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

The environment in which foods are packaged is pretty sterile. There won’t be a lot of microbes in the air. Some products are heated after packaging to destroy any contaminants that may have happened. Some products are injected with nitrogen rather than oxygen, taking away chance for living organisms to be retained.

Once you open the package, you’re introducing mold spores from the air, potentially bacteria, cross contamination, etc.

I drink ultra-high temp pasteurized milk, and it lasts a very long time even after opening because I rarely expose it to the air for more than a quick second.

Lunch meat is easier to contaminate, for example, because some meat remains in the container/bag that you’ve touched already, thereby introducing bacteria.

But anyway- generally there’s just not a lot of exposure to contaminants during the production process; and they have industrial filtering systems in place to reduce the contaminants in the air as much as possible. Items where this is harder to prevent are often heated after packaging to destroy bacteria. (Sorry now I’m repeating myself so I will stop lol!)