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

On some products, you’ll see a note on the label that says “Packaged in a protective atmosphere.” This means that right before it was sealed, it was placed in an oxygen-free environment, so whatever air space is in the container is filled with an inert gas like nitrogen. This is done with products like bread which can’t be re-heated after being cooked, and also products that would oxidise on their own without bacterial intervention, e.g. potato chips.

Other products, often canned goods, are heat-treated (e.g. Pasteurisation) or irradiated after sealing, which sterilises the product within and the can provides the protection. I watched some fascinating YT videos on food irradiation where the operators irradiated themselves by completely ignoring the safety protocols or defeating them entirely (“Balls!”).