Mayonnaise

Unopened commercial mayonnaise keeps for months in the pantry until its date. Once opened, it goes in the refrigerator and keeps about 2 months.

Always use a clean utensil. Double-dipping with a knife carrying bread crumbs or butter is the main way a jar of mayonnaise actually spoils — the mayonnaise itself is quite stable, but the things you introduce into it are not.

Keep the rim and lid clean and screw the cap down properly. Dried mayonnaise around the rim is where mold starts.

Mayonnaise does not freeze. The emulsion breaks irreversibly and it thaws into a separated, curdled mess.

Homemade mayonnaise is a different product and should be refrigerated immediately and used within 3–4 days, since it is made with raw egg.

At a picnic or buffet, the food mixed with mayo — not the jar — is usually what fails the picnic two-hour rule (2 hours, or 1 hour above 90°F).

Mayonnaise has spent a century being blamed for picnic food poisoning, and the reputation is almost entirely undeserved.

Commercial mayonnaise is made with pasteurized eggs and acidified with vinegar or lemon juice to a pH of roughly 3.6–4.0. That is below the threshold at which foodborne pathogens can grow — around pH 4.6 — which makes commercial mayonnaise mildly antimicrobial in its own right. Studies of mayonnaise-containing salads have repeatedly found that adding commercial mayonnaise slows bacterial growth rather than accelerating it.

So when a potato salad or macaroni salad causes illness after a barbecue, the culprit is almost always the potatoes, pasta, eggs, or chicken — all near-neutral, protein- or starch-rich foods — combined with hours at warm temperatures and repeated handling. The mayonnaise is the innocent bystander that happens to be visible.

Homemade mayonnaise is genuinely different, because it uses raw egg with no pasteurization step and often less acid. That carries a real Salmonella risk and warrants the short refrigerated window.

An opened jar fails from contamination introduced on utensils, and from slow oxidation of the oil. Discard on any mold, a sharply rancid or sour smell distinct from the normal tang, a darkened yellow color, or separation that will not stir back together.