Does vinegar go bad?

Short version: vinegar does not go bad in any sense that makes it unsafe to use, opened or not, past whatever date is printed on the cap. It is one of the few things in a pantry built, from the very start, to resist exactly the kind of spoilage most food guidance is about — bacteria, mould and yeast cannot get a foothold in something this acidic, which is the whole reason people have used it to preserve other food for thousands of years. What changes in an old bottle is flavour and appearance, not safety, and even that change is slow enough that most bottles are used up long before it becomes noticeable.

Why vinegar is hostile to spoilage rather than vulnerable to it

Vinegar is dilute acetic acid, typically sitting somewhere around pH 2 to 3 — strongly acidic, in the same rough range as lemon juice. Almost every organism responsible for food spoilage, and the ones responsible for foodborne illness, cannot grow in an environment that acidic. That is not a coincidence or a modern discovery: it is the entire reason pickling exists as a preservation method, and why vinegar shows up as an ingredient specifically to make other, more vulnerable food shelf-stable — cucumbers, onions, eggs, whole fish, all made safer to keep by being submerged in something bacteria cannot tolerate. A bottle of plain vinegar sitting in a cupboard is not waiting for something to go wrong the way a carton of milk is. It is already the environment that stops that from happening.

What actually happens over months and years

Left in a cupboard, even for a long time, a bottle of vinegar mostly just becomes a slightly different version of itself. Flavour can mellow or flatten a little as the most volatile aromatic compounds slowly escape, especially once a bottle has been opened and re-capped many times. Colour can shift subtly, usually darkening a shade. None of this is spoilage in the sense that applies to anything with water and nutrients available for microbes — it is a slow chemical drift closer to what happens to a spice jar over a couple of years than to what happens to leftovers in the fridge. A bottle that has sat at the back of a cupboard for a long time is far more likely to disappoint on flavour than to be unsafe to use, and most people would not notice the difference in a dish it is mixed into anyway.

The cloudy strands some bottles develop

Every so often an opened bottle, especially an unfiltered or raw one like a cloudy apple cider vinegar, grows a stringy, cobweb-like mass floating near the surface or settled at the bottom. This is usually what is called the "mother of vinegar" — a jelly-like structure made of cellulose and acetic acid bacteria, the same organisms that turn alcohol into vinegar in the first place. It looks alarming the first time anyone notices it, closer to something growing in a jar of jam, but it is a natural byproduct of vinegar fermentation rather than a sign the bottle has gone off. It is harmless, and can simply be strained out if the texture is unwelcome, or left in — some people specifically keep a mother around to start a new batch of homemade vinegar. Filtered, pasteurised vinegars like standard white distilled vinegar rarely develop one, since the bacteria that form it were removed or deactivated during processing.

Not every vinegar behaves quite the same way

Plain white distilled vinegar is close to pure acetic acid and water, with almost nothing else in it for anything to act on, which makes it the most stable version on the shelf — it can sit for years with essentially no meaningful change. Wine, cider and rice vinegars carry a little more residual sugar, colour and flavour compound from whatever they were fermented from, so they drift slightly more over time and are the ones more likely to grow a mother if left unfiltered and unrefrigerated. Balsamic vinegar is its own case: a real aged balsamic is designed to change over years, deepening and thickening as it slowly concentrates in wooden barrels, and that transformation is the entire point of it rather than a decline to guard against. None of these differences change the underlying picture — acidity is still doing the same protective job in every one of them — they only change how much the flavour drifts and how fast.

Does it need the fridge?

No, and refrigerating it does not meaningfully extend anything, because there is no spoilage clock running for the cold to slow down. A cool, dark cupboard away from the stove keeps the flavour at its best for longest, in the same way it does for olive oil, mainly by limiting how fast the aromatic compounds fade and by keeping the cap from being exposed to heat that could very slightly accelerate flavour loss. Some people do keep vinegar in the fridge purely as a matter of taste preference, particularly a flavoured or infused vinegar with herbs or fruit added, where the fresh ingredient itself is the part with a real shelf life. That is a choice about the food suspended in the vinegar, not about the vinegar protecting it any less.

What is actually worth checking

• The bottle itself, not a printed date. A "best by" date on vinegar is a quality estimate for peak flavour, not a safety line — plenty of people use a bottle well past it without any issue.
• Cloudy strands are not a reason to throw it out. That is the mother of vinegar, a natural fermentation byproduct, and it can be strained out or ignored.
• What matters is what got into the bottle, not the vinegar itself. A vinegar used repeatedly as a dip, where crumbs or other food have dropped in and stayed submerged, can support growth from that contamination in a way plain vinegar on its own cannot — that is a reason to replace a bottle, and it has nothing to do with how long vinegar itself lasts.
• Flavour, not safety, is the actual test. If an old bottle tastes flat or muted compared to a fresh one, that is a reasonable reason to replace it — it is just not a safety reason.

Because it genuinely does not go bad, vinegar tends to become the opposite kind of pantry problem: not the thing anyone worries is spoiling, but the half-used bottle that gets forgotten behind the oil and quietly duplicated on the next big shop. Munch tracks pantry items by where they are stored and shares that list with everyone in the household who cooks, so a bottle that has been sitting for a year still shows up before a second one gets bought to stand right next to it.