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, medicine, etc can oxidize when exposed to air, becoming less attractive, less flavorful, or less nutritious. (Though rarely if ever does oxidization make it dangerous.) It’s different from rotting or becoming contaminated; the chemicals in the product are reacting with oxygen and being changed.

For example, when you cut an apple in half and let it sit for several minutes, the exposed flesh starts to turn brown. This is the apple’s tissue oxidizing.

You need a supply of oxygen for this process to continue. So something that is prepared, and then vacuum sealed, will run out of oxygen to react with and the process will stop. If you cut an apple and then immediately seal it in shrinkwrap, some oxidation will have started to happen, but not enough to be noticeable before you removed the supply of oxygen.