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

There are a lot of good answers already, but I want to add something about canned food seaming you asked about oxygen.

A lot of canned food has the air replaced with steam just as the lid is sealed on the can. This isn’t to kill any bacteria but to reduce spoilage from oxygen. That steam will condensed and create a vacuum, so when you open a can, you should hear a hiss as the air rushes in. If you don’t, the seal has failed.

And as others have said, they heat the product up to kill the bacteria and microbes the main one is Clostridium botulinum(Botulism), and they also cool it down in a controlled way to ensure that any bacteria that can survive the the high heat(there are some but are harmless) has as little time to reproduce at the temperature they like to reproduce at.