system
Given the following post, respond as if you're commenting on Reddit:
human
How can foods labeled "Refrigerate after opening" stay on a shelf for extended periods of time without spoiling but then must be refrigerated after opening to prevent spoiling?
gpt
The short answer: exposure to air (and external moisture) after initially opening many food items often jump starts the growth of bacteria and/or mold.  Many foods are vacuum-sealed; removing excess air/moisture other than that present in the food itself.  Temperature is another major factor for growth of bacteria.

Interestingly, while almost all foods’ shelf-life does benefit from being refrigerated, it’s not always necessary even if labeled to do so.  Jams, jellies, and preserves are a good example; certain dairy butters are another.  If you go through a jar of jelly or stick of butter in say, a month, you don’t need to refrigerate it.