How long do opened condiments really last in the fridge?

The short version: a bottle of ketchup, mustard or soy sauce keeps for months in the fridge after it has been opened, while a jar of fresh pesto, salsa or mayonnaise-based dressing is realistically finished within a week or two. That is not a difference in quality or brand. It comes down to what is actually in the jar — some condiments are built with their own defence against bacteria, and some are just fresh food that happens to be sold in a jar shape.

Why the label rarely gives you a number

Almost every bottle in the fridge door says some version of "refrigerate after opening" and stops there, without saying for how long. That is not the manufacturer being vague. They genuinely cannot print a countdown, because they do not know when you will open it — the best-before date on the cap describes the sealed bottle, and a sealed bottle of most condiments keeps for a long time in a cupboard. The moment air gets in, a different clock starts, the same way it does for a carton of milk, and nobody can print that date in advance because it depends on the one thing the factory has no way of knowing: the day you actually broke the seal.

What actually separates the two groups

Some condiments were essentially invented as preservation methods, long before refrigeration existed, and they still behave that way.

• Vinegar and salt. Ketchup, mustard, most hot sauces, soy sauce, pickles and pickle brine are acidic or salty enough that bacteria struggle to get established at all. That is why an opened bottle of soy sauce sitting at the back of the fridge for four months is not really taking a risk — the environment inside the bottle is hostile to begin with, and refrigeration is mostly there to protect flavour and colour, not safety.
• Sugar. Jam and thick sweet sauces work the same way from the other direction; a high enough sugar concentration ties up the water bacteria need, which is why an opened jar of jam lasts for months rather than days.
• Oil-and-egg emulsions. Mayonnaise, and anything built on a mayonnaise base — most creamy salad dressings, aioli, some dips — is a different structure entirely. It is a stable emulsion of oil, egg and a little acid, and while the acid does some protective work, it is nowhere near enough to put it in the same league as vinegar-based condiments. A jar of mayonnaise is closer to an egg product than to a pickle, and it is generally best used within a month or two of opening, sooner in a warm kitchen.
• Fresh ingredients in a jar. Pesto, salsa, fresh guacamole-style dips and hummus were never processed to be shelf-stable at all. They are fresh basil, tomato, onion, chickpeas and garlic that happen to be sold pre-made. Once opened, they behave like the fresh food they actually are — usually good for about a week, sometimes less — regardless of how confident the jar on the shelf next to them looks.

The pattern underneath all of this is simple: the more a condiment relies on vinegar, salt or sugar to do the preserving, the longer an opened jar of it holds. The more it relies on oil, egg, dairy or fresh produce, the shorter that window really is, no matter how it is marketed or where it sits on the shelf.

The knife in the jar matters more than the calendar

There is a second factor that has nothing to do with what is inside the jar and everything to do with how it gets used, and it matters especially in a shared kitchen. A knife that spread mayonnaise on a sandwich after cutting raw chicken, or a spoon dipped into salsa after stirring a pot, introduces bacteria that were never part of the original recipe — and that single moment can undo weeks of an otherwise reasonable shelf life. This is why two jars bought on the same day, used by different people in the same house, can end up in very different states: the printed guidance assumes a clean utensil every time, and in practice that assumption is the one that actually gets tested first.

Separation is not spoilage

Oil rising to the top of a salad dressing, or a thin watery layer sitting on top of salsa or nut-based sauces, looks alarming but is usually just physics — ingredients that do not naturally stay mixed have settled back into their separate layers, and a shake or a stir puts them right back together. What actually signals a jar is finished is different: mould anywhere on the surface, a lid that has started to bulge or fizz from gas building up inside, or a smell that has turned distinctly off rather than just faded. For the vinegar-and-salt group especially, those are rare events precisely because the jar's own chemistry works against them — which is also why, when one of those signs does show up, it is worth trusting completely rather than second-guessing.

What actually helps

• Write the day you opened it, not the day you bought it. The same trick that works for milk works here, and arguably matters more, because a bottle of soy sauce can sit in the fridge for so long that "when did I open this" becomes a genuinely hard question by the time it matters.
• Keep the short-window jars where you will actually see them. Mayonnaise, fresh dips and opened dressings belong somewhere you will notice them regularly, not pushed to the back behind the ketchup that is fine to ignore for months.
• Use a clean utensil, every time. This single habit does more for a shared jar's real shelf life than any date on the label.
• Do not let a long shelf life become an excuse to stop checking. A bottle of hot sauce being safe for months is not the same as it being at its best for months — flavour fades well before safety becomes a question.

This is the exact gap Munch's opened-item tracking is built for: marking something as opened switches it onto its own, shorter countdown from that day, separate from the date on the unopened item, so the mayonnaise you opened on Tuesday is not quietly relying on the same clock as the ketchup that has been fine in the door since spring.

None of this needs a memorised table of forty condiments. It needs one habit — noting the day you opened it — and a rough sense of which of the two groups a jar belongs to: built to last, or fresh food that just happens to be in a jar.