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

Chiming in to add another point. I used to operate a brewery canning machine. Before being filled with beer, each can gets purged with carbon dioxide. So before the lid gets popped on and seamed, there’s a cushion of beer foam and CO2, rather than oxygen.