WHAT EXACTLY IS OXIDATION?
If you've ever left the core of an apple sitting out for a couple of hours, then you've seen oxidation at work. As we all know, the apple core will turn brown. And, if you've ever been so bold as to taste an apple after it has turned brown, then you would have discovered that its flavor has changed as well.
What happens is that oxygen which makes up about 20% of the air we breath changes the chemical structure of the apple when its inner part is exposed. This is the same process that turns iron into rust or causes the pages of an old book to become brown.
When the apple has its protective layer of skin intact, the oxygen can not get to the proteins and other elements inside the apple that are subject to rapid oxidation. But once the apple is bitten into and exposed to the air, it does not take long for a noticeable amount of oxidation to occur.
No doubt the apple is a dramatic example of oxidation, as oxidation does not normally occur at such a fast pace with most fruits. None the less, the effects are the same when oxidation is given enough time and opportunity.
HOW DOES OXIDATION AFFECT WINE?
When a wine becomes oxidized it will turn brown - just as the apple did. White wines will start to show an amber tint and red wines will start to develop a brown edge when viewed in a glass that has been tilted. In extreme cases where there is excessive air exposure over longer periods of time, the wine can develop a nutty to caramel aroma, and may also develop slight off-flavors that resemble raisins or cough syrup.
Some wines may develop "bottle sickness". A term used by wineries to describe a flabby character that can come over a wine when it is bottled with too much exposure to air. When excessive amounts of oxygen is thrusted onto a wine in a short period of time, the wine is effected differently than if it was slowly exposed to air over longer periods of time.
Bottle sickness is referred to as a "sickness" because the wine will usually recover if given enough time. Most of the negative effects caused by bottle sickness are usually reduced considerably with 2 or 3 months of aging.
It is also important to note that white wines are effected more easily by oxidation than red wines. This is mainly because red wines have more color pigmentation than white wines. This extra color pigmentation acts as an anti-oxidant, preserving the wine's color and flavor.
Also, it is easier to see discoloration in a white wine than it is in a red. So as one can conclude, while air exposure must be given consideration when dealing with any wine, white wines in particular, must be given special consideration because of their delicate nature.
WHAT CAN I DO TO REDUCE OXIDATION?
There is no reason to take drastic measures or to become obsessed with oxidation. The fact that you are reading this article and are becoming aware of this issue puts you 10 steps ahead of the unsuspecting home winemaker. By getting into good habits and taking some rudimentary precaution, you can easily get oxidation under control.
REDUCING AIR EXPOSURE WHEN SIPHONING
One of the major sources of oxidation comes from splashing the wine during siphoning or bottling. Splashing actually invites the air into the wine. This is much worse than having wine just simply sit while its surface touches air. Wine can sit for several days before oxidation will effect the wine to any notice-able degree.
It is reasonable to say that regardless of how careful you are, some oxygen will get into the wine during these types of process. But by carefully filling the container being siphoned into, from the bottom up you can save the wine from a lot of unnecessary air exposure.
What is meant by "bottom up"? This means to put the siphon hose in the very bottom of the container to be filled. So that, as the wine comes out of the hose it will not be exposed to air, but rather to the wine that has come out of the hose before it. By filling the container in this way, only the first bit of wine coming out of the hose will touch air.
REDUCING AIR EXPOSURE WHEN AGING
A second common source of oxidation comes from improper long-term storage of wine. This long, slow type of oxidation can destroy a wine's character in a none recoverable way.
While the wine is fermenting, oxidation is not an issue. CO2 gasses from the fermentation help to drive away any notable amounts of oxygen that may be in the juice itself or in the head space of the fermentation vessel. So, larger head spaces in containers during and right after fermentation are not a cause of oxidation and should be of no real concern.
But, if you plan to bulk age your wine for a few months after the fermentation has completed, then air contact should be kept to a minimum. Head spaces in containers need to be reduce by either moving the wine to a smaller, more appropriately sized container or topping the wine up with water or another finished wine of similar style.
It is also important to note here that both light and heat will speed up the effects of oxidation. That's why it is always recommended that long term storage of your wine should be done in a cool, dark area.
Adding a dose of Sodium Bisulfite and Campden Tablets right before bulk aging or bottling will also help to reduce oxidation. Either of these will release sulfur into the wine, driving out most of the oxygen. It also fills any small head-space in the container with sulfur gases, again reducing air exposure.
It is also recommended that Ascorbic Acid be added to lighter colored wines at the beginning of fermentation. Ascorbic Acid is an anti-oxidant that helps to reduce the effects of oxidation. About 1/8 teaspoon per gallon is sufficient.
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Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.