Ketchup

An unopened bottle of ketchup keeps about a year in the pantry. Once opened, refrigerate it — it keeps about 6 months in the fridge and around a month at room temperature.

Refrigeration here is mostly about quality rather than safety. Ketchup is acidic enough that it will not become dangerous on a counter, but it darkens, loses its bright tomato flavor, and separates faster when warm. Restaurants keep bottles on tables for exactly this reason — they turn over fast enough that it does not matter.

Keep the cap and nozzle clean. Dried ketchup around the opening is the usual site for mold.

Squeeze bottles keep better than glass jars, since less air enters as the contents go down.

Ketchup does not freeze usefully.

Ketchup is a genuinely well-preserved food, and it gets there through three barriers at once.

Acidity is the main one: vinegar brings the pH to around 3.5–3.9, well below the 4.6 threshold under which foodborne pathogens cannot grow. Sugar is the second — a substantial proportion by weight, which binds water and lowers the water activity available to microbes. Salt contributes a third.

Between them, ketchup is effectively hostile to bacterial growth, which is why an unopened bottle sits happily in a warm warehouse for a year and why it is one of the safest things in a refrigerator door.

What does happen is quality decline. Tomato pigments oxidize and darken, turning bright red toward brown-red. The flavor volatiles degrade. And the suspension separates, so a watery layer appears on top — that separation is entirely normal and shakes back in.

The failures worth acting on are mold around the cap or on the surface, which happens where residue dries and the acidity is diluted; a fermented or yeasty smell, suggesting wild yeast has taken hold in the sugars; and bubbling or a bottle that hisses beyond the expected squeeze-bottle vacuum release.

Darkening alone is not a reason to discard.