Balsamic vinegar doesn't spoil in any sense that makes it unsafe to eat. Its acidity is too hostile an environment for the bacteria and mold that ruin most food, so a bottle that's sat in a cabinet for years is still fine to pour. What changes is flavor, not safety — and a few things that show up in the bottle over time look far worse than they actually are.
Why acidity does the preserving
Vinegar runs around pH 2.5 to 3, acidic enough to kill most bacteria outright and starve the rest of anywhere to grow. It's the same chemistry that makes pickling work: push the pH low enough and food stops being a habitat. Balsamic starts from vinegar itself before grape must and years of aging round it out, so the preservation is built in before the bottle is ever opened.
That's the real line to draw. Spoilage means something is alive and multiplying — bacteria, mold, yeast gone rogue. Balsamic's chemistry won't support that, so sitting on a shelf for a decade doesn't make it dangerous. What it does instead is slowly shed the aromatics that made it taste complex, the same way olive oil turns rancid at the flavor level long before it becomes unsafe to eat.
The stringy mass in the bottle is the mother, not mold
Every so often a bottle grows what looks like a small jellyfish — a translucent, rubbery mat floating near the surface. That's a vinegar "mother," a colony of acetic acid bacteria that builds a cellulose raft as it works. It's the same organism that turns wine or cider into vinegar, and it only shows up when a bottle still has a trace of alcohol or sugar left along with some air.
Commercial balsamic is usually pasteurized and filtered before bottling, which is why most people never see one. Less processed bottles, especially real traditional balsamic that hasn't been stripped of everything alive in it, are more likely to grow one. It isn't dangerous. Strain it out if the texture bothers you, or leave it — either way, the vinegar around it is unaffected.
Grit at the bottom is sediment, not mold either
Balsamic starts as grape must, which makes its sediment closer to wine's than to anything you'd find in a bottle of white vinegar. Tartaric acid, naturally present in grapes, can crystallize out of solution over time into small, harmless flecks winemakers call wine diamonds. Grape pulp that slipped past filtering settles the same way. Give the bottle a shake before you pour, or don't. The sediment just sits there — it isn't doing anything to the liquid around it.
Supermarket balsamic and the traditional kind age differently
The bottle at the grocery store and the small, expensive one behind glass at a specialty shop aren't the same product wearing different price tags. Supermarket balsamic, labeled Aceto Balsamico di Modena, is wine vinegar blended with grape must concentrate and aged for weeks or months, sometimes with caramel added for color. Traditional balsamic, Aceto Balsamico Tradizionale, is nothing but cooked grape must, aged a minimum of twelve years through a succession of progressively smaller wooden barrels, with no vinegar added at all.
That split matters for storage because of what twelve years of aging already does. Traditional balsamic reduces to a syrup along the way, concentrating its sugar until the finished product is nearly as resistant to microbial growth as honey — high sugar, little available water, nothing for anything to grow in. It doesn't need careful handling so much as a bottle that seals well. Supermarket balsamic, thinner and less concentrated, behaves more like ordinary vinegar: stable indefinitely for safety, but its milder flavor fades faster once the bottle is open.
Where to keep it
A cabinet away from the stove, sealed tight, out of direct light. Heat and light both push along the slow oxidation that flattens balsamic's flavor, and a shelf near a hot burner takes more of a beating than most people realize. Skip the refrigerator. Cold thickens balsamic and can nudge a syrupy, sugar-heavy traditional bottle toward crystallizing — a problem that a room-temperature cupboard never creates in the first place.