system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If fruit juice is highly acidic, and the acid in citrus fruits (citric acid) is used as a preservative, why does freshly-squeezed citrus juice only last a couple of days in the fridge?
gpt
It's the oxygen present that allows microbes to metabolise the contents of the juice.

Citric acid is a useful preservative in food packaged with minimal oxygen, but it's pretty ineffective when there's a good supply of oxygen.

That's why things spoil quicker once the packaging has been opened. It's not really to do with introducing microbes, it's to do with introducing fresh air!