If you take a class on wine, perhaps just to learn a little about it or maybe to earn some certification or another, you will at some point learn about wine faults. These are flaws we all should be able to recognize, if only to know when to return a bottle to a store or ask the sommelier for another wine to match our entrees.

It sounds cut-and-dried. Faults are faults, right? Not always. Some are quite subjective, even viewed as positive by some wine lovers, and discussions about them can be ideological. Faults, as we know them, are defined by modern winemaking, which uses science and technology to make wine as clean and technically correct as possible. Many argue that this approach risks stripping the soul and character out of a wine. The natural wine movement is a rejection of modern scientific, technology-driven winemaking. And it embraces some of those characteristics generally considered faults.

“A wine may be flawless yet desperately dull,” the British writer Jamie Goode wrote in his excellent treatise, “Flawless: Understanding Faults in Wine,” published last year by the University of California Press. “Some faults are universally agreed on,” he says, while others “are more a question of degree and personal preference.” Goode cites wabi-sabi, a Japanese worldview centered on the idea that flaws are part of beauty, in contrast to the Western desire for perfection.

Goode analyzes faults that originate in the vineyard as well as in the winery. Some of these are common topics of discussion for wine lovers. Brettanomyces, or brett, for example, is what Goode calls “a rogue yeast” that affects primarily red wines. Brett’s flavors have been described as smoky or leathery, which can be good, especially in the Loire Valley reds with a character I call “grandpop’s library.” Chateau Musar, from Lebanon, has earned cult status in part because it is unabashedly bretty. But brett is also described as horsy, or sweaty saddle, as well as the dreaded barnyard. When it’s that pronounced, the flavor of brett dominates the wine.

Another fault you are likely to encounter at some point is oxidation. “All wines are exposed to oxygen at one stage or another, but it is when the exposure has been too great at the wrong stage, or is inappropriate for the wine style, that we have a problem,” Goode writes. Overexposure to oxygen at any point in a wine’s life can render it tired; think of a half-eaten apple left on your kitchen counter, or guacamole that turns black. Wines can become oxidized through poor storage, from dried out corks and warm temperatures. This is what has happened to those old wines that we’ve kept in our closets or on top of our refrigerators, waiting for a special occasion that never seems to come.

And yet, oxidation is a key characteristic of some wines. Amber or orange wines, whites fermented on their skins, are often exposed to oxygen. The white wines of Lopez de Heredia Viño Tondonia in Spain’s Rioja are noted for their oxidative style. They challenge our modern preconceptions of white wine as fresh and bright. And they can be compelling.

Discussing wine faults can make us sound like nerdy chemists. There’s volatile acidity, which smells like nail polish and makes wine taste a bit like vinegar. In small amounts, proponents say it can “lift,” or enhance, the wine’s aromas. Reduction — the lack of oxygen, leading to volatile sulfur compounds — can smell sulfury or like rotten eggs. Properly managed, though, reduction can help maintain a wine’s freshness.

There’s one fault wine lovers can universally agree on: cork taint. This is the moldy, wet cardboard smell imparted to a wine by a chemical contaminant in cork. The chemical, 2,4,6-trichloroanisole, or TCA, can infect the cork during manufacturing. It’s harmless to humans, but it can be fatal to wines. It can be obvious as soon as you take a sniff, or it can be hard to detect. If you think the wine is muted, the fruit dulled, it may be corked. Luckily, cork taint is dramatically less common now than it was even a decade ago, because of improvements in cork manufacturing and the use of alternatives such as screw caps.

What should you do if you suspect your wine is flawed? Take it back to the store where you bought it; they should offer a refund without question. At restaurants, don’t be shy to ask the sommelier or wine waiter to taste it.

“Feel free to politely express your doubts,” says Erik Segelbaum, former head sommelier for the Starr Restaurant Group, which includes Le Diplomate and St. Anselm’s in Washington. Segelbaum launched his own wine consulting company, Somlyay, earlier this year. “It pains me when my non-industry friends tell me they drank a wine they didn’t like and thought it was flawed, but they didn’t know what to say,” he adds.

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