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

>when they clearly had to be exposed to air before being sealed?

Because when they’re sealed, they’re not exposed to oxygen. It’s either under a vacuum, or they put a gas in there. For example, Pringles tubes are filled with nitrogen. If bacteria need oxygen, and there’s no oxygen, then bacteria don’t do anything.

Another reason is that the bacteria is killed. Canned tuna or milk are examples of this. If the bacteria is killed, it’s harmless to you.

Things like chips and cookies are packaged in a dry environment.

Now once you open the container, it’s exposed to oxygen moisture and bacteria that’s floating around in the air, which will now allow bacteria to grow, food to go stale, etc.