How do things expire once you open them/ expose them to oxygen when they clearly had to be exposed to air before being sealed?

Question

Like milk goes bad a week or two after opening it but if you don’t open it, it will stay good until the expiration date? Like yogurt, sour cream, shredded cheese. All those things. I’m confused

In: Other

Answer 1

Lots of answers explaining a part of it, or why it works in one circumstance. But really the whole thing comes down to one strategy: **minimize the amount of stuff that causes spoliation (like bacteria and oxygen) that exists in the package**.

There are a ton of tactics, depending on what you’re packaging. But a lot has to do with sanitary environments, packaging technology (such as replacing air with oxygen in it with nitrogen before sealing the package), and post-packaging treatments (like pasteurizing something that’s been sealed, so that any bacteria is dead and there’s no path for more until the seal gets broken).