How long does opened pesto really last in the fridge?

The short version: a shop-bought jar of pesto, opened and kept cold with its oil layer more or less intact, is usually good for around a week to ten days. Homemade pesto is a shorter and slightly different question — closer to five days in the fridge, sometimes less — because a homemade jar of basil, garlic and oil is exactly the kind of mixture food-safety guidance singles out by name, for reasons that have nothing to do with whether it still smells like basil.

Why pesto does not behave like ketchup

Ketchup, soy sauce, mustard and most bottled condiments keep for months once opened because they were built to: vinegar, salt, sugar and heat processing all push back against bacteria before the jar ever leaves the factory. Pesto was never built that way. It is fresh basil, garlic, pine nuts or another nut, hard cheese and olive oil, blended and put straight into a jar. Even a commercial jar with a long unopened shelf life is usually relying on refrigeration, a preservative, or both once it is opened — read the lid, because "keep refrigerated after opening" on a jar that sat on a room-temperature shelf before you bought it is not a formality.

The oil on top is doing a job

Olive oil floating over the surface of a jar of pesto is not separation to be embarrassed about. It is a rough barrier between the paste underneath and the air above, and air is what turns bright green basil a dull, brownish olive colour through ordinary oxidation. Pushing the pesto back under its own oil layer after every use, or topping up with a thin film of fresh olive oil if a scoop has broken through it, genuinely slows that down. A spoon dragged straight from a pan of pasta sauce back into the jar defeats the point in one motion, because it introduces both new bacteria and a gap in the oil seal at once.

The one risk that is not about spoilage

This is the part worth actually understanding rather than skimming past. Clostridium botulinum, the bacterium behind botulism, does not need air to grow — it thrives specifically in low-oxygen conditions, and it does not reliably change the smell, taste or look of what it is growing in. Garlic submerged in oil is a textbook example of that kind of environment, which is why food-safety guidance treats any garlic-in-oil mixture — and homemade pesto is one, even though nobody thinks of it that way while blending it — as something to refrigerate promptly, use within about a week, or freeze, rather than something to leave on a counter or keep going indefinitely on the strength of how it smells. Commercial jars get around this with acidification, pasteurisation or preservatives that a home kitchen does not have access to, which is the actual reason a shop-bought jar can carry a longer stated shelf life than a homemade one that looks identical.

What normal aging looks like, and what does not

Browning across the surface, or the basil deepening from bright green toward olive, is oxidation — a quality change you can usually stir back into something perfectly usable, not a sign to throw the jar out. What actually means a jar is finished is different: mould in any colour, anywhere on the surface or up the inside of the glass; a lid that has started to bulge, dome, or hiss gas when opened; or a smell that has turned distinctly sharp or off, rather than just faded and less basil-like than day one. Those signs are rarer in pesto than the browning is, which is exactly why they are worth trusting completely the one time they show up, rather than explaining away.

The freezer is the easy way out of this whole question

Pesto freezes well, and freezing sidesteps the entire clock rather than racing it. Spoon it into an ice-cube tray, freeze until solid, then tip the cubes into a bag — each one is roughly a portion, ready to drop straight into a pan without thawing first. This is worth doing on the day you buy or make a jar you know you will not finish within a week, rather than waiting to decide once the browning has already started.

This is close to the gap Munch's opened-item tracking is built for. Marking a jar as opened starts its own countdown from that day, separate from whatever date is printed on the lid, visible to everyone in the household — so a jar of pesto pushed to the back of the shelf is not relying on anyone's memory of which Tuesday it was actually opened.