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As a student, I drink a lot of bad wine. Probably because I drink a lot of cheap wine. That’s not to say that inexpensive wine is always bad, but there are certainly more misses at the lower end of the price range than at the top. From the convenience store bottle bought at 9:55 pm on a Friday night to the pricier label found for 70% off, I’ve encountered bad wine in many forms.

If anything, I’ve become a connoisseur of wine faults and flaws. The wide variety of characteristics that make wine ‘bad’ are actually quite interesting. And understanding how to identify bad wine not only increases my appreciation for better wine, it helps me avoid similar purchases in the future.

Bad wine can be characterized as either flawed or just off-character. Flawed wine typically results from one four problems.

Heat Damage – Wine stored above proper temperatures may be ‘cooked’ and have a significantly altered taste. Heat damaged wines taste as you would imagine – people often describe hints of prunes, cooked caramel, or candied fruits. They may taste similar to a Port, Madeira, or Sherry.  On the palate, heat damaged wine may seem particularly thin. It can be difficult to determine if a bottle has mild to moderate heat damage, but a completely cooked bottle is hard to miss. A protruding cork is a strong indication that the contents of the bottle expanded under high heat and that the wine will be flawed.

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Cork Taint – Wines bottled with natural cork are susceptible to damage from a fungus that feeds on the cork. This fungus produces a chemical known as TCA, which changes the taste and nose of the wine dramatically.TCA gives corked wine a distinctive, unpleasant smell alternately described as dirty socks, wet cardboard, or mould. It will taste fruitless and possibly somewhat astringent. You cannot discern whether a bottle is corked by smelling the cork. Screw tops, beer bottle tops, and artificial cork are the only surefire protections against cork taint.

Oxidation – Wine exposed to a significant amount of oxygen loses its freshness. This is the same process that occurs once a bottle is opened, but can also happen with a faulty cork. Oxidized wine may be reminiscent of vinegar or Madeira (for which oxidation is an essential part of the winemaking process.)

Refermentation – Sometimes, the dormant or residual yeast in a wine undergo a second fermentation after bottling. This is rare, but results in wine that is fizzy or effervescent. Sparkling wines achieve this effect purposefully.

Beyond these issues, labeling wine as ‘bad’ is fairly subjective. But there are some generally agreed upon characteristics that are not good. Below are a few of the worst qualities that I have encountered the most often.

Ethyl Acetate and Acetic Acid– When wine is reminiscent of nail polish remover (acetone), it has too much ethyl acetate. This problem frequently occurs with a vinegar-like taste and aroma, caused by acetic acid (the compound that gives table vinegar its flavor.) Microbes that can enter the wine when it’s exposed to air convert glucose and ethanol to acetic acid. Acetic acid and ethanol then convert to ethyl acetate at room temperature. A small amount of each can be acceptable and add complexity, but too much makes a bottle undrinkable.

Too much sulfur – Sulfur odor in wine may be due to hydrogen sulfide (H₂S) or sulfur dioxide (SO₂.) The presence of H₂S, which smells like rotten eggs, can occur as a natural yeast byproduct or from antifungal spray residues used on grapes close to harvest. Over time, H₂S can interact with alcohol to produce mercaptans (-SH) and, eventually, disulfides. These are less volatile forms of sulfur and can take on a range of odors, described as everything from garlic to rubber or skunk. It may be possible to remove the scent by aerating the wine or by adding copper or silver (in the form of a pre-1982 penny, a copper wire, or a silver spoon.) If these steps don’t eliminate the smell, the bottle should be tossed.

A burnt matches smell upon opening is likely the result of sulfites (usually SO₂) added during production for their antibiotic and antioxidant properties. Sulfites eliminate the bacteria that produce acetic acid, for example.  The “contains sulfites” line on wine labels refers to this compound. This scent will usually dissipate within a few minutes or through aeration.

Barnyard Funk – A distinctive odor of manure, sweat, wet dog, or leather and sour taste signals the presence of Brettanomyces yeast. Commonly known as Brett, this yeast is considered desirable by some and a contaminant by others. American winemakers and most brewers view Brett as a fault, but the flavors it imparts are acceptable and enjoyable in certain Rhônes, Burgundies, and Belgian-style beers. An antiseptic, medicinal, band-aid-like odor is associated with another less desirable form of Brett.

Wildly imbalanced – Although it’s highly subjective, some wines are just too imbalanced to enjoy. Components like the acid, alcohol, fruit, or oak can be overwhelming. I have seen this most often in inexpensive, mass-produced wines that likely take more winemaking short-cuts.

This is hardly an exhaustive list, but these are the most common issues I have encountered. More detail and a comprehensive list of wine faults are available here