How long does opened mayonnaise last in the fridge?

Opened mayonnaise, kept properly cold, is usually good for somewhere between one and two months when it's a store-bought jar — a long window by condiment-door standards, and one that quietly changes as soon as a spoon has been anywhere near raw meat. Homemade mayonnaise is a completely different food safety-wise, and belongs on a clock closer to three or four days. The date on the lid, either way, describes a jar nobody has opened yet.

Why a store-bought jar starts from such a generous number

Commercial mayonnaise is an emulsion of oil and pasteurised egg held together by vinegar or lemon juice, and that acid does real work: it pulls the jar's overall acidity down to a level most spoilage bacteria cannot get established in. That is the reason a sealed jar can sit on a supermarket shelf rather than in the chiller case, and it is also why an opened jar, kept cold, keeps going for weeks rather than days. The acid does not stop working the moment the lid comes off — it is a property of what is in the jar, not of whether the seal is intact.

What actually shortens it

A knife that has touched something else first. This is the one factor that matters more than every other number in this article combined. A knife or spoon that spread mayonnaise on a sandwich after cutting raw chicken, or after sitting in a bowl of tuna salad, carries bacteria into the jar that the acid was never tested against introducing directly. One careless dip can undo weeks of an otherwise reasonable shelf life, which is why two jars opened on the same day in the same kitchen can end up in genuinely different states a month later.

Where it sits in the fridge. The door feels like the natural home for a condiment jar, but it is also the warmest, least stable part of the fridge — it swings toward room temperature every time the door opens. A middle shelf toward the back holds a steadier temperature, and mayonnaise, like most things with a multi-week shelf life, benefits from that consistency more than a fast-turnover food would.

How warm the kitchen runs. The one-to-two-month figure assumes a fridge doing its job properly. A kitchen that runs warm, or a fridge that sits closer to 7–8°C than to 2–4°C, shortens that window without anything about the jar itself changing.

Homemade, always. A batch whisked from raw egg yolk, oil and whatever acid you happened to add has no factory testing behind its acidity and no pasteurisation behind its egg. Refrigerate it the moment it is made, and treat it as a three-to-four-day food regardless of how the store-bought jar in the same fridge behaves.

How to judge a jar yourself

• A little oil pooling on top, or a slightly runnier texture near the surface, is usually just the emulsion settling and not a sign anything has gone wrong. Stir it back in.
• A distinct yellowing or darkening right at the surface, different from the pale colour underneath, is worth taking seriously — it often shows up before a smell does.
• Any mould, in any amount, means the jar goes. Mould on a moist emulsified food is not contained to one spot the way it can be on a block of hard cheese.
• A sour or distinctly "off" smell, beyond the normal sharp tang of vinegar, is the clearest single signal. Trust it over the date on the lid.

Freezing does not really work here

Unlike a lot of things that go on to live a useful second life in the freezer, mayonnaise is not one of them. Freezing breaks the emulsion — the oil and egg that were holding together separate out on thawing, and no amount of stirring puts a broken emulsion back the way it was. A jar that clearly will not get finished is better used up in cooking over the next week or two — stirred into a sauce, a dressing, a batch of deviled eggs — than frozen for later.

The habit that actually makes the one-to-two-month number reliable is the same one that helps with a carton of milk or a tub of hummus: write down the day you opened it, because "sometime last month" and "a couple of weeks ago" look identical once the jar is half empty. This is close to what Munch's opened-item tracking is built for — marking a jar as opened starts its own, shorter countdown from that date instead of the one printed on the lid, visible to everyone in the household rather than only to whoever unscrewed the cap first.