I’ve seen this in various newspapers today (including online ones). The BBC refers to it as a “Ketchup debate”, but in reality there is nothing to debate. The silly saga has come about because an Asda branch in London has started stacking it in chiller cabinets instead of on the shelves with other condiments, as they have always done until now.
Irrespective of what the so-called experts have said in the articles, which is very little, ketchup is cooked during manufacture and packed into bottles while it is still at about 90°C. It is deaerated first to further reduce the risk of microbial growth, and it contains vinegar, salt, and sugar, which help act as preservatives. The bottles are sealed and cooled, so it is under a partial vacuum until someone opens it to use it. In effect, until the seal is broken, it is pretty much sterile.
To that end, unopened ketchup can – and should – be stored at room temperature, with no adverse effects. Storing it in the fridge is a waste of time and money, and since it uses more energy it’s also negative for the environment.
Once opened, bacteria and yeasts immediately get inside the bottle. It’s even worse if it’s café ketchup, because it will probably be a cheaper brand, and all the dirty scumbags who seem to frequent cafés will have poked their used knives inside to dislodge it (Heinz themselves encouraged this in one of their stupid adverts – ketchup used to be as thick as putty, but subsequent reformulations have rendered it more like thick soup, making knife-poking a pointless exercise).
Dirty practices, natural contamination, and cost-cutting and pseudo-health driven reformulations which have reduced the preserving power of ketchup, mean that it ferments in the bottle once opened – readily when warm, but even when stored in the fridge, though to a much lesser extent. Unless someone has poked something nasty into it with the knife they’ve previously had in their mouth, this fermentation is usually harmless. But it does mean that any ketchup stuck in the neck of a bottle is likely to leap out at you next time you open the lid. It’s happened to me a few times in the past, which is why I always correctly store part-used ketchup in the fridge.
As if this weren’t enough, Heinz themselves state you can store unopened ketchup in a cupboard, but you should refrigerate it once opened and use it within (I think) eight weeks, though eight weeks is a bit conservative in my experience. So what’s to debate? It’s more a case of letting idiots have their say (a lot like Brexit, really), even though anything other than the Heinz advice (or “remain”) is completely wrong.
That’s what’s the matter with the world today! People don’t get told they’re wrong, anymore. They just give dumb opinions which become part of general “knowledge”.