The question: I accidentally left a bottle of wine in the trunk of my car and it froze. Is it still fine to drink?
The answer: Welcome to February in the Great White North. Mercifully, your wine should still be okay.
Many liquids are reasonably tolerant to freezing. Fruit juice and milk, for example, suffer little. It's the same with wine. Some people maintain they can taste a difference, but any change in flavour will be extremely subtle. I've known people to advocate freezing a half-empty bottle as a way to prevent it from spoiling in the presence of air. Frozen wine certainly isn't dead. But I'd be less inclined to store a previously frozen wine in a cellar for the long term.
The more crucial concern in this case is with the seal on the bottle. Wine is mostly water. As water freezes, it expands. This can force the cork up and slightly out of the neck, allowing air to creep in. Because air is corrosive to wine, it's important to drink the bottle sooner rather than later. The same issue applies to screw caps, which may become deformed if the frozen liquid reaches the rim, breaching the airtight seal.
Incidentally, freezing can cause a visible change in the wine in the form of potassium tartrate crystals. Sometimes called "wine diamonds," these resemble little shards of glass but are perfectly harmless. Tartaric acid occurs naturally in wine, and when the liquid is chilled, the acid combines with potassium to precipitate out of solution. The crystals will usually drop to the bottom of the bottle, but occasionally a few will adhere to the surface of a freshly pulled cork.
If you've accidentally frozen a wine, either in the trunk or because you've chilled it too long in the freezer (I've been guilty on both counts), let it thaw at room temperature. Under no circumstances should you microwave it, at least not with the cork still lodged in the neck. If you mistakenly set the timer to, say, two hours rather than two minutes (am I the only person who finds microwave control panels unnecessarily complicated?), you could end up dealing with a mess of real glass crystals.
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