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 is no “expiration date” on many perishable foods like dairy products, only “sell by” dates. This is so you basically know the store is following the rules and not holding on to these items indefinitely when they go past these dates because they haven’t sold them yet. It just keeps the stores honest.

As to why they expire when exposed to oxygen: The Bacteria that can hurt you need oxygen to grow. When you let in the oxygen, you let them grow faster. Eventually, they are in large enough numbers that they can hurt you. The chemicals they release are toxic to you and they smell bad so don’t eat food that smells bad.