At a very basic level, you will know that a wine has gone bad if you do not enjoy the taste.
A wine that has gone bad will have vinegar-like or astringent flavors in the wine, or a strange texture or mouthfeel. The good news is that usually if a wine has gone bad, it will not make you sick.
There are two scenarios to consider: a wine that has not been opened, and a wine that has been opened.
Wine that has been opened will usually only last a day or two after being uncorked. Unopened wine will typically last years, if it was bottled and stored properly. Ultimately, if the taste, smell, or appearance is unsettling, discard the bottle. You should enjoy the wine you’re drinking.
Changes in Flavor
Wine is made through fermenting grapes with yeast. Through the fermentation process, the yeast will eat the sugars that that would typically cause grapes to spoil, producing alcohol as a by-product. This process allows wine to be shelf stable for many years, if the bottles were properly processed and stored.
When wine is bottled, a cork is compressed and forced into the neck of the bottle. These corks contain a certain amount of moisture. Storing the bottle laying down or at a downward angle keeps the cork moist. If the bottle is stored upright, the cork could dry and shrink, allowing air to enter the bottle.
As air is introduced to the bottle of wine, it will react with the alcohols in the wine to create acetaldehyde which in small quantities will create desirable flavors like nutty, roasty and ripe fruit, but in high concentrations will create flat sweet caramel flavors. Bacteria like acetobacter, which naturally occur in the air could also start growing in the wine. In time acetobacter will produce acetic acid, the primary acid in vinegar.
This process happens much more quickly once the bottle is uncorked. Once opened, wine should generally be consumed within a day or two. Options for recorking your wine include spraying a wine preservative into the bottle. These preservers often look like compressed air but are comprised of gases that force oxygen out of the bottle when you reinsert the cork. Another option includes vacuum sealing the bottle with a pump. However, these methods only slightly extend the length of time you could enjoy your bottle.
Often times high-quality bottles of wine will be corked with a natural cork. This cork can be infected with a fungus called trichlororoanisole or TCA commonly refereed to as cork taint. This fungus grows on the cork oak tree, which is where cork for wine bottles is harvested. This fungus can ruin a bottle of wine giving it a wet cardboard, musty basement, or paper pulp smell. Fortunately industry reports say that only about 5-10% of all wines bottled are ruined by this fungus. While natural cork is still preferred, especially for high-quality wines, wines corked with synthetic corks, glass capsules,or screwcaps, are not exposed to the fungus and are therefore not susceptible to the fungus.
Changes in Color
As wines age, the color will typically deepen. Some high-quality wines have lasted hundreds of years with proper storage, but lower quality wines could spoil. Wines are typically stored in colored bottles that prevent light from penetrating through the glass. Sunlight can break down components of wine called antioxidants and tannins. Antioxidants inhibit wine from oxidizing, and like it sounds, oxidation causes acetic acid to build up in the bottle.
Tannins exist naturally in grapes and help give each wine its unique flavor. These tannins can give your mouth a feeling of dryness as you drink. When sunlight hits the tannins, they start to break down and the flavor of the wine degrades.
To store wine for long periods of time, keep the bottles away from sunlight. Wine cellars generally have no windows, or are located underground, to keep daylight from ruining the flavor of the wine.
Issues with Yeast or Bacteria
Sometimes, a wine may go through a secondary fermentation process after it has been bottled. A clue that your wine has undergone a secondary fermentation may be carbonation. Unless your wine is labeled as a sparkling wine or specifically as Champagne, wine should not have any carbonation. Carbonation is the result of carbon dioxide building up in the bottle, similar to how a beer or cider may get its bubbles.
Carbonation is typically a sign that an undesirable yeast or bacteria was introduced to the bottle at some point in the fermentation or bottling process. If you open a bottle that has carbonation when it shouldn’t, it’s best to discard the entire bottle.
Sometimes bacteria like brettanomyces or lactobacillus can cause off flavors or mouthfeels in your wine. If you feel a slimy texture, uncharacteristic bitterness, or a smell of rotten vegetables or earthiness, discard the bottle. These bacteria are rare with commercially produced wine.
High or irregular temperatures can spoil a wine as well. The same way sunlight can degrade tannins in your wine, heat can also degrade them. Additionally, heat could cause the wine and cork to expand, resulting in a leaky bottle of wine. If you notice that the cork has bulged from the neck for the bottle, or wine has seeped up the sides of the cork, the wine has likely spoiled.
Heat will cause your wine to age much more quickly than if stored at recommended temperatures, such as between 53-57F. The flavors of some wines might improve with age, but aging wine through heat can have unpredictable results. It is generally recommended that wine not be stored above 75F for more than a few days, or above 80F for more than a few hours.
In summary, the most common reason for a wine to go “bad” is that oxygen has been introduced to the bottle. To ensure the best flavor of your wine, make sure that you store your bottles between 53-57F, or in your refrigerator at home, laying down and away from direct sunlight. Open your wine and enjoy yourself or with friends within a day or two for the best experience. If you don’t enjoy the flavor, don’t feel obligated to finish a bottle that doesn’t taste right to you.