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

Worked as an engineer for a packaging machinery manufacturer. Almost everything you purchase has the oxygen flushed out with a big puff of nitrogen. Residual oxygen is 1-4% depending on how much the company wants to spend on nitrogen, how fast you package and the “fluffiness” of the product to get flushed with N2.