxygen — for example, by excluding it by using inert gases or dry ice, and using sulfur dioxide at grape reception — because they want to preserve fruity compounds like esters, but in other cases they will let the juice oxidise and turn brown, knowing that it will clean up during fermentation.

The latter process allows a group of compounds called phenolics to oxidize and thus fall out of the must, which then protects the wine later in its life.

The former approach preserves fruity aromas, but also the phenolics, which makes the wine fragile later in its life. In this case, pressing is usually delicate to minimize the phenolics in wine. As we will see later, phenolics are important players in the second form of oxidation, known as chemical oxidation. For red wines, things are quite different.

This is because phenolic compounds have two sides to them as far as oxygen is concerned. They are players in the chemical oxidation process, but they also protect against oxidation because they can scavenge free radicals also baddies when it comes to oxidation and thus play a protective role. Red wines with high phenolic compounds are protected from oxidation, but white wines with high phenolics are more at risk. As you can see, oxidation is actually quite a confusing topic.

One of these is Transition Metal Ions, and the other is Phenolics. With these two dudes in tow, oxygen becomes highly reactive. Free radicals like this are highly reactive short-lived fragments of molecules capable of causing chemical havoc.

This is where we introduce participant three into the story, the phenolic compounds in wine. Phenolic compounds such as catechol are vital players in the oxidation process.

Specifically, we are interested here in phenolics with a particular chemical structure on them. They are known as o-diphenols, and include gallic acid, caffeic acid, caftaric acid, epicatechin and catechin.

They are very reactive with the hydroperoxyl radical, and here the diphenol is oxidised to a semiquinone, and then to a quinone. These quinones will go on to react with wine components. Hydrogen peroxide is also produced in the same process that produces quinones, and this can react with molecules present in the wine. As alcohol is relatively abundant, this reacts with the hydroxyl radical and the result is acetaldehyde. And organic acids will be oxidised to keto acids. Phenolics are contributing to oxidation, but they also have a protective role against oxidation in red wines and skin contact whites, because there are enough polyphenols present that they can effectively put out the fire that the combination of oxygen, phenolics and metal ions started in the first place.

Basically, all oxygen needs is a simple catalyst for the reaction to occur. Anthocyanins and phenols, two components of wine, are very susceptible to oxidation. Same thing goes for oxidation in wine. How do I know if my wine is oxidized?

Well, if the juice is slightly brown in color, with zero aromas of fruit and a lackluster palate, potentially displaying notes of vinegar, then yep, sounds like you got yourself a fully oxidized wine.

So, what can we do to prevent oxidation? There are a number of measures and precautions that can be taken that relate to minimizing air exposure to the must and wine, and sulfite additions that protect against oxidation.

To start, transfer wines quickly, especially white wines because they are most prone to the effects of oxidation as they do not have the polyphenols found in reds that act as natural anti-oxidants. Another way to help keep oxygen out whenever you are transferring your wine is blanketing the container you are moving it into with inert gas beforehand.

Once the wine is in the barrel or carboy, make sure to top up to limit the air space. With barrels it is more difficult as wine breathes through the staves and joints and as wine evaporates it is replaced with air.

Because of this, you should check the level of wine in your barrels twice a month, or more often depending on cellar humidity, and top up to the bung. During racking is another time when wine can pick up excessive oxygen if not done carefully. When racking your wine be mindful to limit oxygen uptake by placing the siphon hose in the bottom of the container to be filled so that the wine does not splash. Using a vacuum pump when filtering will also limit air exposure by drawing air out of the carboy you are pumping wine into.

Regardless of how careful you are, some oxygen will get into your wine during the process, and that is OK as yeast needs a limited amount of oxygen to survive. Excess oxygen after fermentation, especially in whites, is what you want to avoid. SO 2 is used as a preventative against microbial spoilage, but it also reduces the effects of oxidation by scavenging oxygen and its harmful radicals.

Ascorbic acid must be used in combination with potassium metabisulfite Campden tablets or it will act as a pro-oxidant if there is no SO 2 protecting the wine.