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

Things like canned food and jarred stuff are sealed, THEN subjected to heat (or sometimes radiaton), so any bacteria in the product is killed, and since it’s sealed, no more can get in.

Since all the microbes are dead, little to no spoilage of the food happens.. the problem isn’t the air, but what’s in the air.

Once you open it, you reintroduce new microbes and they begin breaking down the product.