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The question: I have sometimes found little crystal deposits stuck to the bottom of a wine cork that look like broken glass. What's the story?
The answer: You'll be relieved to know it's not glass, nor is it dangerous in the least.
Those are tartrate crystals, sometimes called wine diamonds and chemically known as potassium bitartrate. They're formed when tartaric acid, a natural component found in grapes as well as bananas, binds with potassium under cold conditions to form a crystalline salt. Although they adhere to corks, you may sometimes also find them sticking to the sides of your glass with the last pour from the bottle.
Tartrate crystals can be found in both red and white wines. But since whites are served chilled, many wineries prefer to eliminate the aesthetic nuissance by cold stabilizing prior to bottling. The liquid is chilled to near-freezing in tanks, causing the crystals to precipitate out of the solution. Some wineries chill reds prior to bottling, too, anticipating that even a red wine can be inadvertently exposed to low temperatures during transport.
Though they can be off-putting to consumers who may be puzzled, the crystals are in fact an encouraging sign. Makers of high-quality wine prefer to intervene as little as possible with such techniques as cold stabilization and filtering, which can strip wine of subtle flavours. Diamonds are a wine lover's best friend.
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E-mail your wine and spirits questions to Beppi Crosariol. Look for answers to select questions to appear in the Wine & Spirits newsletter and on The Globe and Mail website.