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

Food spoils when bacteria eats it. The bacteria (or at least most types in question here) require oxygen. A sealed gallon of milk only has a very small amount of free air in the bottle which only allows for bacteria to grow for a very short time period of time before depleting the oxygen supply. There’s also very little bacteria initially in the bottle because the milk has been pasteurized. So limited oxygen, minimal microbes to start a colony. When you open the milk, you introduce fresh air and it reinvigorates whatever bacteria is in the milk. You’re also pouring milk out so when you reseal the lid, there’s more oxygen in the bottle than when it was sealed.