Does mayonnaise need to be refrigerated?

An unopened jar of commercial mayonnaise is manufactured to sit on a shelf, and many stores do exactly that with it, next to the ketchup rather than in a chiller case. Once it is opened, the fridge does become the sensible place for it — but not for the reason most people assume. Mayonnaise is not a fragile, high-risk food that spoilage is constantly threatening. It is an emulsion built around a genuinely acidic core, and that acid is doing real protective work before the fridge ever gets involved.

Why the jar is more defended than it looks

Commercial mayonnaise is oil and pasteurised egg held together by vinegar or lemon juice, and the acid is not there only for flavour. It pulls the jar's overall acidity down to a level most spoilage bacteria simply cannot get established in, well below what a lot of fresh food sits at. That is the actual reason a sealed jar can be shelf-stable before opening: manufacturers formulate specifically to land in that acidic range, and it is tested, not guessed at. A homemade batch made to your own taste, with less vinegar or a milder lemon, does not carry the same guarantee — which is part of why the two are really different foods wearing the same name.

The picnic reputation is mostly misplaced

Mayonnaise is the first thing blamed when a potluck goes wrong, and it is usually the wrong culprit. A bowl of potato salad or chicken salad left out on a warm table is a mixture of several ingredients, and the ones actually most hospitable to bacteria are typically the potatoes, the eggs or the chicken — not the acidic dressing binding them together. What actually causes trouble is the whole dish sitting for hours in the range bacteria multiply fastest in, regardless of which ingredient gets pointed at afterward. The acid in the mayonnaise is, if anything, one of the more defended components in the bowl. This does not mean an opened jar left out all afternoon is fine — heat and time work against it too — but it explains why singling out the mayonnaise specifically has never quite matched what is actually going on in that bowl.

What the fridge is doing once the jar is open

Refrigeration after opening is mainly slowing two things: the oil separating out of the emulsion, and the slow decline in flavour and texture that follows air, light and repeated dipping of a knife or spoon into the jar. It also adds a second layer of protection on top of the acid, keeping bacterial growth slower over the weeks a jar realistically sits in a fridge door being used a little at a time. Most commercial jars keep well for a couple of months once opened and kept cold, though a kitchen that runs warm, or a jar used with a utensil that has touched raw meat, will not stretch that far — cross-contamination from a shared knife undoes the acid's advantage in a way the acid itself cannot fix.

Homemade mayonnaise is not the same food

Everything above describes a commercial jar, formulated and tested to hit a specific acidity. A homemade batch — raw egg yolk whisked with oil and whatever vinegar or lemon you happened to add — has no such guarantee behind it. The acidity depends entirely on how it was made, the eggs are not pasteurised the way commercial ones are, and there is no factory testing standing behind the result. Homemade mayonnaise and anything built on it, like a fresh aioli, belongs in the fridge immediately and is realistically a few-day food, not a few-month one. Treating it like the jar from the supermarket door is the one genuine mistake to avoid here.

What actually helps

• An unopened commercial jar can wait in the pantry until you need it; check the label rather than assuming, since some brands and some countries do sell it refrigerated.
• Once opened, the fridge is worth doing — not because the jar is suddenly dangerous, but because cold slows the oxidation and texture change that follow opening.
• Use a clean utensil, every time. This matters more than any number on the label; a knife that touched raw chicken and then the mayonnaise jar has done more damage in one dip than weeks of ordinary storage.
• Homemade mayonnaise runs on its own, much shorter clock — refrigerate it the moment it is made and treat it as a few-day food.

This is the kind of distinction that is easy to know in the abstract and still lose track of in practice, which is where Munch is useful: marking the jar opened starts its own countdown from that date rather than the one printed on the lid, visible to the whole household, so a jar someone opened three weeks ago does not quietly blur into one opened yesterday.

The short version worth keeping: an unopened jar is shelf-stable by design, an opened one belongs in the fridge mostly for quality rather than a safety emergency, and the picnic-table reputation mayonnaise carries has more to do with everything else in the bowl than with the jar itself.