Mustard

Unopened mustard keeps 1–2 years in the pantry. Opened, it keeps about a year refrigerated and several months at room temperature — mustard is among the most durable condiments there is.

Refrigeration is largely about preserving pungency rather than preventing spoilage. The volatile compounds responsible for mustard's heat dissipate over time, faster when warm, so an old jar goes mild rather than dangerous.

Keep the cap clean and closed. Dried mustard around the rim can host mold even though the mustard itself will not.

A dried, darkened layer at the surface is oxidation. Scrape it off or stir it in.

Mustard does not freeze usefully — the emulsion separates.

Mustard is arguably the most self-preserving condiment in a kitchen, protected by three independent mechanisms.

Acidity from vinegar brings it to roughly pH 3.3–3.6, well below the growth threshold for pathogens. Salt lowers water activity. And most distinctively, mustard seeds contain glucosinolates which, on grinding and hydration, release allyl isothiocyanate — the compound responsible for mustard's pungency and a genuinely potent antimicrobial in its own right. Mustard essential oil is effective enough against bacteria to have been studied as a natural food preservative.

Between the three, mustard is close to inhospitable, and there are no documented cases of mustard being a vehicle for foodborne illness.

What does change is flavor. Allyl isothiocyanate is volatile and dissipates with time and warmth, so old mustard loses its heat and turns flat and dull. Refrigeration slows that considerably, which is the real argument for keeping it cold.

The color also darkens through oxidation, and a crust forms where the surface dries.

Discard only on visible mold — usually at the rim where residue has diluted the acidity — or a genuinely rancid or fermented smell. Separation of liquid at the top is normal; stir it back in.