I was just over that threshold of legal drinking age the first time I bought a bottle of wine in a restaurant. The waiter asked if I would like it decanted, and I froze, totally unsure of what criteria should drive this decision. I stared, agape, then glanced at my date. She didn't know, either. "No, no, that's okay," I said, a decline-from-panic, and the waiter gave a shrug that read Hey, you're the boss, hoss.
And now, having decanted innumerable bottles into all manner of receptacles, and in many different arenas, I'm still not sure what exactly drives the decision. At least not in any chemical sense.
I mean, I can totally make that call, and without any panic. In fact, I tend to decant a lot of wines, all types, both young and old, from Barolo to Gruner Veltliner and even the occasional Muscadet. On the average, I think I would describe decanting as a default mode. The un-decantables are exceptions. But not everyone agrees with this attitude.
The most obvious reason to decant—to move the wine from its bottle to another vessel—has to do with removing it from its sediment, those little conglomerated particles of grape matter or the crystallized granules called tartrates (basically bits of tartaric acid, fallen out of solution) which are generally present in wines that are not fined, filtered, or cold-stabilized. Sediment in a slightly different form is also increasingly present in red wines as they age, and can range from sour, milky wisps to bitter, chunky shards. Decanting due to sediment is probably as old as any of our wine rituals.
But sediment isn't the most common reason why I decant. More often than not, I am looking to get the wine to perform in a certain way. I am usually trying to relax the wine, to get it to express itself more fully by exposing it to air. Some reds can be tart and tannic straight from bottle, while whites can be drawn-up, linear, too direct, or obtuse. Decanting a white wine, particularly one with high acidity, like a Chablis or Riesling, can help put meat on its bones, temper it, and show greater depth of its fruit, especially if the wine is very cold.
No one seems to fully understand what decanting does to the wine's inner chemical world. A very common piece of sommelier philosophy says that decanting softens tannins, ostensibly by performing an accelerated version of the aging effect, where tannins form larger chemical complexes and become less astringent. But this answer is too easy, and research doesn't actually support it (in fact, chemists have found that decanting leaves tannins chemically unchanged). That softening process, which we call tannin polymerization, happens all too slowly, and requires more oxygen than a wine can absorb in one night on your dinner table.
There is evidence, however, that decanting eliminates certain highly aromatic compounds—some of which are actually good, like fresh grapefruit notes in New Zealand Sauvignon Blanc—but most of which are not so good, like scents of matchstick or even, well, sewage. Perhaps those pungent notes are hiding the nicer, subtler things in the wine, and the nicer things end up balancing astringent tannins, so they aren't quite as perceptible.
I will spare you any of my own half-baked chemical suppositions, mostly because I think this topic is one where experience, not lab work, rules the day. Those of us who decant readily know for a fact that it changes wine's behavior, and whether these changes appeal to you may just be a matter of taste. The wine begins another evolution, and decanting lets you pick which section of its bandwidth you want to experience. Eyeballing that spot can be tricky, but in the end it is totally up to you. You are, indeed, the boss, hoss.