“Ullage” doesn’t sound like anything you’d want in or near your wine. It sounds more like the final groan of a dying mythical troll, or something that your mechanic will try to charge you $500 for. “See those lines? That means there’s ullage in the sunroof submeter engine.”
Ullage, however, does have a place in wine—old wine, specifically. In the most basic sense of the term, ullage refers to the amount of wine missing from the bottle. Not due to theft or sleep-drinking, but slow evaporation over a long period of time, which is why you’ll see ullage in bottles with years, even decades, on them—and a cork enclosure, allowing for said evaporation.
If you know the term “angel’s share,” you already know what ullage is. The angel’s share is the amount of an aged whiskey that’s lost to evaporation in a given barrel over the period of its maturation. (We assume, and hope, there are some drunk angel’s up there enjoying their take of our hard labors.) A bottle of wine with a porous cork enclosure will experience a similar protracted evaporation.
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And while ullage generally is just a natural byproduct of aging wine, it can potentially endanger good wines, since the more you lose, the more oxygen is in the bottle, the more sped up the aging process. If the ullage goes below the shoulders of the wine (where the wine neck bows out), that might mean too much ullage.
Bear in mind, unless you’re buying older bottles of wine, you won’t likely encounter ullage. But if you are shopping in the aged wine market, you should remember to check on your bottle and the amount of ullage. The “correct” amount depends on the bottle, and even then there’s some debate. So it’s complicated. But the good news is, you get to refer to ullage a lot, and sound really cool.