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

Its not just exposure to oxygen, its continued exposure, Bacteria needs 5 things to live and breed, Oxygen, Moisture, Warmth, Time and Food. Obviously you can’t deny them Time or Food and Moisture is kinda hard as most food items will naturally have some degree of Moisture, so Warmth and Oxygen are the main we ways we can control it.

Which is why Food is either kept in the Fridge or Freezer to inhibit the growth and many items are sealed to prevent more oxygen getting in. You can go further by either Vacumn sealing the items by sucking out all the air so there will be no more oxygen or Nitrogen sealing which I’m not super familiar with but I think it works by sealing the item in a Nitrogen rich environment for the same purpose.

I work in a Kitchen and we have a Vacumn sealer machine and it puts so much life onto our items, especially meat and Fish