MadSci Network: Chemistry

Re: Is it possible for wine to ever go bad?

Date: Wed Aug 1 17:36:13 2001
Posted By: Michael Onken, MadSci Admin
Area of science: Chemistry
ID: 993302470.Ch

Yes and no. Wine can certainly age beyond its peak; however, it doesn't spoil, insofar as spoilage implies the growth of some microbial agent. Nowadays, wine is filtered prior to bottling to remove the fermenting microbes, so that fermentation does not continue after bottling (this is not the case with many sparkling wines that are carbonated through bottle fermentation). Since fermentation is halted prior to bottling, the alcohol content cannot increase with time, and can even decrease after many years in the cellar.

The main issue here is what happens to wine as it ages, and, for that matter, why wine needs to be aged at all. Wine is the perfected union of grape juice and fermentation. Each wine is produced from specific harvests of certain varieties of grapes, which are pressed by varying methods into a juice, strained and fermented. Each step in this process is different for different wines, and each contributes to the final, unique taste of the wine. The flavors and aromas of a wine are produced by several organic compounds, mostly phenolics, terpenoids, alcohols, and esters; however, the overwhelming components of all of the red wines, as well as some whites, are the pigments and tannins extracted from the skins of the grapes during pressing (tannins are also introduced from the oak barrels used for most fermentation). While the other elements add uniqueness to the flavor of the wine, the tannins add acidity and bitterness that often overwhelm the other flavors of the wine. As such, most wine drinkers prefer to eliminate the tannins from their wines before drinking them, so that they can enjoy the other flavors behind the tannins. In the vernacular of the oenologists (wino's), the loss of tannins with the concurrent appearance of the underlying flavors is called "opening up".

Tannins are easily oxidized, so there are two possible means of breaking them down: first, atmosperic oxygen can be used to quickly destroy the tannins; second, the other oxygen containing compounds in the wine can combine with the tannins to slowly remove their stringency. The first method is accomplished through decanting the wine and then allowing it to sit open for a time until it loses its acidity (tannins) - this is often required for younger wines that haven't had time to age. The second method is accomplished through - and is the reason for - aging the wine. As a wine ages, the tannins slowly react with the pigments, flavorants, and other acids, to form a dark precipitate. Since some of the tannins are neutralized by pigments, the color of the wine fades as it ages; since some of the tannins are neutralized by specific types of flavorants, like various alcohols, the wine will take on a distinct character after long aging, and will become slightly less intoxicating. Again, the connoisseurs say that the wine "mellows" with age: referring to the loss of acidity, as well as the change in flavor.

Whether or not a wine should be aged is often a matter of personal preference. The old school of wine drinkers preferred the musty, mellowed flavors of aged wines, while many modern connoisseurs appreciate the bigger, fruitier character of a younger wine, and swear by their decanters. It can also depend on the wine - some wines are incredibly tannic and can lie for 20 years before they open up, while others lose more flavor than tannin during aging and are best drunk young. For more sites on Wines and aging, check the following list:

In answer to the main question, "can aged wine go bad?" we have to look at the differences between decanting and cellaring. Decanting works through contact with air, which oxidizes the tannins very quickly, while cellaring involves years of leaving the wine untouched. One problem with wine is that tannin is not the only compound that can be oxidized by oxygen - over time, many of the pigments and flavorants will also be oxidized, completely altering the color and character of the wine. Wine that has been oxidized to this point is said to have "sherried", and is usually regarded as unpalatable. Obviously, cellaring won't work in the face of oxidation, so winebottles are corked to keep air out. Unfortunately, cork can only form an air-tight seal if it remains moist, and a wine that has been left upright for too long such that the cork has been allowed to dry will often sherry in the bottle. In the days before good filtration methods were available, some bottles would still contain small amounts of the fermenting microbes, which would further ferment the ethanol into acetic acid during the wine's anaerobic aging. This would result in the wine turning to vinegar, which was unfit for drinking but delicious for cooking. So as long as the wine is tannic enough to need aging, the wine is filtered properly, and the cork is kept moist, the aging process should turn an acrid, youthful wine into a fuller, mellower complement to your meal.

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