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Dear Dr. Vinny,

I have noticed that wines made by one of my favorite wineries have recently been prone to crystal formation on the corks. Could this be because they are adding tartaric acid to their wines to increase the acidity? Or can these crystals form naturally?

—Rob, Carlsbad, Calif.

Dear Rob,

There are natural acids found in grapes, including malic, citric and tartaric. Some winemakers do add tartaric acid to raise a wine’s total acidity and lower its pH, as a way to achieve a more balanced wine.

When a wine gets really cold, the tartaric acid can settle out and form tartrate crystals, which can look like rock candy, usually clinging to the bottom of the cork. Once they form, they don’t dissolve back into the wine. These crunchy crystals are safe to consume and don’t affect wine’s flavor. But they can be unpleasant to crunch down on, and they can be worrisome for consumers who don't know what they are.

Some wineries put their wines through cold stabilization to separate the crystals out for cosmetic reasons. But some winemakers believe keeping a wine that cold (about 30° F) for a few weeks can mute its aromas and flavors. For winemakers following a non-interventionist (or "natural") course of winemaking, these crystals can be a badge of honor.

So you’re right that it’s possible that this winery has been adding tartaric acid, or their grapes could just be naturally high in tartaric acid. It could be that they stopped cold stabilizing their wines. Another scenario is that the wine experienced extended cold temperatures in storage, transit, or even in your own wine cellar or refrigerator. The good news is that the presence of tartrate crystals doesn't impact the way a wine smells or tastes.

—Dr. Vinny

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