Even with modern winemaking, storage and shipping technology about five percent of all wines arrive at the table bad to some degree.
Here are some tips about how to spot them.
It all begins with a cork... Even in this day of high-technology plastics, many bottles are still stoppered with an old-fashioned cork. A product of the bark of selected Mediterranean trees, corks have many highly desirable and hard to duplicate attributes. Lightweight, resistant to disease and airflow, flexible and attractive.
But cork, as a natural product, is subject to attack by microorganisms. Certain species of fungus, present anywhere wine is stored, can infiltrate the cork producing a compound called TCA (1,2,4-trichloroanisole, for those interested).
TCA and other factors produce in wine unattractive odors and tastes, similar to wet cardboard, mushrooms, mold and even unwashed socks. Even if the odor is mild the taste may be somewhat bitter and lack fruitiness.
Moving wine from the vineyard to the home or restaurant table involves trips of anywhere from dozens to thousands of miles. Given that it's surprising that only five percent of wine is partially spoiled due to excessive temperatures.
High temperatures cause liquids like wine to expand slightly, which increases the pressure inside a corked bottle. This pressure can cause the cork to get pushed slightly up. When the wine cools down again, air seepage can occur.
If the temperature becomes high enough, the wine can be literally cooked. If cooking has taken place, the taste will be evident. The fruit flavor will convey more stewed prune than fresh berry. Look for corks that sit above the lip of the bottle, or levels of liquid too far near the base of the neck.
Improper storage, such as excessive heat or cold and incorrect humidity levels, can cause corks to shrink or crack. Either of these will cause infiltration of unwanted amounts of air into the bottle, causing oxidation. Oxidation is what it sounds like, oxygen, a volatile compound, combining with a wide variety of wine components and altering them.
Some amount of air, in small amounts over a period of years, may be desirable to ensure proper aging. But even well before producing vinegar, it's possible to spoil a wine from allowing too much air to reach the wine. Wine which has become fruitless and resembles old Madeira (deliberately produced in open air vats) is almost certainly oxidized.
SULPHUR and SEDIMENTS
Sulphur is a common preservative in winemaking which can help stabilize wine. But in excessive concentrations it produces an undesirable aroma and flavor. Its characteristic mothball or burnt matches smell or taste makes it easy to spot.
Some sediment in wine is acceptable and normal. Ports and older wines, even whites, often accumulate material and properly poured or decanted and served are fine. Tartrate crystals too, for example, will naturally form in some wines especially when chilled for shipping and storage. They do no harm, but avoid pouring and tasting them.
In rare cases, it's possible for dormant yeasts to remain in the wine and some amount of additional fermentation to take place during the years of shipping and storage. Champagne, for example, is deliberately re-fermented in the bottle. But for non-sparkling wines, this effervescence is undesirable, but easily spotted.
Techniques of manufacture, bottling, shipping, and storage continue to improve and the odds of encountering one of these conditions is rare. If you find a wine that's distasteful, chances are it simply isn't for you.