Does ketchup need to be refrigerated?

Short answer: not before opening, and not urgently after either, for an ordinary bottle. Ketchup stands on a normal shelf in every grocery store, next to the mustard rather than behind chiller glass, and that is not a technicality anyone is getting away with — the bottle is built to sit there. Tomato concentrate, vinegar and a substantial amount of sugar combine into something most spoilage bacteria find genuinely hostile before refrigeration has done anything at all. The "refrigerate after opening" line printed on the cap is real and worth following, but it is mostly there for a different reason than the one people assume.

Two defences instead of one

Vinegar is doing the same job here it does in a jar of mayonnaise: it pulls the whole bottle's acidity down to a level most spoilage bacteria cannot get established in. Ketchup does not stop there, though. It also carries a substantial amount of sugar, and sugar works the way it does in a jar of jam — drawing water out of anything nearby through osmosis, which starves bacteria of the moisture they need regardless of how acidic things already are. Acid and sugar are two independent mechanisms, not one mechanism described twice, and a bottle of ketchup gets the benefit of both at once. That is a meaningfully better-defended starting position than either butter's low water content alone or mayonnaise's acid alone, and it is the real reason an unopened bottle can travel from factory to shelf to pantry without ever seeing a fridge.

What "refrigerate after opening" is actually protecting

Opening the bottle does not switch off either defence, so what happens afterward on a counter is mostly a quality story rather than a safety one. Air and light reach the ketchup every time the cap comes off, and the surface nearest the opening slowly darkens and flattens in flavour through the same kind of oxidation that turns butter rancid or soy sauce dull. A squeeze bottle used often barely notices this, because the contents turn over before oxidation has time to do much. A bottle that sits open for months develops a browner, less vivid layer near the neck well before anything is actually wrong with it. The one genuine risk worth watching for is mould at the neck or under the cap, which shows up almost exclusively when a used utensil — a fry dipped straight into the bottle and back onto a plate — carries contamination in rather than from the ketchup spoiling on its own. Refrigeration slows the oxidation and keeps that browner layer from forming as fast; it does not do much that acid and sugar were not already doing about anything more serious.

Not every bottle carries the same margin

Everything above describes an ordinary bottle formulated the standard way. Reduced-sugar and no-sugar-added ketchups remove a real part of the second defence, since less sugar means less of that water-starving effect, so they lean more on the fridge once opened than a standard bottle does. Homemade ketchup made from fresh tomatoes, with less vinegar and less sugar than a commercial recipe, is a different food in the same way homemade mayonnaise is — there is no factory testing behind its acidity, and it belongs in the fridge from the moment it is made, treated as a fresh sauce with a life measured in days rather than months. A specialty bottle bought for one recipe and left mostly untouched afterward also loses margin simply by sitting a long time in whatever spot it was put down, particularly a spot near the stove that runs warmer than the rest of the kitchen.

What actually helps

• An unopened bottle is fine in the pantry until you need it — that is genuinely what it was formulated for.
• Once opened, the fridge is worth doing for flavour and colour more than for danger, especially for a bottle that will sit around being used a little at a time rather than emptied quickly.
• Wipe the neck and cap clean occasionally and avoid dipping a used utensil straight into the bottle — that single habit prevents almost all of the mould risk on its own.
• Reduced-sugar, no-sugar-added and homemade ketchup all deserve the fridge without waiting — they are missing part of the defence a standard bottle relies on.

This is really a question of which bottle is in front of you rather than a rule to memorise, and that is the part Munch is built to keep straight without you having to hold it in your head. Marking a bottle opened starts its own countdown from that date rather than the one printed on the cap, visible to everyone in the household, so the specialty bottle bought for one recipe back in the spring does not sit forgotten at the back of a shelf for a year without anyone noticing.

The version worth keeping: an unopened bottle is shelf-stable by design because acid and sugar are both already working on its behalf, an opened one goes in the fridge mainly to protect flavour rather than to prevent anything dangerous, and the bottles that actually need faster attention are the ones — reduced-sugar, homemade — missing one of those two defences from the start.