Many wine lovers find that half a bottle, or maybe even 1 or 2 glasses of wine is sufficient for an evening, especially when dining alone. This level of moderation is safe for a healthy liver, and reserves the rest of the wine for enjoyment later. The down side is that you have the problem of storing opened wine.
Many things can happen to wine after the bottle is opened. Pulling the cork is the point of no return. Once the bottle has been opened, air rushes in. Contact with air can do many things to wine, depending on several things. One of the most important factors to consider is the wines age. Young wines are much less effected by exposure than old wines. For example, a young Cabernet of 5 years old, or less, can actually benefit from a little oxidation. The oxidation process can ease the harshness of the tannins, and allow the bouquet to develop. Older wines are a different story, as they are much more fragile. For a mature wine, even a few minutes exposure to air can cause it to lose its character and bouquet. From this, you might gather that mature wines are best enjoyed on the spot. However, the majority of wines consumed are of the younger, sturdier variety. There are ways to preserve these for short periods of time with little, or no loss in quality and character.
Like many other organic materials, wine can be preserved in the refrigerator. Chemical reactions are slowed down significantly at lower temperatures. Oxidation slows down to a fraction of its normal rate, allowing the wine to be less effected by the process. The lower temperatures also inhibit the action of various bacteria, namely Mycoderma aceti. This is the strain that causes wine to turn into vinegar. Chilling left-over wine is the first, and most important step you can take to preserve its integrity.
Another process that can greatly extend the life of your favorite vintage is to decant it. Decanting means to transfer the wine into a smaller container, just big enough to hold it, with only a small amount of space for expansion. This allows much less oxygen to be stored in the bottle with the wine. There are some that decry this process on the theory that the extra oxygenation resulting from pouring the wine from one bottle to the other outweighs any benefit the smaller amount of oxygen may bestow. As far as scientific evidence, there is none to support either position. Nonetheless, it has been my personal experience that decanting does actually result in a discernable improvement over wine that has only been refrigerated.
For people that want to go one step further, or maybe need to store the wine longer, there are a few, more serious, methods for storing opened wine. The first involves the use of a gadget that can be purchased at most grocery stores. It is a bung that fits into the neck of the wine bottle, and has a small manual vacuum pump on the end. The bung is inserted, and the pump pulled back. This creates a partial vacuum in the bottle, and greatly reduces the amount of oxygen. I have used this method before, and it had very disappointing results. The wine tasted flat, and had lost most of its bouquet. I believe there are two main reasons for this. First, as you pull back on the pump, you may notice small bubbles rising to the surface. This is CO2 (Carbon Dioxide) that was suspended in the wine. It is created during the fermentation and aging process. I am of the opinion that losing the CO2 changes the chemical composition of the wine, and hence the flavor and aroma. Secondly, the bouquet is made up of many volatile compounds such as esters, which are also pulled out of suspension by the reduced pressure. This accounts for the loss of bouquet. I really cannot recommend this process, if you really want to drink your wine later. With a vacuum pump, it will only be fit for cooking (maybe) later.
Another extreme method is to use nitrogen. A short burst from a compressed nitrogen bottle will prevent oxidation. Again, my experience has been that the nitrogen causes the wine to deteriorate in both character and bouquet. I am sure the nitrogen reacts with the other chemicals present in wine, in a less-than-desirable way.
There is one major exception to all of these methods. That is Sparkling Wine, Champaign, Asti Spumonte, and similar carbonated wines. Decanting these wines will deplete the carbonation, and make them go flat very quickly. The best way to preserve them is to cap them with a pressure cap, and refrigerate as soon as possible. There is much less danger of oxidation in carbonated wines because a layer of CO2, which displaces the oxygen in the bottle, protects them. I do not have a lot of experience keeping champagnes. Somehow, someone always shows up to help me finish a bottle.
Regardless of what method you use, the wine should be used within 24 hours. After that, no amount of protection seems to help. Oxidation is inevitable. It has to do with physics, and Molecular Orbital and Valence Bond Theory. All atoms have electron ‘shells’ which have a certain number of allowable electrons. The outermost shell is called the Valence Shell, and determines the atoms chemical properties. When a valence Shell has less than the maximum number of allowable electrons in it, it will freely bind to other atoms to attempt to ‘fill’ the shell, by sharing electrons. Oxygen readily combines with many other molecules because the Valence Shell has 6 electrons out of a possible 8 allowable, so it will attempt to ‘share’ another atoms electrons to complete the outer shell, forming a Chemical Bond. This is the process we call ‘oxidation’.
With a little care, most wines can be preserved, to be enjoyed later in the day, as long as the proper methods are used for storing opened wine.