What you should know about unscrewing your wine

Our waiter was opening a bottle of screw-capped wine when the cap, instead of screwing off, lifted straight off the bottle without any "crack". The waiter, who did not seem very experienced, said he thought it wasn't a problem but we asked for (and got) a replacement bottle. Were we making a fuss about nothing?

The brisk crack we hear when opening a screw-capped wine might be less sonorous than the pop of a cork but it's at least an assurance that the wine has been properly sealed – right? Wrong. Counter-intuitive though it seems, the section of a screw-cap assembly that works hardest to keep a wine bottle sealed is not the bit where you twist the cap. The thing that does the job is inside the cap; it's a thin, slightly squishy disc sitting inside the screw cap where it meets the mouth of the bottle.

When screw-capped wine is bottled, a straight-sided metal sheath comprising the cap and the "skirt" that goes around the neck is dropped over the bottle mouth. A machine delivers a whack to the flat top of the sheath sitting over the mouth. This compresses the disc against the glass lip, creating the seal. Only then does the machine crimp the sides of the cap.

The fact that the whole sheath came away in one piece when the waiter tried to unscrew the cap suggests there was a fault when the sides were crimped. That doesn't necessarily mean the seal at the top was broken. In my years of opening screw-capped wines (all in the name of research) I've had the sheath come away in the manner you describe only twice. Both times the wine seemed unaffected. But I wasn't in a restaurant then, and in the circumstances you describe it was fine to ask for a new bottle. It is possible that the seal was also faulty; you spared yourself a bit of concern about whether the wine had been affected, and the waiter did the right thing in bringing you another.

The thing that most commonly compromises the seal on screw-caps is a knock to the lip of the bottle, which can loosen the compressed seal. When you're at a wine store, avoid bottles with obvious damage to the screw-cap around the lip.