Wine defects: how to recognize them

Who hasn’t been there? You uncork a bottle, pour a few glasses with the best of intentions, and… hmm, this doesn’t taste right. What happened to that wine that winemakers carefully bottled for your enjoyment? Was it poorly made, or did something happen during storage? Is it what people commonly call “corked,” or is it another issue? A corked wine is indeed defective, but not all defective wines are corked.

Things that can happen to wine

First, let’s define what a defective wine is: it’s one that presents undesirable odors due to problems during harvesting, in the winery, or in storage. With that in mind, let’s address two opposing phenomena that are common causes of wine deterioration.

Oxidation occurs when a wine has excessive contact with air. You can recognize it by its sherry-like or rancid taste (though note that rancid wine isn’t necessarily bad—winemakers may intentionally induce oxidative aging to create a low-alcohol, food-friendly wine). An oxidized wine loses its brilliance and often takes on a brownish hue in reds or an orange tint in whites. How does oxidation occur? Likely due to poor storage, prolonged aging, or improper conservation—especially in wines meant to be consumed within a year.

The opposite phenomenon is reduction, where the wine lacks proper oxygen exposure, leading to the formation of sulfur compounds. This results in unpleasant odors: rotten eggs (due to hydrogen sulfide), cooked potatoes or cauliflower (methionol), or onions (ethanethiol). Reduction often occurs in older wines with many years in the bottle, but also in young wines bottled too early without proper aeration. Tip: Decant them.

A quick guide to bad odors

• Vinegar: This is what’s commonly called a “corked” wine. The smell is caused by bacteria that convert alcohol into acetic acid. Every wine contains a small amount of this acid, but if the odor is too strong and unpleasant, it’s likely the bottle has been left open too long, exposed to high temperatures, or stored in overly dry conditions, leading to spoilage.
• Sulfur: Since sulfur dioxide is used in winemaking (the infamous sulfites), sometimes the wine may carry a sharp smell, similar to a lit match. Aerating the wine may help.
• Cork Taint: This is caused by a molecule called trichloroanisole (TCA) and gives the wine a musty smell, like wet cardboard. It’s relatively common for some bottle corks to be affected by this issue.
• Mold: Mold can settle in cork crevices, imparting an unpleasant musty smell to the wine. This is a frequent problem in some low-alcohol wines.

Other wine defects

• Bubbles can indicate an unwanted second fermentation, which may cause bitterness or a vinegary taste. However, it’s often not severe enough to make the wine undrinkable.
• Excessive oak flavors can occur when the wine is overly affected by its wooden container, leading to weakened aromas and sometimes a bittersweet taste.
• Chemical and microbial alterations can cause wine to turn cloudy or overly sour.
• Volatile acidity, which is present in all wines, can give off vinegar or nail polish remover aromas when excessive.
• Brett (Brettanomyces yeast) can produce animal-like aromas. These aren’t always considered a flaw, as they can add complexity to some wines.

False wine defects

Not all apparent alterations mean the wine is defective. Some are simply byproducts of the winemaking process and don’t indicate spoilage:

• Crystals at the bottom of the bottle are harmless. They are crystallized tartaric acid salts (bitartrate), a natural wine component. Many wines undergo cold stabilization to remove them, but high-end wines often skip this process.
• Sediment consists of natural color compounds that settle in the bottle over time. This is not a flaw—rather, it’s a sign that the wine hasn’t been aggressively filtered. Tip: Decant the wine to keep these harmless particles out of your glass.

Now, pour yourself a glass and enjoy! 🍷