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The question

I'm experiencing a lot of oxidized wine due to faulty screw caps. About 10 to 15 per cent of the bottles I purchase appear to taste this way. Is it just me?

The answer

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Hmm. I've got an alternative theory (if I may) that could explain a screwy flavour in your wines. Are you certain they were overexposed to oxygen or might they in fact have been suffering from precisely the opposite problem, which is to say "reductive" aromas and flavours? The latter would be far more likely.

Screw caps are generally considered very good oxygen barriers. They're not perfectly hermetic but they generally provide a much tighter seal than natural cork. This has been chronicled in many studies, including a good, independent one by four enologists at France's Bordeaux Segalen University, published a decade ago in the California-based Practical Winery & Vineyard Journal under the title "Oxygen Transmission Through Different Closures Into Wine Bottles." The researchers tested four brands of screw cap as well as a variety of natural and synthetic cylindrical corks. Result: Screw caps were at the low end of oxygen permeation, with natural corks in the middle and synthetic closures worst of all. (Watch those purple or pink plastic "corks;" they're not for long-term cellaring.)

I should add that there are exceptional cases in which screw caps conspicuously fail the oxygen test. Namely: when the caps have been dented or otherwise damaged, creating a breach in the seal, leading to off-flavours after months or years in the cellar. But I suspect that's not the factor at play in your own displeasure with screw caps, unless you're buying your wine from dubious-looking characters on the side of a highway or at an unclaimed-freight depot. I sample a lot of screw-cap wine every week and don't find much of the bruised-fruit, tired or flat flavours or rusty colours generally associated with oxidation.

The more common screw-cap drawback is a phenomenon chemically known as reduction. Many wines, prior to bottling, contain a substance called hydrogen sulfide. It's often described as the flinty smell of a struck match or, when more serious, rotten egg. Usually (though not always) undesirable, it can sneak into your glass in a variety of ways. When unwanted, it's usually because yeasts in the fermentation tank have been starved of nitrogen, possibly because of poor vineyard management. It's essentially the yeast world's version of a bad-gas stress response. Screw caps, being pretty good closures, trap that gas, to be released only when you twist open the bottle. Standard cork, on the other hand, being more porous, usually enables hydrogen sulfide to escape before the bottle makes it to your dinner table.

The good news is that hydrogen sulfide is normally far from a fatal flaw. Slosh the wine around in your glass to let it blow off and you'll usually be fine.

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.

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