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

Pasteurization is how a lot of that works. That’s how beer, wine, canned goods, milk, and so on last so long.

Some dry goods, however, like potato chips will fill the bag with low humidity nitrogen instead of air. That keeps them fresh for months on the shelf even though they go stale in days if you leave them open.