Most manufacturers advise that, once opened, ketchup should be stored in the fridge to maintain its freshness. However, due to the sugar and vinegar content of the condiment, it will still be safe to eat if it’s kept in the cupboard for a time, but the taste or colour may alter slightly. Ketchup used to contain more salt, which meant it remained well preserved when stored in a cupboard. But with today’s versions containing less salt, putting it in the fridge is now advised.
Answered by Jo Stass for Brain Dump in How It Works issue 101.
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