Tartaric Acid

Have you ever opened up a bottle of wine only to panic when pulling out the cork, because it looks like it’s covered with tiny shards of broken glass? Fearing an unsalvageable bottle, thanks to poor handling or, even worse, user error, you look even closer. Then on closer inspection, with relief, you discover that it’s not glass, but tiny little crystals that have materialized on the cork.

“Wine diamonds,” as they are known to seasoned oenophiles, are actually crystalized compounds of tartaric acid, reacting with potassium. And rather than serving as an indication of flawed wine, these crystals actually serve as proof that a wine has been less heavily manipulated. Even though these tiny crystals, which form spontaneously when tartaric acid and potassium bind under chilly conditions, are harmless, since many people react out of concern when they see them, certain producers act to ensure that they can never, ever materialize.

The process employed is called “cold stabilization,” and it involves dramatically ratcheting down the temperature in steel fermenters once the fermentation process is complete. The wine then sits at a near-freezing temperature for 3-4 days, while the crystals form along the sides of the fermenter. Then, when wine moves from the fermenter into bottles, or into wood barrels for aging, the tartrates are left behind. The catch, according to some winemakers, is that some of the tart flavor that makes the wine unique gets left behind in the fermenter as well.

If you do open up a wine that reveals crystals on the cork or near the mouth of the bottle, there’s an easy solution. (No pun intended.) Simply let the bottle stand upright for a bit, while the sediment drains to the bottom of the bottle, then decant the wine, making sure that you stop pouring before the sediment starts flowing into the decanter.